\ua\d\u)\d\ Not necessarily available on every network.
.parag
\fINote\fR
\ \(em\ A bit which is indicated as \*QX\*U may be set to either 0 or 1 as
indicated in the text.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 17/X.25 [T15.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
\fINote\fR \ \(em\ The \fITOA/NPI address subscription\fR \| facility is
designated in Recommendation\ X.2 for further study (FS). In addition,
there are several
technical items associated with this TOA/NPI address format which are for
further study.
.PP
When transmitting a call set\(hyup or clearing packet, a DTE will use the
TOA/NPI address format if the DTE has subscribed to the \fITOA/NPI address\fR
\fIsubscription\fR facility, the non\(hyTOA/NPI address format if it has not.
.PP
When the address format used by one DTE in a call set\(hyup or clearing
packet is different from the address format used by the remote DTE, the
network (if it supports the TOA/NPI address format) converts from one address
format to the other (see \(sc\ 6.2.8).
.bp
.RT
.sp 1P
.LP
5.2.1.1
\fIFormat of the address block when the A bit is set to 0\fR
\fI(non\(hyTOA/NPI address)\fR
.sp 9p
.RT
.PP
Figure 4/X.25 illustrates the format of the address block when the A\ bit
is set to\ 0.
.RT
.LP
.sp 1
.rs
.sp 22P
.ad r
\fBFigure 4/X.25 (comme tableau) [T16.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.sp 1P
.LP
5.2.1.1.1
\fICalling and called DTE address length fields\fR
.sp 9p
.RT
.PP
These fields are four bits long each and consist of field length
indicators for the called and calling DTE addresses. Bits\ 4, 3, 2 and\ 1
indicate the length of the called DTE address in semi\(hyoctets. Bits\ 8, 7, 6
and\ 5 indicate the length of the calling DTE address in semi\(hyoctets.
Each DTE address length indicator is binary coded and bit\ 1 or\ 5 is the
low order bit of the indicator.
.RT
.sp 1P
.LP
5.2.1.1.2
\fICalled and calling DTE address fields\fR
.sp 9p
.RT
.PP
Each digit of an address is coded in a semi\(hyoctet in binary coded decimal
with bit\ 5 or\ 1 being the low order bit of the digit.
.PP
Starting from the high order digit, a DTE address is coded in
consecutive octets with two digits per octet. In each octet, the higher
order digit is coded in bits\ 8, 7, 6 and\ 5.
.PP
When present, the calling DTE address field starts on the first
semi\(hyoctet following the end of the called DTE address field. Consequently,
when the number of digits of the called DTE address field is odd, the
beginning of the calling DTE address field, when present, is not octet
aligned.
.PP
When the total number of digits in the called and calling DTE address fields
is odd, a semi\(hyoctet with zeros in bits\ 4, 3, 2 and\ 1 will be
inserted after the calling DTE address field in order to maintain octet
alignment.
.PP
Further information on the coding of called and calling DTE address
fields is given in Appendix\ IV.
.bp
.PP
\fINote\fR \ \(em\ These fields may be used for optional addressing facilities
such as abbreviated addressing. The optional addressing facilities employed
as well as the coding of those facilities are for further study.
.RT
.sp 1P
.LP
5.2.1.2
\fIFormat of the address block when the A bit is set to 1\fR
\fI(TOA/NPI address)\fR
.sp 9p
.RT
.PP
Figure 5/X.25 illustrates the format of the address block when the A\ bit
is set to\ 1.
.RT
.LP
.sp 1
.rs
.sp 22P
.ad r
\fBFigure 5/X.25 (comme tableau) [T17.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.sp 1P
.LP
5.2.1.2.1
\fICalled and calling DTE address length fields\fR
.sp 9p
.RT
.PP
These fields are one octet long each and consist of field length
indicators for the called and calling DTE addresses. They indicate the
length of the called DTE address and the calling DTE address, respectively,
in
semi\(hyoctets. Each DTE address length indicator is binary coded and bit\
1 is the low order bit of the indicator.
.PP
The maximum value of a DTE address field length indicator
is\ 17.
.RT
.sp 1P
.LP
5.2.1.2.2
\fICalled and calling DTE address fields\fR
.sp 9p
.RT
.PP
These fields respectively consist of the called DTE address when
present, and the calling DTE address when present.
.PP
Each DTE address field, when present, has three subfields: type of
address subfield (TOA), numbering plan identification subfield (NPI), address
digits subfield. The first two subfields are at the beginning of the address
and are binary coded with the values indicated in Tables\ 18/X.25
and\ 19/X.25.
.RT
.PP
\fINote\ 1\fR \ \(em\ Currently, no non\(hyBCD encodable values have been
allocated for type of address and numbering plan identification
subfields.
.PP
\fINote\ 2\fR \ \(em\ A DTE address containing type of address and numbering
plan identification subfields but no address digits subfield is
invalid.
.bp
.RT
.ce
\fBH.T. [T18.25]\fR
.ce
TABLE\ 18/X.25
.ce
\fBCoding of the type of address subfield\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(36p) | lw(42p) | cw(102p) .
Bits: or Bits: T{
8\ \ 7\ \ 6\ \ 5
\| \
4\ \ 3\ \ 2\ \ 1
T} T{
\| \
Type of address
\| \
\
T}
.T&
cw(78p) | lw(102p) .
(see Note 1)
_
.T&
lw(36p) | lw(42p) | lw(102p) .
0\ \ 0\ \ 0\ \ 0 T{
Network\(hydependent number (see Note 2)
T}
.T&
lw(36p) | lw(42p) | lw(102p) .
0\ \ 0\ \ 0\ \ 1 T{
International number (see Note 3)
T}
.T&
lw(36p) | lw(42p) | lw(102p) .
0\ \ 0\ \ 1\ \ 0 National number (see Note 3)
.T&
lw(36p) | lw(42p) | lw(102p) .
to be defined T{
Complementary address alone (see Note 4)
T}
.T&
lw(36p) | lw(42p) | lw(102p) .
other values T{
Reserved
\fINote\ 1\fR
\ \(em\ The type of address subfield of the called DTE address field uses
bits 8, 7, 6 and 5. The type of address subfield of the calling DTE address
field uses bits\ 4, 3, 2 and\ 1 if the called DTE address field does \fInot\fR
end on an octet boundary; otherwise, it uses bits\ 8, 7, 6 and\ 5.
.parag
\fINote\ 2\fR
\ \(em\ In this case, the address digits subfield present after the type of address and numbering plan identification subfields are organized according to the network numbering plan, e.g., prefix or escape code might be present.
This case is equivalent to the use of the same code point in\ Q.931, where
it is called \*Qunknown\*U.
.parag
\fINote\ 3\fR
\ \(em\ As for Q.931, prefix or escape code shall not be included in the
address digits subfield.
.parag
\fINote\ 4\fR
\ \(em\ See Appendix IV for the definition of a complementary
address.
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 18/X.25 [T18.25] + notes, p.\fR
.sp 1P
.RT
.ad b
.RT
.ce
\fBH.T. [T19.25]\fR
.ce
TABLE\ 19/X.25
.ce
\fBCoding of the numbering plan identification subfield\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(36p) | lw(42p) | cw(102p) .
Bits: or Bits: T{
8\ \ 7\ \ 6\ \ 5
\| \
4\ \ 3\ \ 2\ \ 1
T} T{
\| \
Numbering plan
\| \
\
T}
.T&
cw(78p) | lw(102p) .
(see Note 1)
_
.T&
lw(36p) | lw(42p) | lw(102p) .
0\ \ 0\ \ 1\ \ 1 X.121 (see Note 2)
.T&
lw(36p) | lw(42p) | lw(102p) .
to be defined T{
Network\(hydependent (see Note 3)
T}
.T&
lw(36p) | lw(42p) | lw(102p) .
other values T{
Reserved (see Note 4)
\fINote\ 1\fR
\ \(em\ The numbering plan identification subfield of the called DTE address field uses bits 4, 3, 2 and\ 1. The numbering plan identification subfield of
the calling DTE address field uses bits\ 8, 7, 6 and\ 5 if the called DTE address does \fInot\fR
end on an octet boundary; otherwise, it uses bits\ 4, 3, 2 and\ 1.
.parag
\fINote\ 2\fR
\ \(em\ A mechanism equivalent to that provided by escape digits, as defined in Recommendation\ X.121, is not yet defined for use in conjunction whith the
TOA/NPI capability; such a mechanism will not use the numbering plan
identification subfield. Until the availability of such a mechanism
(potentially, an optional user facility), only the code point for
X.121 shall be used. The X.121 escape codes shall apply and, when they are
used,
the type of address subfield shall indicate network\(hydependent number.
.parag
\fINote\ 3\fR
\ \(em\ In this case, the address digits subfield present after the type
of address and numbering plan identification subfields are organized according to the network numbering plan, e.g., prefix or escape code might be present.
.parag
\fINote\ 4\fR
\ \(em\ Included among the reserved values are those corresponding to
numbering plan identifiers in Q.931 (e.g., F.69, E.164).
.parag
T}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 19/X.25 [T19.25] + notes, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
The other semi\(hyoctets of a DTE address are digits, coded in binary
coded decimal with bit\ 5 or\ 1 being the low order bit of the digit. Starting
from the high order digit, the address digits are coded in consecutive
semi\(hyoctets. In each octet, the higher order digit is coded in bits\ 8, 7, 6
and\ 5.
.PP
When present, the calling DTE address field starts on the first
semi\(hyoctet following the end of the called DTE address field. Consequently,
when the number of semi\(hyoctets of the called DTE address field is odd, the
beginning of the calling DTE address field, when present, is not octet
aligned.
.PP
When the total number of semi\(hyoctets in the called and calling DTE
address fields is odd, a semi\(hyoctet with zeros in bits\ 4, 3, 2 and\
1 will be
inserted after the calling DTE address field in order to maintain octet
alignment.
.PP
Further information on the coding of called and calling DTE address
fields is given in Appendix\ IV.
.PP
\fINote\fR \ \(em\ These fields may be used for optional addressing facilities
such as abbreviated addressing. The optional addressing facilities employed
as well as the coding of those facilities are for further study.
.RT
.sp 1P
.LP
5.2.2
\fICall request and incoming call packets\fR
.sp 9p
.RT
.PP
Figure 6/X.25 illustrates the format of \fIcall request\fR \| and
\fIincoming call\fR \| packets.
.RT
.LP
.sp 1
.rs
.sp 25P
.ad r
\fBFigure 6/X.25 (comme tableau) [T20.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.sp 1P
.LP
5.2.2.1
\fIGeneral format identifier\fR
.sp 9p
.RT
.PP
Bit 8 of octet 1 (A bit) should be set as described in
\(sc\ 5.2.1.
.PP
Bit 7 of octet 1 should be set to 0 unless the mechanism defined in
\(sc\ 4.3.3 is used.
.bp
.RT
.sp 1P
.LP
5.2.2.2
\fIAddress block\fR
.sp 9p
.RT
.PP
The address block is described in \(sc\ 5.2.1.
.RT
.sp 1P
.LP
5.2.2.3
\fIFacility length field\fR
.sp 9p
.RT
.PP
The octet following the address block indicates the length of the facility
field, in octets. The facility length indicator is binary coded and
bit\ 1 is the low order bit of the indicator.
.RT
.sp 1P
.LP
5.2.2.4
\fIFacility field\fR
.sp 9p
.RT
.PP
The facility field is present only when the DTE is using an
optional user facility requiring some indication in the \fIcall request\fR and
\fIincoming call\fR packets.
.PP
The coding of the facility field is defined in \(sc\(sc\ 6
and\ 7.
.PP
The facility field contains an integral number of octets. The actual maximum
length of this field depends on the facilities which are offered by the
network. However, this maximum does not exceed 109\ octets.
.PP
\fINote\fR \ \(em\ It is for further study whether another value should be
defined, relative to the total number of octets in the packet.
.RT
.sp 1P
.LP
5.2.2.5
\fICall user data field\fR
.sp 9p
.RT
.PP
Following the facility field, the call user data field may be
present and has a maximum length of 128\ octets when used in conjunction with
the \fIfast select\fR facility described in \(sc\ 6.16, 16\ octets in the other
case.
.PP
\fINote\fR \ \(em\ Some networks require the call user data field to contain
an integral number of octets (see the note in \(sc\ 3).
.PP
When the virtual call is being established between two packet\(hymode
DTEs, the network does not act on any part of the call user data field. In
other circumstances, see Recommendation\ X.244.
.RT
.sp 1P
.LP
5.2.3
\fICall accepted and call connected packets\fR
.sp 9p
.RT
.PP
Figure 7/X.25 illustrates the format of the \fIcall accepted\fR \| and
\fIcall connected\fR \| packets in the basic or extended format.
.RT
.sp 2P
.LP
5.2.3.1
\fIBasic format\fR
.sp 1P
.RT
.sp 1P
.LP
5.2.3.1.1
\fIGeneral format identifier\fR
.sp 9p
.RT
.PP
Bit 8 of octet 1 (A bit) should be set as described in
\(sc\ 5.2.1
.PP
Bit 7 of octet 1 should be set to\ 0 unless the mechanism defined in
\(sc\ 4.3.3 is used.
.RT
.sp 1P
.LP
5.2.3.1.2
\fIAddress block\fR
.sp 9p
.RT
.PP
The address block is described in \(sc\ 5.2.1.
.PP
The use of the called and calling DTE address length fields in
\fIcall accepted\fR \| packets is only mandatory when the called DTE address
field, the calling DTE address field or the facility length field is
present.
.RT
.sp 1P
.LP
5.2.3.1.3
\fIFacility length field\fR
.sp 9p
.RT
.PP
The octet following the address block indicates the length of the facility
field, in octets. The facility length indicator is binary coded and
bit\ 1 is the low order bit of the indicator.
.PP
The use of the facility length field in \fIcall accepted\fR packets is
only mandatory when the facility field is present.
.bp
.RT
.LP
.rs
.sp 27P
.ad r
\fBFigure 7/X.25 (comme tableau) [T21.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
.sp 1
5.2.3.1.4
\fIFacility field\fR
.sp 9p
.RT
.PP
The facility field is present only when the DTE is using an
optional user facility requiring some indication in the \fIcall accepted\fR and
\fIcall connected\fR packets.
.PP
The coding of the facility field is defined in \(sc\(sc\ 6
and\ 7.
.PP
The facility field contains an integral number of octets. The actual maximum
length of this field depends on the facilities which are offered by the
network. However, this maximum does not exceed 109\ octets.
.PP
\fINote\fR \ \(em\ It is for further study whether another value should be
defined, relative to the total number of octets in the packet.
.RT
.sp 1P
.LP
5.2.3.2
\fIExtended format\fR
.sp 9p
.RT
.PP
The extended format may be used only in conjunction with the \fIfast\fR
\fIselect\fR \| facility described in \(sc\ 6.16. In this case, the called
user data
field may be present and has a maximum length of 128\ octets.
.PP
The calling and called DTE address length fields and the facility
length field must be present when the called user data field is present.
.PP
\fINote\fR \ \(em\ Some networks require the called user data field to
contain an integral number of octets (see the note in \(sc\ 3).
.bp
.PP
When the virtual call is being established between two packet\(hymode
DTEs, the network does not act on any part of the called user data field.
See Recommendation\ X.244.
.RT
.sp 1P
.LP
5.2.4
\fIClear request and clear indication packets\fR
.sp 9p
.RT
.PP
Figure 8/X.25 illustrates the format of \fIclear request\fR \| and
\fIclear indication\fR \| packets, in basic and extended formats.
.RT
.LP
.sp 1
.rs
.sp 31P
.ad r
\fBFigure 8/X.25 (comme tableau) [T22.25], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.sp 2P
.LP
5.2.4.1
\fIBasic format\fR
.sp 1P
.RT
.sp 1P
.LP
5.2.4.1.1
\fIClearing cause field\fR
.sp 9p
.RT
.PP
Octet 4 is the clearing cause field and contains the reason for the clearing
of the call.
.PP
In the \fIclear request\fR \| packets, the clearing cause field should be
set by the DTE to one of the following values:
.RT
.LP
bits:
8\|
7\|
6\|
5\|
4\|
3\|
2\|
1
.LP
value:
0\|
0\|
0\|
0\|
0\|
0\|
0\|
0
.LP
or:
1\|X\|X\|X\|X\|X\|X\|X
.LP
where each X may be independently set to 0 or 1 by the DTE.
.bp
.PP
The DCE will prevent values of the clearing cause field other than those
shown above from reaching the other end of the call by either accepting
the \fIclear request\fR packet and forcing the clearing cause field to
all zeros in the corresponding \fIclear indication\fR packet, or considering
the \fIclear request\fR as an error and following the procedure described
in Annex\ C.
.PP
The coding of the clearing cause field in \fIclear indication\fR \| packets
