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

DCE side DTE side
T10 60s
T11 180s
T12 60s
T13 60s
T20 180s
T21 200s
T22 180s
T23 180s
T28 300s

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

 

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updated:  27-Feb-04