ITU-X25 Packet layer - Packet types and format

Call request / Incoming call

- Octet 1: mandatory
bits 8-5 General format identifier coded XX01 (modulo 8) or XX10 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: 0B H
- Octets 4-n: mandatory
bits 8-1 Address block
- Octet :
bits 8-1 Facility length
- Octet :
bits 8-1 Facilities
- Octet : optional
bits 8-1 Call user data
FIGURE 6/X.25 (ITU)

Call accepted / Call connected

- Octet 1: mandatory
bits 8-5 General format identifier coded XX01 (modulo 8) or XX10 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: 0F H
- Octets 4-n: mandatory
bits 8-1 Address block (a)
- Octet :
bits 8-1 Facility length (a)
- Octet :
bits 8-1 Facilities (a)
- Octet : optional
bits 8-1 Called user data (b)
FIGURE 7/X.25 (ITU)

(a): These fields are not mandatory in the basic format of call accepted packets.

(b): This field may be present only in the extended format.

Clear request / Clear indication

- Octet 1: mandatory
bits 8-5 General format identifier coded X001 (modulo 8) or X010 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: 13 H
- Octet 4: mandatory
bits 8-1 Clearing cause
- Octet 5: mandatory
bits 8-1 Diagnostic code (a)
- Octets 6-n: optional
bits 8-1 Address block (b)
- Octet : optional
bits 8-1 Facility length (b)
- Octet : optional
bits 8-1 Facilities (b)
- Octet : optional
bits 8-1 Clear user data (b)
FIGURE 8/X.25 (ITU)

(a): This field is not mandatory in the basic format of clear request packets .

(b): Used only in the extended format

Clearing cause field (CR/CI)

Octet 4 is the clearing cause field and contains the reason for the clearing of the call.

In the clear request packets, the clearing cause field should be set by the DTE to one of the following values, where each "x" may be independently set to 0 or 1 by the DTE.:

bits 8 7 6 5 4 3 2 1
value 0 0 0 0 0 0 0 0
or 1 x x x x x x x

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 clear request packet and forcing the clearing cause field to all zeros in the corresponding clear indication packet, or considering the clear request as an error and following the procedure described in Annex C (ITU).

The coding of the clearing cause field in clear indication packets is given in Table 20/X.25 (ITU):

bits 8 _ _ _ _ _ _ 1 hex Class
0 0 0 0 0 0 0 0 00 DTE originated
1 x x x x x x x DTE originated (a)
0 0 0 0 0 0 0 1 01 Number busy
0 0 0 0 1 0 0 1 09 Out of order
0 0 0 1 0 0 0 1 11 Remote procedure error
0 0 0 1 1 0 0 1 19 Reverse charging acceptance not subscribed (b)
0 0 1 0 0 0 0 1 21 Incompatible destination
0 0 1 0 1 0 0 1 29 Fast select acceptance not subscribed (b)
0 0 1 1 1 0 0 1 39 Ship absent (c)
0 0 0 0 0 0 1 1 03 Invalid facility request
0 0 0 0 1 0 1 1 0B Acces barred
0 0 0 1 0 0 1 1 13 Local procedure error
0 0 0 0 0 1 0 1 05 Network congestion
0 0 0 0 1 1 0 1 0D Not obtainable
0 0 0 1 0 1 0 1 15 RPOA out of order (b)
TABLE 20/X.25 (ITU)

(a): When bit 8 is set to 1, the bits represented by Xs are those included by the remote DTE in the clearing or restarting cause field of the clear or restart request packet respectively.

(b): May be received only if the corresponding optional user facility is used.

(c): Used in the conjunction with mobile maritime service.

Diagnostic code (CR/CI)

Octet 5 is the diagnostic code and contains additional information on the reason for the clearing of the call.

In a clear request packet, the diagnostic code is not mandatory.

In a clear indication packet, if the clearing cause field indicates "DTE originated", the diagnostic code is passed unchanged from the clearing DTE. If the clearing DTE has not provided a diagnostic code in its clear request packet, then the bits of the diagnostic code in the resulting clear indication packet will all be zero.

When a clear indication packet results from a restart request packet, the value of the diagnostic code will be that specified in the restart request packet, or all zeros in the case where no diagnostic code has been specified in the restart request packet.

When the clearing cause field does not indicate "DTE originated", the diagnostic code in a clear indication packet is network generated. Annex E (ITU) lists the codings for network generated diagnostics. The bits of the diagnostic code are all set to 0 when no specific additional information for the clearing is supplied:

bits 8 _ _ _ _ _ _ 1 dec Class
0 0 0 0 0 0 0 0 000 No additional information
001 Invalid P(S)
002 Invalid P(R)
0 0 0 1 0 0 0 0 016 Packet type invalid
017 Packet type invalid for state r1
018 Packet type invalid for state r2
019 Packet type invalid for state r3
020 Packet type invalid for state p1
021 Packet type invalid for state p2
022 Packet type invalid for state p3
023 Packet type invalid for state p4
024 Packet type invalid for state p5
025 Packet type invalid for state p6
026 Packet type invalid for state p7
027 Packet type invalid for state d1
028 Packet type invalid for state d2
029 Packet type invalid for state d3
0 0 1 0 0 0 0 0 032 Packet not allowed
033 Unidentifiable packet
034 Call on one-way logical channel
035 Invalid packet type on a PVC
036 Packet on unassigned LC
037 Reject not subscribed to
038 Packet too short
039 Packet too long
040 Invalid general format identifier
041 Restart/registration packet. with nonzero bits
042 Pkt type not compatible with facility
043 Unauthorised interrupt confirmation
044 Unauthorised interrupt
045 Unauthorised reject
0 0 1 1 0 0 0 0 048 Time expired
049 Time expired for incoming call
050 Time expired for clear indication
051 Time expired for reset indication
052 Time expired for restart indication
053 Time expired for call deflection
0 1 0 0 0 0 0 0 064 Call set-up/clearing or registration pb.
065 Facility/registration code not allowed
066 Facility parameter not allowed
067 Invalid called DTE address
068 Invalid calling DTE address
069 Invalid facility/registration length
070 Incoming call barred
071 No logical channel available
072 Call collision
073 Duplicate facility requested
074 Non zero address length
075 Non zero facility length
076 Facility not provided when expected
077 Invalid CCITT-specified DTE facility
078 Max. nb of call redir/defl. exceeded
0 1 0 1 0 0 0 0 080 Miscellaneous
081 Improper cause code from DTE
082 Not aligned octet
083 Inconsistent Q bit setting
084 NUI problem
0 1 1 1 0 0 0 0 112 International problem
113 Remote network problem
114 International protocol problem
115 International link out of order
116 International link busy
117 Transit network facility problem
118 Remote network facility problem
119 International routing problem
120 Temporary routing problem
121 Unknown called DNIC
122 Maintenance action
TABLE E-1/X.25 (ITU)

Note: The contents of the diagnostic code field do not alter the meaning of the cause field. A DTE is not required to undertake any action on the contents of the diagnostic code field. Unspecified code combinations in the diagnostic code field shall not cause the DTE to refuse the cause field.

Clear confirmation (DTE and DCE)

- Octet 1: mandatory
bits 8-5 General format identifier coded X001 (modulo 8) or X010 (modulo 128)
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: 17 H
- Octets 4-n: optional
bits 8-1 Address block (a)
- Octet : optional
bits 8-1 Facility length (a)
- Octet : optional
bits 8-1 Facilities (a)
FIGURE 9/X.25 (ITU)

(a): Used only in the extended format of DCE clear confirmation packets.

The extended format may be used for DCE clear confirmation packets only in conjunction with the charging information facility described in § 6.22 Rec. X.25 (ITU). It is not used for DTE clear confirmation packet.

DTE and DCE data

The format of the DTE and DCE data packets depends on the P(S) and P(R) sequence numbering scheme:

Numbering modulo 8

- Octet 1: mandatory
bits 8-5 General format identifier (Q D 0 1)
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-6 P(R)
bits 5 M
bits 4-2 P(S)
bit 1 Packet type identifier = 0
- Octets 4-n: mandatory
bits 8-1 User Data
FIGURE 10/X.25 Modulo 8 (ITU)

With following definitions:

  • D: Delivery confirmation bit
  • M: More data bit
  • Q: Qualifier bit

Numbering modulo 128

- Octet 1: mandatory
bits 8-5 General format identifier (Q D 0 1)
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-2 P(R)
bits 1 Packet type identifier = 0
- Octet 4: mandatory
bits 8-2 P(S)
bit 1 M
- Octets 5-n: mandatory
bits 8-1 User Data
FIGURE 10/X.25 When extended to modulo 128 (ITU)

With following definitions:

  • D: Delivery confirmation bit
  • M: More data bit
  • Q: Qualifier bit

Interrupt (DTE and DCE)

- Octet 1: mandatory
bits 8-5 General format identifier coded X001 (modulo 8) or X010 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: 23 H
- Octets 4-n: optional
bits 8-1 Interrupt user data
FIGURE 11/X.25 (ITU)

Octet 4 and any following octets contain the interrupt user data. This field may contain from 1 to 32 octets.

Note: Some networks require the interrupt user data field to contain an integral number of octets.

Interrupt confirmation (DTE and DCE)

- Octet 1: mandatory
bits 8-5 General format identifier coded X001 (modulo 8) or X010 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: 27 H
FIGURE 12/X.25 (ITU)

RR (DTE and DCE)

The format of the DTE and DCE RR packets depends on the P(R) sequence numbering scheme:

Numbering modulo 8

- Octet 1: mandatory
bits 8-5 General format identifier = 0001
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-6 P(R)
bits 5-1 Packet type identifier = 00001
FIGURE 13/X.25 Modulo 8 (ITU)

Numbering modulo 128

- Octet 1: mandatory
bits 8-5 General format identifier = 0010
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier = 00000001
- Octet 4: mandatory
bits 8-2 P(R)
bit 1 0
FIGURE 13/X.25 When extended to modulo 128 (ITU)

RNR (DTE and DCE)

The format of the DTE and DCE RNR packets depends on the P(R) sequence numbering scheme:

Numbering modulo 8

- Octet 1: mandatory
bits 8-5 General format identifier = 0001
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-6 P(R)
bits 5-1 Packet type identifier = 00101
FIGURE 14/X.25 Modulo 8 (ITU)

Numbering modulo 128

- Octet 1: mandatory
bits 8-5 General format identifier = 0010
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier = 00000101
- Octet 4: mandatory
bits 8-2 P(R)
bit 1 0
FIGURE 14/X.25 When extended to modulo 128 (ITU)

DTE REJ

The format of the DTE and DCE REJ packets depends on the P(R) sequence numbering scheme:

Numbering modulo 8

- Octet 1: mandatory
bits 8-5 General format identifier = 0001
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-6 P(R)
bits 5-1 Packet type identifier = 01001
FIGURE 20/X.25 (ITU)

Numbering modulo 128

- Octet 1: mandatory
bits 8-5 General format identifier = 0010
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bit 1 Packet type identifier= 00001001
- Octet 4: mandatory
bits 8-2 P(R)
bit 1 0
FIGURE 20/X.25 When extended to modulo 128 (ITU)

 P(R) - Packet receive sequence number

Bits 8, 7 and 6 of octet 3, or bits 8 through 2 of octet 4 when extended, are used for indicating the packet receive sequence number P(R). P(R) is binary coded and bit 6, or bit 2 when extended, is the low order bit.

Reset request / Reset indication

- Octet 1: mandatory
bits 8-5 General format identifier coded X001 (modulo 8) or X010 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: 1B H
- Octet 4: mandatory
bits 8-1 Resetting cause
- Octet 5: mandatory
bits 8-1 Diagnostic code (a)
FIGURE 15/X.25 (ITU)

(a): This field is not mandatory in reset request packets.

Resetting cause field (RR/RI)

Octet 4 is the resetting cause field and contains the reason for the reset.

In reset request packets, the resetting cause field should be set by the DTE to one of the following values, where each "x" may be independently set to 0 or 1 by the DTE.:

bits 8 7 6 5 4 3 2 1
value 0 0 0 0 0 0 0 0
or 1 x x x x x x x

The DCE will prevent values of the resetting cause field, other than those shown above, from reaching the other end of the virtual call or permanent virtual circuit by either accepting the reset request packet and forcing the resetting cause field to all zeros in the corresponding reset indication packet, or considering the reset request as an error and following the procedure described in Annex C (ITU).

The coding of the resetting cause field in a reset indication packet is given in Table 21/X.25 (ITU):

bits 8 _ _ _ _ _ _ 1 Class
0 0 0 0 0 0 0 0 DTE originated
1 x x x x x x x DTE originated (a)
0 0 0 0 0 0 0 1 Out or order (b)
0 0 0 0 0 0 1 1 Remote procedure error
0 0 0 0 0 1 0 1 Local procedure error
0 0 0 0 0 1 1 1 Network congestion
0 0 0 0 1 0 0 1 Remote DTE operational (b)
0 0 0 0 1 1 1 1 Network operational (b)
0 0 0 1 0 0 0 1 Incompatible destination
0 0 0 1 1 1 0 1 Network out of order (b)
TABLE 21/X.25 (ITU)

(a): When bit 8 is set to 1, the bits represented by Xs are those indicated by the remote DTE in the resetting cause field (virtual calls and permanent virtual circuits) or the restarting cause field (permanent virtual circuits only) of the reset or restart request packet, respectively.

(b): Applicable to permanent virtual circuits only.

Diagnostic code (RR/RI)

Octet 5 is the diagnostic code and contains additional information on the reason for the reset.

In a reset request packet the diagnostic code is not mandatory.

In a reset indication packet, if the resetting cause field indicates "DTE originated", the diagnostic code has been passed unchanged from the resetting DTE. If the DTE requesting a reset has not provided a diagnostic code in its reset request packet, then the bits of the diagnostic code in the resulting reset indication packet will all be zeros.

When a reset indication packet results from a restart request packet, the value of the diagnostic code will be that specified in the restart request packet, or all zeros in the case where no diagnostic code has been specified in the restart request packet.

When the resetting cause field does not indicate "DTE originated", the diagnostic code in a reset indication packet is network generated. Annex E (ITU) lists the codings for network generated diagnostics. The bits of the diagnostic code are all set to 0 when no specific additional information for the reset is supplied.

Note: The contents of the diagnostic code field do not alter the meaning of the cause field. A DTE is not required to undertake any action on the contents of the diagnostic code field. Unspecified code combinations in the diagnostic code field shall not cause the DTE to not accept the cause field.

Restart request / Restart indication

- Octet 1: mandatory
bits 8-5 General format identifier coded X001 (modulo 8) or X010 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: FB H
- Octet 4: mandatory
bits 8-1 Restarting cause
- Octet 5: mandatory
bits 8-1 Diagnostic code(a)
FIGURE 17/X.25 (ITU)

(a): This field is not mandatory in restart request packets.

Restarting cause field (RaR/RaI)

Octet 4 is the restarting cause field and contains the reason for the restart.

In restart request packets, the restarting cause field should be set by the DTE to one of the following values, where each "x" be independently set to 0 or 1 by the DTE:

bits 8 7 6 5 4 3 2 1
value 0 0 0 0 0 0 0 0
or 1 x x x x x x x

The DCE will prevent values of the restarting cause field, other than those shown above, from reaching the other end of the virtual calls and/or permanent virtual circuits by either accepting the restart request packet and forcing the clearing or resetting cause field to all zeros in the corresponding clear and/or reset indication packets, or considering the restart request as an error and following the procedure described in Annex C (ITU).

The coding of the restarting cause field in the restart indication packets is given in Table 22/X.25 (ITU):

bits 8 _ _ _ _ _ _ 1 Class
0 0 0 0 0 0 0 1 Local procedure error
0 0 0 0 0 0 1 1 Network congestion
0 0 0 0 0 1 1 1 Network operational
0 1 1 1 1 1 1 1 Registration/cancellation confirmed (a)
TABLE 22/X.25 (ITU)

(a): May be received only if the optional on-line facility registration facility is used.

Diagnostic code (RaR/RaI)

Octet 5 is the diagnostic code and contains additional information on the reason for the restart.

In a restart request packet, the diagnostic code is not mandatory. The diagnostic code, if specified, is passed to the corresponding DTEs as the diagnostic code of a reset indication packet for permanent virtual circuits or a clear indication packet for virtual calls.

The coding of the diagnostic code field in a restart indication packet is given in Annex E (ITU). The bits of the diagnostic code are all set to zero when no specific additional information for the restart is supplied.

Note: The contents of the diagnostic code field do not alter the meaning of the cause field. A DTE is not required to undertake any action on the contents of the diagnostic code field. Unspecified code combinations in the diagnostic code field shall not cause the DTE to not accept the cause field.

Restart confirmation (DTE and DCE)

- Octet 1: mandatory
bits 8-5 General format identifier coded 0001 (modulo 8) or 0010 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: FF H
FIGURE 18/X.25 (ITU)

Diagnostic

- Octet 1: mandatory
bits 8-5 General format identifier coded 0001 (modulo 8) or 0010 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: F1 H
- Octet 4: mandatory
bits 8-1 Diagnostic code
- Octets 5-n: mandatory
bits 8-1 Diagnostic explanation.
FIGURE 19/X.25 (ITU)

Diagnostic code field (D)

Octet 4 is the diagnostic code and contains information on the error condition which resulted in the transmission of the diagnostic packet. The coding of the diagnostic code field is given in Annex E (ITU).

Diagnostic explanation field (D)

When the diagnostic packet is issued as a result of the reception of an erroneous packet from the DTE (see Tables C-1/X.25 and C-2/X.25), this field contains the first three octets of header information from the erroneous DTE packet. If the packet contains less than 3 octets, this field contains whatever bits were received.

When the diagnostic packet is issued as a result of a DCE time-out (see Table D-1/X.25), the diagnostic explanation field contains 2 octets coded as follows:

  • bits 8, 7, 6 and 5 of the first octet contain the general format identifier for the interface;
  • bits 4 to 1 of the first octet and bits 8 to 1 of the second octet are all 0 for expiration of time-out T10 and give the number of the logical channel on which the time-out occurred for expiration of time-out T12 or T13.

Registration request

- Octet 1: mandatory
bits 8-5 General format identifier coded 0001 (modulo 8) or 0010 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: F3 H
- Octet 4: mandatory
bits 8-5 DTE address length
bits 4-1 DCE address length
- Octets 5-n: mandatory
bits 8-1 DCE and DTE address(es)
- Octet : mandatory
bit 8 0
bits 7-1 Registration length
- Octet : mandatory
bits 8-1 Registration
FIGURE 21/X.25 (ITU)

Address length fields (RgR)

Octet 4 consists of the field length indicators for the DTE and DCE addresses. Bits 4, 3, 2 and 1 indicate the length of the DCE address in semi-octets. Bits 8, 7, 6 and 5 indicate the length of the DTE address in semi-octets. Each address length indicator is binary coded and bit 1 or 5 is the low order bit of the indicator.

These fields are coded with all zeros under the procedures in this Recommendation.

Address field (RgR)

Octet 5 and the following octets consist of the DCE address, when present, and the DTE address, when present.

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, the address is coded in octet 5 and consecutive octets with two digits per octet. In each octet, the higher order digit is coded in bits 8, 7, 6 and 5.

The address field shall be rounded up to an integral number of octets by inserting zeros in bits 4, 3, 2 and 1 of the last octet of the field when necessary.

This field is not present under the procedures in this Recommendation.

Registration length field (RgR)

The octet following the address field indicates the length of the registration field in octets. The registration length indicator is binary coded and bit 1 is the low order bit of the indicator.

Registration field (RgR)

The registration field is present only when the DTE wishes to request the DCE to agree to, or to stop a previous agreement for, an optional user facility.

The coding of the registration field is defined in § 7.3 Rec. X.25 (ITU).

The registration field contains an integral number of octets. The actual maximum length of this field depends on the network. However, this maximum does not exceed 109 octets.

Registration confirmation

- Octet 1: mandatory
bits 8-5 General format identifier coded 0001 (modulo 8) or 0010 (modulo 128).
bits 4-1 Logical channel group number
- Octet 2: mandatory
bits 8-1 Logical channel number
- Octet 3: mandatory
bits 8-1 Packet type identifier: F3 H
- Octet 4: mandatory
bits 8-1 Cause
- Octet 5: mandatory
bits 8-1 Diagnostic
- Octet 6: mandatory
bits 8-5 DTE address length
bits 4-1 DCE address length
- Octets 7-n: mandatory
bits 8-1 DCE and DTE address(es)
- Octet : mandatory
bit 8 0
bits 7-1 Registration length
- Octet : mandatory
bits 8-1 Registration
FIGURE 22/X.25 (ITU)

Cause field (RgC)

Octet 4 is the cause field and contains the cause of any failure in negotiation of facilities or an indication that the registration field was verified by the DCE.

The coding of the cause field in the registration confirmation packet is shown in Table 23/X.25 (ITU):

bits 8 _ _ _ _ _ _ 1 Class
0 1 1 1 1 1 1 1 Registration/cancellation confirmed
0 0 0 0 0 0 1 1 Invalid facility request
0 0 0 1 0 0 1 1 Local procedure error
0 0 0 0 0 1 0 1 Network congestion
TABLE 23.25 (ITU)

Diagnostic code (RgC)

Octet 5 is the diagnostic code and contains additional information on the reason for failure of facilities negotiation.

Annex E (ITU) lists the coding for diagnostics. The bits of the diagnostic code are all set to 0 when negotiation is successful, or when no additional information is supplied.

Address length fields (RgC)

Octet 6 consists of the field length indicators for the DTE and DCE addresses. Bits 4, 3, 2 and 1 indicate the length of the DCE address in semi-octets. Bits 8, 7, 6 and 5 indicate the length of the DTE address in semi-octets. Each address length indicator is binary coded and bit 1 or 5 is the low order bit of the indicator.

These fields are coded with all zeros under the procedures in this Recommendation.

Address field (RgC)

Octet 7 and the following octets consist of the DCE address, when present, and the DTE address, when present.

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, the address is coded in octet 7 and consecutive octets with two digits per octet. In each octet, the higher order digit is coded in bits 8, 7, 6 and 5.

The address field shall be rounded up to an integral number of octets by inserting zeros in bits 4, 3, 2 and 1 of the last octet of the field when necessary.

This field is not present under the procedures in this Recommendation.

Registration length field (RgC)

The octet following the address field indicates the length of the registration field, in octets. The registration length indicator is binary coded and bit 1 is the low order bit of the indicator.

Registration field (RgC)

The registration field is used to indicate which optional user facilities are available, and which are currently in effect.

The coding of the registration field is defined in § 7.3 Rec. X.25 (ITU).

The registration field contains an integral number of octets. The actual maximum length of this field depends on the network. However, this maximum does not exceed 109 octets.

 

home     Quick start     Specifications     Connections     Features     How to?     Notes     Search     Site Map

updated:  27-Feb-04