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InfoMagic Standards 1994 January
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1988
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.rs
.\" Troff code generated by TPS Convert from ITU Original Files
.\" Not Copyright (~c) 1991
.\"
.\" Assumes tbl, eqn, MS macros, and lots of luck.
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.EN
.nr LL 40.5P
.nr ll 40.5P
.nr HM 3P
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.nr PO 4P
.nr PD 9p
.po 4P
.rs
\v'|.5i'
.LP
\fBMONTAGE : FIN DE LA RECOMMANDATION X.30 EN\(hyT\*\|ETE DE CETTE PAGE\fR
.sp 2P
.LP
\v'30P'
\fBRecommendation\ X.31\fR
.FS
This Recommendation is also included
in the Recommendations of the I\(hyseries under the number I.462.
.FE
.RT
.sp 2P
.sp 1P
.ce 1000
\fBSUPPORT\ OF\ PACKET\ MODE\ TERMINAL\ EQUIPMENT\ BY\ AN\ ISDN\fR
.EF '% Fascicle\ VIII.2\ \(em\ Rec.\ X.31''
.OF '''Fascicle\ VIII.2\ \(em\ Rec.\ X.31 %'
.ce 0
.sp 1P
.ce 1000
\fI(Malaga\(hyTorremolinos, 1984; amended at Melbourne, 1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.sp 2P
.LP
The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR
.sp 9p
.RT
.PP
(a)
that DTEs conforming to Recommendation X.25 will be
used, at least during the evolution of
integrated services digital networks (ISDN)
and possibly thereafter, in conjunction with packet switched data transmission
services (PSDTS) provided on an ISDN or via an ISDN to PSPDNs;
.PP
(b)
that packet\(hymode TE1s conforming to the I\(hyseries
Recommendations (I.430/I.431) at reference points\ S and\ T will be used in
conjunction with PSDTS provided by an ISDN or via an ISDN to PSPDNs;
.PP
(c)
that the functions and protocol defined by this
Recommendation must allow the provision of the network service defined in
Recommendation\ X.213;
.bp
.PP
(d)
that the interworking function between an ISDN and a
PSPDN is defined in Recommendation\ X.325;
.PP
(e)
that the demand access to PSPDNs is defined in
Recommendation\ X.32;
.PP
(f)
that the dedicated access to PSPDNs is defined in
Recommendation\ X.25,
.sp 1P
.LP
\fIunanimously declares\fR
.sp 9p
.RT
.PP
that the following should apply for the support of
packet\(hymode terminal equipment
by an ISDN.
.PP
This Recommendation addresses the following aspects:
.LP
(1)
definition of the aspects of the packet\(hymode services
provided to the ISDN users in accordance with the bearer services defined in
I\(hyseries Recommendations;
.LP
(2)
definition of the procedures at the ISDN user\(hynetwork
interface for accessing packet\(hymode services in alignment with
Recommendations\ I.430, I.431, Q.921 and Q.931;
.LP
(3)
definition of the TA's functions for adapting existing X.25 terminals.
.PP
PADs may be supported within the network, in which case existing Recommendations
shall apply for asynchronous access (e.g.,\ X.3, X.28, X.29,
X.52). The support of asynchronous access by an ISDN or through an ISDN
is not within the scope of this Recommendation.
.ce 1000
CONTENTS
.ce 0
.LP
1
\fIGeneral service aspects\fR
.sp 1P
.RT
.sp 2P
.LP
2\fR \fIReference configurations\fR
.sp 1P
.RT
.sp 1P
.LP
2.1
Configuration when accessing PSPDN services (Case A)
.sp 9p
.RT
.LP
2.2
Configuration for the ISDN virtual circuit service (Case B)
.sp 2P
.LP
\fB3\fR \fIService aspects\fR
.sp 1P
.RT
.sp 1P
.LP
3.1
Access to PSPDN services (Case A)
.sp 9p
.RT
.LP
3.1.1
Service characteristics
.LP
3.1.2
User access capabilities
.LP
3.1.3
Basic rules
.LP
3.1.4
Notification classes
.sp 1P
.LP
3.2
Access to the ISDN virtual circuit service (Case B)
.sp 9p
.RT
.LP
3.2.1
Service characteristics
.LP
3.2.2
User access capabilities
.LP
3.2.2.1
Access through the B\(hychannel
.LP
3.2.2.1.1
Service limitations
.LP
3.2.2.1.2
Basic rules
.LP
3.2.2.2
Access through the D\(hychannel
.LP
3.2.2.2.1
Service limitations
.LP
3.2.2.2.2
Basic rules
.LP
3.2.3
Notification classes for incoming calls
.LP
3.2.3.1
No notification class
.LP
3.2.3.2
Conditional notification class
.LP
3.2.3.3
Unconditional notification class
.LP
3.2.3.4
Information mapping from the X.25
incoming call packet to the Q.931 message
.sp 1P
.LP
3.3
Compatibility checking
.bp
.sp 9p
.RT
.sp 2P
.LP
4
\fIAddressing and routing aspects\fR
.sp 1P
.RT
.sp 1P
.LP
4.1
Terminal interface selection
.sp 9p
.RT
.LP
4.2
Access to PSPDN services (Case A)
.LP
4.2.1
Channel type selection
.LP
4.2.2
Addressing scheme for outgoing calls
.LP
4.3
Access to the ISDN virtual circuit service (Case B)
.LP
4.3.1
Channel type selection
.LP
4.3.2
Addressing scheme for outgoing calls
.sp 2P
.LP
5
\fIInterworking with dedicated networks\fR
.sp 1P
.RT
.sp 1P
.LP
5.1
Circuit\(hymode access to PSPDN services (Case A)
.sp 9p
.RT
.LP
5.2
Access to PSPDNs via virtual circuit service (Case B)
.sp 2P
.LP
6
\fIPacket communications at the S/T reference point\fR
.sp 1P
.RT
.sp 1P
.LP
6.1
Outgoing access
.sp 9p
.RT
.LP
6.1.1
Circuit\(hyswitched access to PSPDN services (Case A)
.LP
6.1.2
Access to the ISDN virtual circuit service (Case B)
.LP
6.1.2.1
B\(hychannel
.LP
6.1.2.2
D\(hychannel
.sp 1P
.LP
6.2
Incoming access
.sp 9p
.RT
.LP
6.2.1
Access from PSPDN services (Case A)
.LP
6.2.1.1
General
.LP
6.2.1.2
Channel negotiation
.LP
6.2.2
Access from the ISDN virtual circuit service (Case B)
.LP
6.2.2.1
B\(hychannel
.LP
6.2.2.2
D\(hychannel
.LP
6.2.2.3
Call offering
.LP
6.2.2.3.1
Channel selection through call offering
.LP
6.2.2.3.2
Information element mapping
.LP
6.2.2.3.3
Channel selection without call offering
.sp 1P
.LP
6.3
Virtual call establishment and release
.sp 9p
.RT
.LP
6.3.1
Link layer establishment and release
.LP
6.3.2
Packet layer virtual call setup and release
.sp 1P
.LP
6.4
Call clearing
.sp 9p
.RT
.LP
6.4.1
B\(hychannel
.LP
6.4.2
D\(hychannel
.LP
6.4.3
Additional error handling information
.LP
6.4.4
Cause mapping
.LP
6.4.4.1
Access to/from PSPDN services (Case A)
.LP
6.4.4.2
Access to/from the ISDN virtual circuit service (Case B)
.sp 1P
.LP
6.5
Access collision
.sp 9p
.RT
.sp 2P
.LP
7
\fITerminal adaptor functionalities\fR
.sp 1P
.RT
.sp 1P
.LP
7.1
General
.sp 9p
.RT
.LP
7.2
Physical interfaces
.LP
7.3
Access through the B\(hychannel
.LP
7.3.1
General
.LP
7.3.2
Rate adaption
.bp
.LP
7.3.3
Signalling
.LP
7.3.3.1
Outgoing call
.LP
7.3.3.1.1
Conditions for initiating B\(hychannel
establishment
.LP
7.3.3.1.2
Options for transferring the ISDN address of the PSPDN port to the TA
.LP
7.3.3.1.3
Mapping of procedures
.LP
7.3.3.1.4
Mapping of Q.931 messages
.LP
7.3.3.1.5
X.25 procedures
.LP
7.3.3.2
Incoming call
.LP
7.3.3.2.1
Q.931 call offering
.LP
7.3.3.2.2
Actions at the reference point
.LP
7.3.3.2.3
X.25 procedures
.LP
7.3.3.3
Call clearing
.LP
7.3.3.3.1
Initiation of call clearing by the DTE
.LP
7.3.3.3.2
Initiation of call clearing by the network
.LP
7.3.3.3.3
Initiation of call clearing by the user
.LP
7.3.4
Synchronization
.sp 1P
.LP
7.4
Access through the D\(hychannel
.sp 9p
.RT
.LP
7.4.1
General
.LP
7.4.2
LAPB\(hyLAPD mapping
.LP
7.4.2.1
Mapping by full link layer termination
.LP
7.4.2.1.1
Information frame address field mapping
.LP
7.4.2.1.2
Information frame control field mapping
.LP
7.4.2.1.3
Information frame check sequence recalculation
.LP
7.4.2.2
Mapping by minimum link layer termination
.LP
7.4.3
Signalling
.LP
7.4.3.1
Outgoing call
.LP
7.4.3.1.1
Conditions for the establishment of a logical link between the DTE and the PH
.LP
7.4.3.1.2
Mapping of link procedures
.LP
7.4.3.1.3
X.25 procedures
.LP
7.4.3.2
Incoming call
.LP
7.4.3.2.1
Q.931 call offering
.LP
7.4.3.2.2
X.25 procedures
.LP
7.4.3.3
Data link disconnection
.LP
7.4.3.3.1
Disconnection by the PH
.LP
7.4.3.3.2
Disconnection by the DTE
.sp 1P
.LP
7.5
Access through the B and D channel
.sp 9p
.RT
.LP
7.5.1
General
.LP
7.5.2
Outgoing call
.LP
7.5.3
Incoming call
.sp 1P
.LP
7.6
Test loops
.sp 9p
.RT
.LP
7.6.1
Test loops for TA with access through the B\(hychannel
.LP
7.6.1.1
Test loop reference configuration
.LP
7.6.1.2
Test loop characteristics
.LP
7.6.1.3
Loop activation/deactivation mechanism
.LP
7.6.1.4
Coding of activation/deactivation control messages
.LP
7.6.2
Test loops for TA with access through the D\(hychannel
.LP
Appendix I
\(em
B\(hychannel TA acting on layer 2 and 3 of X.25
.LP
Appendix II
\(em
Interconnection of packet mode TE2s which use the
circuit\(hymode bearer service of the ISDN
.LP
Appendix III
\(em
Example message flow diagrams and example conditions for cause mapping
.LP
Appendix IV
\(em
D\(hychannel TAs requiring full protocol termination in the TA
.LP
Appendix V
\(em
References
.bp
.sp 2P
.LP
\fB1\fR \fBGeneral service aspects\fR
.sp 1P
.RT
.PP
Two main services for packet switched data transmission are defined for
packet\(hymode terminals connected to the ISDN, namely:
.RT
.LP
Case A\ \(em\ access to a PSPDN (PSPDN services);
.LP
Case B\ \(em\ use of an ISDN virtual circuit service.
.PP
The provision of these services is defined in
Recommendation\ I.230\ series.
.PP
In Case A an ISDN transparent circuit connection, either permanent
(i.e., non\(hyswitched) or demand (i.e.,\ switched), is used. The corresponding
ISDN bearer service is a 64\ kbit/s service as described in
Recommendation\ I.231. The service available to the user is that of the PSPDN
described in X.25 (permanent access) and X.32 (demand access), as well as in
other X\(hyseries\ Recommendations (e.g.,\ X.2, X.121).
.PP
In Case B an ISDN virtual circuit service is used, as described in
Recommendation\ I.231, \(sc\ 3.2.1. The service available is described in
I\(hyseries\ Recommendations.
.PP
In Case A only B\(hychannel can be used to access the packet switched
service at the user\(hynetwork interface, while in Case\ B both B\(hy and
D\(hychannels can be used. The detailed service aspects for both cases
are described in\ \(sc\ 3.
.PP
This Recommendation covers the following procedures at the S/T
reference point:
.RT
.LP
\(em
B\(hy and D\(hychannel access on both basic and primary rate
interfaces. Application to H\(hychannel access is for further study.
.LP
\(em
X.25 LAPB procedures on the B\(hychannel and Q.921 LAPD
procedures on the D\(hychannel. X.25 LAP procedures are not considered here.
.LP
\(em
X.25 packet layer procedures on both B\(hy and D\(hychannels.
.PP
In addition, this Recommendation defines the use of Q.921 and
Q.931 procedures, when appropriate for the establishment and release of a
physical path through the ISDN.
.sp 2P
.LP
\fB2\fR \fBReference configurations\fR
.sp 1P
.RT
.PP
The configurations given below are the basis on which the support of X.25
DTEs and TE1s by the ISDN should be standardized. Interworking
considerations are defined in \(sc\ 5.
.PP
These configurations are also the basis on which the support of
packet mode TEs
by an ISDN has been standardized, since an X.25 DTE and its Terminal Adaptor
(TA) is always equivalent to a packet mode TE1 at the S/T interface. Therefore,
every reference in this Recommendation to the combination of an\ X.25 DTE
and its TA should always be considered as being applicable to a packet
mode TE1. However, some TE1s may have more capability than that
available from an X.25 DTE and its TA. Similarly, this Recommendation covers
the support of NT2s operating in the packet\(hymode.
.PP
Multiple X.25 DTE + TAs or TE1s, or a combination thereof, may be
supported at the customer premises. Multiple X.25 DTEs may be multiplexed at
layer\ 3 by an NT2 onto a single B\(hychannel. Multiple TAs or TE1s are
able to use the B\(hychannel, one at a time, on a per\(hycall basis.
.PP
\fINote\fR \ \(em\ Multiplexing at layer 2 within a B\(hychannel is for further
study.
.PP
This Recommendation only applies to packet mode operation carried out independently
on a single ISDN network connection type (i.e.,\ involving either a B\(hy
or D\(hychannel).
.RT
.sp 1P
.LP
2.1
\fIConfiguration when \fR \fIaccessing PSPDN services\fR \fI(Case A)\fR
.sp 9p
.RT
.PP
This configuration (Figure 2\(hy1/X.31) refers to the service of
Case\ A, thus implying a transparent handling of packet calls through an
ISDN. Only access via the B\(hychannel is possible. In this context, the
only support
that an ISDN gives to packet calls is a physical 64\ kbit/s circuit\(hymode
semi\(hypermanent or demand transparent network connection type between the
appropriate PSPDN port and the X.25\ DTE\ +\ TA or TE1 at the customer
premises.
.PP
In the case of semi\(hypermanent access, the X.25 DTE + TA or TE1 is
connected to the corresponding ISDN port at the PSPDN (AU). The TA, when
present, performs only the necessary physical channel rate adaption between
the user at the R\ reference point and the 64\ kbit/s B\(hychannel rate.
Q.931
messages are not used in this case.
.bp
.RT
.LP
.rs
.sp 33P
.ad r
\fBFigure 2\(hy1/X.31, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
In the case of demand access to PSPDNs, which is illustrated in
the upper portion of Figure\ 2\(hy1/X.31, the X.25\ DTE\ +\ TA or TE1 is
connected to an ISDN port at the PSPDN (AU). The AU is also able to set
up 64\ kbit/s
physical channels through the ISDN.
.PP
In this type of connection, originating calls will be set up over the B\(hychannel
towards the PSPDN port using the ISDN signalling procedure prior to starting
X.25 layer\ 2 and layer\ 3 functions. This can be done by exploiting
either
hot\(hyline
(e.g.,\ direct call) or complete selection methods.
Moreover, the TA, when present, performs user rate adaption at 64\ kbit/s.
Depending on the data rate adaption technique employed, a complementary
function may be needed at the AU of the PSPDN (see \(sc\ 7 on TA rate
adaption).
.PP
In the complete selection case, two separate numbers are used for
outgoing access to the PSPDN:
.RT
.LP
\(em
the ISDN number of the access port of the PSPDN, indicated in the Q.931
SETUP message;
.LP
\(em
the address of the called DTE indicated in the X.25 call
request packet.
.PP
The corresponding service requested in the Q.931 SETUP message is ISDN
circuit\(hymode bearer services.
.PP
For calls originated by the PSPDN, the same considerations as above
apply. In fact, with reference to Figure\ 2\(hy1/X.31, the ISDN port of
the PSPDN includes both rate adaption (if required) and path setting\(hyup
functions.
.bp
.PP
When needed, DTE identification may be provided to the PSPDN by using the
call establishment signalling protocols in Recommendation\ Q.931.
Furthermore, DCE identification may be provided to the DTE, when needed, by
using the same protocols.
.PP
For the demand access case, layer 2 and layer 3 operation in the
B\(hychannel as well as service definitions are given in Recommendation\ X.32.
.PP
Some PSPDNs may operate the additional DTE identification procedures defined
in Recommendation\ X.32 to supplement the ISDN provided information in
Case\ A.
.RT
.sp 1P
.LP
2.2
\fIConfiguration for the\fR
\fIISDN virtual circuit service\fR
\fI(Case B)\fR
.sp 9p
.RT
.PP
This configuration refers to the case where a packet handling
(PH) function is provided within the ISDN. The configuration in
Figure\ 2\(hy2/X.31 relates to the case of X.25 link and packet level procedures
conveyed through the B\(hychannel. In this case, the packet call is routed,
within an ISDN, to some PH function where the complete processing of the
X.25 call can be carried out.
.RT
.LP
.rs
.sp 38P
.ad r
\fBFigure 2\(hy2/X.31, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
The PH function may be accessed in various ways depending on the related
ISDN implementation alternatives. In any case a B\(hychannel connection
is set up to/from a PH port supporting the necessary processing for B\(hychannel
packet calls, standard\ X.25 functions for layer\ 2 and layer\ 3 as well as
possible path setting\(hyup functions for layer\ 1 and possible rate adaption.
.PP
The configuration in Figure 2\(hy3/X.31 refers to the case of X.25 packet
layer procedures conveyed through the D\(hychannel. In this case a number
of DTEs can operate simultaneously through a D\(hychannel by using connection
identifier discrimination at layer\ 2. The accessed port of PH is still
able to support
X.25 packet layer procedures.
.RT
.LP
.rs
.sp 38P
.ad r
\fBFigure 2\(hy3/X.31, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
It is also important to note that the procedures for accessing a PSDTS
through an ISDN user\(hynetwork interface over a\ B\(hy or D\(hychannel
are
independent of where the service provider chooses to locate packet handling
functions,\ i.e.:
.LP
a)
in a remote exchange or packet switching module in an ISDN;
.LP
b)
in the local exchange.
.PP
However, the procedures for packet access through the B\(hychannel or the
D\(hychannel are different (see \(sc\ 6).
.bp
.PP
In both cases of B\(hy and D\(hychannel accesses, in the service of Case
B, the address of the called DTE is contained in the X.25 \fIcall request\fR
packet. The establishment of the physical connection from the TA/TE1 to
the packet
handling functions is done on the basis of the requested bearer service
(ISDN virtual circuit service), therefore, the user does not provide any
addressing information in the Q.931 procedures.
.RT
.sp 2P
.LP
\fB3\fR \fBService aspects\fR
.sp 1P
.RT
.sp 1P
.LP
3.1
\fIAccess to PSPDN services\fR \fI(Case A)\fR
.sp 9p
.RT
.PP
Interworking considerations are defined in \(sc 5.
.RT
.sp 1P
.LP
3.1.1
\fIService characteristics\fR
.sp 9p
.RT
.PP
In this case, the ISDN offers a 64 kbit/s circuit\(hyswitched or
semi\(hypermanent transparent network connection type between the TA/TE1
and the PSPDN port (AU). In the switched access case the AU must be selected
by the
called address in the D\(hychannel signalling protocol when the TA/TE1
sets up the circuit\(hyswitched connection to the AU. In the non\(hyswitched
access case, Q.931 call control messages are not used.
.PP
Since the packet switched service provider is a PSPDN, some DTEs are PSPDN
terminals; they are handled by the PSPDN. Other DTEs may access the PSPDN
without subscribing to the PSPDN permanently.
.PP
In the first case, the same services as PSPDN services are maintained,
including facilities, quality of service (QOS) characteristics and DTE\(hyDCE
interfaces. In the case where a DTE is not subscribing to the PSPDN, it
will be provided with a limited set of PSPDN facilities (see Recommendation\
X.32).
.PP
Every DTE will be associated with one or more ISDN (E.164) numbers. In
addition, a DTE may be associated with one or more X.121 numbers assigned
by
the PSPDN(s) associated by the DTE. The method for X.25 packets to convey
numbers from the ISDN numbering plan and the relationship with X.121 are
described in Recommendation\ E.166.
.RT
.sp 1P
.LP
3.1.2
\fIUser access capabilities\fR
.sp 9p
.RT
.PP
In this case DTEs belonging to user classes of service 8 to 11, 13 and
30 of Recommendation\ X.1 (categories of access Q1 to Q5 of
Recommendation\ X.10) can be supported with no restrictions on the use of
Recommendation\ X.25. The rate adaption mechanism for user classes of
services\ 8 to 11 (categories of access Q1 to Q4) as well as the TA
functionalities are described in \(sc\ 7.
.RT
.sp 1P
.LP
3.1.3
\fIBasic rules\fR
.sp 9p
.RT
.PP
Packet data communications, when using a switched B\(hychannel,
will be established by separating the establishment phase of the
B\(hychannel and the control phase of the virtual circuits using the X.25
protocol (link layer and packet layer).
.PP
In general ISDN has no knowledge of the customers' terminal
equipment or configuration. The incoming B\(hychannel connection establishment
will have to employ the D\(hychannel signalling procedure (see
Recommendation\ Q.931).
.RT
.sp 1P
.LP
3.1.4
\fINotification classes\fR
.sp 9p
.RT
.PP
There is one class in terms of Q.931 procedures to notify the user of incoming
calls. In addition there is a notification class which does not use Q.931
procedures. These two classes may be provided on a subscription basis.
Networks shall provide one or more of these classes. These classes are
defined in \(sc\ 3.2.3.1 and \(sc\ 3.2.3.2 with the following exceptions:
.RT
.LP
\(em
The terms used in \(sc 3.2.3.1 apply by replacing \*QPH\*U with
\*QAU\*U.
.LP
\(em
Only the B\(hychannel access will be used in this case.
.LP
\(em
Mapping of information in the conditional case is restricted to the information
elements available for end\(hyto\(hyend transfer of
information.
.bp
.sp 1P
.LP
3.2
\fIAccess to the ISDN virtual circuit service (Case B)\fR
.sp 9p
.RT
.PP
Interworking considerations are defined in \(sc 5.
.RT
.sp 1P
.LP
3.2.1
\fIService characteristics\fR
.sp 9p
.RT
.PP
The virtual circuit service provided within the ISDN is aligned
with what is described in the
X\(hySeries Recommendations (e.g.,\ in terms of
facilities, quality of service,\ etc.).
.PP
The service and facilities provided as well as the quality of service characteristics
are those of the ISDN. Existing features of the
X\(hySeries\ Recommendation may be enhanced and additional features may also be
developed taking into account the new ISDN customer capabilities. A number
from the ISDN numbering plan will be associated with one or more TA/TE1
(see
Recommendation\ E.164).
.RT
.sp 1P
.LP
3.2.2
\fIUser access capabilities\fR
.sp 9p
.RT
.PP
In this case both B\(hy and D\(hychannels can be used for accessing the
ISDN virtual circuit service.
.RT
.sp 2P
.LP
3.2.2.1
\fIAccess through the B\(hychannel\fR
.sp 1P
.RT
.sp 1P
.LP
3.2.2.1.1
\fIService limitations\fR
.sp 9p
.RT
.PP
In this case DTEs belonging to user classes of service 8 to 11, 13 and
30 of Recommendation\ X.1 (categories of access T1 to T5 and Y1 to Y5
of Recommendation\ X.10) can be supported with no restrictions on the use of
Recommendation\ X.25. The rate adaption mechanisms for user classes of
service\ 8 to 11 (access categories T1 to T4 and Y1 to Y4) as well as the
TA functionalities are described in \(sc\ 7.
.RT
.sp 1P
.LP
3.2.2.1.2
\fIBasic rules\fR
.sp 9p
.RT
.PP
Packet data communications, when using a switched B\(hychannel,
will be established by separating the establishment phase of the
B\(hychannel and the control phase of the virtual circuits using the X.25
protocol (link layer and packet layer).
.PP
In general, an ISDN has no knowledge of the customer's terminal
equipment or configuration. In the demand access case the incoming B\(hychannel
connection establishment will have to employ the signalling procedures
of \(sc\ 6 (see Recommendation\ Q.931).
.RT
.sp 2P
.LP
3.2.2.2
\fIAccess through the D\(hychannel\fR
.sp 1P
.RT
.sp 1P
.LP
3.2.2.2.1
\fIService limitations\fR
.sp 9p
.RT
.PP
In this case DTEs belonging to user classes of service 8 to 10 of Recommendation\
X.1 (categories of access U1 to U4 of Recommendation\ X.10) and except
on basic access user class of service\ 11 of Recommendation\ X.1
(categories of access U5 of Recommendation\ X.10) can be supported subject to
the limitation imposed by LAPD as regards the maximum I\(hyfield length of the
information frames (parameter\ N201 as defined in Recommendation\ Q.921).
In any case, the maximum limit for the size of each frame to be transferred
on the
D\(hychannel shall be 260\ octets.
.RT
.sp 1P
.LP
3.2.2.2.2
\fIBasic rules\fR
.sp 9p
.RT
.PP
The following principles must always be respected in order to offer TE
access to the PSDTS as it is defined in the Series\ X Recommendations,
particularly\ X.25.
.PP
A single SAPI\ =\ 16 LAPD link, as viewed by both the network and the
user, must support multiplexing of logical channels at layer\ 3. Additionally,
because the user may have a multipoint access, and because a single TA
or TE1 is allowed to operate with more than one TEI, the network must support
the
presence of multiple SAPI\ =\ 16\ LAPD logical links simultaneously operating
at layer\ 2. This results in the requirement that the network be able to
support
simultaneous layer\ 2 and layer\ 3 multiplexing for D\(hychannel packet mode
connections.
.bp
.PP
All X.25 packets, including \fIcall request\fR and \fIincoming call\fR
packets, must be transported to and from the TE in numbered information\fR
(I\ frames) in a SAPI\ =\ 16\ LAPD link.
.PP
An \fIincoming call\fR \| packet will be transmitted to a TE only after
the public networks check at least the following:
.RT
.LP
\(em
compatibility of user facilities contained in the \fIincoming\fR \fIcall\fR
packet with the called subscriber profile when present;
.LP
\(em
availability of the logical channel, either two\(hyway or
incoming, on which the \fIincoming call\fR packet is sent.
.sp 1P
.LP
3.2.3
\fINotification classes for incoming calls\fR
.sp 9p
.RT
.PP
There are three classes in terms of Q.931 procedures to notify the user
of incoming calls. These classes may be provided on a subscription basis.
Networks shall provide one or more of these classes.
.RT
.sp 1P
.LP
3.2.3.1
\fINo notification class\fR
.sp 9p
.RT
.PP
The network shall allocate incoming calls to a channel (D/B) using a network
implemented algorithm. No Q.931 procedures are used to notify the
user of incoming calls. Two subclasses are recognized:
.RT
.LP
a)
Semi\(hypermanent (nailed\(hyup) connections to the PH.
An \fIincoming call\fR packet will be directly delivered over
the semi\(hypermanent connection.
.LP
b)
User initiated demand connections (at the called side)
.LP
The user is responsible for initiating channels to the PH
using Q.931 procedures. If the user has not initiated channels to the PH,
the network shall clear incoming calls.
.sp 1P
.LP
3.2.3.2
\fIConditional notification class\fR
.sp 9p
.RT
.PP
Q.931 procedures are only used by the network to activate a channel for
delivery of an incoming call when there is no available channel in the
active state as defined in Recommendation\ Q.931. Subsequent incoming calls
to the same ISDN number will be delivered over this channel without using
Q.931
procedures.
.PP
Some networks may have the ability to maintain information related to the
state of the user's packet access channel. The network may apply an
algorithm to determine that no additional calls should be added to the
active packet access channel. The network may then reject the call immediately
or use Q.931 procedures in an attempt to activate another channel for the
purpose of delivering additional calls.
.PP
\fINote\fR \ \(em\ Some network may also compare the subaddress and use
Q.931 procedure when the ISDN address differs from the ISDN address of
the terminal with the active packet access channel.
.RT
.sp 1P
.LP
3.2.3.3
\fIUnconditional notification class\fR
.sp 9p
.RT
.PP
Q.931 procedures are used by the network to notify the user of each X.25
incoming call. As Table\ 3\(hy1/X.31 notes, all of the information that
is
able to be copied from the X.25 \fIincoming call\fR packet to the Q.931 SETUP
message is copied. This service is provided in order to aid the terminal
equipment in the management of the interface (e.g.,\ compatibility checking,
channel selection).
.RT
.sp 1P
.LP
3.2.3.4
\fIInformation mapping from the X.25 incoming call packet\fR
\fIto the Q.931 message\fR
.sp 9p
.RT
.PP
In case of the conditional notification and unconditional
notification classes, some of the information present in the X.25 \fIincoming\fR
\fIcall\fR packet should be mapped into the Q.931 SETUP message as indicated
in
Table\ 3\(hy1/X.31.
.RT
.sp 1P
.LP
3.3
\fICompatibility checking\fR
.sp 9p
.RT
.PP
This paragraph is relevant for both Case A and Case B services.
.PP
Information subject to compatibility checking in the public
network(s), in the terminal systems, or in both the public network(s) and
the terminal systems when establishing a communication between two systems
can be divided into two basic capabilities:
.RT
.LP
\(em
The transmission capability may include ISDN network
connection types,
bearer service identification information
in relation to layers\ 1 to\ 3 in the terminals, and facilities defined in
Recommendation\ X.2.
.bp
.LP
\(em
The communication capability involves higher layer functions for standardized
applications in relation to telecommunication services. Other information,
which is passed transparently between the terminal systems, may
also form part of the communication capability. The coding of the information
elements for compatibility checking and their relation to the open systems
interconnection (OSI) reference model is in Recommendations\ Q.931 and X.300.
Communication capability checking at the ISDN network connection level is
limited to those parameters conveyable by the X.25 packet layer protocols,
i.e.,\ higher layer compatibility parameters cannot be passed from the
calling user to the called user.
.LP
.sp 1
.rs
.sp 21P
.ad r
\fBCuadro 3\(hy1/X.31 [T1.31], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
.sp 1
The network provides the transmission capability and furnishes the associated
bearer capability information element to the user in the Q.931 SETUP message
when the incoming call is notified to the user. This element and
possibly others are used by the user equipment for compatibility checking
purposes as described in Recommendation\ Q.931, Annex\ B.
.PP
The network does not transmit any communication capability (i.e., the associated
high layer compatibility information element) to the user since an X.25
packet layer protocol cannot transfer such an information element from
the calling to the called user.
.RT
.sp 2P
.LP
\fB4\fR \fBAddressing and routing aspects\fR
.sp 1P
.RT
.sp 1P
.LP
4.1
\fITerminal interface selection\fR
.sp 9p
.RT
.PP
This section describes the information necessary to select a
compatible TA/TE1 for the completion of an incoming call since users may
operate several packet terminals in their multiservice arrangements.
.PP
For data transmission, it is envisaged that an ISDN would identify, by
means of an ISDN address, a specific interface within the subscriber premises.
The transmission capability information may be used by the called TA/TE1
for
compatibility checking purposes.
.PP
\fINote\fR \ \(em\ The terminal identification for PVC services is for
further study.
.bp
.PP
In general, an ISDN number identifies one or more ISDN user\(hynetwork
interfaces. However, some networks may allow an ISDN user\(hynetwork interface
to be allocated more than one ISDN number, thus allowing the identification
of a given terminal within an ISDN user\(hynetwork interface. Furthermore,
a
subaddress, derived from the X.25 address extension facility may be used to
identify a specific terminal within a user installation.
.RT
.sp 2P
.LP
4.2
\fIAccess to PSPDN services (Case A)\fR
.sp 1P
.RT
.sp 1P
.LP
4.2.1
\fIChannel type selection\fR
.sp 9p
.RT
.PP
Packet calls using this bearer service (i.e., circuit\(hymode) will
always use the B\(hychannel.
.RT
.sp 1P
.LP
4.2.2
\fIAddressing scheme for outgoing calls\fR
.sp 9p
.RT
.PP
The Q.931 SETUP message, when used, contains the request for a
circuit\(hymode bearer service. The SETUP message also contains the ISDN
address of the AU of the PSPDN.
.PP
The X.25 \fIcall request\fR packet contains the address of the called
terminal.
.RT
.sp 2P
.LP
4.3
\fIAccess to the ISDN virtual\(hycircuit service (Case B)\fR
.sp 1P
.RT
.sp 1P
.LP
4.3.1
\fIChannel type selection\fR
.sp 9p
.RT
.PP
Two procedures are available regarding the manner in which channel type
selection (i.e.,\ selecting between the B\(hy and D\(hychannel type) can
be
performed:
.RT
.LP
i)
the terminal which is to accept the call will indicate the channel type
to be used;
.LP
ii)
the ISDN has information on which channel type will be used for the incoming
call.
.PP
The various sorts of information that the ISDN may use to
determine the channel may include, but are not limited to:
.LP
a)
subscription time agreements;
.LP
b)
occupancy level on established channels.
.PP
Channel negotiation procedures may be found in \(sc 6.
.sp 1P
.LP
4.3.2
\fIAddressing scheme for outgoing calls\fR
.sp 9p
.RT
.PP
The Q.931 SETUP message, when used, contains the request for the
ISDN virtual circuit service. The SETUP message does not contain an address.
.PP
The X.25 \fIcall request\fR packet contains the address of the called
terminal.
.RT
.sp 2P
.LP
\fB5\fR \fBInterworking with dedicated networks\fR
.sp 1P
.RT
.sp 1P
.LP
5.1
\fICircuit\(hymode access to PSPDN services (Case A)\fR
.sp 9p
.RT
.PP
Interworking by port access
(see Recommendation X.300)
applies, i.e.\ the packet mode terminal accesses the PSPDN access port
(AU) by use of a 64\ kbit/s connection through the ISDN. The AU belongs
to the PSPDN and is functionally equal to the interworking function (IWF)
(see
Recommendation\ X.325).
.RT
.sp 1P
.LP
5.2
\fIAccess to PSPDNs via virtual circuit service (Case B)\fR
.sp 9p
.RT
.PP
Interworking by call control mapping (see Recommendation X.300)
applies, i.e.\ interworking between the ISDN and PSPDN is effected using\
X.75 or a functionally equivalent internal network protocol. In some
implementations, the PH functions logically belonging to the ISDN may
reside physically in a node of the PSPDN. The service provided is still the
ISDN virtual circuit service. In any case, interworking between network
providers is effected through use of X.75. See also Recommendation\ X.325.
.bp
.RT
.sp 2P
.LP
\fB6\fR \fBPacket communications at the S/T reference point\fR
.sp 1P
.RT
.PP
This section describes the information flows necessary to support packet
communication over:
.RT
.LP
a)
circuit mode (Case A) operation on B\(hychannels; and
.LP
b)
packet mode (Case B) operation on B\(hy and D\(hychannels of an
ISDN access line.
.PP
The ISDN TA/TE1 presents an S/T reference point towards the
network and therefore the TA/TE1 implementation should embody the procedures
described in Recommendations\ Q.921 and Q.931 for B\(hy and D\(hychannel
connection
establishment and control. The protocol and the text of \(sc\(sc\ 6.1\(hy6.5
and
Appendix\ II of Recommendation\ Q.931, and \(sc\(sc\ 6.1\(hy6.5 and Appendix\
III of
Recommendation\ X.31 are identical.
.PP
For demand access connections, \(sc\(sc 6.1 through 6.4 apply. Example
message flows for demand access connections are shown in Appendix\ III.
.PP
Two types of semi\(hypermanent connections on B\(hy and D\(hychannels are
covered in this Section:
.RT
.LP
1)
physical layer semi\(hypermanently established between the
terminal and the PH/AU, i.e.,\ the I.430/I.431 physical layer remains activated
and the physical path through the ISDN is connected semi\(hypermanently;
and
.LP
2)
data link and physical layers semi\(hypermanently established between
the terminal and the PH/AU (in this type, the network shall keep the
data link layer in the established state).
.PP
When a PVC is used, there must exist a type 2) semi\(hypermanent
connection.
.PP
In semi\(hypermanent connection type 1), the procedures of \(sc 6.3 are
followed for call establishment and release.
.PP
In semi\(hypermanent connection type 2), only the procedures of \(sc 6.3.2
are followed for call establishment and release.
.PP
When semi\(hypermanent connection type 2) is used for PVCs, none of the
following procedures apply.
.PP
Semi\(hypermanent connections are established via a provisioning process
without Q.931 procedures.
.RT
.sp 1P
.LP
6.1
\fIOutgoing access\fR
.sp 9p
.RT
.PP
If the user selects an already established channel for the outgoing virtual
call, then the procedures described in \(sc\ 6.3 apply. If the selected
channel is not established to the AU/PH, then the procedures for activating
a channel described in the following subsections are to be used before
establishing the virtual call using the procedures of \(sc\ 6.3.
.PP
For outgoing data calls, the user first must decide whether
circuit\(hyswitched (Case\ A) or packet switched services (Case\ B) are
desired from the network. For outgoing circuit calls, the user follows
the procedures of
\(sc\ 6.1.1. For outgoing packet calls, a user decides whether B\(hychannel or
D\(hychannel is to be used for the packet call. If the user decides to use the
B\(hychannel, then the procedures described in \(sc\ 6.1.2.1 are used.
If the user
decides to use the D\(hychannel, then the procedures described in \(sc\
6.1.2.2 are
used.
.PP
\fINote\fR \ \(em\ Some networks may not support every type of access. In the
case of B\(hychannel access, the network will clear a request for unsupported
services by sending a RELEASE COMPLETE message with cause\ ##65, \*Q\fIbearer\fR
\fIservice not implemented\fR \*U. In the case of a request for D\(hychannel
access (an SABME with SAPI\ =\ 16), on a network port which does not support
the service, no response is required of the network.
.RT
.sp 1P
.LP
6.1.1
\fICircuit\(hyswitched access to PSPDN services (Case A)\fR
.sp 9p
.RT
.PP
The B\(hychannel connection between the user and the AU shall be
controlled using the D\(hychannel signalling procedures for call establishment
described in \(sc\ 5.1 of Recommendation\ Q.931. The specific B\(hychannel
to be used as a switched connection is selected using the channel selection
procedures
described in \(sc\ 5.1.2 of Recommendation\ Q.931 and summarized in
Table\ 6\(hy1/X.31.
.bp
.RT
.ce
\fBH.T. [T2.31]\fR
.ce
TABLE\ 6\(hy1/X.31
.ce
\fBUser requested channel and network response\fR
.ce
\fBOutgoing access to either an AU or PH\fR
.ce
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(54p) sw(54p) sw(54p) | cw(66p) , c | c | c | ^ .
T{
Channel indicated in the SETUP message user to
network direction
T} T{
Allowable network response
network\(hyuser
T}
Channel indication Preferred or exclusive D\(hychannel indication
_
.T&
cw(54p) | cw(54p) | cw(54p) | cw(66p) , ^ | c | c | c.
Bi Exclusive No Bi
Preferred No Bi, Bi`
_
.T&
cw(54p) | cw(54p) | cw(54p) | cw(66p) .
Any (Ignore) No Bi`
_
.T&
cw(54p) | cw(54p) | cw(54p) | cw(66p) .
(Absent) T{
Bi`
Bi
the indicated (idle) B\(hychannel
Bi`
any (other) idle B\(hychannel
.parag
\fINote\ 1\ \(em\ \fR
All other encodings are invalid.
.parag
\fINote\ 2\ \(em\ \fR
All columns under the heading \*QChannel indicated in the SETUP
message\*U indicate possible user codings of the Channel indentification
information element contained in the SETUP message sent by the user to the
network requesting a connection to an AU or PH (see Section\ 4.5.13 of
Recommendation\ Q.931). The column under \*QAllowable network response\*U refers to the allowable responses by the network to the user.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 6\(hy1/X.31 [T2.31], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
On the basis of the call set\(hyup information (e.g., called party
number identifying an AU, transit network selection,\ etc.) and/or a
subscription time agreement, the network provides a connection to the
appropriate AU. The bearer capability information element included in the
SETUP message shall be coded with:
.LP
\(em
information transfer capability set to either:
.LP
a)
\*Qunrestricted digital information\*U; or
.LP
b)
\*Qrestricted digital information\*U.
.LP
\(em
transfer mode set to \*Qcircuit mode\*U;
.LP
\(em
information rate set to \*Q64 kbit/s\*U.
.PP
\fINote\fR \ \(em\ Bearer capability information element octets 4a and 4b
shall not be included.
.PP
The user may also specify the layer 1 (e.g., rate adaption), layer
2 (i.e.,\ LAPB), and layer\ 3 (i.e.,\ X.25) information transfer protocols
in the low layer compatibility information element in the SETUP message
(see Annex to Q.931 entitled \*QLow layer information coding principles\*U).
.RT
.sp 2P
.LP
6.1.2
\fIAccess to the ISDN virtual circuit service (Case B)\fR
.sp 1P
.RT
.sp 1P
.LP
6.1.2.1
\fIB\(hychannel\fR
.sp 9p
.RT
.PP
Demand access B\(hychannel connections are controlled using the
D\(hychannel signalling procedures for call establishment described in
\(sc\ 5.1 of
Recommendation\ Q.931 using the messages defined in \(sc\ 3.2 of
Recommendation\ Q.931 with the following exceptions:
.RT
.LP
\(em
The procedures for overlap sending specified in \(sc 5.1.3 of
Recommendation\ Q.931 do not apply.
.LP
\(em
The procedures for call proceeding and overlap sending
specified in \(sc\ 5.1.5.2 of Recommendation\ Q.931 do not apply.
.bp
.LP
\(em
The procedures for notification of interworking at the
origination interface specified in \(sc\ 5.1.6 of Recommendation\ Q.931 do not
apply.
.LP
\(em
The procedures for call confirmation indication specified in \(sc\ 5.1.7
of Recommendation\ Q.931 do not apply.
.LP
\(em
The procedures for call connected specified in \(sc 5.1.8 of
Recommendation\ Q.931 apply as follows:
.LP
\(em
upon accepting the access connection, the network shall
send a CONNECT message across the user\(hynetwork interface to the calling user
and enter the active state;
.LP
\(em
this message indicates to the calling user that an
access connection to the packet handler has been established;
.LP
\(em
on receipt of the CONNECT message, the calling user shall
stop timer\ T310 (see Recommendation\ Q.931), may optionally send a CONNECT
ACKNOWLEDGE message, and shall enter the active state.
.LP
\(em
The procedures for call rejection specified in \(sc 5.1.9 of
Recommendation\ Q.931 apply as follows:
.LP
\(em
when unable to accept the access connection, the
network shall initiate call clearing at the originating user\(hynetwork
interface as described in \(sc\ 5.3 of Recommendation\ Q.931.
.LP
\(em
The procedures for transit network selection specified in
\(sc\ 5.1.10 of Recommendation\ Q.931 do not apply.
.PP
The specific B\(hychannel to be used as a demand connection is
selected using the channel selection procedures described in \(sc\ 5.1.2 of
Recommendation\ Q.931 and summarized in Table\ 6\(hy1/X.31.
.PP
For a demand connection to an ISDN PH, the bearer capability
information element included in the SETUP message shall be coded
with:
.RT
.LP
\(em
information transfer capability set to \*Qunrestricted digital information\*U;
.LP
\(em
transfer mode set to \*Qpacket mode\*U;
.LP
\(em
information transfer rate set to 00000;
.LP
\(em
user information layer 2 protocol set to
\*QRecommendation\ X.25, link layer\*U;
.LP
\(em
user information layer 3 protocol set to
\*QRecommendation\ X.25, packet layer\*U.
.PP
\fINote\fR \ \(em\ Octets 4a, 4b and 5a, 5b, 5c, 5d shall not be included.
.PP
The demand access connection can then be used to support packet
communications according to X.25 link layer and X.25 packet layer procedures
as specified in \(sc\ 6.3.
.RT
.sp 1P
.LP
6.1.2.2
\fID\(hychannel\fR
.sp 9p
.RT
.PP
The D\(hychannel provides a connection which enables the ISDN user
terminal to access a PH function within the ISDN by establishing a link
layer connection (SAPI\ =\ 16) to that function which can then be used
to support
packet communications according to X.25 layer\ 3 procedures as defined
in \(sc\ 6.3. The X.25 packet layer uses the acknowledged information transfer
service
(i.e.,\ I\(hyframes) provided by LAPD (see Recommendation\ Q.920). Consequently
Q.931 procedures are not required to provide D\(hychannel access.
.PP
A number of packet mode user equipment can operate simultaneously
over the D\(hychannel, each using a separate layer\ 2 data link identified
by an
appropriate address (see Recommendation\ Q.921) in frames transferred between
the user and PH.
.RT
.sp 2P
.LP
6.2
\fIIncoming access\fR
.sp 1P
.RT
.sp 1P
.LP
6.2.1
\fIAccess from PSPDN services\fR \fI(Case A)\fR
.sp 9p
.RT
.PP
The ISDN signals the establishment of the circuit\(hymode connection using
the procedures described in \(sc\ 5.2 of Recommendation\ Q.931. The virtual
calls are signalled between the user and the AU using the procedures described
in \(sc\ 6.3.
.RT
.sp 1P
.LP
6.2.1.1
\fIGeneral\fR
.sp 9p
.RT
.PP
The general procedures performed by the AU are those defined in
Recommendation\ X.32.
.bp
.RT
.sp 1P
.LP
6.2.1.2
\fIChannel selection\fR
.sp 9p
.RT
.PP
If the physical circuit desired by the AU does not exist between
the terminal and the AU, the procedures for physical channel establishment
described in the following sections apply.
.PP
The format of the SETUP message sent by the network to the user is in accordance
with \(sc\ 3.1 of Recommendation\ Q.931.
.PP
The bearer capability information element included in SETUP message
shall be coded with:
.RT
.LP
\(em
information transfer capability set to either:
.LP
a)
\*Qunrestricted digital information\*U; or
.LP
b)
\*Qrestricted digital information\*U.
.LP
\(em
transfer mode set to \*Qcircuit mode\*U;
.LP
\(em
information rate set to \*Q64 kbit/s\*U.
.PP
\fINote\fR \ \(em\ Bearer capability information element octets 4a and 4b
shall not be included. The channel identification information element shall
be coded according to Table\ 6\(hy2/X.31.
.ce
\fBH.T. [T3.31]\fR
.ce
TABLE\ 6\(hy2/X.31
.ce
\fBNetwork requested channel and user response\fR
.ce
\fBIncoming access from an AU\fR
.ce
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(54p) sw(54p) sw(54p) | cw(66p) , c | c | c | ^ .
T{
Channel indicated in the SETUP message network
to user direction
T} T{
Allowable user response
user\(hynetwork
\fR
T}
Channel indication Preferred or exclusive D\(hychannel indication
_
.T&
cw(54p) | cw(54p) | cw(54p) | cw(66p) .
Bi Exclusive No Bi
_
.T&
cw(54p) | cw(54p) | cw(54p) | cw(66p) .
Bi Preferred No T{
Bi, Bi` (Note 1)
Bi
indicated (idle) B\(hychannel
Bi`
any another idle B\(hychannel (not permitted for broadcast call
offering)
.parag
\fINote\ 1\ \(em\ \fR
This encoding is not used for broadcast call offering.
.parag
\fINote\ 2\ \(em\ \fR
All other encodings are invalid.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 6\(hy2/X.31 [T3.31], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
The B\(hychannel connection to the called user shall be established by
the network using the signalling procedures described in \(sc\ 5.2 of
Recommendation\ Q.931. The call is offered by sending the SETUP message on a
point\(hyto\(hypoint data link or on the broadcast data link.
.PP
The user responds to the SETUP as specified in \(sc 5 of
Recommendation\ Q.931.
.RT
.sp 1P
.LP
6.2.2
\fIAccess from the ISDN virtual circuit service\fR \fI(Case B)\fR
.sp 9p
.RT
.PP
To offer an incoming call, the network must perform the following steps
in sequence:
.RT
.LP
1)
Channel selection \(em the physical channel/logical link to be used for
the incoming call must be identified. The network may use customer
profile information, network resources,\ etc., to choose the channel, or the
procedures in Step\ 2 below.
.LP
2)
Physical channel/logical link establishment \(em if the
physical B\(hychannel or the logical link of the D\(hychannel have not been
determined by Step\ 1, the network may use the procedures in \(sc\ 6.2.2.3. The
network may then proceed with Step\ 3.
.LP
3)
Virtual call establishment \(em the network establishes the
virtual call using the procedures described in \(sc\ 6.3.
.bp
.PP
In the configuration for the ISDN virtual circuit service, the
choice of channel type to be used for the delivery of a new \fIincoming
call\fR
packet shall be made by the network as described below.
.LP
1)
A new \fIincoming call\fR packet may be indicated to the ISDN
customer by a call offering procedure between the network and all user
packet mode terminals (see \(sc\(sc\ 3.2.3.2 and 3.2.3.3).
.LP
2)
An incoming virtual call directed to a terminal with an
established connection to the PH may be offered directly to the terminal
over the established access connection without the use of Q.931 call offering
procedures (see \(sc\(sc\ 3.2.3.1 and 3.2.3.2 of Recommendation\ X.31).
.sp 1P
.LP
6.2.2.1
\fIB\(hychannel\fR
.sp 9p
.RT
.PP
When calls are to be offered on the B\(hychannels without channel
negotiation, the procedures described in \(sc\ 5.2 of Recommendation\ Q.931
using
the messages of \(sc\ 3.2 of Recommendation\ Q.931 apply with the following
exceptions:
.RT
.LP
\(em
The procedures for overlap receiving specified in \(sc 5.2.4 of Recommendation\
Q.931 do not apply.
.LP
\(em
The procedures for receipt of CALL PROCEEDING and ALERTING
specified in \(sc\ 5.2.5.2 of Recommendation\ Q.931 apply with the following
exception:
.LP
\(em
the receipt of an ALERTING message shall not cause the network to send
a corresponding ALERTING message to the calling user.
.LP
\(em
The procedures for call failure specified in \(sc 5.2.5.3 of
Recommendation\ Q.931 apply with the following note:
.LP
\(em
the network clears the incoming X.25 virtual call
towards the calling X.25 DTE using the appropriate cause from Table\ 6\(hy5/X.31.
.LP
\(em
The procedures for notification of interworking at the
terminating interface specified in \(sc\ 5.2.6 of Recommendation\ Q.931
apply with the following exceptions:
.LP
\(em
the case of the call entering an ISDN environment
during call establishment is not applicable;
.LP
\(em
in the case of a call leaving the ISDN environment
within the called user's premises, no notification is sent to the calling
party;
.LP
\(em
the case of in\(hyband information/patterns is not
applicable.
.LP
\(em
The procedures for active indication specified in \(sc 5.2.8 of Recommendation\
Q.931 apply with the following exception:
.LP
f
.LP
\(em
the network shall not initiate procedures to send a
CONNECT message towards the calling user.
.LP
\(em
The procedures for user notification specified in \(sc 5.2.10 of Recommendation\
Q.931 do not apply.
.PP
Where an established B\(hychannel connection is to be used, the
\fIincoming call\fR packet will be delivered in accordance with \(sc\ 6.3.
.PP
Where a new B\(hychannel connection is to be established, the identity
of the selected user will be associated with the Connection Endpoint Suffix
(CES) from which the first CONNECT message has been received.
.RT
.sp 1P
.LP
6.2.2.2
\fID\(hychannel\fR
.sp 9p
.RT
.PP
The D\(hychannel provides a connection which enables the ISDN PH to
access an ISDN user terminal or vice versa. This access is accomplished by
establishing a link layer connection (SAPI\ =\ 16) to the terminal or network
which can then be used to support packet communications according to X.25
layer\ 3 procedures as defined in \(sc\ 6.3.
.PP
The layer 2 procedures shall be in accordance with
Recommendation\ Q.921. The D\(hychannel provides a semi\(hypermanent connection
for
packet access since all layer\ 2 frames containing a packet mode SAPI\
(16) are routed automatically between the user and the PH function.
.PP
When an incoming call is offered to packet mode user equipment at the user
interface, the channel selection procedures described in \(sc\ 6.2.2.3
shall be used.
.PP
A number of packet mode terminals can operate simultaneously over the D\(hychannel,
each using a separate layer\ 2 link identified by an appropiate TE1 (see
Recommendation\ Q.921) in frames transferred between the terminal and the
network.
.bp
.RT
.sp 2P
.LP
6.2.2.3
\fICall offering\fR
.sp 1P
.RT
.sp 1P
.LP
6.2.2.3.1
\fIChannel selection through call offering\fR
.sp 9p
.RT
.PP
The call offering procedure is performed using the layer 3 messages and
procedures of \(sc\ 5 of Recommendation\ Q.931. The call offering procedure
is integrated into the circuit\(hyswitched call control procedures, signalled
on the D\(hychannel, with the channel selection being accomplished by means
of the
channel selection procedure if offered as a network option.
.PP
As described in \(sc 5 of Recommendation Q.931, the network selects the
first user which responds to the call offering with a CONNECT message.
When the selected user has requested that the X.25 call be set up over
a new B\(hychannel, the network will indicate that the channel is acceptable
by returning a CONNECT ACKNOWLEDGE message to the user. If multiple terminals
have responded
positively to the SETUP message, the network shall clear each of the
non\(hyselected terminals with a RELEASE message containing cause\ ##\ 26,
\*Q\fInon\(hyselected user clearing\fR \*U.
.PP
When the selected user has requested that the X.25 call be set up over
an established B\(hychannel or the D\(hychannel, the network shall respond
to the
CONNECT message with a RELEASE message containing cause\ ##\ 7,
\*Q\fIcall awarded and\fR \fIbeing delivered in an established channel\*U\fR
. The network shall also return a RELEASE message containing cause\ ##\
26, \*Q\fInon\(hyselected user clearing\fR \*U to any other positively
responding terminals. The network will then deliver the X.25 call over
the selected channel.
.PP
\fINote\ 1\fR \ \(em\ There is no time significance between the delivery
of the RELEASE message and the \fIincoming call\fR packet, i.e.,\ either
may occur first.
.PP
\fINote\ 2\fR \ \(em\ The network shall send the RELEASE message(s) and the
user(s) shall respond with RELEASE COMPLETE.
.PP
If the channel indicated by the first positively responding user is
not available, the network will use Q.931 call clearing procedures to clear
the call with cause\ ##\ 6, \*Q\fIchannel unacceptable\fR \*U. If the channel
indicated in the SETUP message is not acceptable to the user, the user
will clear the call with a RELEASE message containing cause\ ##\ 34, \*Q\fIno
circuit/channel available\fR \*U or cause\ ##\ 44, \*Q\fIrequested circuit/channel
not available\fR \*U.
.PP
On the basis of a network option or subscription agreement, the
network may choose the access channel or access channel type (e.g.,\ B
or D) for a particular incoming packet call.
.PP
When the channel indication information element indicates
\fIChannel indication\ =\ No channel\fR , \fIExclusive\fR , and \fID\(hychannel\fR
\fIindication\ =\ Yes\fR , then the bearer capability information element
should be
encoded as follows:
.RT
.LP
\(em
Information transfer capability set to either: \fIUnrestricted\fR \fIdigital
information\fR or \fIrestricted digital information\fR .
.LP
\(em
Transfer mode set to: \fIpacket mode\fR .
.LP
\(em
Information rate set to: \fIpacket mode (00000)\fR .
.LP
\(em
Layer 2 protocol set to: \fIRecommendation Q.921\fR .
.LP
\(em
Layer 3 protocol set to: \fIRecommendation X.25\fR
\fIpacket layer\fR .
.PP
In all other cases, the bearer capability information element
should be encoded as follows:
.LP
\(em
Information transfer capability set to either: \fIUnrestricted\fR \fIdigital
information\fR or \fIrestricted digital information\fR .
.LP
\(em
Transfer mode set to: \fIpacket mode\fR .
.LP
\(em
Information rate set to: \fIpacket mode (00000)\fR .
.LP
\(em
Layer 2 protocol set to: \fIRecommendation X.25\fR
\fIlink layer\fR .
.LP
\(em
Layer 3 protocol set to: \fIRecommendation X.25\fR
\fIpacket layer\fR .
.PP
There exists an understanding that if the terminal responds with D\(hychannel
indication set (see Table\ 6\(hy3/X.31), the Layer\ 2 protocol to be used
is Recommendation\ Q.921 (LAPD).
.PP
The channel selection procedure for incoming calls is independent of the
type of channel selected at the calling end. In this respect, any
combination of channel type used at each end is possible, provided the user
rates and available bandwidth are compatible.
.PP
The channel selection principle to be used in the procedure is shown in
Table\ 6\(hy3/X.31.
.PP
\fINote\ 3\fR \ \(em\ When the incoming SETUP message is sent on a broadcast
data link with a channel identification information element which indicates
an idle B\(hychannel and \*Qpreferred\*U, the called user is not permitted
to respond with a different idle B\(hychannel in the response. The option
to respond with a
different idle channel is restricted to point\(hyto\(hypoint call offerings.
.bp
.PP
\fINote\ 4\fR \ \(em\ Networks providing packet mode call offering shall
provide Q.931 signalling procedures for packet mode calls on SAPI\ =\ 0.
For an interim period, some networks, by subscription agreement, may offer
SAPI\ =\ 16 broadcast call offering procedures for providing Q.931 signalling.
This option shall use all Q.931 procedures for packet mode calls with the
following restriction: All calls will be offered as \*QD\(hychannel exclusive\*U
and will not provide channel
selection procedures. Terminals implementing SAPI\ =\ 16 procedures shall also
implement SAPI\ =\ 0 procedures for portability.
.RT
.ce
\fBH.T. [T4.31]\fR
.ce
TABLE\ 6\(hy3/X.31
.ce
\fBNetwork requested channel and user response\fR
.ce
\fBIncoming access for packet mode\fR
.ce
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(54p) sw(54p) sw(54p) | cw(66p) , c | c | c | ^ .
T{
Channel indicated in the SETUP message network to
user direction
T} T{
Allowable user response
user\(hynetwork
T}
Channel indication Preferred or exclusive D\(hychannel indication
_
.T&
cw(54p) | cw(54p) | cw(54p) | cw(66p) , ^ | ^ | c | c.
Bi Exclusive No Bi
Yes Bi, D
_
.T&
cw(54p) | cw(54p) | cw(54p) | cw(66p) , ^ | ^ | c | c.
Bi Preferred No Bi, Bi`, Bj
Yes Bi, Bi`, Bj, D
_
.T&
cw(54p) | lw(54p) | lw(54p) | lw(66p) , ^ | ^ | l | l
^ | l | l | l.
T{
No channel
Preferred
No
Bj
Yes
Bj, D
Exclusive
Yes
D
Bi
indicated (idle) B\(hychannel
Bi`
any other idle B\(hychannel (not permitted in response to broadcast
call offering)
Bj
an established B\(hychannel under the user's control
D
the D\(hychannel
.parag
\fINote\ \(em\ \fR
All other encodings are invalid.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 6\(hy3/X.31 [T4.31], p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
6.2.2.3.2
\fIInformation element mapping\fR
.sp 9p
.RT
.PP
Some networks may choose to provide a service of mapping some or
all of the information from the \fIincoming call\fR packet into the SETUP
message (see \(sc\ 3.2.3). Table\ 6\(hy4/X.31 shows the mapping of the\
X.25 incoming call
elements to Q.931 information elements. The \fIincoming call\fR packet
will still contain these fields when it is delivered. See \(sc\ 3.2.3 for
mapping
requirements.
.RT
.sp 1P
.LP
6.2.2.3.3
\fIChannel selection without call offering\fR
.sp 9p
.RT
.PP
Where the network and user have agreed beforehand, the network may route
an incoming call to the called user over an established B\(hychannel
connection or D\(hychannel link without the need for any signalling for channel
selection.
.RT
.sp 1P
.LP
6.3
\fIVirtual call establishment and release\fR
.sp 9p
.RT
.PP
In all cases, once the physical channel has been selected and, if necessary,
connected to the\ PH or AU, the virtual call is established according to
the procedures below. Some networks may require some of the terminal
identification procedures of Recommendation\ X.32 as well.
.bp
.RT
.sp 1P
.LP
6.3.1
\fILink layer establishment and release\fR
.sp 9p
.RT
.PP
Link layer (LAPB on the B\(hychannel or LAPD on the D\(hychannel)
establishment shall be initiated by:
.RT
.LP
\(em
the calling terminal in the case of outgoing calls;
.LP
\(em
the AU in the case of incoming calls in Case A; or
.LP
\(em
the PH in the case of incoming calls in Case B.
.PP
Link layer release may be initiated by:
.LP
\(em
the terminal;
.LP
\(em
the AU in Case A; or
.LP
\(em
the PH in Case B.
.sp 1P
.LP
6.3.2
\fIPacket layer virtual call SETUP and RELEASE\fR
.sp 9p
.RT
.PP
The packet layer procedures of X.25 will be used for layer 3 call set\(hyup
and release. The packet layer procedures will additionally be able to
control and monitor the established or released state of the link layer.
.RT
.PP
In Case B, the PH may maintain a timer T320 (defined in
Recommendation\ Q.931). T320, if implemented, is started:
.LP
a)
upon clearance of the last virtual call; or
.LP
b)
upon transmission of a CONNECT message by the network in
case of an outgoing\ B\(hychannel access connection; or
.LP
c)
upon transmission of a CONNECT ACKNOWLEDGE message by the
network in case of an incoming\ B\(hychannel access connection; or
.LP
d)
upon establishment of the link layer for D\(hychannel access
connections.
.PP
T320 is cancelled upon:
.LP
a)
establishment of the first (next) virtual call; or
.LP
b)
receipt of a Q.931 clearing message from the user; or
.LP
c)
disconnection of the SAPI\ =\ 16 link on the D\(hychannel.
.PP
Upon expiry of T320, the PH will release the link layer and, in
the case of B\(hychannel access, initiate clearing of the B\(hychannel.
.PP
X.25 logical channels are associated with their underlying logical
link. Specifically, in case of the use of the B\(hychannel for packet
communication there is an association between the logical channels and
the LAPB logical link below them. Thus the same logical channel number
may be used
simultaneously on each different B\(hychannel.
.RT
.sp 2P
.LP
6.4
\fICall clearing\fR
.sp 1P
.RT
.sp 1P
.LP
6.4.1
\fIB\(hychannel\fR
.sp 9p
.RT
.PP
The clearing of the switched connection shall be effected by using the
D\(hychannel signalling procedures for call clearing as specified in \(sc\
5.3 of Recommendation\ Q.931. For access to PSPDN services, no exceptions
apply. For
the ISDN virtual circuit service, the messages of \(sc\ 3.2 of Recommendation\
Q.931 are used, and the following exceptions apply:
.RT
.LP
\(em
the terms defined in \(sc\ 5.3.1 of Recommendation Q.931
\*QTerminology\*U apply by replacing \*Qcircuit\(hyswitched ISDN connection\*U
with
\*Qdemand packet mode access connection\*U;
.LP
\(em
the exception condition (f) specified in \(sc\ 5.3.2 of
Recommendation\ Q.931 does not apply;
.LP
\(em
the procedures for clearing with tones and announcements
provided in \(sc\ 5.3.4.1 of Recommendation\ Q.931 do not apply.
.PP
The B\(hychannel may be cleared at any time by the user though, in
general, it will be cleared following the clearing of the last virtual call
over that B\(hychannel. In the ISDN virtual circuit service, if the user clears
the B\(hychannel access connection using a Q.931 clearing message while X.25
virtual calls still exist on the B\(hychannel, the network shall clear the X.25
.PP
virtual call(s) with cause\ ##\ 17, \*Q\fIremote procedure error\fR \*U, and
diagnostic\ ##\ 64,
\*Q\fIcall setup, call clearing, or registration problem\fR \*U.
.bp
.RT
.ce
\fBH.T. [T5.31]\fR
.ce
TABLE\ 6\(hy4/X.31
.ce
\fBMapping of X.25 information elements to corresponding Q.931 SETUP\fR
.ce
.ce
\fBmessage information elements in packet\(hymode incoming call\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(24p) | cw(102p) | lw(102p) .
T{
Information elements in X.25
\fIincoming call\fR
\| packet
T} T{
Corresponding information elements
in\ Q.931 SETUP message
T}
.T&
lw(102p) | lw(102p) .
Calling address Calling party number
_
.T&
lw(102p) | lw(102p) .
Called address Called party number
_
.T&
lw(102p) | lw(102p) .
User data (UD) T{
User\(hyuser information (Note\ 2)
T}
_
.T&
lw(102p) | lw(102p) .
A\(hybit (Note\ 3) For further study
_
.T&
lw(102p) | lw(102p) .
D\(hybit T{
Packet layer binary parameters
T}
_
.T&
lw(102p) | lw(102p) .
Modulus T{
Packet layer binary parameters
T}
.T&
lw(24p) | lw(102p) | lw(102p) .
.T&
cw(24p) | lw(102p) | lw(102p) , ^ | l | l
^ | l | l
^ | l | l
^ | l | l
^ | l | l
^ | l | l
^ | l | l
^ | l | l.
T{
X.25
user
facility
Flow control parameter negotiation
Packet size
Packet layer
window size
Throughput class negotiation
Information rate
Fast select
Packet layer binary parameters
Reverse charging
For further study
Closed user group selection
For further study
Closed user group with outgoing access selection
For further study
Bilateral closed user group
For further study
Transit delay selection and indication
Transit delay selection and indication
Call redirection and deflection notification
Redirecting number
T}
_
.T&
lw(228p) .
.T&
cw(24p) | lw(102p) | lw(102p) , ^ | l | l
^ | l | l
^ | l | l
^ | l | l.
T{
DTE
Facility
Calling address extension
Calling party sub\(hyaddress
Called address extension
Called party sub\(hyaddress
End\(hyto\(hyend transit delay
End\(hyto\(hyend transit delay
Minimum throughput class
Information rate
Expedited data negotiation
Packet layer binary parameters
\fINote\ 1\ \(em\ \fR
Mapping is optional or required as indicated in
\(sc\ 3.2.3.
.parag
\fINote\ 2\ \(em\ \fR
The maximum length of the user data within the user\(hyuser information element is network dependent and is either\ 32 or 128\ octets.
.parag
\fINote\ 3\ \(em\ \fR
The need and procedures for A\(hybit mapping is for further
study.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 6\(hy4/X.31 [T5.31], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.PP
In Case B, if a Q.931 RESTART message is received by the PH during the
X.25 data transfer phase, the X.25 virtual calls shall be treated as
follows:
.RT
.LP
\(em
For switched virtual circuits, an X.25 \fIclear indication\fR
packet shall be sent with cause\ ##\ 9, \*Q\fIout of order\fR \*U and diagnostic\
##\ 0,
\*Q\fIno\fR \fIadditional information\fR \*U.
.LP
\(em
For permanent virtual circuits, an X.25 \fIreset\fR packet shall be sent
containing cause\ ##\ 9, \*Q\fIout of order\fR \*U and diagnostic\ ##\
0,
\*Q\fIno additional information\fR \*U.
.PP
At the expiration of timer T320, the network may disconnect the
X.25 link layer and the access connection. B\(hychannel clearing is as
described in \(sc\ 5.3 of Recommendation\ Q.931 with the exceptions above,
with cause\ ##\ 102, \*Q\fIrecovery on time expiry\fR \*U.
.bp
.sp 1P
.LP
6.4.2
\fID\(hychannel\fR
.sp 9p
.RT
.PP
D\(hychannel access connections are cleared using the disconnect
procedures as defined in \(sc\ 6.3.
.RT
.sp 1P
.LP
6.4.3
\fIAdditional error handling information\fR
.sp 9p
.RT
.PP
When call failure occurs, or the X.25 virtual call is cleared
permanently, the rules of \(sc\ 5.8 of Recommendation\ Q.931 shall apply. In
addition, the following rules for determining the appropriate cause to
be used shall apply in order of decreasing priority:
.RT
.LP
1)
If a Q.931 clearing message or RESTART message is received by the PH
during the X.25 data transfer phase, \(sc\ 6.4.1 applies.
.LP
2)
If a call is rejected by the destination user using Q.931
messages, the X.25 virtual call shall be cleared using a \fIclear indication\fR
packet and the appropriate cause from Table\ 6\(hy5/X.31.
.LP
3)
If a condition exists that prevents the Q.931 SETUP message from being
delivered at the user\(hynetwork interface, the X.25 virtual call shall
be cleared using a \fIclear indication\fR packet and a cause shall be selected
appropriate to the condition. Table\ 6\(hy5/X.31 shall serve as a guide to
selecting an appropriate cause, i.e.,\ the X.25 mapping of the Q.931 cause
describing the interface condition shall be used.
.LP
4)
If the Q.931 SETUP message is sent across the user\(hynetwork iterface,
but no response is received prior to the second expiry of timer\ T303 (defined
in Recommendation\ Q.931), rule\ ##\ 3 applies.
.LP
5)
If the Q.931 SETUP message is sent across the user\(hynetwork interface,
and a response is received from a user which results in the clearing of
the call at the user\(hynetwork interface, the X.25 virtual call shall
be
cleared using a \fIclear indication\fR packet containing the appropriate
cause from Table\ 6\(hy5/X.31 relative to the cause received/sent in the
Q.931 clearing
message.
.LP
6)
If an X.25 \fIclear request\fR packet is received from the
originating user prior to the delivery of the X.25 \fIincoming call\fR
packet to
the called user (premature clearing), the PH shall send a \fIclear configuration\fR
packet to the calling user and the access connection shall be treated as
follows:
.LP
\(em
if the Q.931 SETUP message was associated with the
unconditional notification class of service (see \(sc\ 3.2.3), the access
connection, when and if established, shall be cleared. The Q.931 clearing
message shall contain the appropriate cause as described in Table\ 6\(hy6/X.31;
.LP
\(em
if the Q.931 SETUP message was associated with the
Conditional notification class of service (see \(sc\ 3.2.3) and there exists at
least one terminal which responds positively to the Q.931 SETUP message,
then two options are allowed:
.LP
a)
the access connection is cleared as described
for the unconditional class of service; or
.LP
b)
the access connection is established and timer
T320 is started. Upon expiry of timer\ T320, the access connection is cleared
with cause\ ##\ 102, \*Q\fIrecovery on timer expiry\fR \*U and diagnostic
indicating
timer\ T320.
.sp 2P
.LP
6.4.4
\fICause mappings\fR
.sp 1P
.RT
.sp 1P
.LP
6.4.4.1
\fIAccess to/from PSPDN services (Case A)\fR
.sp 9p
.RT
.PP
The AU may choose to follow the procedures in \(sc 6.4.4.2 when
mapping between causes delivered by the ISDN or the PSPDN.
.RT
.sp 1P
.LP
6.4.4.2
\fIAccess to/from the ISDN virtual circuit service (Case B)\fR
.sp 9p
.RT
.PP
There are several cases where it is necessary to map causes between Q.931
and X.25. Networks shall use Table\ 6\(hy5/X.31 and Table\ 6\(hy6/X.31
to map the causes between Q.931 and X.25 messages. The figures in Appendix\
III describe
some example situations.
.RT
.sp 1P
.LP
6.5
\fIAccess collision\fR
.sp 9p
.RT
.PP
When the network offers a packet mode call at the interface
simultaneously with the user requesting a packet mode call, the network
shall give priority to the completion of the incoming call. If the user
determines
that accepting the incoming call would meet the needs of its own outgoing
call request, the user may clear the call request and accept the incoming
call.
.bp
.RT
.ce
\fBH.T. [1T6.31]\fR
.ce
TABLE\ 6\(hy5/X.31
.ce
\fBMapping of Q.931 cause fields to X.25 cause field\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(18p) | cw(42p) | cw(12p) | cw(42p) | cw(42p) | cw(18p) | cw(42p) | cw(12p) .
Item Q.931 cause Code Q. 931 Diagnostic X.25 Cause Code X.25 Diagnostic Code
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
\ 1 T{
Unallocated (unassigned) number
T} \ 1 T{
Condition:
unknown, transient, permanent
T} Not obtainable 13 Invalid called address 67
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
\ 2 No route to destination \ 3 T{
Condition:
unknown, transient, permanent
T} Not obtainable 13 Invalid called address 67
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
\ 3 Channel unacceptable \ 6 (None) Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
\ 4 Normal call clearing 16 T{
Condition:
unknwon, transient, permanent
T} DTE originated \ 0 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
\ 5 User busy 17 (None) Number busy \ 1 No logical channel available 71
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
\ 6 No user responding 18 (None) Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
\ 7 User alerting, no answer 19 (None) Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
\ 8 Call rejected 21 T{
Condition:
unknown, transient, permanent + user applied
diagnostics
T} DTE originated \ 0 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
\ 9 Number changed 22 New destination address Not obtainable 13 Invalid called address 67
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
10 Destination out of order 27 (None) Out of order \ 9 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
11 T{
Invalid number format (incomplete number)
T} 28 (None) Local procedure error 19 Invalid called address 67
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
12 Normal, unspecified 31 (None) DTE originated \ 0 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
13 No circuit/channel available 34 (None) Number busy \ 1 No logical channel available 71
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 6\(hy5/X.31 [1T6.31], p. 9\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [2T6.31]\fR
.ce
TABLE\ 6\(hy5/X.31 \fI(continued)\fR
.ce
\fBMapping of Q.931 cause fields to X.25 cause field\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(18p) | cw(42p) | cw(12p) | cw(42p) | cw(42p) | cw(18p) | cw(42p) | cw(12p) .
Item Q.931 cause Code Q. 931 Diagnostic X.25 Cause Code X.25 Diagnostic Code
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
14 Network out of order 38 (None) Out of order \ 9 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
15 Temporary failure 41 Network identity Out of order \ 9 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
16 T{
Switching equipment congestion
T} 42 Network identity Network congestion \ 5 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
17 T{
Requested circuit/channel not available
T} 44 (None) Number busy \ 1 No logical channel available 71
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
18 T{
Resources unvailable, unspecified
T} 47 (None) Nework congestion \ 5 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
19 Quality of service unvailable 49 T{
Condition:
unknown, transient, permanent
T} Network congestion \ 5 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
20 T{
Bearer capability not authorized
T} 57 T{
Bearer capability information element identifier
T} Incompatible destination 33 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
21 T{
Bearer capability not presently available
T} 58 T{
Bearer capability information element identifier
T} Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
22 T{
Service or option unavailable, unspecified
T} 63 (None) Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
23 T{
Bearer service not implemented
T} 65 Attribute numbers Incompatible destintion 33 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
24 Channel type not implemented 66 Channel type Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
25 T{
Service or option not implemented, unspecified
T} 79 (None) Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 6\(hy5/X.31 [2T6.31], p. 10\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [3T6.31]\fR
.ce
TABLE\ 6\(hy5/X.31 \fI(continued)\fR
.ce
\fBMapping of Q.931 cause fields to X.25 cause field\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(18p) | cw(42p) | cw(12p) | cw(42p) | cw(42p) | cw(18p) | cw(42p) | cw(12p) .
Item Q.931 cause Code Q. 931 Diagnostic X.25 Cause Code X.25 Diagnostic Code
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
26 Invalid call reference value 81 (None) Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
27 T{
Identified channel does not exist
T} 82 Channel identity Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
28 Incompatible destination 88 Incompatible parameter Incompatible destination 33 No additional information \ 0
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
29 Invalid message, unspecified 95 (None) Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
30 T{
Mandatory information element is missing
T} 96 T{
Information element identifier(s)
T} Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
31 T{
Message type non\(hyexistent or not implemented
T} 97 Message type Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
32 T{
Message not compatible with call state or message type
non\(hyexistent or not implemented
T} 98 Message type Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
33 T{
Information element non\(hyexistent or not implemented
T} 99 T{
Information element identifier(s)
T} Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
34 T{
Invalid information element contents
T} 100\ T{
Information element identifier(s)
T} Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 6\(hy5/X.31 [3T6.31], p. 11\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [4T6.31]\fR
.ce
TABLE\ 6\(hy5/X.31 \fI(continued)\fR
.ce
\fBMapping of Q.931 cause fields to X.25 cause field\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(18p) | cw(42p) | cw(12p) | cw(42p) | cw(42p) | cw(18p) | cw(42p) | cw(12p) .
Item Q.931 cause Code Q. 931 Diagnostic X.25 Cause Code X.25 Diagnostic Code
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
35 T{
Message not compatible with call state
T} 101 Message type Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
36 Recovery on timer expiry 102 Timer number Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
37 Protocol error, unspecified 111 (None) Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} 64
_
.T&
cw(18p) | lw(42p) | cw(12p) | lw(42p) | lw(42p) | cw(18p) | lw(42p) | cw(12p) .
38 Interworking, unspecified 127 (None) Remote procedure error 17 T{
Call setup, call clearing or registration
problem
T} T{
64
\fINote\ 1\ \(em\ \fR
When clearing occurs during the X.25 data transfer
phase, the procedure described in \(sc\ 6.4.1 should be
used.
.parag
\fINote\ 2\ \(em\ \fR
When a Q.931 RESTART message is received during
the X.25 data tranfer phase, switched virtual circuits shall
be cleared with a \fIclear indication\fR
packet containing
cause\ ##\ 9, \fI\*QOut of order\*U,\fR
with diagnostic\ ##\ 0,
\fI\*Qno additional information\*U.\fR
Permanent virtual circuits
shall have an X.25\ \fIreset\fR
packet sent with the same cause
and diagnostic.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 6\(hy5/X.31 [4T6.31], p. 12\fR
.ad b
.RT
.ce
\fBH.T. [T7.31]\fR
.ce
TABLE\ 6\(hy6/X.31
.ce
\fBMapping of X.25 cause to Q.931 cause for premature\fR
.ce
\fBclearing of the incoming call\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(18p) | cw(126p) | cw(84p) .
T{
X.25 cause in clear indication packet
T} Q.931 error condition
_
.T&
cw(18p) | cw(48p) | cw(12p) | cw(48p) | cw(18p) | cw(48p) | cw(12p) | cw(24p) .
Item X.25/X.96 cause Code Diagnostic Code Q.931 cause Code Diagnostic
_
.T&
cw(18p) | lw(48p) | cw(12p) | lw(48p) | cw(18p) | lw(48p) | cw(12p) | lw(24p) .
1 DTE originated 0 No additional information 0 Normal call clearing 16 (None)
.T&
cw(18p) | lw(48p) | cw(12p) | lw(48p) | cw(18p) | lw(48p) | cw(12p) | lw(24p) .
.T&
cw(18p) | lw(48p) | cw(12p) | lw(48p) | cw(18p) | lw(48p) | cw(12p) | lw(24p) .
1XX DTE specified XX
_
.T&
cw(18p) | lw(48p) | cw(12p) | lw(48p) | cw(18p) | lw(48p) | cw(12p) | lw(24p) .
2 Network congestion 5 No additional information 0 T{
Switching equipment congestion
T} 42 (None)
_
.T&
cw(18p) | lw(48p) | cw(12p) | lw(48p) | cw(18p) | lw(48p) | cw(12p) | lw(24p) .
3 Out of order 9 No addtional information 0 Destination out of order 27 (None)
_
.T&
cw(18p) | lw(48p) | cw(12p) | lw(48p) | cw(18p) | lw(48p) | cw(12p) | lw(24p) .
4 Remote procedure error 17\ (Any allowed) Protocol error, unspecified 111\ T{
(None)
\fINote\ \(em\ \fR
Instead of providing the above mapping of X.25 to Q.931,
the PH, as a network option, may code the
Q.931\ Cause information element to indicate \*QCCITT Coding Standard\*U
in octet\ 3, \*QX.25\*U in octet\ 3a, and code octets\ 4 and\ 5
according to Recommendation\ X.25, copying the cause from the
X.25\ \fIclear indication\fR
packet rather than mapping it to a
Q.931\ cause.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 6\(hy6/X.31 [T7.31], p. 13\fR
.ad b
.RT
.LP
.bp
.sp 2P
.LP
\fB7\fR \fBTERMINAL ADAPTOR FUNCTIONALITIES\fR
.sp 1P
.RT
.sp 1P
.LP
7.1
\fIGeneral\fR
.sp 9p
.RT
.PP
Terminal Adaptor (TA) functions are needed to support the access
of\ X.25\ DTEs at the S/T reference point (see Figure\ 7\(hy1/X.31).
.RT
.LP
.rs
.sp 12P
.ad r
\fBFIGURE 7\(hy1/X.31, p. \fR
.sp 1P
.RT
.ad b
.RT
.PP
Main functionalities which are provided by the TA are the
following:
.LP
\(em
rate adaption;
.LP
\(em
mapping of signalling information and procedures between the S/T and
the reference point;
.LP
\(em
synchronization;
.LP
\(em
maintenance.
.PP
In the following, these main functionalities are described
depending on the access types (B\(hychannel and/or D\(hychannel access),
highlighting the differences between the two services defined in this Recommendation
(Case\ A and Case\ B).
.PP
The procedures at the S/T reference point are described in \(sc 6.
.RT
.sp 1P
.LP
7.2
\fIPhysical interfaces\fR
.sp 9p
.RT
.PP
The physical interfaces supported at the R\ reference point are
those defined in Recommendation\ X.25 Section\ 1, and Recommendation\ X.32.
.RT
.sp 2P
.LP
7.3
\fIAccess through the B\(hychannel\fR
.sp 1P
.RT
.sp 1P
.LP
7.3.1
\fIGeneral\fR
.sp 9p
.RT
.PP
This part defines the functionalities to be supported by the TA
when the access through the B\(hychannel is used. Both service Cases\ A
and B are covered and differences, if any, are shown in the appropriate
subparagraphs.
.RT
.sp 1P
.LP
7.3.2
\fIRate adaption\fR
.sp 9p
.RT
.PP
Rate adaption can be performed in two ways:
.RT
.LP
1)
Packet mode of operation (Case\ B) by using HDLC interframe flag stuffing.
.LP
In this case, packet mode terminals operating at data
signalling rates lower than\ 64\ kbit/s at the R\ reference point can no
longer be distinguished by the network from packet mode terminals operating
at a data
rate of\ 64\ kbit/s at the R\ interface.
.LP
Therefore, the D\(hychannel signalling procedures will indicate
the data signalling rate of\ 64\ kbit/s rather than the user data signalling
rate at the R\ reference point. In addition, a throughput class may be
indicated in the D\(hychannel incoming call signalling procedures.
.LP
It should be noted that the packet handling in the ISDN will
be optimized for DTEs generating HDLC structured traffic at\ 64\ kbit/s.
In such an ISDN, flag stuffing is the preferred method for rate adaption.
.LP
In order to avoid unnecessary retransmission on the
B\(hychannel, the TA implementation could have a buffer capacity which
is related to the layer\ 2 window size and maximum frame length or may
flow control at
layer\ 2.
.bp
.LP
2)
Circuit mode of operation (Case\ A) by using the method
indicated in Recommendation\ X.30/I.461.
.LP
In this case, the D\(hychannel signalling procedures shall
indicate the data signalling rate being used by the DTE connected to the
R\ reference point (this will be lower than\ 64\ kbit/s).
.LP
As an alternative to HDLC interframe flag stuffing, this bit
rate adaption method may be supported by some network in case of access to
PSPDN services.
.LP
\fINote\fR \ \(em\ The use of V\(hySeries specification is for further
study.
.sp 1P
.LP
7.3.3
\fISignalling\fR
.sp 9p
.RT
.PP
This part defines the functionalities to be supported by the TA to establish,
maintain and release a B\(hychannel connection to the PH/AU. These
functionalities require a different degree of capabilities by the TA on the
basis of the different implementation of\ X.25 procedures in the DTE. Two
cases can be identified, namely:
.RT
.LP
Case\ 1:
TA acts only on level 1
.LP
Case\ 2:
TA acts also on level 2 and/or 3
.PP
The first case applies to\ X.25 DTEs which can disconnect at the
physical level, when no VCs are in progress.
.PP
For X.25 DTEs which are not able to disconnect at the physical level or
even require an active link, the consequence of the first case may be the
automatic allocation of the B\(hychannel immediately after power on. To
avoid this situation with a permanent allocated B\(hychannel, an alternative
configuration is presented in Appendix\ I.
.PP
This section refers to signalling mapping of the first case.
.RT
.sp 1P
.LP
7.3.3.1
\fIOutgoing call\fR
.sp 9p
.RT
.PP
To provide a physical connection by means of a B\(hychannel to the PH or
PSPDN\ AU the TA shall provide;
.RT
.LP
\(em
a method to indicate that the TA should start the B\(hychannel establishment
procedure at the S/T reference point. The options available are described
in \(sc\ 7.3.3.1.1,
.LP
\(em
a method to transfer address information to the TA which is needed by
the B\(hychannel establishment procedure. The options available are
described in \(sc\ 7.3.3.1.2.
.sp 1P
.LP
7.3.3.1.1
\|\fIConditions for initiating B\(hychannel establishment\fR
.sp 9p
.RT
.PP
Two situations can be identified to categorize the conditions which may
cause the TA to attempt to establish a B\(hychannel connection.
.RT
.LP
a)
\fI(semi\(hy) permanent B\(hychannel\fR
.LP
In this case, the B\(hychannel is always available. No TA
functionality is required to initiate the establishment of the B\(hychannel
connection.
.LP
b)
\fIB\(hychannel establishment is initiated by actions at the\fR
\fIR\(hyreference point (DTE/TA interface)\fR
.LP
Two conditions are possible. See Table\ 7\(hy1/X.31.
.LP
1)
Hot\(hyline access at the R\ reference point
.LP
In case of hot\(hyline access at the R\ reference point
the detection of the following appropriate interface conditions shall cause
the TA to establish the B\(hychannel with the PH/PSPDN.
.LP
i)
For X.25 level 1 interfaces\ \(em\
a transition from OFF to ON on the control lead (in case of\ X.21 leased
circuit procedures) or circuit\ 108 (in case of\ X.21\|\fIbis\fR or V\(hyseries
interface procedures).
.LP
ii)
For X.21 interfaces\ \(em\ direct call signal (C\ =\ ON).
.LP
The DTE will wait for I\ =\ ON before starting
transmission.
.LP
iii)
For the X.21\|\fIbis\fR interface\ \(em\ direct call signal
(108\ =\ ON).
.LP
The DTE will wait for 107\ =\ ON before starting
transmission.
.LP
iv)
For the V.25\|\fIbis\fR interface\ \(em\ direct call signal
(108\ =\ ON).
.LP
The DTE will wait for 107\ =\ ON before starting
transmission.
.bp
.LP
2)
Full circuit\(hyswitched selection access
.LP
Full circuit\(hyswitched selection procedure (X.21, X.21\|\fIbis\fR or
V.25\|\fIbis\fR ) may be used at the DTE/TA interface to request the establishment
of the B\(hychannel connection to a PSPDN or PH. The TA will establish
the
B\(hychannel connection to a PSPDN or PH. The TA will establish the B\(hychannel
in accordance with the procedures described in Section\ 6. The address
provided may be used to identify the PSPDN port and full\ X.25 procedures
must be used
following the establishment of the B\(hychannel connection to identify
the called packet mode DTE.
.LP
In case of full circuit\(hyswitched selection, the following
operating modes of Recommendation\ X.21, X.21\|\fIbis\fR and V.25\|\fIbis\fR
at the DTE/TA interface shall cause the TA to establish the B\(hychannel
with the PH/PSPDN.
.LP
i)
For X.21 circuit\(hyswitched interfaces\ \(em\ X.21 call
control phase.
.LP
ii)
For X.21\|\fIbis\fR circuit\(hyswitched interfaces\ \(em\ use
of\ X.21\|\fIbis\fR automatic address call facility.
.LP
iii)
For V.25\|\fIbis\fR circuit\(hyswitched
interfaces\ \(em\ V.25\|\fIbis\fR addressed call mode.
.PP
\fINote\fR \ \(em\ The user may cause the TA to attempt to establish a
B\(hychannel connection by manual actions (e.g.,\ by pressing a button) at the
human/machine interface of the TA. Subsequently the TA may emulate the
incoming call towards the DTE.
.sp 1P
.LP
7.3.3.1.2
\|\fIOptions for transferring the ISDN address of the PSPDN port\fR \fIto
the TA\fR
.sp 9p
.RT
.PP
Four options exist to handle address information of the PSPDN port at the TA:
.RT
.LP
a)
(Semi\(hy) permanent B\(hychannel at the S/T reference point.
.LP
In this case the TA has no need for address information,
i.e.,\ no functionality is required in the TA to obtain an address.
.LP
b)
The address is conveyed across the R\ reference point.
.LP
In this case the circuit\(hyswitched procedures described in
\(sc\ 7.3.3.1.1\ b)\ 2) are required.
.LP
c)
The address is conveyed across the human/machine interface of the\ TA.
.LP
Manual procedures are used (e.g.,\ by means of a keypad) at the human/machine
interface of the TA. The address may be input each time the B\(hychannel
is requested. Alternatively the address may be stored at the TA
(e.g.,\ in the case of hot line operation at the R\ reference point).
.LP
d)
The address is downloaded by the network via the S/T
reference point.
.LP
The need for this option is for further study.
.PP
\fINote\ 1\fR \ \(em\ The address information may be for example a full
ISDN address and abbreviated ISDN address, which is used by hot\(hyline
access
procedures at the S/T reference point, or an abbreviated address which is
interpreted by the TA and expanded to an (abbreviated) ISDN address using
pre\(hyrecorded information in the TA.
.sp 1P
.LP
7.3.3.1.3
\|\fIMapping of procedures\fR
.sp 9p
.RT
.PP
The list of supported combinations and the appropriate procedures are given
in Table\ 7\(hy2/X.31.
.PP
Following the establishment of the connection, the TA should place the
R\ reference point in the appropriate condition for data transfer at
layer\ 1.
.RT
.sp 1P
.LP
7.3.3.1.4
\|\fIMapping of the Q.931 messages\fR
.sp 9p
.RT
.PP
The procedures between the TA and the network are the same as
described in \(sc\ 6. The choice of the requested service will be made by the
appropriate coding of the bearer capability.
.PP
In Case A the ISDN address of the PSPDN port will be introduced as the
destination in the\ Q.931 message while in Case\ B no address is contained.
.RT
.sp 1P
.LP
7.3.3.1.5
\|\fIX.25 procedures\fR
.sp 9p
.RT
.PP
In the data transfer phase, the TA may be transparent to layer\ 2
and layer\ 3 of the\ X.25 procedures. However, some realizations of\ X.25
terminals may require full or partial termination of layer\ 2 within the
TA to accommodate existing LAPB establishment procedures (see Appendices\
I
and\ IV).
.bp
.RT
.ce
\fBH.T. [T8.31]\fR
.ce
TABLE\ 7\(hy1/X.31
.ce
\fBDTE/TA Layer 1 specifications and procedures to\fR
.ce
\fBinitiate B\(hychannel establishment\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(36p) | cw(66p) | cw(66p) | cw(60p) .
Condition T{
DTE/TA layer 1 specification
T} T{
Events at the R reference point
T} Procedures according to:
_
.T&
lw(228p) .
Unable to convert table
.TE
.nr PS 9
.RT
.ad r
\fBTableau 7\(hy1/X.31 [T8.31], p. 15\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 17P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T9.31]\fR
.ce
TABLE\ 7\(hy2/X.31
.ce
\fBTA functionality to control B\(hychannel\fR
.ce
\fBestablishment\fR
.ce
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(12p) | lw(45p) sw(39p) | lw(132p) , l | c | l | ^ .
TA functions T{
Conditions for initiation of B\(hychannel
establishment (\(sc\ 7.3.3.1.1)
T}
T{
Transfer of address information to the TA (\(sc\ 7.3.3.1.2)
T} Description of procedures
_
.T&
cw(12p) | lw(45p) | lw(39p) | lw(132p) .
1 Condition a Option a T{
(Semi\(hy) permanent B\(hychannel. No signalling functions for
layer\ 1 are needed in the TA.
T}
_
.T&
cw(12p) | lw(45p) | lw(39p) | lw(132p) .
2 Condition b1 i Option c T{
The DTE sets C = ON or circuit\ 108 = ON. When\ C
(or circuit\ 108) becomes ON, and the manual selection has been
made at the TA, the TA then initiates, using the D\(hychannel
procedures, the establishment of an ISDN B\(hychannel to provide
a connection to the PSPDN. When the B\(hychannel is completely
established at the S/T reference point, the TA
sets I\ =ON (or circuit\ 107\ =\ ON).
T}
_
.T&
cw(12p) | lw(45p) | lw(39p) | lw(132p) .
3 T{
Any of conditions b1 ii, iii, iv
See Note in \(sc\ 7.3.3.1.1
T} Option c Option c T{
When the manual selection has been made at the TA, the TA may
emulate an incoming call at the R\ reference point. If the DTE accepts
this incoming call, the TA places the R\ reference point in the DCE
waiting state at layer\ 1 and the initiates, using the
D\(hychannel procedures, the establishment of an ISDN B\(hychannel to
provide a connection to the PSPDN. When the B\(hychannel is completely
established at the S/T\ reference point, the TA signals ready for
data at the R\ reference point.
T}
_
.T&
cw(12p) | lw(45p) | lw(39p) | lw(132p) .
4 Any of condition b2 Option b T{
When the DTE has requested the layer\ 1 connection and provided
address information to the TA, the TA initiates, using the
D\(hychannel procedures, the establishment of an ISDN B\(hychannel.
When the B\(hychannel is completely established at the
S/T\ reference point, the TA signals ready for data, using
the appropriate procedure at the R\ reference point.
T}
_
.T&
cw(12p) | lw(45p) | lw(39p) | lw(132p) .
5 Condition b1 Option a T{
In this case, hot\(hyline access is applied at the R\ reference
point as well as the S/T\ reference point. No address information
is therefore required by the TA. When the DTE presents the call
request, the TA attempts to establih a B\(hychannel. When the
B\(hychannel is completely set\(hyup, the TA signals ready for data
at the R\ reference point.
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 7\(hy2/X.31 [T9.31], p. 16\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 2
.sp 2P
.LP
7.3.3.2
\fIIncoming call\fR
.sp 1P
.RT
.sp 1P
.LP
7.3.3.2.1
\|Q.931 \fICall offering\fR
.sp 9p
.RT
.PP
The incoming call, in both Cases\ A and\ B, is first offered
using\ Q.931 procedures for setting up the B\(hychannel connection.
.bp
.RT
.sp 1P
.LP
7.3.3.2.2
\|\fIActions at the reference point\fR
.sp 9p
.RT
.PP
The TA shall not accept an incoming call from the network unless
the R\ reference point is in one of the following states;
.RT
.LP
\(em
the ready state for an R\ reference point conforming to\ X.21 circuit\(hyswitched
procedures,
.LP
\(em
the ready or send data state for an R\ reference point
conforming to\ X.21 leased circuit procedures,
.LP
\(em
circuits\ 125 and\ 108\ ON with 107\ OFF for an R\ reference point conforming
to\ X.21\|\fIbis\fR procedures.
.PP
If the R\ reference point is in, or can be placed in, the
appropriate state defined above, the TA shall respond to the SETUP message
(when compatibility checking has been successful) by returning a CONNECT
message in accordance with the procedures of \(sc\ 6 and shall then wait for
either a CONNECT ACKNOWLEDGE or RELEASE signalling message from the network.
(The TA may also reject the SETUP message by responding with a RELEASE
COMPLETE message.)
.PP
\fINote\fR \ \(em\ Recommendation\ Q.931 does not oblige the TA to return an
ALERTING message prior to the return of a CONNECT.
.PP
If the R\ reference point is not and can not be placed in the
appropriate states defined above, then the TA shall respond to the SETUP
message in accordance with the negative response to the incoming call
procedures defined in \(sc\ 6.
.PP
The receipt of a CONNECT ACKNOWLEDGE message causes the TA to initiate
the appropriate procedures described in Recommendation\ X.30/I.461 leading
to
the placing of the R\ reference point into the appropriate condition for data
transfer, and to begin transmission of information in the B\(hychannel.
.PP
\fINote\fR \ \(em\ The DTE/TA interface shall not be placed in the data
transfer state before the B\(hychannel is completely set up at the S/T
reference point (see \(sc\ 7.3.4).
.RT
.sp 1P
.LP
7.3.3.2.3
\|X.25 \fIProcedures\fR
.sp 9p
.RT
.PP
In the data transfer phase, the TA may be transparent to layer\ 2
and layer\ 3 of the\ X.25 procedures. However, some realizations of\ X.25
terminals may require full or partial termination of layer\ 2 within the
TA to accommodate existing LAPB establishment procedures (see Appendix\
I).
.RT
.sp 1P
.LP
7.3.3.3
\fICall clearing\fR
.sp 9p
.RT
.PP
To initiate the clearing of the B\(hychannel it is necessary to detect
the clearing of the last virtual call on the B\(hychannel. Three parties
can
detect the clearing of the B\(hychannel;
.RT
.LP
1)
the DTE; initiating clearing via the R\ reference point,
.LP
2)
the network (PH or AU); initiating clearing via the S/T
reference poin,
.LP
3)
the user, initiating clearing manually via the human/machine interface.
.PP
Before clearing of the B\(hychannel is initiated, the layer\ 2
connection between the network and the DTE should be cleared.
.sp 1P
.LP
7.3.3.3.1
\|\fIInitiation of call clearing by the DTE\fR
.sp 9p
.RT
.PP
The conditions of the R\ reference point which cause the TA to
attempt to disconnect the B\(hychannel connection are;
.RT
.LP
\(em
for X.21 circuit\(hyswitched interface\|\(em\|DTE clear request
signal,
.LP
\(em
for X.21 leased circuit interface\|\(em\|a transition from ON to OFF
on the control lead,
.LP
\(em
for X.21\|\fIbis\fR interface\|\(em\|DTE clear request signal
(circuit\ 108 from ON to OFF).
.PP
When one of these conditions occur, the TA will disconnect the
internal rate adapting connection between the R and the S/T reference point
(see \(sc\ 7.3.2) and will try to disconnect the B\(hychannel applying the
procedures of \(sc\ 6.4.
.bp
.sp 1P
.LP
7.3.3.3.2
\|\fIInitiation of call clearing by the network\fR
.sp 9p
.RT
.PP
For the clearing of the B\(hychannel the network applies the
procedures of \(sc\ 6.4. The receipt of a DISCONNECT or RELEASE message
shall cause the TA to disconnect the internal rate adapting connection
between the R and
the S/T reference point and to take on the R\ reference point the appropriate
action as described below;
.RT
.LP
\(em
for X.21 circuit\(hyswitched interface\|\(em\|signal a DCE clear
indication,
.LP
\(em
for X.21 leased circuit interface\|\(em\|signal a DCE ready
condition,
.LP
\(em
for X.21\|\fIbis\fR interface\|\(em\|set circuit 107 OFF.
.PP
See Recommendation X.30 for further details.
.PP
\fINote\fR \ \(em\ RELEASE may be a common response to a CONNECT message
in the case where more than one packet mode terminal is present at the
customer
premises and the incoming call has been globally offered using the point to
multipoint procedures of Recommendation\ Q.921.
.RT
.sp 1P
.LP
7.3.3.3.3
\|\fIInitiation of call clearing by the user\fR
.sp 9p
.RT
.PP
After the manual notification of the clearing of the last virtual call
by the user, the TA disconnects the internal connection between the R and
S/T reference point and applies the procedures of \(sc\ 6.4 for the clearing
of the B\(hychannel. On the R\ reference point it takes the appropriate
action as described below;
.RT
.LP
\(em
for X.21 circuit\(hyswitched interface\|\(em\|signal a DCE clear
indication,
.LP
\(em
for X.21 leased circuit interface\|\(em\|signal a DCE ready
condition,
.LP
\(em
for X.21\|\fIbis\fR interface\|\(em\|set circuit 107\ OFF.
.PP
See Recommendation X.30 for further details.
.sp 1P
.LP
7.3.4
\fISynchronization\fR
.sp 9p
.RT
.PP
The TA should effect synchronization between the D
(Q.931\ activities) and the B (X.25\ activities) channel activities.
.PP
Synchronization between TA and PH/AU is provided by the exchange of
synchronization pattern. Continuous flag transmission shall be used when
flag stuffing rate adaption is used. For class\ 30, synchronization will
be between the DTE and the PH/AU. The\ X.30 scheme will be used when the\
X.30 rate adaption is used.
.RT
.sp 2P
.LP
7.4
\fIAccess through the D\(hychannel\fR
.sp 1P
.RT
.sp 1P
.LP
7.4.1
\fIGeneral\fR
.sp 9p
.RT
.PP
This part defines the functionalities to be supported by a TA when access
through a D\(hychannel is used. This applies only to Case\ B (access to
the ISDN virtual circuit service).
.RT
.sp 1P
.LP
7.4.2
\fILAPB\(hyLAPD mapping\fR
.sp 9p
.RT
.PP
The rate adaption is inherent to the contention mechanism for
accessing the D\(hychannel. In particular, the contiguous flag transmission
perceived at the R\ reference point shall not be repeated at the S/T reference
point.
.RT
.sp 1P
.LP
7.4.2.1
\fIMapping by full link layer termination\fR
.sp 9p
.RT
.PP
Figure 7\(hy2/X.31 shows the mapping architecture between the LAPB
link at the R\ and the LAPD logical link at the S/T reference point, based on
full termination of both link layer protocols in the TA. This figure is
intended to describe the functionality to be provided by the TA. However,
this figure should not constrain any specific implementation.
.bp
.RT
.LP
.rs
.sp 12P
.ad r
\fBFIGURE 7\(hy2/X.31, p. \fR
.sp 1P
.RT
.ad b
.RT
.PP
The supervisory and unnumbered frames of the LAPB and LAPD
procedures have local significance (i.e.,\ only relevant for that link)
and need therefore not to be mapped to the other link. However, it is possible
that the receipt of a supervisory or unnumbered frame should result in
the transmission of such a frame on the other link; for instance a SABM(E)
frame (when both
links are in the disconnected state) or an RR frame may result in the
transmission of an equivalent frame on the other link.
.PP
The information transfer frames have to be mapped if both links are in
the transfer phase. The following mapping functions for these frames can
be
distinguished:
.RT
.LP
\(em
Address field mapping
.LP
\(em
Control field mapping
.LP
\(em
Frame check sequence recalculation
.PP
These functions are subject to the following sections.
.sp 1P
.LP
7.4.2.1.1
\|\fIInformation frame address field mapping\fR
.sp 9p
.RT
.PP
The LAPB address length is 1\ octet.
.PP
The LAPD address length is 2\ octets.
.RT
.LP
.rs
.sp 11P
.ad r
\fBFIGURE 7\(hy3/X.31, p. \fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
7.4.2.1.2
\|\fIInformation frame control field mapping\fR
.sp 9p
.RT
.PP
LAPB sequence numbering of the I\(hyframes is in general modulo\ 8,
though it might be modulo\ 128.
.PP
LAPD sequence numbering of the I\(hyframe is modulo\ 128.
.PP
The sequence numbers N(S) and N(R) on the LAPB and LAPD link are
independent.
.bp
.PP
In the case where the LAPD window size of\ 8 would fulfill (e.g., the throughput
requirements), the maximum number of I\(hyframes to be buffered in the
TA's relay function is\ 7 in each direction.
.PP
The use of the P/F\(hybit in the LAPB link is independent of the use of
that bit in the LAPD link.
.RT
.sp 1P
.LP
7.4.2.1.3
\|\fIInformation frame check sequence recalculation\fR
.sp 9p
.RT
.PP
The FCS values on the LAPB and LAPD link are independent. For every frame
the FCS values need to be calculated.
.RT
.sp 1P
.LP
7.4.2.2
\fIMapping by minimum link layer termination\fR
.sp 9p
.RT
.PP
In addition a simpler implementation of the LAPB\(hyLAPD mapping is
possible, without implementing the flow control and error recovery procedures.
Such a TA has as a minimum to implement the following mapping
functions;
.RT
.LP
\(em
frame type recognition,
.LP
\(em
mapping of address field,
.LP
\(em
mapping of control field,
.LP
\(em
administration of the state variables (V(S) and V(R) at both interfaces),
.LP
\(em
FCS handling.
.sp 1P
.LP
7.4.3
\fISignalling\fR
.sp 9p
.RT
.PP
This part defines the functionalities to be supported by the TA to establish,
maintain and release a LAPD, SAPI\ =\ 16, logical link to the PH at
the S/T reference point and a LAPB link at the R\ reference point. These
functionalities require a different degree of capabilities by the TA on the
bases of the different implementation of\ X.25 procedures in the DTE.
.PP
Several types may be identified, depending on the criteria at the
R\ reference point to initiate call set up:
.RT
.LP
type\ 1:
Establishment of a logical, SAPI\ =\ 16, link in the
D\(hychannel upon recognition of the receipt of a SABM frame
.LP
type\ 2:
idem on receipt of an I\ frame
.LP
type\ 3:
idem on receipt of \fIcall request\fR \|packet
.PP
\fINote\fR \ \(em\ Alternatively, initiation of a call set\(hyup can be
carried out by manual means, for example a push button on the TA.
.PP
Type 1 is the most simple one because a minimum mapping between LAPB and
LAPD is implemented in the TA. This type will be described in
\(sc\(sc\ 7.4.3.1, 7.4.3.2 and 7.4.3.3. Types\ 2 and\ 3, which require full
protocol termination in the TA (as described in \(sc\ 7.4.2) are more
complicated, but application of these types may be necessary depending
on the needs of existing\ X.25 DTEs. More details about these types are
given in
Appendix\ IV.
.RT
.sp 1P
.LP
7.4.3.1
\fIOutgoing call\fR
.sp 9p
.RT
.PP
At the outgoing call the TA initiates the establishment of a
D\(hychannel SAPI\ =\ 16 link to the\ PH.
.PP
The several conditions which force the TA to initiate this
establishment are described in \(sc\ 7.4.3.1.1. The establishment of the
D\(hychannel link is in accordance with the procedures described in
\(sc\ 6.1.2.2.
.RT
.sp 1P
.LP
7.4.3.1.1
\|\fIConditions for the establishment of a logical link between\fR \fIthe
DTE and the PH\fR
.sp 9p
.RT
.PP
Two situations can be identified to categorize the condition which may
cause the TA to attempt to establish a D\(hychannel SAPI\ =\ 16 logical
link:
.RT
.LP
a)
(semi\(hy) permanent logical link.
.LP
In this case the logical link is always available. No TA
functionality is required to initiate the establishment of a logical link.
.LP
b)
Logical link establishment is initiated by actions at the
R\ reference point.
.LP
The layer\ 1 interface at the R\ reference point is in
accordance with\ X.21, X.21\|\fIbis\fR or\ V.25\|\fIbis\fR interface procedures.
Subsequently the DTE will establish the LAPB link at the R\ reference point
and as a consequence the TA will activate the LAPD link (SAPI\ =\ 16) at
the S/T
reference point.
.PP
\fINote\fR \ \(em\ In principle it is also possible to activate the link
between DTE and PH as a result of a manual action at the TA.
.bp
.sp 1P
.LP
7.4.3.1.2
\|\fIMapping of link procedures\fR
.sp 9p
.RT
.PP
The mapping between the LAPD logical link at the S\ reference point and
the LAPB link at the R\ reference point is described in \(sc\ 7.4.2.
.RT
.sp 1P
.LP
7.4.3.1.3
\|X.25 \fIProcedures\fR
.sp 9p
.RT
.PP
After the establishment of a LAPD link at the S/T, and a LAPB link at the
R\ reference point and the concatenation of both links via a mapping
function in the TA, X.25 layer\ 3 procedures are possible between the DTE and
the PH. The TA is transparent for these\ X.25 layer\ 3 procedures.
.RT
.sp 2P
.LP
7.4.3.2
\fIIncoming call\fR
.sp 1P
.RT
.sp 1P
.LP
7.4.3.2.1
\|Q.931 \fICall offering\fR
.sp 9p
.RT
.PP
When notification of the incoming call applies (see \(sc\ 3.2.3),
the incoming call is first offered using the Q.931 procedures described in
\(sc\ 6.2.2.3.1 (channel selection through call offering).
.PP
The TA accepts the incoming call when the R\ reference point is or can
be placed in one of the following states;
.RT
.LP
\(em
the ready or send data state for an R\ reference point
conforming to\ X.21 procedures,
.LP
\(em
circuits 125 and 108 ON with 107 OFF for an R\ reference point conforming
to X.21\|\fIbis\fR and V.25\|\fIbis\fR procedures.
.PP
The TA shall respond to the offered call according to the same
\(sc\ 6.2.2.3.1.
.PP
After a successful incoming call procedure, the PH will initiate the establishment
of a LAPD SAPI\ =\ 16 link between the PH and the TA. The LAPB link at
the R\ reference point will be established at the same time via the mapping
procedures described in \(sc\ 7.4.2.
.PP
The link level layers are now in the data transfer phase.
.RT
.sp 1P
.LP
7.4.3.2.2
\|X.25 \fIProcedures\fR
.sp 9p
.RT
.PP
After the establishment of a LAPD link at the S/T, and a LAPB link at the
R\ reference point and the concatenation of both links via a mapping
function in the TA, X.25 layer\ 3 procedures are possible between the DTE and
the PH. The TA is transparent for these X.25 layer\ 3 procedures.
.RT
.sp 1P
.LP
7.4.3.3
\fIData link disconnection\fR
.sp 9p
.RT
.PP
To initiate the clearing of a D\(hychannel, SAPI\ =\ 16 logical link, it
is necessary to detect the clearing of the last virtual call on that logical
link. This detection can be done by two parties:
.RT
.LP
a)
the PH\ \(em\ clearing of the LAPD logical link is initiated by the PH,
.LP
b)
the DTE\ \(em\ clearing is initiated by actions at the
R\ reference point.
.PP
\fINote\fR \ \(em\ In principle it is possible to clear the link between
the DTE and the PH as a result of a manual action at the TA.
.sp 1P
.LP
7.4.3.3.1
\|\fIDisconnection by the PH\fR
.sp 9p
.RT
.PP
The PH clears the LAPD logical link in accordance with the
procedures described in \(sc\ 6.4.2. Via the mapping function between that
LAPD logical link and an LAPB link at the R\ reference point, clearing of the
format will be followed by clearing of the latter, applying the appropriate
LAPD procedures. After having cleared the LAPB link, the TA will disconnect
the layer\ 1 interface at the R\ reference point by either:
.RT
.LP
\(em
A transition from ON to OFF on the I\(hylead in case of an X.21 interface;
the DTE has to respond with an ON to OFF transition of the C\(hylead. After
that the interface is in the ready state, or
.LP
\(em
A transition from ON to OFF on circuit\ 107 in case of an
X.21\|\fIbis\fR or a V.25\|\fIbis\fR interface; the DTE has to respond
with an ON to OFF transition on circuit\ 108.
.sp 1P
.LP
7.4.3.3.2
\|\fIDisconnection by the DTE\fR
.sp 9p
.RT
.PP
The DTE indicates to the TA that the logical link between the TA
and the PH has to be cleared, by clearing the LAPB link at the R\ reference
point between the DTE and the TA.
.bp
.PP
The appropriate LAPB procedures apply. Following that DTE action, the TA
will clear the logical link between the TA and the PH.
.PP
After having completed the disconnection procedure of the LAPB link, the
DTE disconnects the layer\ 1 interface at the R\ reference point, by
either:
.RT
.LP
\(em
A transition from ON to OFF on the C\(hylead in case of an X.21 interface;
the TA has to respond with an ON to OFF transition on the I\(hylead.
After that the interface is in the ready state, or
.LP
\(em
A transition from ON to OFF on circuit\ 108 in case of an
X.21\|\fIbis\fR or a V.25\|\fIbis\fR interface; the TA has to respond with
an ON to OFF transition on circuit\ 107.
.sp 2P
.LP
7.5
\fIAccess through the B and D channel\fR
.sp 1P
.RT
.sp 1P
.LP
7.5.1
\fIGeneral\fR
.sp 9p
.RT
.PP
This part defines the functionalities to be supported by a TA when the
access through both\ B and\ D channels is used. This applies only to Case\
B (access to the ISDN virtual circuit service). Everything described in
\(sc\(sc\ 7.3 and\ 7.4 is applicable except the following.
.RT
.sp 1P
.LP
7.5.2
\fIOutgoing call\fR
.sp 9p
.RT
.PP
When the PH can be accessed by either the B\(hy or the D\(hychannel, the
call request will be sent by a\ Q.931 message with the resquest for packet
mode bearer service.
.PP
The TA may express its preference for a particular channel based on
preselected criteria such as\ X.25 packet size (limited in the D\(hychannel to
256\ bytes) or throughput requirements (limited to less than 16\ kbit/s on the
basic access D\(hychannel).
.PP
If the \*Qany channel\*U option is retained by the TA, the network will
allocate a B\(hychannel based on\ X.25 quality of service requirements that are
assumed a priori.
.RT
.sp 1P
.LP
7.5.3
\fIIncoming call\fR
.sp 9p
.RT
.PP
The network will issue a call offering on the D\(hychannel according to\
Q.931 procedures. The TA will proceed according to the procedures defined
in previous sections (see \(sc\ 6.2).
.RT
.sp 1P
.LP
7.6
\fITest loops\fR
.sp 9p
.RT
.PP
The maintenance concept of the TA shall comply with the maintenance concept
of the ISDN subscriber access and subscriber installation as defined in
Recommendation of the I.600\ Series and in Recommendation\ I.430 on ISDN
subscriber access and installation maintenance. The test loops are specified
in these Recommendations. The ISDN communication architecture enables
communication of maintenance information over bearer connections between
network service access points (NSAPs). Accordingly, bearer service may
be used on either a B\(hy or the D\(hychannel to transport the protocol.
.PP
Maintenance entities can choose to communicate information about
performance management, fault management, configuration and naming management,
etc., using an application layer protocol using OSI. The specification
of these management capabilities to be supported by TAs is for further
study.
.RT
.sp 2P
.LP
7.6.1
\fITest loops for TA with access through the B\(hychannel\fR
.sp 1P
.RT
.sp 1P
.LP
7.6.1.1
\fITest loop reference configuration\fR
.sp 9p
.RT
.PP
Figure 7\(hy4/X.31 shows the location of test loops within the
TA.
.RT
.LP
.rs
.sp 7P
.ad r
\fBFIGURE 7\(hy4/X.31, p. \fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
Loop 4 shall be allocated close to the S/T reference point. Loop\ 5 shall
be allocated close to the R\ reference point. Loop\ A shall be allocated
close to the S/T reference point.
.sp 1P
.LP
7.6.1.2
\fITest loop characteristics\fR
.sp 9p
.RT
.PP
The test loop characteristics for loops 4, 5 and A are defined in Recommendation\
I.430 and the I.600\ Series.
.RT
.sp 1P
.LP
7.6.1.3
\fILoop activation/deactivation mechanism\fR \v'3p'
.sp 9p
.RT
.LP
(i)
Test loop 4
.LP
Test loop 4 being controlled from the network side of the TA is activated
either via an application layer protocol on the B/D\(hychannel or via a
layer\ 1 message on the selected B\(hychannel after a connection has been
established from the control point to the TA. Selection of the B\(hychannel
to be looped is part of the call set\(hyup procedure. During the loop is
established the following states shall apply at the R\ reference point
(X.21):
.LP
towards the terminal R\ =\ 0/1\|.\|.\|., I\ =\ OFF (DCE controlled not
ready) shall apply.
.LP
(ii)
Test loop 5
.LP
For activation/deactivation of test loop\ 5, the
definitions as under\ (i) apply. Since the test loop\ 5 is close to the
R\ reference point, the loop point is located within the R\ reference point
circuitry and not within the B\(hychannel. Due to the rate adaption mechanism
the composition of the bit\(hystream received at the TA and the composition
of the
bit\(hystream which is looped and sent back on the B\(hychannel may not
be identical at the S/T reference point. At the loop point, however, the
incoming and
outgoing (looped) bit\(hystreams are identical.
.LP
During the loop is established the states as defined in\ X.21 for loop\
2b shall apply.
.LP
\fINote\fR \ \(em\ The possible use of logical loops by means of
layer\ 2 frames in the B\(hychannel requires further study.
.LP
iii)
Test loop A
.LP
Test loop\ A is activated/deactivated by procedures defined in Recommendations\
X.21/X.21\|\fIbis\fR .
.LP
\fINote\ 1\fR \ \(em\ Since selection of a specific B\(hychannel is
not part of\ X.21/X.21\|\fIbis\fR , the subject of B\(hychannel selection
within test
loop\ A, if required, remains for further study.
.LP
\fINote\ 2\fR \ \(em\ Loop activation/deactivation for the above\ 3
test loops can optionally as an alternative also be provided manually.
.sp 1P
.LP
7.6.1.4
\fICoding of activation/deactivation control messages\fR \v'3p'
.sp 9p
.RT
.LP
\(em
loop 4 control via B\(hy or D\(hychannel application layer
protocol: for further study
.LP
\(em
loop 4 control via B\(hychannel layer\ 1 message: for further
study
.LP
\(em
loop 5 control via B\(hy or D\(hychannel application layer
protocol: for further study
.LP
\(em
loop 5 control via B\(hychannel layer\ 1 message: as in
X.21/X21\|\fIbis\fR
.LP
\(em
loop A: as in X.21/X.21\|\fIbis\fR .
.sp 1P
.LP
7.6.2
\fITest loops for TA with access through the D\(hychannel\fR
.sp 9p
.RT
.PP
For further study.
.RT
.LP
.bp