ITU-X25 Packet layer
X25 Packet structure: fields
| - Octet 1: |
mandatory |
| bits 8-5 |
General format identifier or GFI |
| bits 4-1 |
Logical channel group number: |
| - Octet 2: |
mandatory |
| bits 8-1 |
Logical channel number |
| - Octet 3: |
Packet type identifier |
| bits 8-1 |
Packet type identifier. |
|
X25 Packet structure: General format identifier
| General format identifier |
8 7 6 5 bits |
| Call set–up packets |
Numbering modulo 8 |
x x 0 1 |
|
Numbering modulo 128 |
x x 1 0 |
| Clearing packets |
Numbering modulo 8 |
x 0 0 1 |
|
Numbering modulo 128 |
x 0 1 0 |
Flow control, interrupt, reset, restart,
registration and diagnostic packets |
Numbering modulo 8 |
0 0 0 1 |
|
Numbering modulo 128 |
0 0 1 0 |
| Data packets |
Numbering modulo 8 |
x x 0 1 |
|
Numbering modulo 128 |
x x 1 0 |
| General format identifier extension |
|
0 0 1 1 |
| Reserved for other applications |
|
* * 0 0 |
| TABLE 16/X.25 (ITU) |
|
Note : A bit which is indicated as "x"
may be set to either 0 or 1, as indicated:
| Bit number |
indication |
| bit 8 |
used for the Qualifier bit in data packets,
for the Address bit in call set–up and clearing packets, and
is set to 0 in all other packets. |
| bit 7 |
used for the delivery confirmation procedure in
data and call set–up packets and is set to 0 in all
other packets. |
| bits 6-5 |
encoded for four possible indications.
Two of the codes are used to distinguish packets using modulo
8 sequence numbering from packets using modulo 128 sequence
numbering.
The third code is used to indicate an extension to an expanded
format for a family of general format identifier codes which
are a subject of further study.
The fourth code is reserved for other applications. |
|
X25 Packet structure: logical channels
To enable simultaneous virtual calls and/or permanent virtual circuits,
logical channels are used. For virtual calls, a logical channel group number and
a logical channel number are assigned during the call set-up phase. The range of
logical channels used for virtual calls is agreed with the Administration at the
time of subscription to the service (see Annex A ITU). For permanent virtual
circuits, logical channel group numbers and logical channel numbers are assigned
in agreement with the Administration at the time of subscription to the service
(see Annex A ITU).
Logical channel group number
The logical channel group number appears in every packet except restart,
diagnostic and registration packets in bit position 4, 3, 2 and 1 of
octet 1. For each logical channel, this number has local significance at the DTE/DCE
interface.
This field is binary coded and bit 1 is the low order bit of the logical
channel group number. In restart, diagnostic and registration
packets, this field is coded all zeros.
Logical channel number
The logical channel number appears in every packet except restart,
diagnostic and registration packets in all bit positions of octet 2.
For each logical channel, this number has local significance at the DTE/DCE
interface.
This field is binary coded and bit 1 is the low order bit of the logical
channel number. In restart, diagnostic and registration
packets, this field is coded all zeros.
X25 Packet structure: list of types (see details)
| Packet type |
|
8 _ _ _ _ _ _ 1 bits |
| From DCE to DTE |
From DTE to DCE |
|
| Call set–up and clearing |
|
| Incoming call |
Call request |
0 0 0 0 1 0 1 1 |
0B |
| Call connected |
Call accepted |
0 0 0 0 1 1 1 1 |
0F |
| Clear indication |
Clear request |
0 0 0 1 0 0 1 1 |
13 |
| DCE clear confirmation |
DTE clear confirmation |
0 0 0 1 0 1 1 1 |
17 |
| Data and interrupt |
|
| DCE data |
DTE data |
x x x x x x x 0 |
|
| DCE interrupt |
DTE interrupt |
0 0 1 0 0 0 1 1 |
23 |
| DCE interrupt confirm. |
DTE interrupt confirm. |
0 0 1 0 0 1 1 1 |
27 |
| Flow control and reset |
|
| DCE RR (modulo 8) |
DTE RR (modulo 8) |
x x x 0 0 0 0 1 |
x1 |
| DCE RR (modulo 128) * |
DTE RR (modulo 128) * |
0 0 0 0 0 0 0 1 |
01 |
| DCE RNR (modulo 8) |
DTE RNR (modulo 8) |
x x x 0 0 1 0 1 |
x5 |
| DCE RNR (mod. 128) * |
DTE RNR (mod. 128) * |
0 0 0 0 0 1 0 1 |
05 |
| DTE REJ (modulo 8)* |
|
x x x 0 1 0 0 1 |
x9 |
| DTE REJ (mod. 128)* |
|
0 0 0 0 1 0 0 1 |
09 |
| Reset indication |
Reset request |
0 0 0 1 1 0 1 1 |
1B |
| DCE reset confirmation |
DTE reset confirmation |
0 0 0 1 1 1 1 1 |
1F |
| Restart |
|
| Restart indication |
Restart request |
1 1 1 1 1 0 1 1 |
FB |
| DCE restart confirm. |
DTE restart confirm. |
1 1 1 1 1 1 1 1 |
FF |
| Diagnostic |
|
| Diagnostic * |
|
1 1 1 1 0 0 0 1 |
F1 |
| Registration |
|
|
Registration request |
1 1 1 1 0 0 1 1 |
F3 |
| Registration confirm. |
|
1 1 1 1 0 1 1 1 |
F7 |
| TABLE 17/X.25 (ITU) |
|
(*): The types are not available in all networks.
Note: A bit which is indicated as "x" may be
set to either 0 or 1, as indicated in the text.
List sorted by the hexa value
| DCE RR (modulo 128) * |
DTE RR (modulo 128) * |
0 0 0 0 0 0 0 1 |
01 |
| DCE RNR (mod. 128) * |
DTE RNR (mod. 128) * |
0 0 0 0 0 1 0 1 |
05 |
| DTE REJ (mod. 128)* |
|
0 0 0 0 1 0 0 1 |
09 |
| Incoming call |
Call request |
0 0 0 0 1 0 1 1 |
0B |
| Call connected |
Call accepted |
0 0 0 0 1 1 1 1 |
0F |
| Clear indication |
Clear request |
0 0 0 1 0 0 1 1 |
13 |
| DCE clear confirmation |
DTE clear confirmation |
0 0 0 1 0 1 1 1 |
17 |
| Reset indication |
Reset request |
0 0 0 1 1 0 1 1 |
1B |
| DCE reset confirmation |
DTE reset confirmation |
0 0 0 1 1 1 1 1 |
1F |
| DCE interrupt |
DTE interrupt |
0 0 1 0 0 0 1 1 |
23 |
| DCE interrupt confirm. |
DTE interrupt confirm. |
0 0 1 0 0 1 1 1 |
27 |
| Diagnostic * |
|
1 1 1 1 0 0 0 1 |
F1 |
|
Registration request |
1 1 1 1 0 0 1 1 |
F3 |
| Registration confirm. |
|
1 1 1 1 0 1 1 1 |
F7 |
| Restart indication |
Restart request |
1 1 1 1 1 0 1 1 |
FB |
| DCE restart confirm. |
DTE restart confirm. |
1 1 1 1 1 1 1 1 |
FF |
| DCE data |
DTE data |
x x x x x x x 0 |
|
| DCE RR (modulo 8) |
DTE RR (modulo 8) |
x x x 0 0 0 0 1 |
x1 |
| DCE RNR (modulo 8) |
DTE RNR (modulo 8) |
x x x 0 0 1 0 1 |
x5 |
| DTE REJ (modulo 8)* |
|
x x x 0 1 0 0 1 |
x9 |
|
X25 Packet structure: address block format (call set–up and clearing)
The call set–up and clearing packets contain an address block. This address
block has two possible formats: a non-TOA/NPI address format and a TOA/NPI
address format. These two formats are distinguished by bit 8 of the general
format identifier (A bit). When the A bit is set to 0, the
non-TOA/NPI address format is used. When the A bit is set to 1, the TOA/NPI
address format is used.
The non-TOA/NPI address format is supported by all networks. The TOA/NPI
address format may be supported by some networks, in particular by those
networks wishing to communicate with ISDNs for which the non-TOA/NPI address
format provides insufficient addressing capacity.
Note: Prior to 1997, packet-mode DTEs operating according to
case B of Recommendation X.31 (ISDN virtual circuit bearer service) will be
addressed by a maximum 12 digit address from the E.164 numbering plan. After
1996, such a packet-mode DTE may have 15 digit E.164 address TOA/NPI address
procedures will be required to address these DTEs. Recommendations E.165 and
E.166 provide further guidance.
When transmitting a call set–up or clearing packet, a DCE will use the TOA/NPI
address format if the DTE has subscribed to the TOA/NPI address subscription
facility (see § 6.28 Rec. X.25 ITU), the non-TOA/NPI address format if it has
not.
Non-TOA/NPI format of the address block when
the A bit is set to 0
| bits |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
|
Calling DTE address length |
Called DTE address length |
|
a |
a |
a |
a |
a |
a |
a |
a |
|
a |
a |
a |
a |
b |
b |
b |
b |
|
b |
b |
b |
b |
0 |
0 |
0 |
0 |
| FIGURE 4/X.25 (ITU) |
|
Note: The figure is drawn assuming the number of address digits
present in the called DTE address field (a) is odd and the number of address
digits present in the calling DTE address field (b) is even.
Calling and called DTE address length fields
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-octets. Bits 8, 7, 6 and 5 indicate the length
of the calling DTE address in semi-octets. Each DTE address length indicator is
binary coded and bit 1 or 5 is the low order bit of the indicator.
Called and calling DTE address fields
Each digit of an address is coded in a semi-octet in binary coded decimal
with bit 5 or 1 being the low order bit of the digit.
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.
When present, the calling DTE address field starts on the first semi-octet
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.
When the total number of digits in the called and calling DTE address fields
is odd, a semi-octet with zeros in bits 4, 3, 2 and 1 will be inserted after the
calling DTE address field in order to maintain octet alignment.
Further information on ther coding of called and calling DTE address fields
is given in Appendix IV (ITU).
Note: 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.
TOA/NPI format of the address block when the
A bit is set to 1
| bits |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
|
Called DTE address length |
Calling DTE address length |
|
a |
a |
a |
a |
a |
a |
a |
a |
|
a |
a |
a |
a |
b |
b |
b |
b |
|
b |
b |
b |
b |
0 |
0 |
0 |
0 |
| FIGURE 5/X.25 (ITU) |
|
Note: The figure is drawn assuming the number of address digits
present in the called DTE address field (a) is odd and the number of address
digits present in the calling DTE address field (b) is even.
Called and calling DTE address length fields
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-octets. Each DTE
address length indicator is binary coded and bit 1 is the low order bit of the
indicator.
The maximum value of a DTE address field length indicator is 17.
Called and calling DTE address fields
These fields respectively consist of the called DTE address when present, and
the calling DTE address when present.
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 (ITU).
Note 1: Currently, no non-BCD encodable values have been
allocated for type of address and numbering plan identification subfields.
Note 2: A DTE address containing type of address and numbering
plan identification subfields but no address digits subfield is invalid.
X25 Packet structure: facilities list (see details)
| bits 8 _ _ _ _ _ _ 1 |
hex |
facilities type |
| 0 0 0 0 0 0 0 0 |
00 |
Marker:
When several categories of facilities are simultaneously present,
used to separate these categories from each other. |
| 0 0 0 0 0 0 0 1 |
01 |
Reverse charging / fast select |
| 0 0 0 0 0 0 1 0 |
02 |
Throughput class negociation |
| 0 0 0 0 0 0 1 1 |
03 |
Closed user group selection (basic format) |
| 0 0 0 0 1 0 0 0 |
08 |
Called line address modified notification |
| 0 0 0 0 1 0 0 1 |
09 |
Closed user group with outgoing access selection
(basic format) |
| 0 1 0 0 0 0 0 1 |
41 |
Bilateral closed user group selection |
| 0 1 0 0 0 0 1 0 |
42 |
Flow control parameter negotiation
Packet size |
| 0 1 0 0 0 0 1 1 |
43 |
Flow control parameter negotiation
Window size |
| 0 1 0 0 0 1 0 0 |
44 |
RPOA selection (basic format) |
| 0 1 0 0 1 0 0 1 |
49 |
Transit delay selection and indication |
| 1 1 0 0 0 0 1 1 |
C3 |
Call redirection or deflection notification |
| 1 1 0 1 0 0 0 1 |
D1 |
Call deflection selection |
| TABLE 29/X.25 (ITU) |
|
The coding of the facility field is defined in § 7.2 Rec. X.25 (ITU).
X25 Packet structure: facilities format
| bits 8 _ _ _ _ _ _ 1 |
Class |
Facilities code: class coding |
| 0 0 x x x x x x |
A |
single octet parameter field |
| 0 1 x x x x x x |
B |
double octet parameter field |
| 1 0 x x x x x x |
C |
triple octet parameter field |
| 1 1 x x x x x x |
D |
variable length parameter field |
| TABLE 28/X.25 (ITU) |
|
| bits 8 _ _ _ _ _ _ 1 |
facilities field: class A format |
| 0 0 x x x x x x |
Facility code |
| octet 1 |
Facility / registration parameter field |
| bits 8 _ _ _ _ _ _ 1 |
facilities field: class B format |
| 0 1 x x x x x x |
Facility code |
| octets 1-2 |
Facility / registration parameter field |
| bits 8 _ _ _ _ _ _ 1 |
facilities field: class C format |
| 1 0 x x x x x x |
Facility code |
| octets 1-3 |
Facility / registration parameter field |
| bits 8 _ _ _ _ _ _ 1 |
facilities field: class D format |
| 1 1 x x x x x x |
Facility code |
| octet 1
octets 2-n
|
Facility / registration parameter field length
Facility / registration parameter field
|
|
X25 L3 parameters: time-limit list
Time-limit T10
| T10 |
DCE side |
DTE side |
| State |
r3 |
|
| Start |
DCE issues a restart indication |
|
| Stop |
DCE leaves the r3 state (i.e.,the restart confirmation
or restart request is received) |
|
|
Time-limit T11
| T11 |
DCE side |
DTE side |
| State |
p3 |
|
| Start |
DCE issues an incoming call |
|
| Stop |
DCE leaves the p3 state (e.g., the call accepted, clear
request or call request is received) |
|
|
Time-limit T12
| T12 |
DCE side |
DTE side |
| State |
d3 |
|
| Start |
DCE issues a reset indication |
|
| Stop |
DCE leaves the d3 state (e.g., the reset confirmation
or reset request is received) |
|
|
Time-limit T13
| T13 |
DCE side |
DTE side |
| State |
p7 |
|
| Start |
DCE issues a clear indication |
|
| Stop |
DCE leaves the p7 state (e.g., the clear confirmation
or clear request is received) |
|
|
Time-limit T20
| T20 |
DCE side |
DTE side |
| State |
|
r2 |
| Start |
|
DTE issues a restart request |
| Stop |
|
DTE leaves the r2 state (i.e., the restart confirmation
or restart indication is received) |
|
Time-limit T21
| T21 |
DCE side |
DTE side |
| State |
|
p2 |
| Start |
|
DTE issues a call request |
| Stop |
|
DTE leaves the p2 state (e.g., the call connected,
clear indication or incoming call is received) |
|
Time-limit T22
| T22 |
DCE side |
DTE side |
| State |
|
d2 |
| Start |
|
DTE issues a reset request |
| Stop |
|
DTE leaves the d2 state (e.g., the reset confirmation
or reset indication is received) |
|
Time-limit T23
| T23 |
DCE side |
DTE side |
| State |
|
p6 |
| Start |
|
DTE issues a clear request |
| Stop |
|
DTE leaves the p6 state (e.g., the clear confirmation
or clear indication is received) |
|
Time-limit T28
| T28 |
DCE side |
DTE side |
| State |
|
any |
| Start |
|
DTE issues a registration request |
| Stop |
|
DTE receives the registration confirmation or a diagnostic
packet |
|
| 
