home *** CD-ROM | disk | FTP | other *** search
Wrap
Text File | 1991-12-22 | 171.5 KB | 7,408 lines
.rs .\" Troff code generated by TPS Convert from ITU Original Files .\" Not Copyright (~c) 1991 .\" .\" Assumes tbl, eqn, MS macros, and lots of luck. .TA 1c 2c 3c 4c 5c 6c 7c 8c .ds CH .ds CF .EQ delim @@ .EN .nr LL 40.5P .nr ll 40.5P .nr HM 3P .nr FM 6P .nr PO 4P .nr PD 9p .po 4P .rs \v'|.5i' .sp 2P .LP \fBRecommendation\ X.10\fR .RT .sp 2P .ce 1000 \fBCATEGORIES\ OF\ ACCESS\ FOR\ DATA\ TERMINAL\ EQUIPMENT\ (DTE)\fR .EF '% Fascicle\ VIII.2\ \(em\ Rec.\ X.10'' .OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.10 %' .ce 0 .sp 1P .ce 1000 \fBTO\ PUBLIC\ DATA\ TRANSMISSION\ SERVICES\fR .ce 0 .sp 1P .ce 1000 \fI(Malaga\(hyTorremolinos, 1984; amended at Melbourne, 1988)\fR .sp 9p .RT .ce 0 .sp 1P .LP The\ CCITT, .sp 1P .RT .sp 1P .LP \fIconsidering\fR .sp 9p .RT .PP (a) that Recommendation X.1 defines the international user classes of service in public data networks (PDNs) and ISDN; .PP (b) that Recommendation X.2 defines the international user services and facilities in PDNs; .PP (c) that Recommendation X.3 defines the Packet Assembly/Disassembly facility (PAD) in packet switched public data networks; .PP (d) that Recommendation I.411 defines the reference configurations for access to ISDN services, including Terminal Adaptor (TA) functional grouping; .PP (e) that Recommendations X.30 (I.461), X.31 (I.462) and the I.230\ series, define the circuit switched and packet switched data transmission services available from ISDN (including through Terminal Adaptors); .PP (f ) the desirability for the users to have defined the various possibilities and requirements for accessing the different public data transmission services, .sp 1P .LP \fIunanimously recommends\fR .sp 9p .RT .PP that categories of access for data terminal equipment (DTE) to the data transmission services provided by PDNs and by ISDNs through Terminal Adaptors should be as defined in this Recommendation. .sp 2P .LP \fB1\fR \fBScope\fR .sp 1P .RT .PP This Recommendation defines the different categories of access for data terminal equipment to the different data transmission services provided by public data networks (PDNs) as defined in Recommendation\ X.2 and by ISDNs (including through Terminal Adaptors) as defined in Recommendations\ X.30 (I.461) X.31 (I.462) and the I.230 series, namely: .RT .LP i) circuit switched data transmission services ; .LP ii) packet switched data transmission services ; .LP iii) leased circuit data transmission services . .PP The categories of access described in this Recommendation take into account direct connections (see Note) to public data networks and ISDNs and the various access cases where interworking with other public networks is involved. Access to the packet switched data transmission service via the PAD function as defined in Recommendation\ X.3 is also covered in this Recommendation. .PP \fINote\fR \ \(em\ Direct connections may be provided by means of leased circuits or by dedicated access circuits. .RT .sp 2P .LP \fB2\fR \fBGeneral\fR .sp 1P .RT .PP Access for data terminal equipment to data transmission services may be achieved by either of the following (see Note): .RT .LP a) by direct connection of the DTE to the public data network or ISDNs; .LP b) or by switched connection of the DTE to a PDN via an intermediate public network of another type (including a PDN, PSTN or ISDN); .LP c) or by switched connection of the DTE to an ISDN (including through a Terminal Adaptor) via an intermediate public network of another type. .bp .PP For example, packet mode terminals may access the public packet switched data transmission service, in user classes of service 8 to 11, either directly or via a switched connection. The switched connection will be established using a circuit switched data network or a public switched telephone network. In both switched cases an interworking function will be required to access the packet switched data transmission service. .PP \fINote\fR \ \(em\ It is not mandatory for Administrations to provide all the categories of access contained in this Recommendation. .RT .sp 2P .LP \fB3\fR \fBCategories of access\fR .sp 1P .RT .PP \(sc\ 3.1 specifies the categories of access to the data transmission services provided by PDNs for the direct connection case. .PP \(sc\ 3.2 specifies the categories of access to the data transmission services provided by PDNs for the switched connection case. .PP \(sc\ 3.3 specifies the categories of access to the data transmission services provided by ISDNs through Terminal Adaptors for the direct connection case. .PP \(sc\ 3.4 specifies the categories of access to the data transmission services provided by ISDNs through Terminal Adaptors for the switched connection case. .RT .sp 1P .LP 3.1 \fIDirect connection to data transmission services provided by public\fR \fIdata networks\fR .sp 9p .RT .ce \fBH.T. [T1.10]\fR .ce TABLE\ 1/X.10 .ce \fBStart\(hystop direct connection to a circuit switched\fR .ce \fBdata transmission service\fR .ce (see Notes 1 and 2) .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | cw(48p) | lw(84p) . A1 A2 50 to 200 bit/s 300 bit/s T{ See Recommendations X.20 and X.20\|\fIbis\fR T} _ .TE .nr PS 9 .RT .ad r \fBTable 1/X.10 [T1.10], p.\fR .sp 1P .RT .ad b .RT .ce \fBH.T. [T2.10]\fR .ce TABLE\ 2/X.10 .ce \fBSynchronous direct connection to a circuit switched\fR .ce \fBdata transmission service\fR .ce (see Note 1) .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | cw(48p) | cw(84p) . B1 \ \ \|600 bit/s .T& cw(48p) | cw(48p) | cw(84p) . B2 \ 2\|400 bit/s .T& cw(48p) | cw(48p) | lw(84p) . B3 \ 4\|800 bit/s T{ See Recommendations X.21 and X.21\|\fIbis\fR T} .T& cw(48p) | cw(48p) | lw(84p) . B4 \ 9\|600 bit/s .T& cw(48p) | cw(48p) | lw(84p) . B5 48\|000 bit/s .T& cw(48p) | cw(48p) | lw(84p) . B6 64\|000 bit/s _ .TE .nr PS 9 .RT .ad r \fBTable 2/X.10 [T2.10], p.\fR .sp 1P .RT .ad b .RT .LP .bp .ce \fBH.T. [T3.10]\fR .ce TABLE\ 3/X.10 .ce \fBStart\(hystop direct connection to a packet switched\fR .ce \fBdata transmission service\fR .ce (see Notes 1 and 2) .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | rw(48p) | lw(84p) . C1 110 bit/s .T& cw(48p) | rw(48p) | lw(84p) . C2 200 bit/s .T& cw(48p) | rw(48p) | lw(84p) . C3 300 bit/s See Recommendation X.28 .T& cw(48p) | rw(48p) | lw(84p) . C4 1\|200 bit/s .T& cw(48p) | rw(48p) | lw(84p) . C5 75/1\|200 bit/s .T& cw(48p) | rw(48p) | lw(84p) . C6 2\|400 bit/s _ .TE .nr PS 9 .RT .ad r \fBTableau 3/X.10 [T3.10], p. 3\fR .sp 1P .RT .ad b .RT .ce \fBH.T. [T4.10]\fR .ce TABLE\ 4/X.10 .ce \fBSynchronous direct connection to a packet switched\fR .ce \fBdata transmission service\fR .ce (see Note 1) .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | cw(48p) | lw(84p) . D1 D2 D3 D4 D5 T{ \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s 64\|000 bit/s T} T{ See Recommendations X.25 and X.31 (case A) (see Note 3) T} _ .TE .nr PS 9 .RT .ad r \fBTableau 4/X.10 [T4.10], p. 4\fR .sp 1P .RT .ad b .RT .ce \fBH.T. [T5.10]\fR .ce TABLE\ 5/X.10 .ce \fBStart\(hystop direct connection to a leased circuit\fR .ce \fBdata transmission service\fR .ce (see Note 2) .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | cw(48p) | lw(84p) . E1 E2 50 to 200 bit/s 300 bit/s T{ See Recommendations X.20 and X.20\|\fIbis\fR T} _ .TE .nr PS 9 .RT .ad r \fBTableau 5/X.10 [T5.10], p. 5\fR .sp 1P .RT .ad b .RT .LP .bp .ce \fBH.T. [T6.10]\fR .ce TABLE\ 6/X.10 .ce \fBSynchronous direct connection to a leased circuit\fR .ce \fBdata transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | cw(48p) | cw(84p) . F1 \ \ \|600 bit/s .T& cw(48p) | cw(48p) | cw(84p) . F2 \ 2\|400 bit/s .T& cw(48p) | cw(48p) | lw(84p) . F3 \ 4\|800 bit/s T{ See Recommendations X.21 and X.21\|\fIbis\fR T} .T& cw(48p) | cw(48p) | lw(84p) . F4 \ 9\|600 bit/s .T& cw(48p) | cw(48p) | lw(84p) . F5 48\|000 bit/s _ .TE .nr PS 9 .RT .ad r \fBTableau 6/X.10 [T6.10], p. 6\fR .sp 1P .RT .ad b .RT .LP .sp 3 .sp 1P .LP 3.2 \fISwitched connection to data transmission services provided by\fR \fIpublic data networks\fR .sp 9p .RT .LP .sp 3 .ce \fBH.T. [T7.10]\fR .ce TABLE\ 7/X.10 .ce \fBSynchronous switched connection by means of the PSTN\fR .ce \fB to a circuit switched data transmission service\fR .ce For further study. .ce .ce \fBH.T. [T8.10]\fR .ce TABLE\ 8/X.10 .ce \fBStart\(hystop switched connection by means of a CSPDN\fR .ce .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | cw(48p) | lw(84p) . K1 300 bit/s See Recommendation X.28 _ .TE .nr PS 9 .RT .ad r \fBTable 7/X.10 [T7.10], p.\fR .sp 1P .RT .ad b .RT .LP .sp 3 .ce \fBH.T. [T8.10]\fR .ce TABLE\ 8/X.10 .ce \fBStart\(hystop switched connection by means of a CSPDN\fR .ce .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | cw(48p) | lw(84p) . K1 300 bit/s See Recommendation X.28 _ .TE .nr PS 9 .RT .ad r \fBTable 8/X.10 [T8.10], p.\fR .sp 1P .RT .ad b .RT .LP .bp .ce \fBH.T. [T9.10]\fR .ce TABLE\ 9/X.10 .ce \fBStart\(hystop switched connection by means of the PSTN\fR .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | rw(48p) | lw(84p) . L1 110 bit/s .T& cw(48p) | rw(48p) | lw(84p) . L2 200 bit/s .T& cw(48p) | rw(48p) | lw(84p) . L3 300 bit/s See Recommendation X.28 .T& cw(48p) | rw(48p) | lw(84p) . L4 1\|200 bit/s .T& cw(48p) | rw(48p) | lw(84p) . L5 75/1\|200 bit/s .T& cw(48p) | rw(48p) | lw(84p) . L6 2\|400 bit/s _ .TE .nr PS 9 .RT .ad r \fBTableau 9/X.10 [T9.10], p. 9\fR .sp 1P .RT .ad b .RT .LP .sp 1 .ce \fBH.T. [T10.10]\fR .ce TABLE\ 10/X.10 .ce \fBSynchronous switched connection by means of a CSPDN\fR .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | cw(48p) | lw(84p) . O1 O2 O3 O4 O5 T{ \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s 64\|000 bit/s T} See Recommendation X.32 _ .TE .nr PS 9 .RT .ad r \fBTableau 10/X.10 [T10.10], p. 10\fR .sp 1P .RT .ad b .RT .LP .sp 1 .ce \fBH.T. [T11.10]\fR .ce TABLE\ 11/X.10 .ce \fBSynchronous switched connection by means of the PSTN\fR .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(84p) . Category of access Data signalling rate T{ DTE/DCE interface requirements T} _ .T& cw(48p) | cw(48p) | lw(84p) . P1 P2 P3 P4 T{ 1\|200 bit/s 2\|400 bit/s 4\|800 bit/s 9\|600 bit/s T} See Recommendation X.32 _ .TE .nr PS 9 .RT .ad r \fBTableau 11/X.10 [T11.10], p. 11\fR .sp 1P .RT .ad b .RT .LP .bp .ce \fBH.T. [T12.10]\fR .ce TABLE\ 12/X.10 .ce \fBSynchronous switched access by means of an ISDN B channel\fR .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(66p) sw(66p) , ^ | ^ | c | c. Category of access Data signalling rate T{ DTE/DCE interface requirements at T} Reference point S/T Reference point R _ .T& cw(48p) | cw(48p) | lw(66p) | lw(66p) . Q1 Q2 Q3 Q4 Q5 T{ \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s 64\|000 bit/s T} T{ See Recommendation X.31 (case A) and Recommendation X.32 T} See Recommendation X.32 _ .TE .nr PS 9 .RT .ad r \fBTableau 12/X.10 [T12.10], p. 12\fR .sp 1P .RT .ad b .RT .LP .sp 4 .sp 1P .LP 3.3 \fIDirect connection to data transmission services provided by ISDNs\fR \fI(including through Terminal Adaptors)\fR .sp 9p .RT .LP .sp 4 .ce \fBH.T. [T13.10]\fR .ce TABLE\ 13/X.10 .ce \fBSynchronous direct connection to a circuit switched\fR .ce \fBdata transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(66p) sw(66p) , ^ | ^ | c | c. Category of access Data signalling rate T{ DTE/DCE interface requirements at T} Reference point S/T Reference point R _ .T& cw(48p) | cw(48p) | lw(66p) | lw(66p) . S1 S2 S3 S4 S5 S6 T{ \ \ \|600 bit/s \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s 64\|000 bit/s T} See Recommendation X.30 T{ See Recommendations X.21 and X.21\|\fIbis\fR T} _ .TE .nr PS 9 .RT .ad r \fBTableau 13/X.10 [T13.10], p. 13\fR .sp 1P .RT .ad b .RT .LP .bp .ce \fBH.T. [T14.10]\fR .ce TABLE\ 14/X.10 .ce \fBSynchronous direct connection via the ISDN B channel\fR .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(66p) sw(66p) , ^ | ^ | c | c. Category of access Data signalling rate T{ DTE/DCE interface requirements at T} Reference point S/T Reference point R _ .T& cw(48p) | cw(48p) | lw(66p) | lw(66p) . T1 T2 T3 T4 T5 T{ \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s 64\|000 bit/s T} T{ See Recommendation X.31 (case B) T} See Recommendation X.25 _ .TE .nr PS 9 .RT .ad r \fBTableau 14/X.10 [T14.10], p. 14\fR .sp 1P .RT .ad b .RT .ce \fBH.T. [T15.10]\fR .ce TABLE\ 15/X.10 .ce \fBSynchronous direct connection via the ISDN D channel\fR .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(66p) sw(66p) , ^ | ^ | c | c. Category of access Data signalling rate T{ DTE/DCE interface requirements at T} Reference point S/T Reference point R _ .T& cw(48p) | cw(48p) | lw(66p) | lw(66p) . U1 U2 U3 U4 . U5 T{ \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s (see Note 4) 64\|000 bit/s (see Note 4) T} See Recommendation X.31 See Recommendation X.25 _ .TE .nr PS 9 .RT .ad r \fBTableau 15/X.10 [T15.10], p. 15\fR .sp 1P .RT .ad b .RT .sp 1P .LP 3.4 \fISwitched connection to data transmission services provided by ISDNs\fR \fI(including through Terminal Adaptors)\fR .sp 9p .RT .ce \fBH.T. [T16.10]\fR .ce TABLE\ 16/X.10 .ce \fBStart\(hystop switched connection by means of the PSTN\fR .ce \fBto a packet switched data transmission service\fR .ce For further study. .ce .ce \fBH.T. [T17.10]\fR .ce TABLE\ 17/X.10 .ce \fBSynchronous switched connection by means of a CSPDN\fR .ce \fBto a packet switched data transmission service\fR .ce For further study. .ce .ce \fBH.T. [T18.10]\fR .ce TABLE\ 18/X.10 .ce \fBSynchronous switched connection by means of the PSTN\fR .ce \fBto a packet switched data transmission service\fR .ce For further study. .ce .ce \fBH.T. [T19.10]\fR .ce TABLE\ 19/X.10 .ce \fBSynchronous switched connection by means of an ISDN B channel\fR .ce .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(66p) sw(66p) , ^ | ^ | c | c. Category of access Data signalling rate T{ DTE/DCE interface requirements at T} Reference point S/T Reference point R _ .T& cw(48p) | cw(48p) | lw(66p) | lw(66p) . Y1 Y2 Y3 Y4 Y5 T{ \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s 64\|000 bit/s T} T{ See Recommendation X.31 (case\ B) T} See Recommendation X.25 .TE .LP \fINote 1\fR \ \(em\ Direct connections may be provided by means of leased circuits or by dedicated access circuits. .LP \fINote 2\fR \ \(em\ Some Administrations may offer the categories of access of 600\ bit/s, 1200\ bit/s, 2400\ bit/s, 4800\ bit/s and 9600\ bit/s. .LP \fINote 3\fR \ \(em\ Recommendation X.31 (case A) is appropriate at the S/T reference point when category of access D5 is provided by means of ISDN B channel. .LP \fINote 4\fR \ \(em\ For 64 kbit/s D channel only. .nr PS 9 .RT .ad r \fBTable 16/X.10 [T16.10], p.\fR .sp 1P .RT .ad b .RT .LP .bp .ce \fBH.T. [T17.10]\fR .ce TABLE\ 17/X.10 .ce \fBSynchronous switched connection by means of a CSPDN\fR .ce \fBto a packet switched data transmission service\fR .ce For further study. .ce .ce \fBH.T. [T18.10]\fR .ce TABLE\ 18/X.10 .ce \fBSynchronous switched connection by means of the PSTN\fR .ce \fBto a packet switched data transmission service\fR .ce For further study. .ce .ce \fBH.T. [T19.10]\fR .ce TABLE\ 19/X.10 .ce \fBSynchronous switched connection by means of an ISDN B channel\fR .ce .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(66p) sw(66p) , ^ | ^ | c | c. Category of access Data signalling rate T{ DTE/DCE interface requirements at T} Reference point S/T Reference point R _ .T& cw(48p) | cw(48p) | lw(66p) | lw(66p) . Y1 Y2 Y3 Y4 Y5 T{ \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s 64\|000 bit/s T} T{ See Recommendation X.31 (case\ B) T} See Recommendation X.25 .TE .LP \fINote 1\fR \ \(em\ Direct connections may be provided by means of leased circuits or by dedicated access circuits. .LP \fINote 2\fR \ \(em\ Some Administrations may offer the categories of access of 600\ bit/s, 1200\ bit/s, 2400\ bit/s, 4800\ bit/s and 9600\ bit/s. .LP \fINote 3\fR \ \(em\ Recommendation X.31 (case A) is appropriate at the S/T reference point when category of access D5 is provided by means of ISDN B channel. .LP \fINote 4\fR \ \(em\ For 64 kbit/s D channel only. .nr PS 9 .RT .ad r \fBTable 17/X.10 [T17.10], p.\fR .sp 1P .RT .ad b .RT .LP .sp 3 .ce \fBH.T. [T18.10]\fR .ce TABLE\ 18/X.10 .ce \fBSynchronous switched connection by means of the PSTN\fR .ce \fBto a packet switched data transmission service\fR .ce For further study. .ce .ce \fBH.T. [T19.10]\fR .ce TABLE\ 19/X.10 .ce \fBSynchronous switched connection by means of an ISDN B channel\fR .ce .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(66p) sw(66p) , ^ | ^ | c | c. Category of access Data signalling rate T{ DTE/DCE interface requirements at T} Reference point S/T Reference point R _ .T& cw(48p) | cw(48p) | lw(66p) | lw(66p) . Y1 Y2 Y3 Y4 Y5 T{ \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s 64\|000 bit/s T} T{ See Recommendation X.31 (case\ B) T} See Recommendation X.25 .TE .LP \fINote 1\fR \ \(em\ Direct connections may be provided by means of leased circuits or by dedicated access circuits. .LP \fINote 2\fR \ \(em\ Some Administrations may offer the categories of access of 600\ bit/s, 1200\ bit/s, 2400\ bit/s, 4800\ bit/s and 9600\ bit/s. .LP \fINote 3\fR \ \(em\ Recommendation X.31 (case A) is appropriate at the S/T reference point when category of access D5 is provided by means of ISDN B channel. .LP \fINote 4\fR \ \(em\ For 64 kbit/s D channel only. .nr PS 9 .RT .ad r \fBTable 18/X.10 [T18.10], p.\fR .sp 1P .RT .ad b .RT .LP .sp 3 .ce \fBH.T. [T19.10]\fR .ce TABLE\ 19/X.10 .ce \fBSynchronous switched connection by means of an ISDN B channel\fR .ce .ce \fBto a packet switched data transmission service\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(66p) sw(66p) , ^ | ^ | c | c. Category of access Data signalling rate T{ DTE/DCE interface requirements at T} Reference point S/T Reference point R _ .T& cw(48p) | cw(48p) | lw(66p) | lw(66p) . Y1 Y2 Y3 Y4 Y5 T{ \ 2\|400 bit/s \ 4\|800 bit/s \ 9\|600 bit/s 48\|000 bit/s 64\|000 bit/s T} T{ See Recommendation X.31 (case\ B) T} See Recommendation X.25 .TE .LP \fINote 1\fR \ \(em\ Direct connections may be provided by means of leased circuits or by dedicated access circuits. .LP \fINote 2\fR \ \(em\ Some Administrations may offer the categories of access of 600\ bit/s, 1200\ bit/s, 2400\ bit/s, 4800\ bit/s and 9600\ bit/s. .LP \fINote 3\fR \ \(em\ Recommendation X.31 (case A) is appropriate at the S/T reference point when category of access D5 is provided by means of ISDN B channel. .LP \fINote 4\fR \ \(em\ For 64 kbit/s D channel only. .nr PS 9 .RT .ad r \fBTable 19/X.10 [T19.10], p.\fR .sp 1P .RT .ad b .RT .LP .bp .sp 1P .ce 1000 \v'3P' SECTION\ 2 .ce 0 .sp 1P .ce 1000 \fBINTERFACES\fR .ce 0 .sp 1P .sp 2P .LP \fBRecommendation\ X.20\fR .RT .sp 2P .ce 1000 \fBINTERFACE\ BETWEEN\ DATA\ TERMINAL\ EQUIPMENT\ (DTE)\fR .EF '% Fascicle\ VIII.2\ \(em\ Rec.\ X.20'' .OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.20 %' .ce 0 .ce 1000 \fBAND\ DATA\ CIRCUIT\(hyTERMINATING\ EQUIPMENT\ (DCE)\ FOR\fR .ce 0 .sp 1P .ce 1000 \fBSTART\(hySTOP\ TRANSMISSION\ SERVICES\ ON\ PUBLIC\ DATA\ NETWORKS\fR .ce 0 .sp 1P .ce 1000 \fI(Geneva, 1972; amended at Geneva, 1976 and 1980,\fR .sp 9p .RT .ce 0 .sp 1P .ce 1000 \fIMalaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR .ce 0 .sp 1P .sp 2P .LP The\ CCITT, .sp 1P .RT .sp 1P .LP \fIconsidering\fR .sp 9p .RT .PP (a) that Recommendations X.1 and X.2 define the services and facilities to be provided by a public data network; .PP (b) that Recommendation X.92 defines the hypothetical reference connections for public synchronous data networks; .PP (c) that Recommendation X.96 defines \fIcall progress\fR \| signals; .PP (d) that the necessary elements for an interface Recommendation should be defined in architectural levels; .PP (e) that it is desirable for characteristics of the interface between the DTE and DCE of a public data network to be standardized, .sp 1P .LP \fIunanimously recommends\fR .sp 9p .RT .PP that the interface between the DTE and DCE in public data networks for user classes of service employing start\(hystop transmission should be as defined in this Recommendation. .sp 2P .LP \fB1\fR \fBScope\fR .sp 1P .RT .PP 1.1 This Recommendation defines the physical characteristics and call control procedures for a general purpose interface between DTE and DCE for user classes of service, as defined in Recommendation\ X.1, employing start\(hystop transmission. .sp 9p .RT .PP 1.2 The formats and procedures for \fIselection\fR , \fIcall progress\fR \|and \fIDCE provided information\fR are included in this Recommendation. .PP 1.3 The provision for duplex operation is covered. .sp 2P .LP \fB2\fR \fBDTE/DCE physical interface elements\fR .sp 1P .RT .sp 1P .LP 2.1 \fIInterchange circuits\fR .sp 9p .RT .PP A list of the interchange circuits concerned is presented in Table\ 1/X.20. Definitions of these interchange circuits are given in Recommendation\ X.24. .bp .RT .ce \fBH.T. [T1.20]\fR .ce TABLE\ 1/X.20 .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(48p) | cw(48p) sw(48p) , ^ | ^ | c | c. Interchange circuit Interchange circuit name Direction to DCE from DCE _ .T& cw(48p) | lw(48p) | lw(48p) | lw(48p) . G (see Note) T{ Signal ground or common return T} .T& cw(48p) | lw(48p) | cw(48p) | lw(48p) . G a DTE common return X .T& cw(48p) | lw(48p) | cw(48p) | cw(48p) . G b DCE common return X .T& cw(48p) | lw(48p) | cw(48p) | cw(48p) . T Transmit X .T& cw(48p) | lw(48p) | cw(48p) | cw(48p) . R Receive X .TE .LP \fINote\fR \ \(em\ This conductor may be used to reduce environmental signal interference at the interface. In case of shielded interconnecting cable, the additional connection considerations are part of Recommendations\ X.24 and ISO\ 4903. .nr PS 9 .RT .ad r \fBTable 1/X.20 [T1.20], p.\fR .sp 1P .RT .ad b .RT .sp 1P .LP 2.2 \fIElectrical characteristics\fR .sp 9p .RT .PP The electrical characteristics of the interchange circuits at the DCE side of the interface will comply with Recommendation\ X.26. .PP The electrical characteristics at the DTE side of the interface may be applied according to Recommendations\ X.26, X.27 (without cable termination in the load), or Recommendation\ V.28. .PP For interworking between a V.28\(hyDTE and a X.26\(hyDCE refer to Recommendation X.26 and ISO\ 4903. .RT .sp 1P .LP 2.3 \fIMechanical characteristics\fR .sp 9p .RT .PP Refer to ISO 4903 (15\(hypole DTE/DCE interface connector and contact number assignments) for mechanical arrangements. .RT .sp 1P .LP 2.4 \fIFault conditions of interchange circuits\fR .sp 9p .RT .PP For the association of the receiver circuit\(hyfailure detection to particular interchange circuits in accordance with the type of failure detection, see Recommendation\ X.26, \(sc\ 11 and Recommendation\ X.27, \(sc\ 9. .RT .sp 1P .LP 2.4.1 \fICircuit R in failure state\fR .sp 9p .RT .PP The DTE should interpret a fault condition on circuit R as r\ =\ 0, using failure detection type\ 2. When the electrical characteristics are applied according to Recommendation\ V.28, the DTE should interpret generator in power\(hyoff condition or open\(hycircuited interconnecting cable as a binary\ 0. .RT .sp 1P .LP 2.4.2 \fICircuit T in failure state\fR .sp 9p .RT .PP The DCE will interpret a fault condition on circuit T as t\ =\ 0, using failure detection type\ 2. .RT .sp 2P .LP \fB3\fR \fBCall control characters and \fBerror checking\fR .sp 1P .RT .PP All characters for call control purposes are selected from International Alphabet No.\ 5 according to Recommendation\ T.50. .PP Even parity according to Recommendation\ X.4 applies for IA5 characters interchanged for call control purposes. .bp .RT .sp 2P .LP \fB4\fR \fBElements of the\fR \fBcall control phase\fR \fBfor\fR \fBcircuit\fR \fBswitched service\fR .sp 1P .RT .PP The state diagram provided in Figure A\(hy1/X.20 shows the relationship between the various \fIcall control\fR phase states as defined below, together with the recognized transactions between these states under normal operating conditions. Illustrated examples of the time sequence relationships between these states and associated time\(hyout operation are provided in Figure\ B\(hy1/X.20. .PP The \fIcall control\fR \|phase can be terminated by either the DTE or the DCE by \fIclearing\fR as defined in \(sc\ 6 below. .RT .sp 1P .LP 4.1 \fIEvents of the call control procedures\fR .sp 9p .RT .PP (See Figure A\(hy1/X.20.) .RT .sp 1P .LP 4.1.1 \fIReady (state 1)\fR .sp 9p .RT .PP Circuits T and R show binary 0. .RT .sp 1P .LP 4.1.2 \fICall request (state 2)\fR .sp 9p .RT .PP The calling DTE shall indicate a request for a call by signalling steady binary condition t\ =\ 1 provided that it was previously signalling \fIDTE\fR \fIready\fR (t\ =\ 0). .RT .sp 1P .LP 4.1.3 \fIProceed\(hyto\(hyselect (state 3)\fR .sp 9p .RT .PP When the network is prepared to receive selection information, the DCE will signal steady binary con dition\ r\ =\ 1. .PP The \fIproceed\(hyto\(hyselect\fR \|signal will start within 6 seconds of the \fIcall request\fR being sent. .RT .sp 1P .LP 4.1.4 \fISelection signal sequence (state 4)\fR .sp 9p .RT .PP The \fIselection\fR \|signal sequence shall be transmitted by the DTE on circuit\ T. .PP The format of \fIselection\fR \|signal sequence is defined in \(sc 4.6.1 below. .PP The information content and coding of the \fIselection\fR \|signal sequence is contained in Annex\ G and Recommendation\ X.121. .PP The \fIselection\fR \|signal sequence shall start within 6 seconds of the \fIproceed\(hyto\(hyselect\fR \| being received and shall be completed within 36 seconds. .PP The maximum permissible interval between individual selection characters is 6\ seconds. .RT .sp 1P .LP 4.1.5 \fIDTE waiting (state 5)\fR .sp 9p .RT .PP During \fIDTE waiting\fR \|, the DTE signals steady binary condition t\ =\ 1. .RT .sp 1P .LP 4.1.6 \fIIncoming call (state 8)\fR .sp 9p .RT .PP The DCE will indicate an incoming call by signalling steady binary condition r\ =\ 1. .RT .sp 1P .LP 4.1.7 \fICall accepted (state 9)\fR .sp 9p .RT .PP The DTE shall accept the incoming call not later than 600\ ms by signalling the steady state binary condition t\ =\ 1. .PP 10\(hy100\ ms thereafter, the DTE transmits the call control character 0/6 (ACK). .RT .sp 1P .LP 4.1.8 \fICall not accepted (state 18)\fR .sp 9p .RT .PP If the DTE does not wish to accept the incoming call it shall signal this not later than 600\ ms by changing circuit T to steady binary condition\ 1. .PP 10\(hy100 ms thereafter, the DTE shall transmit the call control character\ 1/5 (NAK) followed by \fIDTE clear request\fR (state 13). .bp .RT .sp 1P .LP 4.1.9 \fICall progress signal\fR \fIsequence (state 6)\fR .sp 9p .RT .PP The \fIcall progress\fR \|signal sequence will be transmitted by the DCE to the calling DTE on the R circuit when an appropriate condition is encountered by the network. .PP A \fIcall progress\fR \|signal sequence will consist of one or more \fIcall progress\fR \|signal blocks. A \fIcall progress\fR signal block will consist of\ one or more \fIcall progress\fR signals. .PP The format of the \fIcall progress\fR \|signal sequence is defined in \(sc\ 4.6.2 below. .PP The coding of \fIcall progress\fR \|signals is provided in Annex\ E. .PP The description of \fIcall progress\fR \|signals is provided in Recommendation\ X.96. .PP A \fIcall progress\fR \|signal sequence will be transmitted by the DCE within 60\ seconds of: 1) the \fIend\(hyof\(hy\fR \fIselection\fR \ signal or\ 2) in case of \fIdirect call\fR , the \fIproceed\(hyto\(hyselect\fR signal being sent by the DTE. The \fIcall progress\fR signal sequence, however, will not be sent by the DCE before the reception of the \fIend\(hyof\(hyselection\fR signal except in the case of expiration of time\(hyouts described in \(sc\ 4.1.4 where there may be a \fIcall progress\fR signal sequence followed by the \fIclear indication\fR . .PP \fINote\fR \ \(em\ When an error is detected in a received \fIcall progress\fR \| signal sequence, the DTE may choose to either ignore the signal or attempt a new call after clearing. .RT .sp 1P .LP 4.1.10 \fIDCE provided information sequence (states 7A and 7B)\fR .sp 9p .RT .PP The \fIDCE provided information\fR \|sequences will be transmitted by a DCE to the calling DTE (state\ 7A) or a called DTE (state\ 7B) on circuit\ R. .PP A \fIDCE provided information\fR \|sequence will consist of 1 or more \fIDCE\fR \fIprovided information\fR blocks. Each \fIDCE provided information\fR block will be limited to a maximum length of 128\ characters. .PP The format of the \fIDCE provided information\fR \|sequences is defined in \(sc\ 4.6.3\ below. .PP The information content of \fIDCE provided information\fR \|is given in Annex\ G. .PP A \fIDCE provided information\fR \|sequence (state 7B) will be sent to the called DTE within 60\ seconds of the \fIcall accepted\fR signal being sent. .RT .sp 1P .LP 4.1.10.1 \fILine identification\fR .sp 9p .RT .PP \fICalling\fR and \fIcalled line identification\fR \|is an optional additional facility. .PP The information content of \fIcalling\fR \|and \fIcalled line identification\fR is provided in Annex\ G. .PP \fICalling\fR and \fIcalled line identification\fR \|will be transmitted by the DCE on the R circuit during states 7B or 7A respectively. .PP When provided, \fIcalled line identification\fR \|(state 7A) will be transmitted by the DCE to the calling DTE after all \fIcall progress\fR signals, if any. .PP When provided, \fIcalling line identification\fR \|(state 7B) will be transmitted by the DCE to the called DTE after \fIcall accepted\fR has been sent by the DTE. .PP In the case where the \fIcalling line identification\fR \|facility is not provided by the originating network, or the \fIcalled line identification\fR facility is not provided by the destination network, a \fIdummy line\fR \fIidentification\fR shall be provided by the DCE to the DTE. .RT .sp 1P .LP 4.1.10.2 \fICharging information\fR .sp 9p .RT .PP \fICharging information\fR \|is an optional additional facility provided during state 7B. .PP Upon completion of clearing the call for which \fIcharging information\fR has been requested, the DCE will, within 200\ ms after entering \fIready\fR (state\ 1), establish an incoming call to the DTE for the purpose of giving \fIcharging information\fR . .PP \fICharging information\fR \|will be transmitted by the DCE on circuit\ R. .PP The DCE will send \fIclear indication\fR (state 16) upon sending the last \fIcharging information\fR block. The DTE should send \fIclear request\fR (state\ 13) when it has correctly received the \fIcharging information\fR \|signal, if the DCE has not previously signalled the \fIclear indication\fR . .PP The format of \fIcharging information\fR \|is defined in \(sc\ 4.6.3 below. .bp .RT .sp 1P .LP 4.1.11 \fIConnected (state 10)\fR .sp 9p .RT .PP The DCE signals that the connection is being established by the transmission of the call control character 0/6\ (ACK) on circuit\ R. Because of possible switching delays in the network, the DTE must keep circuit\ T in steady binary condition\ 1 during this state. .RT .sp 1P .LP 4.1.12 \fIReady for data (state 11)\fR .sp 9p .RT .PP 20 ms after the reception of the call control character 0/6 (ACK) in state\ 10, the connection is available for data transfer between both DTEs. .RT .sp 2P .LP 4.1.13 \fIEvents of the call control procedures for\fR \fImultipoint\fR \fIcircuit\(hyswitched service\fR .sp 1P .RT .sp 1P .LP 4.1.13.1 \fIReady (state 1)\fR .sp 9p .RT .PP See \(sc 4.1.1. .RT .sp 1P .LP 4.1.13.2 \fICall request (state 2)\fR .sp 9p .RT .PP See \(sc 4.1.2. .RT .sp 1P .LP 4.1.13.3 \fIProceed to select (state 3)\fR .sp 9p .RT .PP See \(sc 4.1.3. .RT .sp 1P .LP 4.1.13.4 \fISelection signal sequence (state 4)\fR .sp 9p .RT .PP See \(sc 4.1.4. .PP A \fIfacility request\fR \|signal is used to indicate the category of the point to multipoint service which is required. .PP The coding is defined in Annex F. .RT .sp 1P .LP 4.1.13.5 \fIDTE waiting (state 5)\fR .sp 9p .RT .PP See \(sc 4.1.5. .RT .sp 1P .LP 4.1.13.6 \fIIncoming call (state 8)\fR .sp 9p .RT .PP See \(sc 4.1.6. .RT .sp 1P .LP 4.1.13.7 \fICall accepted (state 9)\fR .sp 9p .RT .PP See \(sc 4.1.7. .RT .sp 1P .LP 4.1.13.8 \fICall not accepted (state 18)\fR .sp 9p .RT .PP See \(sc 4.1.8. .RT .sp 1P .LP 4.1.13.9 \fICall progress signal sequence (state 6)\fR .sp 9p .RT .PP See \(sc 4.1.9. .PP The \fIcall progress\fR \|signals related to each of the called DTEs are transmitted and then, in the same order, the \fIcalled line identification\fR signals of the different called DTEs. .PP When no specific \fIcall progress\fR \|signals are necessary for a given called DTE, then the call progress signal \*Q00\*U is used for this DTE so that the order could be kept. .RT .sp 1P .LP 4.1.13.10 \fIDCE provided information sequence (states 7A and 7B)\fR .sp 9p .RT .PP The DCE provided information sequences will be transmitted by a DCE to the calling DTE (state\ 7A) or a called DTE (state\ 7B) on circuit\ R. .bp .PP A DCE provided information sequence will consist of one or more \fIDCE\fR \fIprovided information\fR blocks. Each \fIDCE provided information\fR block will be limited to a maximum length of 128\ characters, except for \fIcalled line\fR \fIidentification\fR in case of multipoint calls where the maximum length is 512\ characters. .PP The format of the \fIDCE provided information\fR \|sequences is defined in \(sc\ 4.6.3 below. .PP The information content of DCE provided information is given in Annex\ G. .PP A \fIDCE provided information\fR \|sequence will be sent to the called DTE within 60\ seconds of the call accepted signal being sent. .RT .sp 1P .LP 4.1.13.10.1\ \ \fILine identification\fR .sp 9p .RT .PP See \(sc\ 4.1.10.1. .PP The \fIcalled line identification\fR \|related to the different called DTEs are provided in sequence. .RT .sp 1P .LP 4.1.13.10.2\ \ \fICharging information\fR .sp 9p .RT .PP See \(sc\ 4.1.10.2. .RT .sp 1P .LP 4.1.13.11\ \ \fIConnected (state 10)\fR .sp 9p .RT .PP See \(sc\ 4.1.11. .RT .sp 1P .LP 4.1.13.12\ \ \fIReady for data (state 11)\fR .sp 9p .RT .PP See \(sc\ 4.1.12. .RT .sp 1P .LP 4.2 \fIUnsuccessful call\fR .sp 9p .RT .PP If the required connection cannot be established, the DCE will indicate this and the reason to the calling DTE by means of a \fIcall progress\fR signal. Afterwards the DCE will signal \fIDCE clear indication\fR (state\ 16). .RT .sp 1P .LP 4.3 \fICall collision\fR \fI(state 19)\fR .sp 9p .RT .PP A \fIcall collision\fR \|is detected by the DCE when it receives \fIcall\fR \fIrequest\fR in response to an \fIincoming call\fR . The DCE may either accept the \fIcall request\fR or may perform \fIDCE clearing\fR . .RT .sp 1P .LP 4.4 \fIDirect call\fR .sp 9p .RT .PP For the \fIdirect call\fR \|facility, \fIselection\fR signals (state 4) are always bypassed. .PP \fINote\fR \ \(em\ The \fIdirect call\fR \|facility can only be provided on a subscription basis and not on a per\(hycall basis. .RT .sp 1P .LP 4.5 \fIFacility registration/cancellation\fR \fIprocedure\fR .sp 9p .RT .PP Registration/cancellation of optional user facilities shall be carried out by a DTE in accordance with normal call establishment procedures using the \fIselection\fR sequence, which is defined in \(sc\ 4.6.1\ below. .PP The format of the \fIfacility registration/cancellation\fR \| signal is defined in \(sc\ 4.6.1.3 below. .PP The \fIfacility registration/cancellation\fR \|procedure shall not be combined with establishment of a normally addressed call, but shall be taken as an independent procedure. .PP In response to acceptance or rejection of the \fIfacility\fR \fIregistration/cancellation\fR \| procedure, the network will provide the appropriate \fIcall progress\fR signal followed by \fIclear indication\fR . .RT .sp 1P .LP 4.6 \fISelection, call progress and DCE provided information formats\fR .sp 9p .RT .PP (See also Annex D for a syntactic description of the formats.) .bp .RT .sp 1P .LP 4.6.1 \fIFormat of\fR \fIselection sequence\fR .sp 9p .RT .PP A selection sequence shall consist of a \fIfacility request\fR \|block, or an \fIaddress\fR \| block, or a \fIfacility request\fR block followed by an \fIaddress\fR block, or a \fIfacility registration/cancellation\fR block. .RT .sp 1P .LP 4.6.1.1 \fIFacility request block\fR .sp 9p .RT .PP A \fIfacility request\fR \|block shall consist of one or more \fIfacility request\fR \| signals. .PP Multiple \fIfacility request\fR \|signals shall be separated by character\ 2/12 (\*Q\|,\|\*U). .PP A \fIfacility request\fR \|signal shall consist of a \fIfacility\fR \fIrequest\fR code and may contain one or more \fIfacility\fR parameters . The \fIfacility request\fR code, \fIfacility\fR parameter and subsequent \fIfacility\fR parameters shall be separated by character\ 2/15 (\*Q\|/\|\*U). For an interim period the 2/15 (\*Q\|/\|\*U) separator will not be used in some networks. .PP The end of a \fIfacility request\fR \|block shall be indicated by character\ 2/13 (\*Q\(em\*U). .PP The coding of \fIfacility request\fR \|, indicator and parameter is provided in Annex\ F. .RT .sp 1P .LP 4.6.1.2 \fIAddress block\fR .sp 9p .RT .PP An \fIaddress\fR \|block shall consist of one or more \fIaddress\fR signals. .PP An \fIaddress\fR \|signal shall consist of either a \fIfull address\fR signal or an \fIabbreviated address\fR signal. .PP Start of an \fIabbreviated address\fR \|signal shall be indicated by a prefix character\ 2/14 (\*Q\|.\|\*U). .PP Multiple \fIaddress\fR \|signals shall be separated by character\ 2/12 (\*Q\|,\|\*U). .RT .sp 1P .LP 4.6.1.3 \fIFacility registration/cancellation block\fR .sp 9p .RT .PP A \fIfacility registration/cancellation\fR \|block shall consist of one or more \fIfacility registration/cancellation\fR signals. .PP A \fIfacility registration/cancellation\fR \|signal shall consist of up to four elements in order: \fIfacility request\fR code, \fIindicator\fR , \fIregistration\fR parameter, \fIaddress\fR signal. .PP The elements of a \fIfacility registration/cancellation\fR \|signal shall be separated by character\ 2/15 (\*Q\|/\|\*U). .PP If a \fIfacility registration/cancellation\fR \|signal contains less than four of the elements, the elements should be eliminated in reverse order (e.g. a two\(hyelement \fIfacility registration/cancellation\fR signal will contain the \fIfacility request\fR code \*Q\|/\|\*U \fIindicator\fR ). If any element to be sent within the sequence is not required, a 3/0\ (\*Q0\*U) character should be inserted in the position of each missing element (e.g. \fIfacility request\fR code /0/0/\fIAddress\fR signal). .PP Multiple \fIfacility registration/cancellation\fR \|signals shall be separated by character\ 2/12 (\*Q\|,\|\*U). .PP The end of a \fIfacility registration/cancellation\fR \|block shall be indicated by character 2/13\ (\*Q\(em\*U) followed by character 2/11\ (\*Q+\*U). .RT .sp 1P .LP 4.6.1.4 \fIEnd of selection sequence\fR .sp 9p .RT .PP The end of a selection sequence shall be indicated by character\ 2/11 (\*Q+\*U). .RT .sp 1P .LP 4.6.2 \fIFormat of a\fR \fIcall progress sequence\fR .sp 9p .RT .PP A \fIcall progress\fR \|block shall consist of one or more \fIcall\fR \fIprogress\fR signals. .PP Each \fIcall progress\fR \|signal need not be repeated. .PP Multiple \fIcall progress\fR \|signals shall be separated by characters 0/13 (\*QCR\*U) and 0/10 (\*QLF\*U). .PP The end of a \fIcall progress\fR \|block shall be indicated by character\ 2/11 (\*Q+\*U). .bp .RT .sp 1P .LP 4.6.3 \fIFormats of DCE provided information\fR .sp 9p .RT .PP The following formats are specified for \fIDCE provided information\fR \| signals which have been identified. .PP The \fIDCE provided information\fR \|shall be preceded by the IA5 characters 0/13 (\*QCR\*U), 0/10 (\*QLF\*U), and except for \fIcalling\fR and \fIcalled line\fR \fIidentification\fR , by the IA5 character\ 2/15 (\*Q\|/\|\*U). To distinguish between different types of \fIDCE provided information\fR , the prefix should be followed by one or more numerical characters followed by the character\ 2/15 (\*Q\|/\|\*U) before the actual information is presented. The end of a \fIDCE provided information\fR block shall be indicated by character 2/11\ (\*Q+\*U). The order in which the \fIDCE\fR \fIprovided information\fR blocks are presented to the DTE is variable. .RT .sp 1P .LP 4.6.3.1 \fIFormat of called and calling line identification\fR .sp 9p .RT .PP \fICalling line identification\fR \| block and \fIcalled line\fR \fIidentification\fR \| block shall be preceded by character 2/10\ (\*Q\|*\|\*U). .PP When a \fIcalling\fR \|or \fIcalled line identification\fR \|block contains Data Network Identification Codes (DNIC) or Data Country Codes (DCC), the blocks shall be preceded by 2\ characters 2/10 (\*Q\|**\|\*U). .PP A \fIcalled line identification\fR \|block shall consist of one or more \fIcalled line identification\fR \| signals. .PP Multiple \fIcalled line identification\fR \|signals shall be separated by characters 0/13 (\*QCR\*U) and 0/10 (\*QLF\*U). .PP The end of \fIcalling line identification\fR \|signal and \fIcalled line\fR \fIidentification\fR \| block shall be indicated by character\ 2/11 (\*Q+\*U). .PP The \fIdummy line identification\fR \|block shall be indicated by character\ 2/10 (\*Q\|*\|\*U) followed by 2/11 (\*Q+\*U). .RT .sp 1P .LP 4.6.3.2 \fIFormat of charging information\fR .sp 9p .RT .PP The \fIcharging information\fR \|block will be preceded by characters 0/13 (\*QCR\*U), 0/10 (\*QLF\*U), and 2/15 (\*Q\|/\|\*U) followed by a second IA5 numerical character (1 or 2 or 3) followed by character 2/15 (\*Q\|/\|\*U). The end of the \fIcharging information\fR block shall be indicated by character 2/11 (\*Q+\*U). .RT .sp 2P .LP \fB5\fR \fBData transfer phase\fR .sp 1P .RT .sp 1P .LP 5.1 \fIData transfer (state 12), point\(hyto\(hypoint circuit switched service\fR .sp 9p .RT .PP The events during \fIdata transfer\fR \|are the responsibility of the DTE. .RT .sp 2P .LP 5.2 \fIData transfer, leased circuit service\fR .sp 1P .RT .sp 1P .LP 5.2.1 \fIReady\fR .sp 9p .RT .PP Circuits T and R show binary\ 1. .RT .sp 1P .LP 5.2.2 \fISend data (state 12S)\fR .sp 9p .RT .PP Data transmitted by the DTE on circuit T are delivered to the remote DTE on circuit\ R. .RT .sp 1P .LP 5.2.3 \fIReceive data (state 12R)\fR .sp 9p .RT .PP Data transmitted by a distant DTE are received on circuit\ R. .RT .sp 1P .LP 5.2.4 \fIData transfer (state 12)\fR .sp 9p .RT .PP Data are transferred on circuits T and R. .RT .sp 1P .LP 5.2.5 \fITermination of data transfer\fR .sp 9p .RT .PP The termination of \fIdata transfer\fR \|is the responsibility of the DTE. .RT .sp 2P .LP 5.3 \fIData transfer (state 12), centralized multipoint circuit\(hyswitched\fR \fIservice\fR .sp 1P .RT .PP The events during data transfer are the responsibility of the DTE. .bp .RT .sp 2P .LP \fB6\fR \fBClearing phase\fR .sp 1P .RT .PP In centralized multipoint calls: .RT .LP \(em clearing by the central DTE implies clearing of the call; .LP \(em clearing by a remote DTE clears the call for this DTE, and has no effect on the calls which remain established for the other remote DTEs; .LP \(em clearing by the last remote DTE which is still in the call leads to the clearing of the call. .sp 1P .LP 6.1 \fIClearing by the DTE (states 13, 14, 15)\fR .sp 9p .RT .PP The DTE should indicate clearing by signalling the steady binary condition t\ =\ 0, \fIDTE clear request\fR (state\ 13) for more than 210\ ms. .PP The DCE will respond within 6 seconds by signalling the steady state condition r\ =\ 0, \fIDCE clear confirmation\fR (state\ 14), for more than 210\ ms and will not reverse circuit R to binary\ 1 before \fIDCE ready\fR (state\ 1). .PP Within 210\(hy490 ms after the beginning of DCE \fIclear confirmation\fR the DTE shall be ready to accept an \fIincoming call\fR , i.e. it shall be in the state\ 15, \fIDTE ready\fR . .RT .sp 1P .LP 6.2 \fIClearing by the DCE (states 16, 17, 15)\fR .sp 9p .RT .PP The DCE will indicate clearing to the DTE by signalling the steady binary condition r\ =\ 0, \fIDCE clear indication\fR (state 16) for more than 210\ ms. .PP Within 210\(hy490 ms after the beginning of \fIDCE clear indication\fR \|, the DTE should signify \fIDTE clear confirmation\fR (state 17) by signalling the steady binary condition t\ =\ 0 for more than 210\ ms. .PP Within 490\ ms after the beginning of the \fIDTE clear confirmation\fR , the DTE shall be ready to accept an \fIincoming call\fR , i.e. it shall be in the state\ 15, \fIDTE ready\fR . .RT .sp 1P .LP 6.3 \fIDCE ready (state 1)\fR .sp 9p .RT .PP 490 ms after the beginning of \fIDCE\fR \|or \fIDTE clear confirmation\fR , respectively, the DCE is ready to accept a new \fIcall request\fR . .RT .sp 1P .LP 6.4 \fIClear collision\fR .sp 9p .RT .PP In case \fIDTE clear request\fR \|and \fIDCE clearing\fR \|occur at the same instant or during an overlapping time of 210\ ms, the DTE shall proceed in its clearing procedure. .RT .sp 2P .LP \fB7\fR \fBTest loops\fR .sp 1P .RT .PP The definitions of test loops and the principles of maintenance testing using the test loops are provided in Recommendation\ X.150. .RT .sp 1P .LP 7.1 \fIDTE test loop\fR \fI\(em type 1 loop\fR .sp 9p .RT .PP This loop is used as a basic test of the operation of the DTE, by looping back the transmitted signals inside the DTE for checking. The loop should be set up inside the DTE as close as possible to the DTE/DCE interface. .PP Circuit T is connected to circuit R of the DTE while the DTE is in test condition. .PP Loop 1 may be established from either the \fIdata transfer\fR \|or \fIready\fR state. .PP In some networks, for short routine tests during the \fIdata transfer\fR state, the DTE should maintain the same status on the interchange circuits as before the test. .PP If the loop is established from the \fIdata transfer\fR \|state, the DCE may continue to deliver data to the DTE during the test as though the DTE were in normal operation. It will be the responsibility of the DTEs to recover from any errors that might occur while the test loop is activated. .bp .RT .sp 1P .LP 7.2 \fILocal test loop\fR \fI\(em type 3 loop\fR .sp 9p .RT .PP Local test loops (type 3 loops) are used to test the operation of the DTE, the interconnecting cable and either all or parts of the local DCE, as discussed below. .PP Loop 3 may be established from any state. .PP For testing on leased circuits and for short duration testing on circuit\(hyswitched connections the DCE should continue to present toward the line the conditions that existed before the test (e.g.\ either \fIdata\fR \fItransfer\fR or \fIready\fR state). Where this is not practical (e.g.\ in some cases for loop\ 3a) or desirable (e.g.\ for long duration testing in circuit\(hyswitched applications) the DCE should terminate an existing call. .PP Manual control should be provided on the DCE for activation of the test loop. .PP The precise implementation of the test loop within the DCE is a national option. At least one of the following local loops should be implemented: .RT .sp 1P .LP 7.2.1 \fILoop 3d\fR .sp 9p .RT .PP This loop is used to test the operation of the DTE, including the interconnecting cable, by returning transmitted signals to the DTE for checking. The loop is set up inside the local DCE and does not include interchange circuit generators and loads. .PP Circuit T is connected to circuit R inside of the DCE while the DCE is in test condition. .PP \fINote\fR \ \(em\ While test loop 3d is operated, the effective length of the interface cable is doubled. Therefore, to ensure proper operation of loop\ 3d, the maximum DTE/DCE interface cable length should be one\(hyhalf the length normally appropriate for the data signalling rate in use. .RT .sp 1P .LP 7.2.2 \fILoop 3c\fR .sp 9p .RT .PP This loop is used to test the operation of the DTE, including the interconnecting cable and DCE interchange circuit generators and loads. .PP The configuration is identical to that given for loop 3d in \(sc 7.2.1 with the exception that the looping of circuit\ T to circuit\ R includes the interchange circuit generators and loads. The note concerning restriction of interface cable length is not applicable. .RT .sp 1P .LP 7.2.3 \fILoop 3b\fR .sp 9p .RT .PP This loop is used as a test of the operation of the DTE and the line coding and control logic and circuitry of the DCE. It includes all the circuitry of the DCE with the exclusion of the line signal conditioning circuitry (e.g. impedance matching transformers, amplifiers, equalizers, etc.). .PP The configuration is identical to that given for loop 3c in \(sc 7.2.2 except for the location of the point of loopback. .PP \fINote\fR \ \(em\ In some networks the setting of loop 3b will cause clearing of existing connections. .RT .sp 1P .LP 7.2.4 \fILoop 3a\fR .sp 9p .RT .PP This loop is used to test the operation of the DTE and the DCE. The loop should include the maximum amount of circuitry used in DCE working including, in particular, the line signal conditioning circuitry. It is recognized that, in some cases, the inclusion of devices (e.g. attenuators, equalizers or test loop translators) may be necessary in the loopback path. The subscriber line is suitably terminated during a loop\ 3a test condition. .PP The configuration is identical to that given for test loop\ 3b in \(sc\ 7.2.3 except for the location of the point of loopback. .PP \fINote\fR \ \(em\ In some networks the setting of loop 3a will cause clearing of existing connections. .bp .RT .sp 1P .LP 7.3 \fINetwork test loop\fR \fI\(em type 2 loop\fR .sp 9p .RT .PP Network test loops (type 2 loops) are used by the Administration's test centre to test the operation of the leased line or subscriber line and either all or part of the DCE, as discussed below. .RT .sp 1P .LP 7.3.1 \fIGeneral\fR .sp 9p .RT .PP Loop 2 may be controlled manually on the DCE or automatically from the network. The control of the loop and the method used for automatic control, when implemented, is a national option. .PP In case of a collision between \fIcall request\fR and the activation of the loop, the loop activation command will have priority. .PP When the test is in progress, the DCE will signal r\ =\ 0. .RT .sp 1P .LP 7.3.2 \fIImplementation of type 2 loops\fR .sp 9p .RT .PP The precise implementation of the test loop within the DCE is a national option. At least one of the following network test loops should be implemented. .RT .sp 1P .LP 7.3.2.1 \fILoop 2b\fR .sp 9p .RT .PP This loop is used by the Administration's test centre(s) and/or the remote DTE to test the operation of the subscriber line and all the circuitry of the DCE with the exception of interchange circuit generators and loads. .PP Circuit R is connected to circuit T inside of the DCE while the DCE is in loop\ 2b test condition. .PP At the interface, the DCE signal r = 0. .RT .sp 1P .LP 7.3.2.2 \fILoop 2a\fR .sp 9p .RT .PP This loop is used by either the Administration's test centre(s) or the remote DTE to test the operation of the subscriber line and the entire DCE. .PP The configuration is identical to that given for loop 2b in \(sc\ 7.3.2.1 except for the location of the point of loopback. .RT .LP .rs .sp 21P .ad r Blanc .ad b .RT .LP .bp .ce 1000 ANNEX\ A .ce 0 .ce 1000 (to Recommendation X.20) .sp 9p .RT .ce 0 .ce 1000 \fBInterface signalling state diagrams\fR .sp 1P .RT .ce 0 .sp 1P .ce 1000 Definition of symbols used in state diagrams .sp 9p .RT .ce 0 .sp 1P .LP .rs .sp 47P .ad r \fBFigure A\(hy1/X.20, p.\fR .sp 1P .RT .ad b .RT .LP .bp .LP .rs .sp 24P .ad r \fBFigure A\(hy2/X.20, p\fR .sp 1P .RT .ad b .RT .ce 1000 ANNEX\ B .ce 0 .ce 1000 (to Recommendation X.20) .sp 9p .RT .ce 0 .ce 1000 \fBInterface signalling sequence diagrams and time\(hyout operations\fR .sp 1P .RT .ce 0 .ce 1000 Definition of symbols used to illustrate time\(hyout operation in the signalling sequence diagrams .sp 9p .RT .ce 0 .LP .rs .sp 15P .ad r \fBFIGURE CCITT 25052, p.\fR .sp 1P .RT .ad b .RT .LP .bp .LP .rs .sp 47P .ad r \fBFIGURE B\(hy1/X.20, p.\fR .sp 1P .RT .ad b .RT .LP .bp .LP .rs .sp 47P .ad r \fBFIGURE B\(hy2/X.20, p.\fR .sp 1P .RT .ad b .RT .LP .bp .ce 1000 ANNEX\ C .ce 0 .ce 1000 (to Recommendation X.20) .sp 9p .RT .ce 0 .ce 1000 \fBDTE time\(hylimits and DCE time\(hyouts\fR .sp 1P .RT .ce 0 .LP C.1 \fIDTE time\(hylimits\fR .sp 1P .RT .PP Under certain circumstances this Recommendation requires the DCE to respond to a signal from the DTE within a stated maximum time. If any of these maximum times is exceeded, the DTE should initiate the action indicated in Table\ C\(hy1/X.20. To maximize efficiency, the DTE should incorporate time\(hylimits to send the appropriate signal under the defined circumstances summarized in Table\ C\(hy1/X.20. The time\(hylimits given in the first column are the maximum times allowed for the DCE to respond and are consequently the lower limits of the times a DTE must allow for proper network operation. A time\(hylimit longer than the time shown may be optionally used in the DTE; for example, all DTE .PP time\(hylimits could have one single value equal to or greater than the longest time\(hylimit shown in this table. However, the use of a longer time\(hylimit will result in reduced efficiency of network utilization. The actual DCE response time should be as short as is consistent with the implementing technology and in normal operation should be well within the specified time\(hylimit. The rare situation where a time\(hylimit is exceeded should only occur when there is a failure in DCE operation. .RT .sp 1P .LP C.2 \fIDCE\fR \fItime\(hyouts\fR .sp 9p .RT .PP Under certain circumstances this Recommendation requires the DTE to respond to a signal from the DCE within a stated maximum time. If any of these maximum times is exceeded, a time\(hyout in the DCE will initiate the actions summarized in Table\ C\(hy2/X.20. These constraints must be taken into account in the DTE design. The time\(hyouts given in the first column of the table are the minimum time\(hyout values used in the DCE for the appropriate DTE response and are consequently the maximum times available to the DTE for response to the indicated DCE action. The actual DTE response time should be as short as is consistent with the implementing technology and in normal operation should be within the specified time\(hyout. The rare situation where a time\(hyout is exceeded should only occur when there is a failure in the DTE operation. .RT .LP .rs .sp 24P .ad r Blanc .ad b .RT .LP .bp .ce \fBH.T. [T2.20]\fR .ce TABLE\ C\(hy1/X.20\fR .ce \fBDTE time\(hylimits\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(36p) | cw(30p) | cw(54p) | cw(54p) | cw(54p) . Time\(hylimit Time\(hylimit number Started by Normally terminated by T{ Preferred action to be taken when time\(hylimit exceeded T} _ .T& cw(36p) | cw(30p) | lw(54p) | lw(54p) | lw(54p) . \ \ 6 s T1 T{ Signalling of \fIcall request\fR (state 2) T} T{ Reception of \fIproceed\(hyto\(hyselect\fR (state 3) T} T{ DTE signals \fIDTE ready\fR (state 1) T} _ .T& cw(36p) | cw(30p) | lw(54p) | lw(54p) | lw(54p) . \ 60 s T2 T{ Signalling \fIend\(hyof\(hyselection\fR or \fIDTE waiting\fR (direct call) (state 5) T} T{ Reception of \fIcall progress\fR signals, \fIDCE\(hyprovided information,\fR \fIconnected\fR or \fIDCE clear indication\fR (states 6, 7A, 10 or 16), reset by additional \fIcall progress\fR signals (state 6) T} T{ DTE signals \fIDTE clear request\fR (state 13) T} _ .T& cw(36p) | cw(30p) | lw(54p) | lw(54p) | lw(54p) . \ \ 2 s T3 T{ Change of state to \fIDTE clear request\fR (state 13) T} T{ Change of state to \fIDCE clear confirmation\fR (state 14) or \fIDCE ready\fR (state 1) T} T{ DTE regards the DCE as DCE not ready and signals \fIDTE ready\fR (state\ 15) T} .T& lw(174p) . .T& cw(36p) | cw(30p) | lw(54p) | lw(54p) | lw(54p) . \ 60 s T4 T{ Change of state to \fIcall accepted\fR (state 9) T} T{ Reception of connected or \fIDCE clear indication\fR (state 10 or 16), reset by \fIDCE provided information\fR (state 7B) T} _ .T& cw(36p) | cw(30p) | lw(54p) | lw(54p) | lw(54p) . 200 ms T5 T{ Change of state to \fIready\fR (state 1) when \fIcharge information\fR has been requested T} T{ Reception of \fIincoming call\fR (state 8) T} T{ DTE returns to normal operation and may note absence of \fIcharge\fR \fIinformation\fR T} _ .TE .nr PS 9 .RT .ad r \fBTableau C\(hy1/X.20 [T2.20], p. 26\fR .sp 1P .RT .ad b .RT .LP .rs .sp 12P .ad r Blanc .ad b .RT .LP .bp .ce \fBH.T. [T3.20]\fR .ce TABLE\ C\(hy2/X.20\fR .ce \fBDTE time\(hyouts\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(36p) | cw(30p) | cw(54p) | cw(54p) | cw(54p) . Time\(hyout Time\(hyout number Started by Normally terminated by T{ Action to be taken when time\(hyout expires T} _ .T& cw(36p) | cw(30p) | lw(54p) | lw(54p) | lw(54p) , c | c | l | l | ^ , c | c | l | l | ^ . 36 s T11 (see Note) T{ DCE signalling of \fIproceed\(hyto\(hyselect\fR (state 3) T} T{ DCE reception of \fIend\(hyof\(hyselection\fR signal T} T{ DCE will signal \fIDCE clear indication\fR (state 16) or transmit appropriate call progress signal followed by \fIDCE clear indication\fR (state 16) T} \ 6 s T12 T{ DCE signalling of \fIproceed\(hyto\(hyselect\fR (state 3) T} T{ DCE reception of first selection character or in the case of \fIdirect call,\fR \fIDTE waiting\fR (state 5) T} \ 6 s T13 (see Note) T{ DCE reception of nth selection character (state 4) T} T{ DCE reception of (n+1)th selection character or \fIend\(hyof\(hyselection\fR signal T} .T& lw(228p) . .T& cw(36p) | cw(30p) | lw(54p) | lw(54p) | lw(54p) . 600 ms T14 T{ DCE signalling of \fIincoming call\fR (state 8) T} T{ Change of state to \fIcall accepted\fR (state 9) or \fIcall not accepted\fR (state 18) T} T{ The DTE is noted as not answering. The DCE will signal \fIDCE ready\fR (state 1) T} _ .T& cw(36p) | cw(30p) | lw(54p) | lw(54p) | lw(54p) . 500 ms T15 T{ Change of state to \fIDCE clear indication\fR (state 16) T} T{ Change of state to \fIDTE clear confirmation\fR (state 17) T} T{ DCE will signal \fIDCE ready\fR and mark \fIDTE uncontrolled not\fR \fIready\fR T} .TE .LP \fINote\fR \ \(em\ T11, T12 and T13 do not apply in the case of a direct call. .nr PS 9 .RT .ad r \fBTableau C\(hy2/X.20 [T3.20], p. 27\fR .sp 1P .RT .ad b .RT .LP .rs .sp 17P .ad r Blanc .ad b .RT .LP .bp .ce 1000 ANNEX\ D .ce 0 .ce 1000 (to Recommendation X.20) .sp 9p .RT .ce 0 .ce 1000 \fBFormats of Selection, Call Progress, .sp 1P .RT .ce 0 .ce 1000 \fBand DCE\(hyprovided information signals\fR .ce 0 .PP The following description uses Backus Normal Form as the formalism for syntactic description. A vertical line \*Q\||\|\*U separates alternatives. .sp 1P .RT .LP <LF> \ :\ :\ =\ IA\ 5 character 0/10 .LP <CR> \ \|:\ :\ =\ IA\ 5 character 0/13 .LP <*> \ :\ :\ =\ IA\ 5 character 2/10 .LP <+> \ :\ :\ =\ IA\ 5 character 2/11 .LP <\|,\|> \ :\ :\ =\ IA\ 5 character 2/12 .LP <\(em> \ :\ :\ =\ IA\ 5 character 2/13 .LP <\|.\|> \ :\ :\ =\ IA\ 5 character 2/14 .LP <\|/\|> \ :\ :\ =\ IA\ 5 character 2/15 .LP <\(*y> \ :\ :\ =\ IA\ 5 characters 3/0\(hy3/9 .LP <\|:\|> \ :\ :\ =\ IA\ 5 character 3/10 .LP <Facility request signal>\ :\ :\ =\ See Annex F .LP <Facility parameter>\ :\ :\ =\ See Annex F .LP <Full address signal>\ :\ :\ =\ See Recommendation X.121 .LP <Abbreviated address signal>\ :\ :\ =\ National option .LP <Calling line identification signal>\ :\ :\ =\ See Annex G .LP <Called line identification signal>\ :\ :\ =\ See Annex G .LP <Charging information>\ :\ :\ =\ See Annex G .LP <Indicator>\ :\ :\ =\ See Annex F .LP <Facility request code>\ :\ :\ =\ See Annex F .LP <Registration parameters>\ :\ :\ =\ See Annex F .LP <Call progress signal>\ :\ :\ =\ See Annex E .PP The above signals are combined as follows: .RT .LP <Address signal>\ :\ :\ = <Full address signal> | <\|.\|> <Abbreviated address signal> .LP <Address block>\ :\ :\ = <Address signal> | <Address block> <\|,\|> <Address signal> .LP <Facility\ registration/cancellation signal>\ :\ :\ = <Facility request code> <\|/\|> <Indicator> <\|/\|> <Registration parameter> <\|/\|> <Address signal> .LP <Facility\ registration/cancellation block>\ :\ :\ = <Facility registration/cancellation signal> | <Facility registration/cancellation block> <\|,\|> <Facility registration/cancellation signal> .LP <Facility request signal>\ :\ :\ = <Facility request code> | <Facility request signal> <\|/\|> <Facility parameter> .LP <Facility request block>\ :\ :\ = <Facility request signal> | <Facility request block> <\|,\|> <Facility request signal> .bp .LP <Selection sequence>\ :\ :\ = <Facility request block> <\(em> <Address block> <+> | <Facility request block> <\(em> <+> | <Address block> <+> | <Facility registration/cancellation block> <\(em> <+> .LP <Call progress signal>\ :\ :\ = <Call progress code> | <Call progress code> <\(em> <indicator> .LP <Call progress block>\ :\ :\ = <CR> <LF> <Call progress signal> <+> | <Call progress signal> <\|,\|> <Call progress block> .LP <Calling line identification>\ :\ :\ = <CR> <LF> <*> <Calling line identification signal> <+> .LP <Calling line identification (with DNIC or DCC)>\ :\ :\ = <CR> <LF> <**> <Calling line identification signal> <+> .LP <Called line identification>\ :\ :\ = <CR> <LF> <*> <Called line identification block> <+> .LP <Called line identification block>\ :\ :\ = <Called line identification signal> | <Called line identification block> <CR> <LF> <Called line identification signal> .LP <Called line identification (with DNIC or DCC)>\ :\ :\ = <CR> <LF> <**> <Called line identification block> <+> .LP <Dummy line identification>\ :\ :\ = <CR> <LF> <*> <+> .LP <DCE\(hyprovided information block>\ :\ :\ = <DCE\(hyprovided information signal> <+> | <DCE\(hyprovided information signal> <\|,\|> <DCE\(hyprovided information block> (see Note) .PP \fINote\fR \ \(em\ For \fIDCE\(hyprovided information\fR \|signals and blocks other than \fIcalling\fR \| or \fIcalled line identification\fR signals and blocks. .LP .rs .sp 24P .ad r Blanc .ad b .RT .LP .bp .ce 1000 ANNEX\ E .ce 0 .ce 1000 (to Recommendation X.20) .sp 9p .RT .ce 0 .ce 1000 \fBCoding of call progress signals\fR .sp 1P .RT .ce 0 .ce \fBH.T. [T4.20]\fR .ce TABLE\ E\(hy1/X.20 .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(36p) | cw(30p) | cw(54p) | cw(54p) | cw(54p) . Code group (see Note 1) Code Indicator Significance Category _ .T& cw(36p) | cw(30p) | cw(54p) | lw(54p) | lw(54p) . 0 00 01 02 03 \(em \(em \(em \(em T{ See Note 2 Terminal called Redirected call Connect when free T} Without clearing _ .T& cw(36p) | cw(30p) | cw(54p) | lw(54p) | lw(54p) . 2 20 21 22 23 \(em \(em \(em \(em T{ No connection Number busy Selection signals procedure error Selection signals transmission error T} T{ With clearing due to short\(hyterm conditions T} _ .T& cw(36p) | cw(30p) | cw(54p) | lw(54p) | lw(54p) . 3 \(em \(em \(em Unassigned _ .T& cw(36p) | cw(30p) | cw(54p) | lw(54p) | lw(54p) . 4 and 5 T{ 41 42 43 44 45 45 46 47 48 49 51 52 T} T{ \(em \(em \(em \(em \(em YY\(hyMM\(hyDD\(hyhh:mm \(em \(em \(em \(em \(em \(em T} T{ Access barred Changed number Not obtainable Out of order Controlled not ready DTE inactive until .\|.\|. Uncontrolled not ready DCE power off Invalid facility request Network fault in local loop Call information service Incompatible user class of service T} T{ With clearing due to long\(hyterm conditions T} _ .T& cw(36p) | cw(30p) | cw(54p) | lw(54p) | lw(54p) . 6 61 \(em Network congestion T{ With clearing due to network short\(hyterm conditions T} _ .T& cw(36p) | cw(30p) | cw(54p) | lw(54p) | lw(54p) . 7 71 72 \(em \(em T{ Long\(hyterm network congestion RPOA out of order T} T{ With clearing due to network long\(hyterm conditions T} _ .T& cw(36p) | cw(30p) | cw(54p) | lw(54p) | lw(54p) . 8 81 82 83 \(em \(em \(em T{ Registration/cancellation confirmed Redirection activated Redirection deactivated T} T{ With clearing due to network procedure T} _ .TE .nr PS 9 .RT .ad r \fBTable E\(hy1/X.20 [T4.20], p.\fR .sp 1P .RT .ad b .RT .LP .bp .ce 1000 ANNEX\ F .ce 0 .ce 1000 (to Recommendation X.20) .sp 9p .RT .ce 0 .ce 1000 \fBFacility request, indicator, and parameter coding\fR .sp 1P .RT .ce 0 .ce 1000 (for use as appropriate in \fIfacility request\fR \|signals and .sp 9p .RT .ce 0 .ce 1000 \fIfacility registration/cancellation\fR \| signals) .ce 0 .LP .sp 1 .ce \fBH.T. [T5.20]\fR .ce TABLE\ F\(hy1/X.20 .ce (see Annex D for formats and Note 1 below) .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | cw(66p) . Facility request code Facility parameter Indicator Registration parameter Address Facility _ .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . \ 0 \(em \(em \(em \(em T{ Reserved for future use (may be combined with second character) T} .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . \ 1 XX (see Note 2) \(em \(em \(em T{ Closed user group (other than preferential) T} .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . \ 2 \(em \(em \(em \(em Unassigned .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . \ 3 \(em \(em \(em \(em Unassigned .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 45 \(em 1 YY\(hyMM\(hyDD\(hyhh:mm \(em DTE inactive registration .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 45 \(em 2 \(em \(em DTE inactive cancellation .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . \ 4 \(em \(em \(em \(em Reserved .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 50 \(em \(em \(em \(em Reserved .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 51 \(em \(em \(em \(em Reserved .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 53 \(em \(em \(em \(em Reserved .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 60 0, 1, 2, 3, 4 \(em \(em \(em Multiple address calling .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 61 \(em \(em \(em \(em Charging information .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 62 \(em \(em \(em \(em Called line identification .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 63 \(em 1 \(em \(em T{ Activation of redirection of call T} .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 63 \(em 2 \(em \(em T{ Cancellation of redirection of call T} .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 63 \(em 3 \(em \(em Status of redirection of call .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 64 \(em \(em \(em \(em Reverse charging .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 65 \(em 1 \(em AS Direct call registration .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 65 \(em 2 \(em \(em Direct call cancellation .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 66 \(em 1 AAS AS T{ Abbreviated address registration T} .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 66 \(em 2 AAS \(em T{ Abbreviated address cancellation T} .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . 68 \(em \(em \(em \(em Reserved .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . \ 7 \(em \(em \(em \(em Reserved .T& cw(36p) | cw(36p) | cw(24p) | cw(42p) | cw(24p) | lw(66p) . \ 8 \(em \(em \(em \(em Reserved .TE .nr PS 9 .RT .ad r \fBTable F\(hy1/X.20 [T5.20], p.\fR .sp 1P .RT .ad b .RT .LP .bp .LP F.1 \fIMultiple address calling\fR .sp 1P .RT .PP This facility provides the DTE with the capability to request a category of point to multipoint service. .PP The coding is as follows: \v'6p' .RT .sp 1P .ce 1000 <60> <\|/\|> <i> <\(em> <Address block> <+> .ce 0 .sp 1P .LP .sp 1 where i is numerical character with the following significance: .LP 0\ Reserved .LP 1\ Reserved .LP 2\ Reserved .LP 3\ Centralized multipoint .LP 4\ Reserved .LP 5\ Reserved .sp 1P .LP F.2 \fICharging information\fR .sp 9p .RT .PP This facility enables the DTE to request at the call establishment phase that charging information for the call be provided at the end of the call: \v'6p' .RT .sp 1P .ce 1000 <61> <\(em> <Address> <+> .ce 0 .sp 1P .LP .sp 1 .sp 1P .LP F.3 \fIRedirection of call\fR .sp 9p .RT .PP This facility enables the DTE to request the network to route its incoming calls towards another address. The use of this facility is assigned for an agreed contractual period. .PP \fIActivation of redirection of call\fR \ \(em\ The activation of this facility is coded as follows: \v'6p' .RT .sp 1P .ce 1000 <63> <\|/\|> <1> <\(em> <+> .ce 0 .sp 1P .PP .sp 1 \fICancellation of redirection of call\fR \ \(em\ The cancellation of this facility is coded as follows: \v'6p' .sp 1P .ce 1000 <63> <\|/\|> <2> <\(em> <+> .ce 0 .sp 1P .PP .sp 1 \fIStatus of redirection of call\fR \ \(em\ The DTE has the capability to ask the network for the status of its redirection. The coding is as follows: \v'6p' .sp 1P .ce 1000 <63> <\|/\|> <3> <\(em> <+> .ce 0 .sp 1P .LP .sp 1 F.4 \fIReverse charging\fR .sp 9p .RT .PP This facility enables the DTE to request that reverse charging be applied for the call. .PP The coding is as follows: \v'6p' .RT .sp 1P .ce 1000 <64> <\(em> <Address> <+> .ce 0 .sp 1P .LP .sp 1 .sp 1P .LP F.5 \fIAbbreviated address calling\fR .sp 9p .RT .PP This facility enables the DTE to define a full address by an abbreviated address. .PP The registration coding of an abbreviated address is as follows: \v'6p' .RT .sp 1P .ce 1000 <66> <\|/\|> <1> <\|/\|> <xy> <\|/\|> <Address> <\(em> <+> .ce 0 .sp 1P .LP .sp 1 where .LP <xy> =\ abbreviated address corresponding to the full address, .LP <address> =\ full address .bp .PP \fICancellation\fR \ \(em\ The coding of the cancellation of an abbreviated address is as follows: \v'6p' .sp 1P .ce 1000 <66> <\|/\|> <2> <\|/\|> <xy> <\|/\|> <\(em> <+> .ce 0 .sp 1P .PP .sp 1 The coding of the abbreviated address is as follows: \v'6p' .sp 1P .ce 1000 <\|.\|> <xy> <+> .ce 0 .sp 1P .LP .sp 1 .sp 1P .LP F.6 \fIDTE inactive registration/cancellation\fR .sp 9p .RT .PP This facility enables the DTE to inform the network about a period of time during which the DTE is unable to accept incoming calls for circuit\(hyswitched service. .PP \fIDTE inactive registration\fR \ \(em\ The activation of this facility is as follows: \v'6p' .RT .sp 1P .ce 1000 <45> <\|/\|> <1> <\|/\|> <YY\(hyMM\(hyDD\(hyhh:mm> <\(em> <+> .ce 0 .sp 1P .LP .sp 1 where .LP YY: year, MM: month, DD: day, hh: hour, mm: minute .LP IA5 characters are used for \*QYY\*U, \*QMM\*U, \*QDD\*U, \*Qhh\*U, \*Qmm\*U, \*Q\(em\*U, and \*Q\|:\|\*U. .PP \fIDTE inactive cancellation\fR \(em The coding is as follows: \v'6p' .sp 1P .ce 1000 <45> <\|/\|> <2> <\(em> <+> \v'6p' .ce 0 .sp 1P .LP .sp 1 .ce 1000 ANNEX\ G .ce 0 .ce 1000 (to Recommendation X.20) .sp 9p .RT .ce 0 .ce 1000 \fBInformation content of DCE provided information\fR .sp 1P .RT .ce 0 .LP G.1 \fIGeneral\fR .sp 1P .RT .PP Except for the \fIcalling\fR \|and \fIcalled line identification\fR , the general format for \fIDCE\(hyprovided information\fR , as defined in \(sc\ 4.6.3 should apply. .PP The coding of the numerical character used to distinguish between different types of \fIDCE\(hyprovided information\fR is indicated in Table\ H\(hy1/X.21. .RT .sp 1P .LP G.2 \fIInformation content of calling and called line identification\fR .sp 9p .RT .PP Two formats are defined: .RT .LP i) \fICalling\fR \|and \fIcalled line identification\fR \|consist of the international data number as defined in Recommendation\ X.121 preceded by \fItwo\fR prefixes\ 2/10 (\*Q\|**\|\*U). .LP In the case where the originating network does not provide \fIcalling line identification\fR , only the Data Network Identification Code (DNIC) part of the international data number preceded by two prefixes\ 2/10 (\*Q\|**\|\*U) may be sent in place of the \fIdummy line identification\fR . .LP ii) Calling and called line identification consist of the National Number (NN) or Network Terminal Number (NTN) preceded by the prefix\ 2/10\ (\*Q\|*\|\*U). .bp .sp 1P .LP G.3 \fIGeneral coding of the DCE provided information\fR .sp 9p .RT .LP .sp 2 .ce \fBH.T. [T6.20]\fR .ce TABLE\ G\(hy1/X.20 .ce \fB .ce \fBCoding of DCE provided information\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(48p) | cw(66p) | cw(66p) . Identifier Meaning Remarks _ .T& cw(48p) | lw(66p) | cw(66p) . 0 Reserved _ .T& cw(48p) | lw(66p) | lw(66p) . 1 Charging information See details in \(sc\ G.3 .T& cw(48p) | lw(66p) | lw(66p) . 2 Charging information .T& cw(48p) | lw(66p) | lw(66p) . 3 Charging information _ .T& cw(48p) | lw(66p) | lw(66p) . 4 _ .T& cw(48p) | lw(66p) | lw(66p) . 5 Date and time indication See details in \(sc\ G.4 _ .T& cw(48p) | lw(66p) | lw(66p) . 6 Characteristics of the call See details in \(sc\ G.5 _ .T& cw(48p) | lw(66p) | lw(66p) . 7 Type of call indication See details in \(sc G.6 _ .T& cw(48p) | lw(66p) | lw(66p) . 8 Reserved _ .T& cw(48p) | lw(66p) | lw(66p) . 9 Reserved _ .TE .nr PS 9 .RT .ad r \fBTable G\(hy1/X.20 [T6.20], p.\fR .sp 1P .RT .ad b .RT .LP .sp 2 .sp 1P .LP G.4 \fIInformation content of charging information\fR .sp 9p .RT .PP The \fIcharging information\fR \|will inform the subscriber of either the monetary charges for a call, the duration of the call, or the number of units used during the call. .PP When \fIcharging information\fR \|is given in monetary charges for the call, n\ =\ 1 and the information shall consist of x\ number of integer digits optionally followed by a colon and two digits representing the fraction. The format applied is as follows: \v'6p' .RT .sp 1P .ce 1000 <\|/\|> <1> <\|/\|> <X\|.\|.\|.\|> .ce 0 .sp 1P .ce 1000 <\|/\|> <1> <\|/\|> <X\|.\|.\|.\|> <\|:\|> <yy> .ce 0 .sp 1P .LP .sp 1 .PP When the \fIcharging information\fR \|is presented as the duration of a call, n\ =\ 2 and the information shall consist of x\ number of integer digits representing seconds. The format applied is as follows: \v'6p' .sp 1P .ce 1000 <\|/\|> <2> <\|/\|> <X\|.\|.\|.\|> .ce 0 .sp 1P .PP .sp 1 When the \fIcharging information\fR \|is presented as the number of units used, n\ =\ 3, and the information shall consist of x\ number of integer digits representing the units. The format applied is as follows: \v'6p' .sp 1P .ce 1000 <\|/\|> <3> <\|/\|> <X\|.\|.\|.\|> .ce 0 .sp 1P .LP .sp 1 .bp .sp 1P .LP G.5 \fIDate and time indication\fR .sp 9p .RT .PP The \fIdate and time indication\fR \|will inform the subscriber of the date and time the call is established. .PP The format for the \fIdate and time indication\fR \|is as follows: \v'6p' .RT .sp 1P .ce 1000 <\|/\|> <5> <\|/\|> <YY\(hyMM\(hyDD\(hyhh:mm> .ce 0 .sp 1P .LP .sp 1 where .LP <5> is the DCE\(hyprovided information identification number .LP YY: year, MM: month, DD: day, hh: hour, mm: minute .PP IA5 characters are used for \*QYY\*U, \*QMM\*U, \*QDD\*U, \*Qhh\*U, \*Qmm\*U, \ \*Q\(em\*U and \*Q:\*U. .sp 1P .LP G.6 \fICharacteristics of the call\fR .sp 9p .RT .PP The \fIcharacteristics of the call\fR \|will inform the called DTE of the different facilities that have been requested by the calling DTE. .PP The format of the \fIcharacteristic of the call\fR is as follows: \v'6p' .RT .sp 1P .ce 1000 <\|/\|> <6> <\|/\|> <xy> .ce 0 .sp 1P .LP .sp 1 where .LP x and y are two numerical characters. .PP Table G\(hy2/X.20 indicates the allocation of values of these two characters to facilities. .LP .sp 1 .ce \fBH.T. [T7.20]\fR .ce TABLE\ G\(hy2/X.20 .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(60p) | lw(60p) . 00 Reserved .T& cw(60p) | lw(60p) . 01 Reverse charging .T& cw(60p) | lw(60p) . 02 Reserved .T& cw(60p) | lw(60p) . 03 Reserved _ .TE .nr PS 9 .RT .ad r \fBTable G\(hy2/X.20 [T7.20], p.\fR .sp 1P .RT .ad b .RT .LP .sp 1 .sp 1P .LP G.7 \fIType of call indication\fR .sp 9p .RT .PP The \fItype of call indication\fR \| will inform the called DTE of the configuration of the incoming call. .PP The format of the \fItype of call indication\fR \| is as follows: \v'6p' .RT .sp 1P .ce 1000 <\|/\|> <7> <\|/\|> <xy> .ce 0 .sp 1P .LP .sp 1 where .LP x and y are two numerical characters. .PP Table G\(hy3/X.20 indicates the allocation of values of those two characters to different configurations of calls. .bp .ce \fBH.T. [T8.20]\fR .ce TABLE\ G\(hy3/X.20 .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(60p) | lw(60p) . 00 Reserved .T& cw(60p) | lw(60p) . 01 Reserved .T& cw(60p) | lw(60p) . 02 Reserved .T& cw(60p) | lw(60p) . 03 Centralized multipoint .T& cw(60p) | lw(60p) . 04 Reserved _ .TE .nr PS 9 .RT .ad r \fBTable G\(hy3/X.20 [T8.20], p.\fR .sp 1P .RT .ad b .RT .sp 1P .LP G.8 \fIClosed user group\fR \fIindication\fR .sp 9p .RT .PP The \fIclosed user group indication\fR \|will inform the called DTE to which closed user group the indication call belongs. .PP The format of the \fIclosed user group indication\fR \|is as follows: \v'6p' .RT .sp 1P .ce 1000 <\|/\|> <81> <\|/\|> <xxxx\|.\|.\|.\|x> .ce 0 .sp 1P .LP .sp 1 where .LP <x> is the closed user group index number. .sp 1P .LP G.8.1 \fIClosed user group outgoing access\fR \fIindication\fR .sp 9p .RT .PP The \fIclosed user group outgoing access\fR \|indication will inform the called DTE from a DTE belonging to a closed user group with outgoing access facility. If the called DTE belongs to the same closed user group, the local user group index number will be indicated. In other cases no indication will be given. .PP The format of the \fIclosed user group outgoing access indication\fR \|is as follows: \v'6p' .RT .sp 1P .ce 1000 <\|/\|> <82> <\|/\|> <xx\|.\|.\|.\|x> .ce 0 .sp 1P .LP .sp 1 where .LP <x> is the closed user group index number. .sp 2P .LP \fBRecommendation\ X.20\fR \|\fIbis\fR .RT .sp 2P .ce 1000 \fBUSE\ ON\ PUBLIC\ DATA\ NETWORKS\ OF\ DATA\ TERMINAL\fR .EF '% Fascicle\ VIII.2\ \(em\ Rec.\ X.20^bis'' .OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.20^bis %' .ce 0 .ce 1000 \fBEQUIPMENT\ (DTE)\ WHICH\ IS\ DESIGNED\ FOR\ INTERFACING\fR .ce 0 .sp 1P .ce 1000 \fBTO\ ASYNCHRONOUS\ DUPLEX\ V\(hySERIES\ MODEMS\fR .ce 0 .sp 1P .ce 1000 \fI(Geneva, 1976; amended at Geneva, 1980,\fR .sp 9p .RT .ce 0 .sp 1P .ce 1000 \fIMalaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR .ce 0 .sp 1P .sp 2P .LP The\ CCITT, .sp 1P .RT .sp 1P .LP \fIconsidering\fR .sp 9p .RT .PP (a) that the interface between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) for start\(hystop transmission on public data networks is specified in Recommendation\ X.20, .PP (b) that several Administrations are planning to provide as an interim measure the connection to public data networks of start\(hystop DTEs which are designed for interfacing to V\(hySeries modems for start\(hystop transmission, .bp .sp 1P .LP \fIunanimously declares\fR .sp 9p .RT .PP that the interface between a V\(hySeries type DTE and a DCE in public data networks for user classes of service employing start\(hystop transmission should be as defined in this Recommendation. .sp 2P .LP \fB1\fR \fBScope\fR .sp 1P .RT .PP This Recommendation applies to the interface between a DTE designed for interfacing to duplex V\(hySeries modems for start\(hystop transmission and a DCE on public data networks. .PP The operation is limited to start\(hystop transmission at data signalling rates and character structures specified for start\(hystop transmission in Recommendation\ X.1. .PP The application comprises: .RT .LP a) circuit switched service, .LP b) leased circuit service (point\(hyto\(hypoint and centralized multipoint). .sp 2P .LP \fB2\fR \fBInterchange circuits\fR .sp 1P .RT .sp 1P .LP 2.1 \fIFunctional characteristics\fR .sp 9p .RT .PP The functional characteristics of the interchange circuits concerned (see Table\ 1/X.20\|\fIbis\fR ) comply with Recommendation\ V.24. .RT .ce \fBH.T. [T9.20]\fR .ce TABLE\ 1/X.20\|\fIbis\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(180p) . Interchange circuit _ .T& cw(36p) | cw(144p) . Number Designation _ .T& lw(36p) | lw(144p) . 102 T{ Signal ground or common return T} .T& lw(36p) | lw(144p) . 103 Transmitted data .T& lw(36p) | lw(144p) . 104 Received data .T& lw(36p) | lw(144p) . 106 Ready for sending .T& lw(36p) | lw(144p) . 107 Data set ready .T& lw(36p) | lw(144p) . 108/1\ua\d\u)\d Connect data set to line .T& lw(36p) | lw(144p) . 108/2\ub\d\u)\d Data terminal ready .T& lw(36p) | lw(144p) . 109 T{ Data channel received line signal detector T} .T& lw(36p) | lw(144p) . 125\uc\d\u)\d Calling indicator .T& lw(36p) | lw(144p) . 141\ud\d\u)\d Local loop back .T& lw(36p) | lw(144p) . 142 Test indicator .TE .LP \ua\d\u)\d\ Used in case of automatic control of the direct call facility. .LP \ub\d\u)\d\ Used in case of switched data network service. .LP \uc\d\u)\d\ Not provided in leased circuit service. .LP \ud\d\u)\d\ Not provided in those networks which do not provide automatic activation of the test loops. .nr PS 9 .RT .ad r \fBTable 1/X.20\|bis [T9.20], p.\fR .sp 1P .RT .ad b .RT .sp 1P .LP 2.2 \fIElectrical characteristics\fR .sp 9p .RT .PP The electrical characteristics of the interchange circuits comply with Recommendation\ V.28, using the 25\(hypole DTE/DCE interface connector and contact number assignments in ISO Standard\ 2110. .bp .RT .sp 2P .LP \fB3\fR \fBUse of interchange circuits\fR .sp 1P .RT .sp 1P .LP 3.1 \fIOperation of interchange circuit 107 \(em Data set ready\fR .sp 9p .RT .PP This circuit is used to indicate the operational functions given in Table\ 2/X.20\|\fIbis\fR . .RT .LP .sp 2 .ce \fBH.T. [T10.20]\fR .ce TABLE\ 2/X.20\|\fIbis\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(60p) | cw(120p) . Condition of circuit 107 Meaning in the data network _ .T& lw(60p) | lw(120p) . ON Ready for data (see Note) .T& lw(60p) | lw(120p) . OFF DCE clear indication .T& lw(60p) | lw(120p) . OFF DCE clear confirmation .TE .LP \fINote\fR \ \(em\ Since no circuit 105 is operated, the ON condition on circuit 106 is applied\ 0 to 20\ milli seconds after circuit\ 107 is turned ON. .nr PS 9 .RT .ad r \fBTable 2/X.20\|bis [T10.20], p.\fR .sp 1P .RT .ad b .RT .LP .sp 2 .sp 2P .LP 3.2 \fIUse of interchange circuits 108/1 and 108/2\fR .sp 1P .RT .sp 1P .LP 3.2.1 \fICircuit 108/1 \(em Connect data set to line\fR .sp 9p .RT .PP This circuit is used alternatively to circuit 108/2. The operational functions given in Table\ 3/X.20\|\fIbis\fR should be indicated. .RT .LP .sp 2 .ce \fBH.T. [T11.20]\fR .ce TABLE\ 3/X.20\|\fIbis\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(60p) | cw(120p) . Condition of circuit 108/1 Meaning in the data network _ .T& lw(60p) | lw(120p) . ON T{ Call request for direct call (see \(sc 3.4.1) T} .T& lw(60p) | lw(120p) . ON Call accepted .T& lw(60p) | lw(120p) . OFF DTE clear request .T& lw(60p) | lw(120p) . OFF T{ DTE clear confirmation (see \(sc 3.4.4) T} _ .TE .nr PS 9 .RT .ad r \fBTable 3/X.20\|bis [T11.20], p.\fR .sp 1P .RT .ad b .RT .LP .sp 2 .sp 1P .LP 3.2.2 \fICircuit 108/2 \(em Data terminal ready\fR .sp 9p .RT .PP This circuit is used alternatively to circuit 108/1. The operational functions given in Table\ 4/X.20\|\fIbis\fR should be indicated. .bp .RT .ce \fBH.T. [T12.20]\fR .ce TABLE\ 4/X.20\|\fIbis\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(60p) | cw(120p) . Condition of circuit 108/2 Meaning in the data network _ .T& lw(60p) | lw(120p) . ON Call accepted .T& lw(60p) | lw(120p) . OFF DTE clear request .T& lw(60p) | lw(120p) . OFF T{ DTE clear confirmation (see \(sc 3.4.4) T} _ .TE .nr PS 9 .RT .ad r \fBTable 4/X.20\|bis [T12.20], p.\fR .sp 1P .RT .ad b .RT .sp 1P .LP 3.3 \fICircuit 125 \(em Calling indicator\fR .sp 9p .RT .PP The ON condition indicates \fIincoming call\fR . The circuit will be turned OFF as follows: .RT .LP \(em in conjunction with circuit 107 turned ON, or .LP \(em \fIDCE ready\fR \| is received from the network, or .LP \(em \fIDCE clear indication\fR \| is received from the network. .sp 2P .LP 3.4 \fIOperational requirements for circuits 106, 107, 108/1, 108/2\fR \fIand 109\fR .sp 1P .RT .sp 1P .LP 3.4.1 \fICall request for direct call\fR .sp 9p .RT .PP For a \fIdirect call\fR \|facility the DTE indicates a call request by turning circuit 108/1 ON. Circuit 108/2 cannot be used for this purpose. .RT .sp 1P .LP 3.4.2 \fICall accepted\fR .sp 9p .RT .PP A DTE receiving an \fIincoming call\fR \|should turn circuit 108/1 or 108/2 from OFF to ON within 500\ ms to indicate \fIcall accepted\fR , otherwise the call will be cleared. A DCE presenting an \fIincoming call\fR to a DTE which already has circuit\ 108/2 ON will regard the ON condition on circuit\ 108/2 as an indication of \fIcall accepted\fR . .PP Optionally when a DTE does not provide circuit 108/1 or 108/2, the \fIcall accepted\fR \|signal to the network would be generated within the DCE as an answer to the \fIincoming call\fR signal received from the network. However, it may also be possible to signal to the network a \fIDTE controlled not ready\fR by a manual action on the DCE. .RT .sp 1P .LP 3.4.3 \fIOperation of interchange circuits 109 and 106\fR .sp 9p .RT .PP The DCE switches circuit 109 to ON together with circuit 107. Circuit\ 106 is put to ON 0 to 20\ ms after the appearance of the ON condition on circuit\ 107. .PP The circuits 109 and 106 are switched to OFF either when circuit 108 is switched to OFF or when circuit\ 108 is ON and the DCE signals \fIDCE clear\fR \fIindication\fR (see \(sc\ 3.4.4). .RT .sp 1P .LP 3.4.4 \fIDCE clear indication and DTE clear confirmation\fR .sp 9p .RT .PP \fIDCE clear indication\fR \|is signalled to the DTE by turning circuit 107 OFF. The \fIDTE clear confirmation\fR , when implemented, should be given by the DTE turning OFF circuit 108/1 or 108/2 within 500\ ms after the \fIDCE clear\fR \fIindication\fR is signalled on circuit\ 107. Otherwise, the DCE may consider the DTE as being \fIuncontrolled not ready\fR until circuit\ 108/1 or 108/2 is turned OFF or a \fIready\fR signal is generated by a manual action on the DCE. .PP Circuit 108/1 should always be able to give \fIDTE clear confirmation\fR . .PP Optionally, when a DTE does not turn circuit 108/2 OFF for \fIDTE clear\fR \fIconfirmation\fR \| this would be automatically generated within the DCE as an answer to the \fIclear indication\fR received from the network and the DTE will be considered in the \fIready\fR condition. .bp .PP In the case when the DTE expects to have circuit 107 OFF only as a response to circuit\ 108/1 or 108/2 OFF, the DCE will not turn circuit\ 107 OFF as a \fIDCE clear indication\fR and in this case the DCE indication will not be signalled to the DTE across the interface. The necessary \fIDTE clear\fR \fIconfirmation\fR signal will then be automatically generated within the DCE as an answer to the \fIclear indication\fR signal received from the network. The DTE may be regarded as \fIuncontrolled not ready\fR until circuit 108/1 or 108/2 is turned OFF. .RT .sp 1P .LP 3.4.5 \fICentralized multipoint operation\fR .sp 9p .RT .PP As the circuits 106 and 109 are always in the ON condition, the transmission disciplines must be determined by end\(hyto\(hyend control procedures of the DTEs. .RT .sp 2P .LP \fB4\fR \fBCall progress signals and DCE provided information\fR .sp 1P .RT .PP \fICall progress signals\fR \| and \fIDCE provided information\fR \|cannot be handled by V\(hySeries DTEs. .RT .sp 2P .LP \fB5\fR \fBFailure detection and isolation\fR .sp 1P .RT .sp 1P .LP 5.1 \fIFault conditions of interchange circuits\fR .sp 9p .RT .PP If the DTE or DCE is unable to determine the condition of circuits 107, 108/1 or 108/2 and possibly circuits 103 and\ 104, it shall interpret this as an OFF condition or binary\ 1 (circuits 103 and\ 104) as specified in the relevant electrical interface specifications. .RT .sp 1P .LP 5.2 \fIDCE fault conditions\fR .sp 9p .RT .PP If the DCE is unable to provide service (e.g. loss of incoming line signal) for a period longer than a fixed duration it will turn circuit\ 107 to the OFF condition. The value of this duration is network dependent. .RT .sp 1P .LP 5.3 \fITest loops\fR .sp 9p .RT .PP The definitions of the test loops and the principles of maintenance testing using the test loops are provided in Recommendation\ X.150. .RT .sp 1P .LP 5.3.1 \fIDTE test loop\fR \fI\(em type 1 loop\fR .sp 9p .RT .PP This loop is used as a basic test of the operation of the DTE, by looping back the transmitted signals inside the DTE for checking. The loop should be set up inside the DTE as close as possible to the DTE/DCE interface. .PP Except as noted below, while the DTE is in loop 1 test condition: .RT .LP \(em circuit 103 is connected to circuit 104 inside of the DTE; .LP \(em circuit 103 as presented to the DCE must be in the binary 1 condition; .LP \(em circuit 108/1 or 108/2 may be in the same condition as it was before the test; .LP \(em circuits 140 and 141, if implemented, must be in the OFF condition. .PP The conditions of the other interchange circuits are not specified but they should if possible permit normal working. .PP Loop 1 may be established from either the \fIdata transfer\fR \|phase or the \fIidle\fR phase. .PP If the loop is established from the \fIdata transfer\fR \|phase, the DCE may continue to deliver data to the DTE during the test as though the DTE were in normal operation. It will be the responsibility of the DTE to recover from any errors that might occur while the test loop is activated. .PP If the loop is established from the \fIidle\fR \|phase, the DTE should continue to monitor circuit\ 125 so that an incoming call may be given priority over a routine test. .bp .RT .sp 1P .LP 5.3.2 \fILocal test loop\fR \fI\(em type 3 loop\fR .sp 9p .RT .PP Local test loops (type 3 loops) are used to test the operation of the DTE, the interconnecting cable and either all or parts of the local DCE, as discussed below. .PP Where allowed by national testing principles loop 3 may be established from any state. .PP For testing on leased circuits and for short duration testing, on circuit\(hyswitched connections the DCE should continue to present toward the line the conditions that existed before the test (e.g. either \fIdata\fR \fItransfer\fR or \fIready\fR state). Where this is not practical (e.g. in some cases for loop\ 3a) or desirable (e.g. for long duration testing in circuit\(hyswitched applications) the DCE should terminate an existing call. .PP Manual control should be provided on the DCE for activation of the test loop. .PP The automatic activation on this loop, if provided, should be controlled by circuit\ 141. .PP The precise implementation on the test loop within the DCE is a national option. At least one of the following local test loops should be implemented. .RT .sp 1P .LP 5.3.2.1 \fILoop 3d\fR .sp 9p .RT .PP This loop is used to test the operation of the DTE, including the interconnecting cable, by returning transmitted signals to the DTE for checking. The loop is set up inside the local DCE and does not include interchange circuit generators and loads. .PP While the DCE is in the loop 3d test condition: .RT .LP \(em circuit 103 is connected to the circuit 104; .LP \(em circuits 107 and 142 are placed in the ON condition. .PP \fINote\fR \ \(em\ While test loop 3d is operated, the effective length of the interconnecting cable is doubled. Therefore, to insure proper operation of loop\ 3d, the maximum DTE/DCE interface cable length should be one\(hyhalf the length normally appropriate for the data signalling rate in use. .sp 1P .LP 5.3.2.2 \fILoop 3c\fR .sp 9p .RT .PP This loop is used to test the operation of the DTE, including the interconnecting cable and DCE interchange circuit generators and loads. .PP The configuration is identical to that given for loop 3d in \(sc\ 5.3.2.1 with the exception that the looping of circuit\ 103 to circuit\ 104 includes the interchange circuit generators and loads. The notes concerning restriction of interface cable length and load input impedance are not applicable. .RT .sp 1P .LP 5.3.2.3 \fILoop 3b\fR .sp 9p .RT .PP This loop is used as a test of the operation of the DTE and the line coding and control logic and circuitry of the DCE. It includes all the circuitry of the DCE with the exclusion of the line signal conditioning circuitry (e.g., impedance matching transformers, amplifiers, equalizers, etc.). The delay between transmitted and received test data is a few octets. (See Note). .PP The configuration is identical to that given for loop 3c in \(sc\ 5.3.2.2 except for the location of the point of loopback. .PP \fINote\fR \ \(em\ In some networks the setting of loop 3b will cause clearing of existing connections. .RT .sp 1P .LP 5.3.2.4 \fILoop 3a\fR .sp 9p .RT .PP This loop is used to test the operation of the DTE and the DCE. The loop should include the maximum amount of circuitry used in DCE working including, in particular, the line signal conditioning circuitry. It is recognized that, in some cases, the inclusion of devices (e.g., attenuators, equalizers or test loop translators) may be necessary in the loopback path. The subscriber line is suitably terminated during a loop\ 3a test condition. .bp .PP The configuration is identical to that given for test loop 3b in \(sc\ 5.3.2.3 except for the location of the point of loopback. .PP \fINote\fR \ \(em\ In some networks the setting of loop 3a will cause clearing of existing connections. .RT .sp 1P .LP 5.3.3 \fINetwork test loop\fR \fI\(em type 2 loop\fR .sp 9p .RT .PP Network test loops (test 2 loops) are used by the Administration's test centre to test the operation of the leased line or subscriber line and either all or part of the DCE, as discussed below. .PP Where, allowed by national testing principles loop 2 may be used by a DTE, as follows: .RT .LP a) In the case of switched circuit networks in the \fIdata\fR \fItransfer\fR \| phase to test the operation of the network connection including the remote DCE. It should be possible to reenter the \fIdata transfer\fR phase after completion of the network loop test. .LP b) In the case of leased lines in the \fIidle\fR \| phase to test the operation of the line including the remote DCE. When the test is in progress the DCE will return circuits\ 107 and\ 109 in the OFF condition, circuit\ 104 in the binary\ 1 condition and circuit\ 142 in the ON condition. .PP The loop may be controlled manually by a switch on the DCE or automatically by the network. The control of the loop and the method used for automatic control, when implemented, is a national option. .PP In case of a collision between a \fIcall request\fR \|and the activation of the loop, the loop activation command will have priority and the call request is ignored. .PP The precise implementation of the test loop within the DCE is a national option. One of the following network test loops should be implemented: .RT .sp 1P .LP 5.3.3.1 \fILoop 2b\fR .sp 9p .RT .PP This loop is used by the Administration's test centre(s) and/or the remote DTE to test the operation of the subscriber line and all the circuitry of the DCE with the exception of interchange circuit generators and loads. .PP While the DCE is in the loop 2b test condition: .RT .LP \(em circuit 104 is connected to circuit 103 inside of the DCE, .LP \(em at the interface, the DCE places circuit 104 in the binary\ 1 condition and circuit\ 109 in the OFF condition, or alternatively, may present an open circuit or power off condition on circuits\ 104 and\ 109, .LP \(em circuits 106, 107 and 125 to the DTE are placed in the OFF condition, .LP \(em circuit 142 to the DTE is placed in the ON condition. .sp 1P .LP 5.3.3.2 \fILoop 2a\fR .sp 9p .RT .PP This loop is used by either the Administration's test centre(s) or the remote DTE to test the operation of the subscriber line and the entire DCE. .PP The configuration is identical to that given for loop 2b in \(sc\ 5.3.3.1 except for the location of the point of loopback. .RT .sp 1P .LP 5.3.4 \fISubscriber\(hyline test loop\fR \fI\(em type 4 loop\fR .sp 9p .RT .PP Subscriber\(hyline test loops (type 4 loops) are provided for the maintenance of lines by the Administrations. .RT .sp 1P .LP 5.3.4.1 \fILoop 4a\fR .sp 9p .RT .PP This loop is only provided in the case of 4\(hywire subscriber lines. Loop\ 4a is for the maintenance of lines by Administrations. When receiving and transmitting pairs are connected together, the resulting circuit cannot be considered normal. Loop\ 4a may be established inside the DCE or in a separate device. .bp .PP While the DCE is in the loop 4a test condition: .RT .LP \(em circuit 104 to the DTE is placed in the binary 1 condition, .LP \(em circuits 106, 107, 109 and 125 to the DTE are placed in the OFF condition, .LP \(em circuit 142 to the DTE is placed in the ON condition. .sp 1P .LP 5.3.4.2 \fILoop 4b\fR .sp 9p .RT .PP This loop is used by Administrations to test the operation of the subscriber line including the line signal conditioning circuitry in the DCE. When the receiving and transmitting circuits are connected at this point, loop\ 4b provides a connection that can be considered as normal; however, some impairment of the performance is expected since the DCE does not perform a complete signal regeneration. .PP The configuration is identical to that given for loop 4a in \(sc\ 5.3.4.1 except for the location of the point of loopback. \v'1P' .RT .sp 2P .LP \fBRecommendation\ X.21\fR .RT .sp 2P .ce 1000 \fBINTERFACE\ BETWEEN\ DATA\ TERMINAL\ EQUIPMENT\ (DTE)\fR .EF '% Fascicle\ VIII.2\ \(em\ Rec.\ X.21'' .OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.21 %' .ce 0 .ce 1000 \fBAND\ DATA\ CIRCUIT\(hyTERMINATING\ EQUIPMENT\ (DCE)\ FOR\fR .ce 0 .sp 1P .ce 1000 \fBSYNCHRONOUS\ OPERATION\ ON\ PUBLIC\ DATA\ NETWORKS\fR .ce 0 .sp 1P .ce 1000 \fI(Geneva, 1972; amended at Geneva, 1976 and 1980,\fR .sp 9p .RT .ce 0 .sp 1P .ce 1000 \fIMalaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR .ce 0 .sp 1P .ce 1000 CONTENTS .ce 0 .sp 1P .LP Preface .sp 1P .RT .sp 2P .LP 1 Scope .sp 1P .RT .LP 2 DTE/DCE physical interface elements .LP 3 Alignment of call control characters and error checking .LP 4 Elements of the call control phase for circuit switched service .LP 5 Data transfer phase .LP 6 Clearing phase .LP 7 Test loops .sp 2P .LP \fIAnnex\ A\fR \ \(em Interface signalling state diagrams .sp 1P .RT .LP \fIAnnex\ B\fR \ \(em Interface signalling sequence diagrams and time\(hyout operation .LP \fIAnnex\ C\fR \ \(em DTE time\(hylimits and DCE time\(hyouts .LP \fIAnnex\ D\fR \ \(em Formats of Selection, Call progress and line identification signals .LP \fIAnnex\ E\fR \ \(em Interworking between DTEs conforming to Recommendations\ X.21 and X.21\|\fIbis\fR .LP \fIAnnex\ F\fR \ \(em Coding of Call progress signals .LP \fIAnnex\ G\fR \ \(em Facility request, Indicator and Parameter coding .LP \fIAnnex\ H\fR \ \(em Information content of DCE\(hyprovided information .LP \fIAnnex\ I\fR \ \(em Cross reference and transition tables .bp .LP \fBPreface\fR .sp 1P .RT .sp 2P .LP The\ CCITT, .sp 1P .RT .sp 1P .LP \fIconsidering\fR .sp 9p .RT .PP (a) that Recommendations X.1 and X.2 define the services and facilities to be provided by a public data network; .PP (b) that Recommendation X.92 defines the hypothetical reference connections for synchronous public data networks; .PP (c) that Recommendation X.96 defines \fIcall progress\fR \| signals ; .PP (d) that the necessary elements for an interface Recommendation should be defined in architectural levels; .PP (e) that it is desirable for characteristics of the interface between the DTE and DCE of a public data network to be standardized, .sp 1P .LP \fIunanimously declares the view\fR .sp 9p .RT .PP that the interface between the DTE and DCE in public data networks for user classes of service employing synchronous transmission should be as defined in this Recommendation. .sp 2P .LP \fB1\fR \fBScope\fR .sp 1P .RT .PP 1.1 This Recommendation defines the physical characteristics and call control procedures for a general purpose interface between DTE and DCE for user classes of service, as defined in Recommendation\ X.1, employing synchronous transmission. .sp 9p .RT .PP 1.2 The formats and procedures for \fIselection\fR , \fIcall progress\fR \|and \fIDCE\(hyprovided information\fR \| are included in this Recommendation. .PP 1.3 The provision for duplex operation is covered. .PP 1.4 The operation of the interface for half duplex operation when the data circuit interconnects with Recommendation\ X.21\|\fIbis\fR DTEs is described in Annex\ E. Half duplex operation between X.21 DTEs is for further study when such new facilities are identified. .LP \fB2\fR \fBDTE/DCE physical interface elements\fR .sp 1P .RT .sp 2P .LP 2.1 \fIElectrical characteristics\fR .sp 1P .RT .sp 1P .LP 2.1.1 \fIData signalling rates of 9600 bit/s and below\fR .sp 9p .RT .PP The electrical characteristics of the interchange circuits at the DCE side of the interface will comply with Recommendation\ X.27 without cable termination in the load. The electrical characteristics at the DTE side of the interface may be applied according to either Recommendation\ X.27 either with or without cable termination in the load, or Recommendation\ X.26. The B` leads of receivers in an X.26 DTE must be brought out to the interface individually and not connected together. (See \(sc\ 2.2 below.) .PP \fINote\fR \ \(em\ In certain instances where X.27 circuits are implemented on both sides of the interface, it may be necessary to add either serial impedance matching resistors or parallel cable terminating resistors as specified in X.27 to assure proper operation of the interchange circuits. .RT .sp 1P .LP 2.1.2 \fIData signalling rates above 9600 bit/s\fR .sp 9p .RT .PP The electrical characteristics of the interchange circuits at both the DCE side and the DTE side of the interface will comply with Recommendation\ X.27 with or without implementation of the cable termination in the load. .bp .RT .sp 1P .LP 2.2 \fIMechanical characteristics\fR .sp 9p .RT .PP Refer to ISO 4903 (15\(hypole DTE/DCE interface connector and contact number assignments) for mechanical arrangements. .RT .sp 1P .LP 2.3 \fIFunctional characteristics of\fR \fIinterchange circuits\fR .sp 9p .RT .PP Definitions of the interchange circuits concerned (see Table\ 1/X.21) are given in Recommendation\ X.24. .PP In this Recommendation, signal conditions on interchange circuits T, C, R, and I are designated by t, c, r, and i, respectively. .PP Signal conditions on circuit C (\fIControl\fR ) and I (\fIIndication\fR ) refer to continuous ON (significant level binary\ 0) and continuous OFF (significant level binary\ 1) conditions. .RT .LP .sp 1 .ce \fBH.T. [T1.21]\fR .ce TABLE\ 1/X.21 .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(42p) | cw(60p) | cw(30p) sw(30p) | cw(42p) , ^ | ^ | c | c | ^ . Interchange circuit Name Direction Remarks to DCE from DCE _ .T& cw(42p) | lw(60p) | lw(30p) | lw(30p) | cw(42p) . G T{ Signal ground or common return T} See Note 1 .T& cw(42p) | lw(60p) | cw(30p) | lw(30p) | cw(42p) . G a DTE common return X .T& cw(42p) | lw(60p) | cw(30p) | lw(30p) | cw(42p) . T Transmit X .T& cw(42p) | lw(60p) | cw(30p) | cw(30p) | cw(42p) . R Receive X .T& cw(42p) | lw(60p) | cw(30p) | cw(30p) | cw(42p) . C Control X .T& cw(42p) | lw(60p) | cw(30p) | cw(30p) | cw(42p) . I Indication X .T& cw(42p) | lw(60p) | cw(30p) | cw(30p) | cw(42p) . S Signal element timing X See Note 2 .T& cw(42p) | lw(60p) | cw(30p) | cw(30p) | cw(42p) . B Byte timing X See Note 3 .T& cw(42p) | lw(60p) | cw(30p) | cw(30p) | cw(42p) . X DTE signal element timing X See Note 4 .TE .LP \fINote 1\fR \ \(em\ This conductor may be used to reduce environmental signal interference at the interface. In the case of shielded interconnecting cable, the additional connection considerations are part of Recommendation\ X.24 and ISO\ 4903. .LP \fINote 2\fR \ \(em\ Timing for continuous isochronous data transmission will be provided. .LP \fINote 3\fR \ \(em\ May be provided as an optional additional facility (see \(sc\ 3.1.1 below). .LP \fINote 4\fR \ \(em\ The use and the termination of this circuit by the DCE is a national matter. .nr PS 9 .RT .ad r \fBTable 1/X.21 [T1.21], p.\fR .sp 1P .RT .ad b .RT .LP .sp 1 .sp 1P .LP 2.4 \fIPhysical link control\fR \fIconditions\fR .sp 9p .RT .PP The DTE and DCE shall be prepared to send steady binary conditions\ 0 and\ 1 on circuit R or T, together with associated conditions on circuit\ C or I, for a period of at least 24\ bit intervals. Detection of steady binary\ 0 or\ 1 on circuit\ R or\ T for 16 contiguous bit intervals with the associated condition on circuit\ I or\ C may be interpreted by the DTE or DCE as a steady state condition. .PP If the DTE (or DCE) recognizes that the device on the other side of the interface is signalling recognition of the current state, then the DTE (or DCE) may begin signalling the next valid state. If the DTE (or DCE) is not ready to begin signalling the next valid state, it is obliged to continue signalling the current state until it is so ready. .PP \fINote\fR \ \(em\ As for state 12, \(sc 5.1 has precedence over this \(sc\ 2.4. .bp .RT .sp 1P .LP 2.5 \fIQuiescent phase\fR .sp 9p .RT .PP During the quiescent phase, the DTE and the DCE signal their ability to enter operational phases such as the call control phase or the data transfer phase as defined for the appropriate service. The basic quiescent signals of the DTE and the DCE can appear at the interface in various combinations which result in different interface states as defined below and shown in Figure\ A\(hy1/X.21. .RT .sp 2P .LP 2.5.1 \fIDTE quiescent signals\fR .sp 1P .RT .sp 1P .LP 2.5.1.1 \fIDTE ready\fR .sp 9p .RT .PP The DTE indicates its readiness to enter operational phases, according to the appropriate service, by signalling t\ =\ 1, c\ =\ OFF. .RT .sp 1P .LP 2.5.1.2 \fIDTE uncontrolled not ready\fR .sp 9p .RT .PP The DTE indicates that it is unable to enter operational phases, according to the appropriate service, generally because of abnormal operating conditions, by signalling t\ =\ 0, c\ =\ OFF. .PP For leased circuit service point\(hyto\(hypoint when the DTE enters \fIDTE\fR \fIuncontrolled not ready\fR , the remote interface may signal r\ =\ 0, i\ =\ OFF. Additional actions to be taken by the DCE are for further study. .PP For leased circuit\(hycentralized multipoint when a DTE enters \fIDTE\fR \fIuncontrolled not ready\fR , no indication of this signal will be given at the other connected DTE/DCE interfaces. .RT .sp 1P .LP 2.5.1.3 \fIDTE controlled not ready\fR .sp 9p .RT .PP \fIDTE controlled not ready\fR \|indicates that, although the DTE is operational, it is temporarily unable to accept incoming calls for circuit switched service. .PP This signal is indicated by t\ =\ 01 .\|.\|. (alternate bits are binary\ 0 and binary\ 1), c\ =\ OFF. This signal shall persist for a minimum of 24\ bit intervals. .PP \fINote\fR \ \(em\ \fIDTE controlled not ready\fR \|is normally entered from the \fIready\fR state, as defined in \(sc\ 2.5.3.1 below. In some networks, the DCE may not recognize the \fIDTE controlled not ready\fR signal if the DTE does not first signal \fIDTE ready\fR at the same time the DCE signals \fIDCE ready\fR . .RT .sp 2P .LP 2.5.2 \fIDCE quiescent signals\fR .sp 1P .RT .sp 1P .LP 2.5.2.1 \fIDCE ready\fR .sp 9p .RT .PP The DCE indicates its readiness to enter operational phases, according to the appropriate service, by signalling r\ =\ 1, i\ =\ OFF. .RT .sp 1P .LP 2.5.2.2 \fIDCE not ready\fR .sp 9p .RT .PP \fIDCE not ready\fR \|indicates that no service is available and will be signalled whenever possible during network fault conditions and when test loops are activated. This signal is indicated by r\ =\ 0, i\ =\ OFF. .RT .sp 1P .LP 2.5.2.3 \fIDCE controlled not ready\fR .sp 9p .RT .PP \fIDCE controlled not ready\fR \|indicates that, although the DCE is operational, it is temporarily unable to render service. .PP This signal is indicatd by r = 01\ .\|.\|.\ (alternate bits are binary 0 and binary\ 1), i\ =\ OFF. This signal shall persist for a minimum of 24\ bit intervals. .PP \fINote\ 1\fR \ \(em\ \fIDCE controlled not ready\fR \|may be entered from any state. .PP \fINote\ 2\fR \ \(em\ \fIDCE controlled not ready\fR \|may be provided as an optional facility. .bp .RT .sp 2P .LP 2.5.3 \fIQuiescent states\fR \|(see Figure A\(hy1/X.21) .sp 1P .RT .sp 1P .LP 2.5.3.1 \fIReady (state 1)\fR .sp 9p .RT .PP \fIReady\fR \|is entered when the DTE and the DCE simultaneously signal \fIDTE ready\fR \| and \fIDCE ready\fR , respectively. .RT .sp 1P .LP 2.5.3.2 \fIState 14\fR .sp 9p .RT .PP State 14 is entered when the DTE and the DCE simultaneously signal \fIDTE controlled not ready\fR \| and \fIDCE ready\fR , respectively. .RT .sp 1P .LP 2.5.3.3 \fIState 18\fR .sp 9p .RT .PP State 18 is entered when the DTE and the DCE simultaneously signal \fIDTE ready\fR \| and \fIDCE not ready\fR , respectively. .RT .sp 1P .LP 2.5.3.4 \fIState 22\fR .sp 9p .RT .PP State 22 is entered when the DTE and the DCE simultaneously signal \fIDTE uncontrolled not ready\fR \| and \fIDCE not ready\fR , respectively. .RT .sp 1P .LP 2.5.3.5 \fIState 23\fR .sp 9p .RT .PP State 23 is entered when the DTE and the DCE simultaneously signal \fIDTE controlled not ready\fR \| and \fIDCE not ready\fR , respectively. .RT .sp 1P .LP 2.5.3.6 \fIState 24\fR .sp 9p .RT .PP State 24 is entered when the DTE and the DCE simultaneously signal \fIDTE uncontrolled not ready\fR \| and \fIDCE ready\fR , respectively. .RT .sp 2P .LP 2.6 \fIFailure detection\fR .sp 1P .RT .sp 1P .LP 2.6.1 \fIFault conditions of interchange circuits\fR .FS For the association of the receiver circuit\(hyfailure detection to particular interchange circuits in accordance with the type of failure detection, see Recommendation\ X.26, \(sc\ 11 and Recommendation\ X.27, \(sc\ 9. .FE .sp 9p .RT .PP The DTE should interpret a fault condition on circuit R as r\ =\ 0, using failure detection type\ 2, a fault condition on circuit\ I as i\ =\ OFF, using failure detection type\ 1, and a fault condition on both circuits\ R and\ I as r\ =\ 0, i\ =\ OFF, \fIDCE not ready\fR . Alternatively, a fault condition on one of these circuits, R\ or\ I, may be interpreted by the DTE as \fIDCE not ready\fR , r\ =\ 0, i\ =\ OFF using failure detection type\ 3. .PP The DCE will interpret a fault condition on circuit T as t\ =\ 0, using failure detection type\ 2, a fault condition on circuit C as c\ =\ OFF, using failure detection type\ 1, and a fault condition on both circuits\ T and\ C as t\ =\ 0, c\ =\ OFF, \fIDTE uncontrolled not ready\fR . Alternatively, a fault condition on one of these circuits, T\ or\ C, may be interpreted by the DCE as \fIDTE\fR \fIuncontrolled not ready\fR , t\ =\ 0, c\ =\ OFF using failure detection type\ 3. .RT .sp 1P .LP 2.6.2 \fIDCE fault conditions\fR .sp 9p .RT .PP If the DCE is unable to provide service (e.g., loss of alignment or loss of incoming line signal) after a period longer than a fixed duration, it will indicate \fIDCE not ready\fR by signalling r\ =\ 0, i\ =\ OFF (see \(sc\ 2.5.2.2 above). The value of that duration is network dependent. Prior to this \fIDCE\fR \fInot ready\fR signal, the DTE should be prepared to receive garbled signals or contiguous binary\ 1 on circuit\ R with i\ =\ ON. .RT .sp 1P .LP 2.6.3 \fISignal element timing\fR \fIprovision\fR .sp 9p .RT .PP The signal element timing signal is delivered to the DTE on circuit\ S whenever possible, even when the DCE loses alignment or the incoming line signal. The signal element timing rate should in no case deviate from the nominal value by more than \(+-\|1%. .bp .RT .sp 2P .LP \fB3\fR \fBAlignment of call control characters and error checking\fR .sp 1P .RT .PP All characters for call control purposes are selected from International Alphabet No.\ 5 according to Recommendation\ T.50. .RT .sp 1P .LP 3.1 \fICharacter alignment\fR .sp 9p .RT .PP For the interchange of information between the DTE and the DCE for call control purposes, it is necessary to establish correct alignment of characters. Each sequence of call control characters to and from the DCE shall be preceded by two or more contiguous 1/6 (SYN) characters. .RT .PP 3.1.1 Certain Administrations will require the DTE to align call control characters transmitted from the DTE to either SYN characters delivered to the DTE or to signals on the byte timing interchange circuit. .PP Administrations who require this alignment shall provide the byte timing interchange circuit, but its use and termination by the DTE shall not be mandatory. .PP 3.1.2 Certain Administrations will permit call control characters to be transmitted from the DTE independently of the SYN characters delivered to the DTE. .PP 3.1.3 Additionally, for an intermediate period (see Note), Administrations will provide connection to the public data network of DTEs operating as described in \(sc\ 3.1.2\ above. .PP \fINote\fR \ \(em\ The intermediate period would be determined by customer demand and other relevant factors as interpreted by individual Administrations. .sp 1P .LP 3.2 \fIError checking\fR .sp 9p .RT .PP Odd parity according to Recommendation X.4 applies for IA5 characters interchanged for call control purposes. .RT .sp 2P .LP \fB4\fR \fBElements of the\fR \fBcall control phase for circuit switched\fR \fBservice\fR .sp 1P .RT .PP The state diagram provided in Figure A\(hy2/X.21, shows the relationship between the various \fIcall control\fR \| phase states as defined below, together with the recognized transactions between these states under normal operating conditions. Illustrated examples of the time sequence relationships between these states and associated time\(hyout operation are provided in Figures\ B\(hy1/X.21 and\ B\(hy2/X.21. .PP States which are indicated by an IA5 character on circuits T and R shall be entered and exited on a character boundary. At this time, in some networks, the transition from state\ 6 to state\ 11, or state\ 6 to state\ 12 may not be on a character boundary. .PP Once character alignment has been established by the DCE in response to an outgoing call request, or for presentation of an incoming call, the alignment will be maintained until entering \fIconnection in progress\fR , state\ 11 or \fIready for data\fR if state\ 11 is by\(hypassed. This implies that all IA5 character sequences transmitted on circuit\ R, such as 2/11\ (\*Q+\*U), \fIcall\fR \fIprogress\fR signals, \fIDCE\(hyprovided information\fR , etc., appear on the same character boundary even if they are separated by two or more SYN characters. .PP The call control phase can be terminated by either the DTE or the DCE by \fIclearing\fR \| as defined in \(sc\ 6\ below. .RT .sp 2P .LP 4.1 \fIEvents of the call control procedures\fR \|(see Figure A\(hy2/X.21) .sp 1P .RT .sp 1P .LP 4.1.1 \fICall request\fR \fI(state 2)\fR .sp 9p .RT .PP The calling DTE shall indicate a request for a call by signalling steady binary condition t\ =\ 0, c\ =\ ON, provided that it was previously signalling \fIDTE ready\fR . .PP The change of state from \fIready\fR \|(t\ =\ 1, c\ =\ OFF) to \fIcall request\fR (t\ =\ 0, c\ =\ ON) shall be such that the transition to t\ =\ 0 occurs within a maximum of 7\ bit intervals of the transition to c\ =\ ON. Either transition may occur first. .PP \fINote\fR \ \(em\ When optimizing the use of the byte timing circuit B, the transition to t\ =\ 0 shall occur within the same bit interval as the transition to c\ =\ ON. This might become a requirement for use with special user facilities which might arise from further study. .PP If the DTE signals \fIcall request\fR \|(state 2) and the DCE simultaneously signals r\ =\ 0, i\ =\ OFF, the DCE will be assumed to be in state\ 19 \fI(DCE clear indication)\fR . .bp .RT .sp 1P .LP 4.1.2 \fIProceed to select\fR \fI(state 3)\fR .sp 9p .RT .PP When the network is prepared to receive selection information the DCE will transmit continuously character 2/11\ (\*Q+\*U) preceded by 2 or more contiguous characters 1/6\ (\*QSYN\*U) on the R\ circuit with i\ =\ OFF. .PP \fIProceed\(hyto\(hyselect\fR \|is maintained until receipt of the \fIend\(hyof\(hyselection\fR \| signal, or in the case of \fIdirect call\fR , receipt of \fIDTE waiting\fR . .PP The \fIproceed\(hyto\(hyselect\fR \|signal will start within 3 seconds of the \fIcall request\fR \| being sent. .RT .sp 1P .LP 4.1.3 \fISelection signal sequence\fR \fI(state 4)\fR .sp 9p .RT .PP The \fIselection\fR \|signal sequence shall be transmitted by the DTE on the T\ circuit with c\ =\ ON and shall be preceded by two or more contiguous 1/6 (\*QSYN\*U) characters with c\ =\ ON. .PP The format of the \fIselection\fR \|signal sequence is defined in \(sc\ 4.6.1\ below. .PP The information content and coding of the \fIselection\fR \|signal sequence is contained in Annex\ G and Recommendation\ X.121. .PP The \fIselection\fR \|signal sequence shall start within 6\ seconds of \fIproceed\(hyto\(hyselect\fR being received and shall be completed within 36\ seconds. .PP The maximum permissible interval between individual selection characters is 6\ seconds. .PP The period, if any, between individual selection characters shall be filled by character 1/6\ (\*QSYN\*U) with c\ =\ ON. .RT .sp 1P .LP 4.1.4 \fIDTE waiting\fR \fI(state 5)\fR .sp 9p .RT .PP During \fIDTE waiting\fR , the DTE signals steady binary condition t\ =\ 1, c\ =\ ON. (See also \(sc\ 4.4 below for \fIdirect call\fR conditions.) .RT .sp 1P .LP 4.1.5 \fIIncoming call\fR \fI(state 8)\fR .sp 9p .RT .PP The DCE will indicate an incoming call by continuous transmission of character 0/7\ (\*QBEL\*U) preceded by two or more contiguous 1/6\ (\*QSYN\*U) characters on the R\ circuit with i\ =\ OFF. .PP If the DCE signals \fIincoming call\fR \|and the DTE simultaneously signals t\ =\ 0, c\ =\ OFF, the DTE will be assumed to be in state\ 16 \fI(clear request)\fR . .PP The connection of incoming calls will be inhibited when the DTE signals either \fIDTE uncontrolled not ready\fR \| or \fIDTE controlled not ready\fR . .RT .sp 2P .LP 4.1.6 \fICall accepted\fR \fI(state 9)\fR .sp 1P .RT .sp 1P .LP 4.1.6.1 \fIGeneral case\fR .sp 9p .RT .PP The DTE shall accept the incoming call as soon as possible by signalling the steady state binary condition t\ =\ 1, c\ =\ ON. .RT .LP 1) The DCE will return to \fIDCE ready\fR \|if the incoming call is not accepted within 500\ milliseconds, .LP or, where manual answering is permitted, .LP 2) the DCE will return to \fIDCE ready\fR \|if the incoming call is not accepted within 60\ seconds. .sp 1P .LP 4.1.6.2 \fISub\(hyaddressing\fR .sp 9p .RT .PP Sub\(hyaddressing is an optional procedure. .PP Two procedures for sub\(hyaddressing at the called end are defined: the simple sub\(hyaddressing procedure and the enhanced sub\(hyaddressing procedure. Choice of the provision of these procedures is a national matter. .bp .RT .sp 1P .LP 4.1.6.2.1 \fISimple sub\(hyaddressing procedure\fR (see Figure\ A\(hy6/X.21) .sp 9p .RT .PP The procedure is as follows: .RT .LP a) \fICall accept\fR , see \(sc\ 4.1.6.1. .LP b) \fIDCE waiting\fR , see \(sc\ 4.1.7. .LP c) \fIDCE\(hyprovided information:\fR \| when provided, sub\(hyaddressing information will be transmitted by the DCE to the called DTE after \fIcall accepted\fR has been sent by the DTE, and prior to any other \fIDCE\(hyprovided information\fR if any. .LP The format of sub\(hyaddressing information is defined in \(sc\ 4.6.3 below. .LP The information content of sub\(hyaddressing information is defined in Annex\ H. .LP d) \fIConnection in progress\fR , see \(sc\ 4.1.10. .LP e) \fIReady for data\fR , see \(sc\ 4.1.11. .sp 2P .LP 4.1.6.2.2\ \ \fIEnhanced sub\(hyaddressing procedure\fR \|(see Figures\ A\(hy5/X.21 and B\(hy3/X.21) .sp 1P .RT .sp 1P .LP 4.1.6.2.2.1\ \ \fIProceed with\fR \fIcall information\fR \fI(state\ 9B)\fR .sp 9p .RT .PP \fINote\fR \ \(em\ State 9B is only entered in the case of a DTE with sub\(hyaddressing. .PP The DTE shall accept the incoming call as soon as possible by transmitting continuously character\ 2/10 (\*Q*\*U) preceded by 2 or more contiguous characters\ 1/6 (\*QSYN\*U) on the T circuit with c\ =\ OFF. .PP \fIProceed with call information\fR \|is maintained until receipt of \fIend\fR \fIof call information\fR \| signal. The \fIproceed with call information\fR signal will start within 500\ ms of the \fIincoming call\fR signal being received. .RT .sp 1P .LP 4.1.6.2.2.2\ \ \fICall information signal sequence (state 10C)\fR .sp 9p .RT .PP \fINote\fR \ \(em\ State 10C is only entered in the case of a DTE with sub\(hyaddressing. .PP The \fIcall information signal\fR \|sequence\fR will be transmitted by the DCE on the R\ circuit with i\ =\ OFF and will appear on the same boundary as it was entered in state\ 8. .PP The call information signal may be preceded by two or more contiguous\ 1/6 (\*QSYN\*U) characters. .PP The \fIcall information\fR \|is also a \fIDCE\(hyprovided information\fR (see also \(sc\ 4.1.9). .PP The information content and coding of the \fIcall information signal\fR sequence is contained in Annex\ H and Recommendation\ X.121. .PP The \fIcall information signal\fR \|sequence will start and will be completed within 2\ seconds after the \fIproceed with call information\fR being sent. .PP The maximum period, if any, between individual call information characters will be filled by character 1/6\ (\*QSYN\*U) with i\ =\ OFF. .PP The format of \fIcall information\fR \|is the same as the format of \fIDCE\(hyprovided information\fR \| and is defined in \(sc\ 4.6.3\ below. .RT .sp 1P .LP 4.1.6.2.2.3\ \ \fIDCE waiting (states 6D and 6C)\fR .sp 9p .RT .PP During \fIDCE waiting\fR , the DCE will signal 2 or more contiguous 1/6 (\*QSYN\*U) characters with i\ =\ OFF. .RT .sp 1P .LP 4.1.6.2.2.4\ \ \fIDTE\(hyprovided information (state 25)\fR .sp 9p .RT .PP The \fIDTE\(hyprovided information\fR \|shall be transmitted by the DTE with sub\(hyaddressing on circuit\ T with c\ =\ OFF. .PP A \fIDTE\(hyprovided information\fR \|shall consist of 1 or more \fIDTE\(hyprovided information\fR \| blocks. \fIEach DTE\(hyprovided information\fR block shall be limited to a maximum length of 128\ characters. .PP \fINote\fR \ \(em\ The figure of 128 characters maximum length is provisional and may be changed to another value in the future. .bp .PP The format of the \fIDTE\(hyprovided information\fR \|is defined in \(sc\ 4.6.4 below. .PP The coding of \fIDTE\(hyprovided information\fR \|should be in line with Annex\ F and Recommendation\ X.96. .PP The \fIDTE\(hyprovided information\fR \|shall be sent on the same character boundary as it was entered in state\ 9B. \fIDTE\(hyprovided information\fR blocks within a \fIDTE\(hyprovided information\fR may be preceded and separated by 1/6\ (\*QSYN\*U) characters. .PP A \fIDTE\(hyprovided information\fR \|(state 25) shall be sent to the DCE within 20\ seconds after the \fIend of call information\fR signal, character 2/11 (\*Q+\*U) being received. .PP After reception of a \fIDTE\(hyprovided information\fR \|block, the DCE will reset and restart time\(hyout T14C. .RT .sp 1P .LP 4.1.7 \fIDCE waiting (state 6A and state 6B)\fR .sp 9p .RT .PP During \fIDCE waiting\fR , the DCE will signal two or more contiguous\ 1/6 (\*QSYN\*U) characters on the R circuit with i\ =\ OFF. In the state diagram, Figure\ A\(hy2/X.21, state\ 6A applies to calling procedures and state\ 6B applies to called procedures. .RT .sp 1P .LP 4.1.8 \fICall progress signal\fR \fI(state 7)\fR .sp 9p .RT .PP The \fIcall progress\fR \|signal will be transmitted by the DCE to the calling DTE on the R\ circuit with i\ =\ OFF when an appropriate condition is encountered by the network. .PP A \fIcall progress\fR \|signal will consist of 1 or more \fIcall\fR \fIprogress\fR \| signal blocks. A \fIcall progress\fR \| signal block will consist of\ 1 or more \fIcall progress\fR signals. .PP The format of the \fIcall progress\fR \|signal sequence is defined in \(sc\ 4.6.2\ below. .PP The coding of \fIcall progress\fR \|signals is provided in Annex\ F. .PP The description of \fIcall progress\fR \|signals is provided in Recommendation\ X.96. .PP A \fIcall progress\fR \|signal will be preceded by two or more contiguous 1/6 (\*QSYN\*U) characters sent during state\ 6A. The period between these blocks will also be filled by \fIDCE waiting\fR (state\ 6A). .PP A \fIcall progress\fR \|signal will be transmitted by the DCE within 20\ seconds of: (1)\ the \fIend\(hyof\(hyselection\fR \| signal or (2)\ in the case of \fIdirect call\fR , the \fIDTE waiting\fR signal being sent by the DTE. The \fIcall progress\fR signal, however, will not be sent by the DCE before the reception of the \fIend\(hyof\(hyselection\fR signal or \fIDTE waiting\fR signal is sent by the DTE, except in the case of expiration of time\(hyout\ T11, T12, or\ T13 where there may be a \fIcall progress\fR signal followed by \fIclear indication\fR . .PP \fINote\fR \ \(em\ When an error is detected in a received \fIcall progress\fR signal sequence, the DTE may choose to either ignore the signal or attempt a new call after clearing. .RT .sp 1P .LP 4.1.9 \fIDCE\(hyprovided information\fR \fI(states 10A and\fR \fI10B\fR ) .sp 9p .RT .PP The \fIDCE\(hyprovided information\fR \|will be transmitted by a DCE to the calling DTE (state\ 10A) or a called DTE (state\ 10B) on circuit\ R with i\ =\ OFF. .PP A \fIDCE\(hyprovided information\fR \|will consist of 1 or more \fIDCE\(hyprovided information\fR \| blocks. Each \fIDCE\(hyprovided information\fR block will be limited to a maximum length of 128\ characters. .PP The format of the \fIDCE\(hyprovided information\fR \|is defined in \(sc\ 4.6.3\ below. .PP The information content of \fIDCE\(hyprovided information\fR \|is provided in Annex\ H. .PP The \fIDCE\(hyprovided information\fR \|will be preceded by two or more contiguous 1/6\ (\*QSYN\*U) characters. \fIDCE\(hyprovided information\fR blocks within a \fIDCE\(hyprovided information\fR will be separated by 1/6\ (\*QSYN\*U) characters (the time between blocks to be filled by two or more SYN characters is for further study). In the case of a calling DTE (state\ 10A), the preceding and separating SYN characters will be from \fIDCE waiting\fR (state\ 6A). In the case of a called DTE (state\ 10B), the preceding SYN characters and the separating SYN characters will be from \fIDCE waiting\fR (state\ 6B). .PP In certain circumstances, SYN characters may be inserted between characters within a \fIDCE\(hyprovided information\fR block. Each insertion shall contain at least 2\ SYN characters, and the inserted SYN characters will be counted as part of the maximum limit of 128\ characters per block. In any case, the insertion of SYN characters should be rare and minimized. .bp .PP A \fIDCE\(hyprovided information\fR \|(state 10B) will be sent to the called DTE within 6\ seconds of the \fIcall accepted\fR signal being sent. After reception of a \fIDCE\(hyprovided information\fR block, the called DTE should reset time\(hylimit\ T4B. .RT .sp 1P .LP 4.1.9.1 \fILine identification\fR .sp 9p .RT .PP \fICalling\fR \|and \fIcalled line identification\fR \|is an optional additional facility. .PP The information content of \fIcalling\fR \|and \fIcalled line identification\fR \|is provided in Annex\ H. .PP \fICalling\fR \|and \fIcalled line identification\fR \|will be transmitted by the DCE on the R circuit with i\ =\ OFF during states\ 10B or\ 10A, respectively. .PP When provided, \fIcalled line identification\fR \|(state 10A) will be transmitted by the DCE to the calling DTE after all \fIcall progress\fR signals, if any. .PP When provided, \fIcalling line identification\fR \|(state 10B) will be transmitted by the DCE to the called DTE after \fIcall accepted\fR has been sent by the DTE. .PP In the case where the \fIcalling line identification\fR \|facility is not provided by the originating network, or the \fIcalled line identification\fR facility is not provided by the destination network, a \fIdummy line\fR \fIidentification\fR shall be provided by the DCE to the DTE. In some networks, when the \fIcalling line identification\fR is not provided by the originating network, the DNIC will be provided by the DCE to the DTE in place of the \fIdummy line identification\fR . .RT .sp 1P .LP 4.1.9.2 \fICharging information\fR .sp 9p .RT .PP \fICharging information\fR \|is an optional additional facility provided during state\ 10B. .PP Upon completion of clearing the call for which \fIcharging information\fR \|has been requested, the DCE will, within 200\ ms after entering \fIready\fR (state\ 1), establish an incoming call to the DTE for the purpose of giving \fIcharging information\fR . .PP \fINote\fR \ \(em\ The DTE is advised not to signal \fIcall request\fR \|or \fInot\fR \fIready\fR during the above 200\ ms period. If the DTE does, the \fIcharging\fR \fIinformation\fR will not be transmitted to the DTE. .PP \fICharging information\fR \|will be transmitted by the DCE on R circuit with i\ =\ OFF. .PP The DCE will send \fIclear indication\fR \|(state 19) upon sending the last \fIcharging information\fR block. The DTE should send \fIclear request\fR (state\ 16) when it has correctly received the \fIcharging information\fR signal, if the DCE has not previously signalled \fIclear indication\fR . .PP The format of \fIcharging information\fR \|is defined in \(sc\ 4.6.3 below. .RT .sp 1P .LP 4.1.10 \fIConnection in progress\fR \fI(state 11)\fR .sp 9p .RT .PP While the connection process is in progress, the DCE will indicate \fIconnection in progress\fR \| (state\ 11) by signalling r\ =\ 1, i\ =\ OFF. .PP In some circumstances, \fIconnection in progress\fR \|(state 11) may be bypassed. .RT .sp 1P .LP 4.1.11 \fIReady for data\fR \fI(state 12)\fR .sp 9p .RT .PP When the connection is available for data transfer between both DTEs, the DCE will indicate \fIready for data\fR (state\ 12) by signalling r\ =\ 1, i\ =\ ON. .RT .LP 1) \fIReady for data\fR \|will be indicated by the DCE to the calling DTE within 6\ seconds of the last \fIcall progress\fR signal or \fIDCE\(hyprovided information\fR signals being received by the DTE or within 20\ seconds of the \fIend\(hyof\(hyselection\fR signal being signalled by the DTE, .LP or, when manual answering is permitted at the called DTE. .LP 2) \fIReady for data\fR \|will be indicated by the DCE to the calling DTE within 60\ seconds of the appropriate \fIcall progress\fR signal being received or within 20\ seconds of the \fIend\(hyof\(hyselection\fR signal being received. .LP It will be indicated to the called DTE within 6 seconds of \fIcall\fR \fIaccepted\fR being signalled by the DTE or receipt of \fIDCE\(hyprovided information\fR signal. .LP Subsequent procedures are described in \(sc\ 5 below, \fIdata transfer\fR phase. .bp .sp 2P .LP 4.1.12 \fIEvents of the call control procedure for centralized\fR \fImultipoint circuit\(hyswitched service\fR .sp 1P .RT .sp 1P .LP 4.1.12.1 \fICall request (state 2)\fR .sp 9p .RT .PP See \(sc 4.1.1. .RT .sp 1P .LP 4.1.12.2 \fIProceed to select request (state 3)\fR .sp 9p .RT .PP See \(sc 4.1.2. .RT .sp 1P .LP 4.1.12.3 \fISelection signal sequence (state 4)\fR .sp 9p .RT .PP See \(sc 4.1.3. .PP A \fIfacility request\fR \|signal is used to indicate the category of point\(hyto\(hymultipoint service which is required. .PP The coding is defined in Annex\ G. .RT .sp 1P .LP 4.1.12.4 \fIDTE waiting (state 5)\fR .sp 9p .RT .PP See \(sc 4.1.4. .RT .sp 1P .LP 4.1.12.5 \fIIncoming call (state 8)\fR .sp 9p .RT .PP See \(sc 4.1.5. .RT .sp 1P .LP 4.1.12.6 \fICall accepted (state 9)\fR .sp 9p .RT .PP See \(sc 4.1.6.1. .RT .sp 1P .LP 4.1.12.7 \fIDCE waiting (state 6A and state 6B)\fR .sp 9p .RT .PP See \(sc 4.1.7. .RT .sp 1P .LP 4.1.12.8 \fICall progress signal sequence (state 7)\fR .sp 9p .RT .PP See \(sc 4.1.8. .PP In a point\(hyto\(hymultipoint call , the \fIcall progress\fR \|signals related to each of the called DTEs are transmitted and then in the same order, the \fIcalled line identification\fR signals of the different called DTEs. .PP When no specific call progress signals are necessary for a given called DTE, then the call progress signal \*Q00\*U is used for this DTE so that the order can be kept. .RT .sp 1P .LP 4.1.12.9 \fIDCE\(hyprovided information sequence (states 10A and 10B)\fR .sp 9p .RT .PP The \fIDCE\(hyprovided information\fR \|sequences will be transmitted by a DCE to the calling DTE (state\ 10A) or a called DTE (state\ 10B) on circuit\ R with i\ =\ OFF. .PP A \fIDCE\(hyprovided information\fR \|sequence will consist of one or more \fIDCE\(hyprovided information\fR blocks. Each \fIDCE\(hyprovided information\fR block will be limited to a maximum length of 128\ characters, except for called line identification in case of point\(hyto\(hymultipoint calls where the maximum length is 512\ characters. .PP The format of the \fIDCE\(hyprovided information\fR \|sequence is defined in \(sc\ 4.6.3 below. .PP The information content of \fIDCE\(hyprovided information\fR \|is provided in Annex\ H. .PP The \fIDCE\(hyprovided information\fR \|sequence will be preceded by two or more contiguous\ 1/6 (\*QSYN\*U) characters. \fIDCE\(hyprovided information\fR blocks within a \fIDCE\(hyprovided information\fR sequence will be separated by\ 1/6 (\*QSYN\*U) characters (the time between blocks to be filled by two or more SYN characters is for further study). In the case of a calling DTE (state\ 10A), the preceding and separating SYN characters will be from \fIDCE waiting\fR (state\ 6A). In the case of a called DTE (state\ 10B), the preceding SYN characters and the separating SYN characters will be from \fIDCE waiting\fR (state\ 6B). .bp .PP In certain circumstances, SYN characters may be inserted between characters within a \fIDCE\(hyprovided information\fR block. Each insertion shall contain at least 2\ SYN characters, and the inserted SYN characters will be counted as part of the maximum limit of 128\ characters per block. In any case, the insertion of SYN characters should be rare and minimized. .PP A \fIDCE\(hyprovided information\fR \|sequence (state 10B) will be sent to the called DTE within 6\ s of the \fIcall accepted\fR signal being sent. After reception of a \fIDCE\(hyprovided information\fR block, the called DTE should reset time\(hylimit\ T4. .RT .sp 1P .LP 4.1.12.9.1\ \ \fR \fILine identification\fR .sp 9p .RT .PP The \fIcalled line identification\fR \|related to the different called DTEs is provided in sequence. .PP See \(sc 4.1.9.1. .RT .sp 1P .LP 4.1.12.9.2\ \ \fICharging information\fR .sp 9p .RT .PP See \(sc 4.1.9.2. .RT .sp 1P .LP \fR 4.1.12.10\ \ \fIConnection in progress (state 11)\fR .sp 9p .RT .PP See \(sc 4.1.10. .RT .sp 1P .LP \fR 4.1.12.11\ \ \fIReady for data (state 12)\fR .sp 9p .RT .PP See \(sc 4.1.11. .RT .sp 1P .LP 4.2 \fIUnsuccessful call\fR .sp 9p .RT .PP If the required connection cannot be established, the DCE will indicate this and the reason to the calling DTE by means of a \fIcall progress\fR signal. Afterwards the DCE will signal \fIDCE clear indication\fR (state\ 19). .RT .sp 1P .LP 4.3 \fICall collision\fR \fI(state 15)\fR .sp 9p .RT .PP A \fIcall collision\fR \|is detected by a DTE when it receives \fIincoming\fR \fIcall\fR in response to \fIcall request\fR . The DTE shall not deliberately cause a \fIcall collision\fR by responding to \fIincoming call\fR with \fIcall request\fR . .PP A \fIcall collision\fR \|is detected by a DCE when it receives \fIcall\fR \fIrequest\fR in response to \fIincoming call\fR . .PP When a \fIcall collision\fR \|is detected by the DCE, the DCE will indicate \fIproceed\(hyto\(hyselect\fR (state\ 3) and cancel the incoming call. .RT .sp 1P .LP 4.4 \fIDirect call\fR .sp 9p .RT .PP For a \fIdirect call\fR \|facility, the entering of \fIDTE waiting\fR \| (state\ 5) directly upon receipt of \fIproceed\(hyto\(hyselect\fR (state\ 3) indicates the request for the direct call. When the \fIdirect call\fR facility is provided on a per\(hycall basis, the DTE may choose either an addressed call by presenting \fIselection\fR signal (state\ 4) or a direct call by presenting \fIDTE waiting\fR (state\ 5). When the \fIdirect call\fR facility only is provided on a subscription basis, \fIselection\fR signals (state\ 4) are always bypassed. .RT .sp 1P .LP 4.5 \fIFacility registration/cancellation\fR \fIprocedure\fR .sp 9p .RT .PP Registration/cancellation of optional user facilities shall be accomplished by a DTE using normal call establishment procedures using the \fIselection\fR sequence which is defined in \(sc\ 4.6.1\ below. .PP The format of the \fIfacility registration/cancellation\fR \|signal is defined in \(sc\ 4.6.1.3\ below. .PP The \fIfacility registration/cancellation\fR \|procedure shall not be combined with establishment of a normally addressed call, but shall be taken as an independent procedure. .PP In response to acceptance or rejection of the \fIfacility\fR \fIregistration/cancellation\fR \| actions, the network will provide the appropriate \fIcall progress\fR signal followed by \fIclear indication\fR . .bp .RT .sp 1P .LP 4.6 \fISelection, call progress and DCE provided information formats\fR .sp 9p .RT .PP (See also Annex\ D for a syntactic description of the formats.) .RT .sp 1P .LP 4.6.1 \fIFormat of selection sequence\fR .sp 9p .RT .PP A \fIselection\fR \|sequence shall consist of a \fIfacility request\fR \| block, or an \fIaddress\fR \| block, or a \fIfacility request\fR block followed by an \fIaddress\fR block, or a \fIfacility registration/cancellation\fR block. .RT .sp 1P .LP 4.6.1.1 \fIFacility request block\fR .sp 9p .RT .PP A \fIfacility request\fR \|block shall consist of one or more \fIfacility\fR \fIrequest\fR \| signals. .PP Multiple \fIfacility request\fR \|signals shall be separated by character 2/12\ (\*Q\|,\|\*U). .PP A \fIfacility request\fR \|signal shall consist of a \fIfacility request\fR \|code and may contain one or more \fIfacility\fR parameters. The \fIfacility request\fR code, \fIfacility\fR parameter and subsequent \fIfacility\fR parameters shall be separated by character 2/15\ (\*Q\|/\|\*U). For an interim period the 2/15\ (\*Q\|/\|\*U) separator will not be used in some networks. .PP The end of a \fIfacility request\fR \|block shall be indicated by character 2/13\ (\*Q\(em\*U). .RT .sp 1P .LP 4.6.1.2 \fIAddress block\fR .sp 9p .RT .PP An \fIaddress\fR \|block shall consist of one or more \fIaddress\fR \| signals. .PP An \fIaddress\fR \|signal shall consist of either a \fIfull address\fR \| signal or an \fIabbreviated address\fR signal. .PP Start of an \fIabbreviated address\fR \|signal shall be indicated by a prefix character\ 2/14\ (\*Q\|.\|\*U). .PP Multiple \fIaddress\fR \|signals shall be separated by character\ 2/12 (\*Q\|,\|\*U). .RT .sp 1P .LP 4.6.1.3 \fIFacility registration/cancellation block\fR .sp 9p .RT .PP A \fIfacility registration/cancellation\fR \|block shall consist of one or more \fIfacility registration/cancellation\fR signals. .PP A \fIfacility registration/cancellation\fR \|signal shall consist of up to four elements in order: \fIfacility request\fR code, \fIindicator\fR , \fIregistration\fR parameter, \fIaddress\fR signal. .PP The elements of a \fIfacility registration/cancellation\fR \|signal shall be separated by character\ 2/15\ (\*Q\|/\|\*U). .PP If a \fIfacility registration/cancellation\fR \|signal contains less than four of the elements, the elements should be eliminated in reverse order (e.g.,\ a two\(hyelement \fIfacility registration/cancellation\fR signal will contain the \fIfacility request\fR code\ \*Q\|/\|\*U \fIindicator\fR ). If any element to be sent within the sequence is not required, a\ 3/0\ (\*Q0\*U) character should be inserted in the position of each missing element (e.g.,\ \fIfacility request\fR code\ /0/0/ \fIaddress\fR signal). .PP Multiple \fIfacility registration/cancellation\fR \|signals shall be separated by character\ 2/12\ (\*Q\|,\|\*U). .PP The end of a \fIfacility registration/cancellation\fR \|block shall be indicated by character\ 2/13\ (\*Q\(em\*U) and shall be followed by an end of selection. .RT .sp 1P .LP 4.6.1.4 \fIEnd of selection\fR .sp 9p .RT .PP The end of \fIselection\fR \|shall be indicated by character 2/11\ (\*Q+\*U). .RT .sp 1P .LP 4.6.2 \fIFormat of a call progress block\fR .sp 9p .RT .PP A \fIcall progress\fR \|block shall consist of one or more \fIcall\fR \fIprogress\fR \| signals. .PP A call progress signal shall consist of a call progress code and may contain an indicator. .bp .PP The call progress code and the indicator shall be separated by character 2/13\ (\*Q\(em\*U). .PP Each \fIcall progress\fR \|signal need not be repeated. .PP Multiple \fIcall progress\fR \|signals shall be separated by character\ 2/12 (\*Q\|,\|\*U). .PP The end of a \fIcall progress\fR \|block shall be indicated by character\ 2/11\ (\*Q+\*U). .RT .sp 1P .LP 4.6.3 \fIFormats of DCE\(hyprovided information\fR .sp 9p .RT .PP The following formats are specified for \fIDCE\(hyprovided information\fR \| signals which have been identified. .PP The \fIDCE\(hyprovided information\fR \|shall, except for \fIcalling\fR \|and \fIcalled line identification\fR , be started by the IA5 character 2/15\ (\*Q\|/\|\*U). To distinguish between different types of \fIDCE\(hyprovided information\fR the prefix should be followed by one or more numerical characters, followed by the character\ 2/15\ (\*Q\|/\|\*U) before the actual information is presented. The end of a \fIDCE\(hyprovided information\fR block shall be indicated by character 2/11\ (\*Q+\*U). The order in which the \fIDCE\(hyprovided information\fR blocks are presented to the DTE is variable. .PP Multiple \fIDCE\(hyprovided information\fR \|signals shall be separated by character 2/12 (\*Q,\*U). .PP A dummy \fIDCE\(hyprovided information\fR \|block may be sent in the case of sub\(hyaddressing and will be indicated by the prefix as described above (\*Q/\*U, \*Q4\*U, \*Q/\*U) followed by 2/11 (\*Q+\*U). .PP The dummy sub\(hyaddress should be sent if the network supports sub\(hyaddressing but no sub\(hyaddress has been sent by the calling DTE. .RT .sp 1P .LP 4.6.3.1 \fIFormat of called and calling line identification\fR .sp 9p .RT .PP \fICalling line identification\fR \|block and \fIcalled line\fR \fIidentification\fR \| block shall be preceded by character 2/10\ (\*Q\|*\|\*U). .PP When a \fIcalling\fR \|or \fIcalled line identification\fR \|block contains Data Network Identification Codes (DNIC) or Data Country Codes (DCC), the block shall instead of one character\ 2/10 (\*Q\|*\|\*U) be preceded by 2\ characters 2/10\ (\*Q\|**\|\*U). .PP A \fIcalled line identification\fR \|block shall consist of one or more \fIcalled line identification\fR \| signals. .PP Multiple \fIcalled line identification\fR \|signals shall be separated by character\ 2/12\ (\*Q\|,\|\*U). .PP End of \fIcalling line identification\fR \|block and \fIcalled line\fR \fIidentification\fR \| block shall be indicated by character 2/11\ (\*Q\|+\|\*U). .PP The \fIdummy line identification\fR \|block shall be indicated by character\ 2/10 (\*Q\|*\|\*U) followed by 2/11\ (\*Q+\*U). .RT .sp 1P .LP 4.6.3.2 \fIFormat of charging information\fR .sp 9p .RT .PP The \fIcharging information\fR \|block will be preceded by character\ 2/15 (\*Q\|/\|\*U) followed by a second IA5 numerical character, followed by character 2/15\ (\*Q\|/\|\*U). The end of \fIcharging information\fR block shall be indicated by character\ 2/11\ (\*Q+\*U). .RT .sp 1P .LP 4.6.4 \fIFormat of DTE\(hyprovided information\fR .sp 9p .RT .PP A \fIDTE\(hyprovided information\fR \|block shall consist of 1 or more \fIDTE\(hyprovided information\fR \| signals. .PP Each \fIDTE\(hyprovided information\fR \|signal need not be repeated. .PP Multiple \fIDTE\(hyprovided information\fR \|signals shall be separated by character 2/12 (\*Q,\*U). .PP End of \fIDTE\(hyprovided information\fR \|block shall be indicated by character 2/11 (\*Q+\*U). .RT .sp 2P .LP \fB5\fR \fBData transfer phase\fR .sp 1P .RT .PP During the data transfer phase, any bit sequence may be sent by either DTE. .PP For the interchange of information between one DTE and another DTE during the data transfer phase, the DTEs will be responsible for establishing their own alignment. .bp .PP The byte timing interchange circuit, when implemented, may be utilized by the DTEs for mutual character alignment. .PP Data link control procedures and any other DTE\(hyto\(hyDTE protocols are not the subject of this Recommendation. .RT .sp 1P .LP 5.1 \fICircuit\(hyswitched service\fR .sp 9p .RT .PP All bits sent by a DTE after indication of \fIready for data\fR \|for 16\(hybit intervals (see \(sc\ 2.4) and before sending \fIDTE clear request\fR will be delivered to the corresponding DTE after that corresponding DTE has received \fIready for data\fR and before it has received \fIDCE clear indication\fR (provided that the corresponding DTE does not take the initiative of clearing). .PP All bits received by a DTE, after indication of \fIready for data\fR \|for 16\(hybit intervals (see \(sc\ 2.4) and before receiving \fIDCE clear indication\fR or receiving \fIDCE clear confirmation\fR , were sent by the corresponding DTE. Some of those bits may have originated as \fIDTE waiting\fR before that corresponding DTE has received \fIready for data\fR ; those bits are binary\ 1. .PP During \fIdata transfer\fR \|(state 13), c = ON, i = ON and data are transferred on circuits\ T and\ R. .PP \fIData transfer\fR \|may be terminated by \fIclearing\fR , as defined in \(sc\ 6 below, by either: .RT .LP i) the DCE, or .LP ii) any connected DTE. .PP The action to be taken when circuit C is turned OFF during \fIdata\fR \fItransfer\fR \|(state 13), except when the DTE is signalling \fIDTE clear request\fR (state\ 16) by t\ =\ 0, c\ =\ OFF, is for further study except for the procedures for half\(hyduplex operation between DTEs conforming to Recommendations\ X.21 and X.21\|\fIbis\fR as described in Annex\ E. .sp 1P .LP 5.2 \fILeased circuit service\fR \fI\(em point\(hyto\(hypoint\fR \|(see Figure A\(hy3/X.21) \fIand\fR \fIpacket\(hyswitched service\fR \|(Recommendation\ X.25, level\ 1) .sp 9p .RT .PP In this section, for the case of packet\(hyswitched service, one of the DTEs must be understood as being the packet network data switching exchange (DSE). .PP Data transmitted by the DTE on circuit T with c = ON are delivered to the remote DTE on circuit R with i\ =\ ON. .PP Both DTEs may employ duplex operation for the exchange of data. .PP Any bit sequence may be sent by either DTE during the ON\(hycondition of its circuit\ C. .PP \fINote\fR \ \(em\ The entering of the DTE/DCE\(hyinterface of \fIDTE uncontrolled\fR \fInot ready\fR \|(state 22) will be signalled on the remote end as \fIDCE not ready\fR (state\ 18). .PP In Figure\ A\(hy3/X.21 a state diagram indicating a possible data connection is shown. Apart from state\ 13, two additional states, 13S and\ 13R, can be identified. .RT .sp 1P .LP 5.2.1 \fISend data (state 13S)\fR .sp 9p .RT .PP Data transmitted by the DTE on circuit T with c\ =\ ON are delivered to the remote DTE on circuit\ R with i\ =\ ON. .RT .sp 1P .LP 5.2.2 \fIReceive data (state 13R)\fR .sp 9p .RT .PP Data transmitted by a distant DTE with c\ =\ ON are received on circuit\ R with i\ =\ ON. .RT .sp 1P .LP 5.2.3 \fIData transfer (state 13)\fR .sp 9p .RT .PP When c\ =\ ON, i\ =\ ON, data are transferred on circuits T and\ R. .bp .RT .sp 1P .LP 5.2.4 \fITermination of data transfer\fR .sp 9p .RT .PP The DTE signals the termination of \fIdata transfer\fR \|by signalling t\ =\ 1, c\ =\ OFF. The DCE indicates termination of \fIdata transfer\fR by signalling r\ =\ 1, i\ =\ OFF. .PP \fINote\fR \ \(em\ The action taken by the DCE when the DTE signals c\ =\ OFF and t\ does not equal\ 1, is for further study except for the \fIDTE uncontrolled\fR \fInot ready\fR \|procedures described in \(sc\ 2.5.1.2 above. .RT .LP 5.3 \fILeased circuit service \(em centralized multipoint\fR \|(see Figure A\(hy3/X.21) .sp 1P .RT .sp 2P .LP 5.3.1 \fICentral DTE data transfer\fR .sp 1P .RT .sp 1P .LP 5.3.1.1 \fISend data (state 13S)\fR .sp 9p .RT .PP Data transmitted by the central DTE on circuit T with c\ =\ ON are delivered to all remote DTEs on circuit\ R with i\ =\ ON. .RT .sp 1P .LP 5.3.1.2 \fIReceive data (state 13R)\fR .sp 9p .RT .PP Data transmitted by any remote DTE with c\ =\ ON (one at a time as determined by the data link protocol) during state\ 13S are delivered to the central DTE on circuit\ R with i\ =\ ON. .RT .sp 1P .LP 5.3.2 \fIRemote DTE data transfer\fR .sp 9p .RT .PP Data transmitted by a remote DTE are not delivered to other remote DTEs. .PP \fINote\fR \ \(em\ Transmission of data by two or more remote DTEs at the same time may result in unsatisfactory conditions. .RT .sp 1P .LP 5.3.2.1 \fISend data (state 13S)\fR .sp 9p .RT .PP Data transmitted by remote DTEs with c\ =\ ON (one at a time as determined by the data link protocol) are delivered to the central DTE on circuit\ R with i\ =\ ON. .RT .sp 1P .LP 5.3.2.2 \fIReceive data (state 13R)\fR .sp 9p .RT .PP Data transmitted by the central DTE with c\ =\ ON are delivered to the remote DTE on circuit\ R with i\ =\ ON. .RT .sp 1P .LP 5.3.3 \fIData transfer (state 13)\fR .sp 9p .RT .PP When c\ =\ ON, i\ =\ ON data transmitted by the central DTE are delivered to all remote DTEs, and data transmitted by a remote DTE (one at a time as determined by data link protocol) are delivered to the central DTE. A remote DTE may send (one at a time as determined by the data link protocol) while the central DTE is sending to all remote DTEs. .RT .sp 1P .LP 5.4 \fICircuit\(hyswitched service \(em Point\(hyto\(hymultipoint service\fR .sp 9p .RT .PP The different configurations of point\(hyto\(hymultipoint are defined in Recommendation\ X.2. .RT .sp 1P .LP 5.4.1 \fICentralized multipoint\fR .sp 9p .RT .PP All bits sent by the central DTE after indication of \fIready for\fR \fIdata\fR \|for 16\(hybit intervals (see \(sc\ 2.4) and before clearing procedure has been started by the central DTE or by the network, will be delivered to all remote DTEs after they have received \fIready for data\fR and before they have received \fIDCE clear indication\fR (provided that the remote DTEs do not take the initiative of clearing). .PP All bits received by a remote DTE, after indication of \fIready for\fR \fIdata\fR \| for 16\(hybit intervals (see \(sc\ 2.4) and before clearing procedure has been started by the central DTE, by the network or the remote DTE, were sent by the central DTE. Some of those may have originated as \fIDTE waiting\fR before the central DTE has received \fIready for data\fR ; those bits are binary\ 1. .bp .PP All bits sent by a remote DTE, after indication of \fIready for data\fR for 16\(hybit intervals (see \(sc\ 2.4) and before clearing procedure has been started by the central DTE, by the network or by the remote DTE, will be delivered to the central DTE after that central DTE has received \fIready for data\fR and before it has received \fIDCE clear confirmation\fR or \fIDCE clear indication\fR , provided that the the other remote DTEs are transmitting binary\ 1. .PP All bits received by the central DTE, after receiving \fIready for\fR \fIdata\fR for 16\(hybit intervals (see \(sc\ 2.4) and before receiving \fIDCE clear\fR \fIindication\fR or \fIDCE clear confirmation\fR , were sent by one of the remote DTEs. Some of these may have originated as \fIDTE waiting\fR before the remote DTEs have received \fIready for data\fR ; those bits are binary\ 1. .PP During \fIdata transfer\fR \|(state 13), c = ON, i = ON and data are transferred on circuits T and\ R. .PP \fIData transfer\fR \|may be terminated by clearing, as defined in \(sc\ 6 below, by either: .RT .LP i) the central DTE or, .LP ii) all the remote DTEs or, .LP iii) the DCEs. .sp 2P .LP \fB6\fR \fBClearing phase\fR (see Figure A\(hy4/X.21) .sp 1P .RT .PP In centralized multipoint calls : .RT .LP \(em clearing by the central DTE imply clearing of the call; .LP \(em clearing by a remote DTE clears the call for this DTE, and has no effect on the calls which remain established for the other remote DTEs; .LP \(em clearing by the last remote DTE still in the call leads to the clearing of the call. .sp 1P .LP 6.1 \fIClearing by the DTE (states 16, 17, 21\fR ) .sp 9p .RT .PP The DTE should indicate clearing by signalling the steady binary condition t\ =\ 0, c\ =\ OFF, \fIDTE clear request\fR (state\ 16). .PP The DCE will respond by signalling the steady state condition r\ =\ 0, i\ =\ OFF, \fIDCE clear confirmation\fR \|(state\ 17), followed by the steady binary condition r\ =\ 1, i\ =\ OFF, \fIDCE ready\fR (state\ 21). The \fIDCE ready\fR signal will be sent within 2 seconds after the receipt of the \fIDTE clear request\fR signal. .PP The DTE shall recognize DCE clear confirmation and, except as noted below, shall then respond to DCE ready, when presented, within 100\ millisecondes by signalling t\ =\ 1, c\ =\ OFF, ready (state\ 1). .PP In the case where DCE clear confirmation is either not presented by the DCE or not recognized by the DTE, the DTE shall remain in the DTE clear request state for a minimum of 2\ seconds and then go to DTE ready. In this case, the DTE may not respond to DCE ready within the 100\ milliseconds stipulated above and may be considered by the DCE to be uncontrolled not ready (state\ 24) for a finite period of time (until it goes to DTE ready). .RT .sp 1P .LP 6.2 \fIClearing by the DCE (states 19, 20, 21)\fR .sp 9p .RT .PP The DCE will indicate clearing to the DTE by signalling the steady binary condition r\ =\ 0, i\ =\ OFF, \fIDCE clear indication\fR (state\ 19). .PP The DTE should signify \fIDTE clear confirmation\fR \|(state 20) by signalling the steady binary condition t\ =\ 0, c\ =\ OFF, within 500\ milliseconds. The DCE will signal r\ =\ 1, i\ =\ OFF, \fIDCE ready\fR (state\ 21) within 2\ seconds of receiving \fIDTE clear confirmation\fR . .PP The DTE should respond to \fIDCE ready\fR \|within 100 milliseconds by signalling t\ =\ 1, c\ =\ OFF, \fIready\fR (state\ 1). .RT .sp 2P .LP \fB7\fR \fBTest loops\fR .sp 1P .RT .PP The definitions of the test loops and the principles of maintenance testing using the test loops are provided in Recommendation\ X.150. .RT .sp 1P .LP 7.1 \fIDTE test loop\fR \fI\ \(em\ type 1 loop\fR .sp 9p .RT .PP This loop is used as a basic test of the operation of the DTE, by looping back the transmitted signals inside the DTE for checking. The loop should be set up inside the DTE as close as possible to the DTE/DCE interface. .bp .PP While the DTE is in the loop 1 test condition: .RT .LP \(em circuit T is connected to circuit R inside of the DTE; .LP \(em circuit C is connected to circuit I inside of the DTE; .LP \(em the DCE continues to present signal element timing on circuit S and, if implemented, byte timing on circuit\ B. The DTE need not make use of the timing information. .PP Loop 1 may be established from either the \fIdata transfer\fR \|or \fIready\fR \| state. .PP In some networks, for short routine tests during the \fIdata transfer\fR state, the DTE should either maintain the same status on the interchange circuits as before the test or send the \fIcontrolled not ready\fR signal. If the loop is established from the \fIdata transfer\fR state, the DCE may continue to deliver data to the DTE during the test as though the DTE were in normal operation. It will be the responsibility of the DTEs to recover from any errors that might occur while the test loop is activated. .PP If the loop is established from the \fIready\fR \|state, the DTE should signal one of the \fInot ready\fR \| states. .RT .sp 1P .LP 7.2 \fILocal test loop\fR \fI \(em type 3 loop\fR .sp 9p .RT .PP Local test loops (type 3 loops) are used to test the operation of the DTE, the interconnecting cable and either all or parts of the local DCE, as discussed below. .PP Loop 3 may be established from any state. .PP For testing on leased circuits and for short duration testing on circuit\(hyswitched connections the DCE should either continue to present toward the line the conditions that existed before the test (e.g.\ either \fIdata\fR \fItransfer\fR or \fIready\fR \|state) or send the \fIcontrolled not ready\fR state to the remote DTE. Where this is not practical (e.g.\ in some cases for loop\ 3a) or desirable (e.g.\ for long duration testing in circuit\(hyswitched applications) the DCE should terminate an existing call and, if possible, signal toward the subscriber\(hyline one of the \fInot ready\fR states. .PP Manual and/or automatic control should be provided on the DCE for activation of the test loop. .PP The precise implementation of the test loop within the DCE is a national option. At least one of the following local loops should be implemented: .RT .sp 1P .LP 7.2.1 \fILoop 3d\fR .sp 9p .RT .PP This loop is used to test the operation of the DTE, including the interconnecting cable, by returning transmitted signals to the DTE for checking. The loop is set up inside the local DCE and does not include interchange circuit generators and loads. .PP While the DCE is in the loop 3d test condition: .RT .LP \(em circuit T is connected to circuit R inside of the DCE; .LP \(em circuit C is connected to circuit I inside of the DCE; .LP \(em the DCE continues to present signal element timing on circuit S and, if implemented, byte timing on circuit\ B. The DTE must make use of the timing information. .PP \fINote\fR \ \(em\ While test loop 3d is operated, the effective length of the interface cable is doubled. Therefore, to insure proper operation of loop\ 3d, the maximum DTE/DCE interface cable length should be one\(hyhalf the length normally appropriate for the data signalling rate in use. .sp 1P .LP 7.2.2 \fILoop 3c\fR .sp 9p .RT .PP This loop is used to test the operation of the DTE, including the interconnecting cable and DCE interchange circuit generators and loads. .PP The configuration is identical to that given for loop\ 3d in \(sc\ 7.2.1 with the exception that the looping of circuit\ T to circuit\ R and the looping of circuit\ C to circuit\ I includes the interchange circuit generators and loads. The note concerning restriction of interface cable length is not applicable. .RT .sp 1P .LP 7.2.3 \fILoop 3b\fR .sp 9p .RT .PP This loop is used as a test of the operation of the DTE and the line coding and control logic and circuitry of the DCE. It includes all the circuitry of the DCE with the exclusion of the line signal conditioning circuitry (e.g.\ impedance matching transformers, amplifiers, equalizers,\ etc.). The delay between transmitted and received test data is a few octets (see Note). .bp .PP The configuration is identical to that given for loop 3c in \(sc\ 7.2.2 except for the location of the point of loopback. .PP \fINote\fR \ \(em\ In some DCEs, the setting of loop 3b will result in momentary loss of envelope alignment causing random signals to appear on the receiving interchange circuit for a period of time. This may impact upon the DTE test procedure. In some networks the setting of loop 3b will cause clearing of existing connections. .RT .sp 1P .LP 7.2.4 \fILoop 3a\fR .sp 9p .RT .PP This loop is used to test the operation of the DTE and the DCE. The loop should include the maximum amount of circuitry used in DCE working including, in particular, the line signal conditioning circuitry. It is recognized that, in some cases, the inclusion of devices (e.g.\ attenuators, equalizers or test loop translators) may be necessary in the loopback path. The subscriber line is suitably terminated during a loop 3a test condition. The delay between transmitted and received test data is a few octets (see Note). .PP The configuration is identical to that given for test loop 3b in \(sc\ 7.2.3 except for the location of the point of loopback. .PP \fINote\fR \ \(em\ In some DCEs, the setting of loop 3a will result in momentary loss of envelope alignment causing random signals to appear on the receiving interchange circuit for a period of time. This may impact upon the DTE test procedure. In some networks the setting of loop\ 3a will cause clearing of existing connections. .RT .sp 1P .LP 7.2.5 \fIAutomatic operation of test loop\fR \fI3\fR \| (see Figure\ A\(hy9/X.21) .sp 9p .RT .PP The procedure provides for transparent loop testing and may be entered from any state. .RT .sp 1P .LP 7.2.5.1 \fISend loop 3 command (state L31)\fR .sp 9p .RT .PP The testing DTE shall indicate a request for a local loop 3 by signalling t\ =\ 00001111 (alternate bit quadruples are binary\ 0 and binary\ 1), c\ =\ OFF. The loop command shall persist until the DCE signals r\ =\ 00001111\ .\|.\|., i\ =\ OFF, but not longer than 6\ s. Some networks may require the loop command to be preceded by\ 2 or more contiguous\ 1/6 (\*QSYN\*U) characters and to appear on a character boundary 00001111. Some networks will not require the preceding SYN\(hycharacters to be sent, i.e.\ they will treat the loop command as a bit pattern which, therefore, does not need to appear on a character boundary. However, this would impose no restrictions on DTEs sending the loop command on a character boundary with preceding SYN\(hycharacters. .RT .sp 1P .LP 7.2.5.2 \fILoop 3 confirmation (state L32)\fR .sp 9p .RT .PP The DCE indicates that the local loop 3 is established by signalling r\ =\ 00001111, i\ =\ OFF. .PP \fINote\fR \ \(em\ This bit pattern received on circuit R is the loop command reflected by the local loop\ 3. .RT .sp 1P .LP 7.2.5.3 \fITest data (state L33)\fR .sp 9p .RT .PP The DTE enters the transparent \fItest data\fR \|state by turning c\ =\ ON. During the test any bit sequence may be sent by the DTE. The looped back data are received on circuit\ R with i\ =\ ON. .RT .sp 1P .LP 7.2.5.4 \fILoop 3 clear request (state L34A)\fR .sp 9p .RT .PP The DTE signals the termination of the test data by turning c\ =\ OFF. .RT .sp 1P .LP 7.2.5.5 \fILoop 3 clear request (state L34B)\fR .sp 9p .RT .PP In cases where the DTE does not wish to enter state L33 it may leave state L32 by signalling t\ \(!=\ 00001111, c\ =\ OFF for a minimum time of 24\ bits. .bp .RT .sp 1P .LP 7.2.5.6 \fILoop 3 clear confirmation (state L35)\fR .sp 9p .RT .PP Release of the loop is confirmed by the DCE signalling r\ \(!=\ 00001111, i\ =\ OFF. .RT .sp 1P .LP 7.3 \fINetwork test loop\fR \fI\ \(em\ type 2 loop\fR .sp 9p .RT .PP Network test loops (type 2 loops) are used by the Administration's test centre to test the operation of the leased line or subscriber line and either all or part of the DCE, as discussed below. .RT .sp 1P .LP 7.3.1 \fIGeneral\fR .sp 9p .RT .PP Loop 2 may be controlled manually on the DCE or automatically from the network or where allowed by national testing principles automatically from the remote DTE. .PP In case of a collision between call request and the activation of the loop, the loop activation command will have priority. .PP When the test is in progress, the DCE will signal r\ =\ 0, i\ =\ OFF or r\ =\ 0101, i\ =\ OFF. The choice as to which of these signals is sent is a national matter. .RT .sp 1P .LP 7.3.2 \fIAutomatic operation of test loop 2\fR \|(see Figures\ A\(hy7/X.21 and A\(hy8/X.21) .sp 9p .RT .PP The procedure provides for transparent loop testing and is entered from the \fIdata transfer\fR \|phase, state\ 13 in circuit switched service point\(hyto\(hypoint. It may also be entered from any state in leased circuit service. .RT .sp 1P .LP 7.3.2.1 \fISend loop 2 command\fR (state L21) .sp 9p .RT .PP The testing DTE shall indicate a request for a remote loop 2 by signalling t\ =\ 00110011 (alternate bit pairs are binary\ 0 and binary\ 1), c\ =\ OFF. The loop command shall persist until the DCE signals r\ =\ 00110011\ .\|.\|.\ , i\ =\ OFF, but not longer than 2\ s. Some networks may require the loop command to be preceded by\ 2 or more contiguous\ 1/6 (\*QSYN\*U) characters and to appear on a character boundary\ 00110011. Some networks will not require the preceding SYN\(hycharacters to be sent, i.e.\ they will treat the loop command as a bit pattern which, therefore, does not need to appear on a character boundary. However, this would impose no restrictions on DTEs sending the loop command on a character boundary with preceding SYN\(hycharacters. .RT .sp 1P .LP 7.3.2.2 \fILoop 2 confirmation (state L22)\fR .sp 9p .RT .PP The DCE indicates that the remote loop 2 is established by signalling r\ =\ 001100\ .\|.\|.\ , i\ =\ OFF. .PP \fINote\fR \ \(em\ This bit pattern received on circuit R is the loop command reflected by the remote loop\ 2. .RT .sp 1P .LP 7.3.2.3 \fITest data (state L23)\fR .sp 9p .RT .PP The DTE enters the transparent \fItest data\fR \| state by turning c\ =\ ON. .PP During the test any bit sequence may be sent by the DTE. The looped back data are received on circuit R with i\ =\ ON. .PP \fINote\fR \ \(em\ Allowance must be made for propagation delay in the network. .RT .sp 1P .LP 7.3.2.4 \fILoop 2 clear request (state L24A)\fR .sp 9p .RT .PP The DTE signals the termination of the \fItest data\fR \|state by turning c\ =\ OFF. .PP In the case where the DTE wishes to clear the connection it signals t\ =\ 0, c\ =\ OFF (state\ 16). .PP In the case where the DTE wishes to re\(hyenter the \fIdata transfer\fR \|phase it signals t\ \(!=\ 0\ \(!=\ 0011, c\ =\ OFF until the DCE signals state\ L25 \fIloop\ 2 clear confirmation\fR , r\ \(!=\ 0\ \(!=\ 0011, i\ =\ OFF. After that, the DTE re\(hyenters the \fIdata transfer\fR \|phase by signalling t\ =\ D, c\ =\ ON. .bp .RT .sp 1P .LP 7.3.2.5 \fILoop 2 clear request (state L24B)\fR .sp 9p .RT .PP In cases where the DTE does not wish to enter state L23 it may leave state\ L22 by signalling t\ \(!=\ 0011, c\ =\ OFF for a minimum time of 24\ bits. .PP In the case where the DTE wishes to clear the connection it signals t\ =\ 0, c\ =\ OFF (state\ 16). .PP In the case where the DTE wishes to re\(hyenter the data transfer phase it signals t\ \(!=\ 0\ \(!=0011, c\ =\ OFF until the DCE signals state\ L25 loop\ 2 clear confirmation, r\ \(!=\ 0\ \(!=\ 0011, i\ =\ OFF. After that, the DTE re\(hyenters the data transfer phase by signalling t\ =\ D, c\ =\ ON. .RT .sp 1P .LP 7.3.2.6 \fILoop 2 clear confirmation (state L25)\fR .sp 9p .RT .PP Release of the loop is confirmed by the DCE signalling r\ \(!=\ 0011\ .\|.\|.\ , i\ =\ OFF. .PP \fINote\fR \ \(em\ The DCE signalling of r = 0, i = OFF must be interpreted by the DTE as \fIloop clear confirmation\fR \| and \fIDCE clear indication\fR (state\ 19). .RT .sp 1P .LP 7.3.2.7 \fIReceive loop 2 command (state L26)\fR .sp 9p .RT .PP The DCE will indicate the receipt of a \fIloop command\fR \| by transmission of 0011\ .\|.\|. on circuit\ R with i\ =\ OFF. .PP \fINote\fR \ \(em\ In some networks state L26 will be bypassed if the loop\ 2 is operated from the Administration's test centre. .RT .sp 1P .LP 7.3.2.8 \fILoop 2 consent (state L26A)\fR .sp 9p .RT .PP The DTE of the tested station will indicate its readiness for a loop test by signalling t\ =\ X, c\ =\ OFF. .PP \fINote\fR \ \(em\ Some networks may require this state for additional security against malicious test loop operation. .RT .sp 1P .LP 7.3.2.9 \fIDCE controlled not ready (state L27)\fR .sp 9p .RT .PP When the DCE has closed the loop 2 it transmits 0101\ .\|.\|.\ on circuit\ R and i\ =\ OFF to the DTE. .PP This DTE should not interpret this state as a \fIclear\fR \|indication, if it was in the \fIdata transfer\fR \|phase before the test procedure began. .RT .sp 1P .LP 7.3.2.10 \fILoop 2 released (state L28)\fR .sp 9p .RT .PP When the DCE stops transmitting 0101\ .\|.\|. for more than 24 bits on circuit R, the DTE is informed that it may continue with the state it had left at the beginnng of state\ L27. .RT .sp 1P .LP 7.3.2.11 \fIDCE not ready (state L29)\fR .sp 9p .RT .PP In the case when the DCE is not able to send \fIDCE controlled not\fR \fIready\fR , it will signal \fIDCE not ready\fR \|(state\ L29). This state will persist until the loop is released. .PP \fINote\fR \ \(em\ It is not possible to re\(hyenter the \fIdata transfer\fR \|phase in this case. .RT .sp 1P .LP 7.3.3 \fIImplementation of type 2 loops\fR .sp 9p .RT .PP The precise implementation of the test loop within the DCE is a national option. At least one of the following network test loops should be implemented: .RT .sp 1P .LP 7.3.3.1 \fILoop 2b\fR .sp 9p .RT .PP This loop is used by either the Administration's test centre(s) and/or the remote DTE to test the operation of the subscriber line and all the circuitry of the DCE with the exception of interchange circuit generators and loads. .bp .PP While the DCE is in the loop 2b test condition: .RT .LP \(em circuit R is connected to circuit T inside of the DCE, .LP \(em circuit I is connected to circuit C inside of the DCE, .LP \(em at the interface, the DCE signals r\ =\ 0, i\ =\ OFF, or where provided r\ =\ 0101\ .\|.\|., i\ =\ OFF, .LP \(em the DCE provides timing information on circuits S and, if implemented, on circuit\ B. .sp 1P .LP 7.3.3.2 \fILoop 2a\fR .sp 9p .RT .PP This loop is used by either the Administration's test centre(s) or the remote DTE to test the operation of the subscriber line and the entire DCE. .PP The configuration is identical to that given for loop 2b in \(sc\ 7.3.3.1 except for the location of the point of loop back. Alternatively, the DCE may present an open circuit or power off condition on circuits\ R and\ I. .RT .sp 1P .LP 7.4 \fISubscriber\(hyline test loop\fR \fI\ \(em\ type 4 loop\fR .sp 9p .RT .PP Subscriber\(hyline test loops (type 4 loops) are provided for the maintenance of lines by the Administrations. .PP \fINote\fR \ \(em\ In the case of loops 4 and 2 (see \(sc\ 7.3 above) the DCE may signal the local DTE in such a manner that the DTE can distinguish a test mode from a network failure. This is for further study. .RT .sp 1P .LP 7.4.1 \fILoop 4a\fR .sp 9p .RT .PP This loop is only provided in the case of 4\(hywire subscriber lines. Loop\ 4a is for the maintenance of lines by Administrations. When receiving and transmitting pairs are connected together, the resulting circuit cannot be considered normal. Loop 4a may be established inside the DCE or in a separate device. .PP While the DCE is in the loop 4a test condition: .RT .LP \(em the DCE signals to the local DTE r = 0, i = OFF, or where provided r\ =\ 0101\ .\|.\|., i\ =\ OFF. .LP \(em the DCE provides timing information on circuit S and, if implemented, circuit\ B. .sp 1P .LP 7.4.2 \fILoop 4b\fR .sp 9p .RT .PP This loop is used by Administrations to test the operation of the subscriber line including the line signal conditioning circuitry in the DCE. When the receiving and transmitting circuits are connected at this point, loop\ 4b provides a connection that can be considered as normal; however, some impairment of the performance is expected since the DCE does not perform a complete signal regeneration. .PP The configuration is identical to that given for loop 4a in \(sc\ 7.4.1 except for the location of the point of the loopback. .RT .sp 1P .LP 7.5 \fISignal element timing provision\fR .sp 9p .RT .PP The provision of signal element timing to the DTE is maintained when any of the loops, described above, are activated. .PP When test loops are activated, the signal element timing should in no case deviate from the nominal value by more than\ \(+-\|1%. .RT .LP .rs .sp 7P .ad r Blanc .ad b .RT .LP .bp