ISO/IEC 8208:2000

INTERNATIONAL ISO/IEC
STANDARD 8208
Fourth edition
2000-11-01
Information technology — Data
communications — X.25 Packet Layer
Protocol for Data Terminal Equipment
Technologies de l'information — Communication de données —
Protocole X.25 de couche paquet pour terminal de données
Reference number
©
ISO/IEC 2000
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ISO/IEC 8208 : 2000(E)
CONTENTS
Page
Foreword . vi
1 Scope .1
2 Normative references .1
2.1 Identical Recommendations | International Standards .1
2.2 Paired Recommendations | International Standards equivalent in technical content.2
2.3 Additional references .2
3 General considerations .2
3.1 Compatibility with versions of Recommendation X.25 .3
3.2 Environments.5
3.3 Differences in DTE/DTE and DTE/DCE operation .5
3.4 Operation over circuit-switched connections.6
3.5 Provision of the OSI Network Service.7
3.6 External Packet Layer interactions.7
3.7 Logical channels .7
3.8 Packet Layer entity.8
3.9 Packet types .9
3.10 Procedures for initialization.9
4 Procedures for restart.9
4.1 Originating a restart request.10
4.2 Receiving a restart indication.12
4.3 Restart collision .12
4.4 Restart confirmation. 12
4.5 Determining “DTE” or “DCE” characteristics.12
5 Procedures for Virtual Call setup and clearing .13
5.1 Ready state.13
5.2 Procedures for Virtual Call setup.13
5.3 Rejecting a call.15
5.4 Aborting a call request.15
5.5 Procedures for Virtual Call clearing .15
6 Procedures for data and interrupt transfer.16
6.1 States for data and interrupt transfer .17
6.2 Maximum User Data Field length of DATA packets .17
6.3 Delivery Confirmation bit.17
6.4 More Data mark . 18
6.5 Complete packet sequence .18
6.6 Qualifier bit.18
6.7 Fragmentation and reassembly of messages.19
6.8 Procedures for interrupt .20
6.9 Transit delay of DATA packets.21
7 Procedures for flow control.21
7.1 Flow control.22
7.2 Throughput characteristics and throughput classes.25
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ISO/IEC 8208 : 2000(E)
8 Procedures for reset . . 25
8.1 Originating a reset request .27
8.2 Receiving a reset indication . 27
8.3 Reset collision. 27
8.4 Reset confirmation. 27
9 Effects of clear, reset, and restart procedures on the transfer of packets . 27
10 Effects of Layers 1 and 2 on the Packet Layer. 28
11 Error handling . . 28
11.1 The DIAGNOSTIC packet. 29
11.2 Nonreceipt of window-rotation information . 29
11.3 Receipt of erroneous DATA packets . 30
12 Packet formats. 31
12.1 General. 31
12.2 Call setup and call clearing packets . 33
12.3 DATA and interrupt packets. 42
12.4 Flow control packets. 44
12.5 Reset packets . 45
12.6 Restart packets . 47
12.7 DIAGNOSTIC packet. 48
12.8 REJECT packet. 49
12.9 Registration packets. 50
13 Procedures for optional user facilities . 52
13.1 On-line Facility Registration. 52
13.2 Extended and Super Extended Packet Sequence Numbering . 59
13.3 D-bit Modification. 60
13.4 Packet Retransmission . 60
13.5 Incoming Calls Barred . 61
13.6 Outgoing Calls Barred . 61
13.7 One-way Logical Channel Outgoing. 61
13.8 One-way Logical Channel Incoming . 61
13.9 Nonstandard Default Packet Sizes . 61
13.10 Nonstandard Default Window Sizes . 61
13.11 Default Throughput Classes Assignment. 62
13.12 Flow Control Parameter Negotiation . 62
13.13 Throughput Class Negotiation Facilities. 63
13.14 Closed User Group related facilities . 64
13.15 Bilateral Closed User Group related facilities. 68
13.16 Fast Select. 69
13.17 Fast Select Acceptance . 70
13.18 Reverse Charging. 70
13.19 Reverse Charging Acceptance . 70
13.20 Local Charging Prevention . 70
13.21 Network User Identification (NUI) related facilities. 71
13.22 Charging Information. 71
13.23 ROA related facilities .73
13.24 Hunt Group. 73
13.25 Call Redirection and Call Deflection related facilities. 73
13.26 Called Line Address Modified Notification. 76
13.27 Transit Delay Selection and Indication. 76
13.28 Alternative Addressing Related Facilities. 76
13.29 TOA/NPI address subscription . 78
13.30 Reference Number . 78
14 Procedures for optional ITU-T specified DTE facilities. 80
14.1 Calling Address Extension. 80
14.2 Called Address Extension. 80
14.3 Minimum Throughput Class Negotiation . 80
14.4 End-to-End Transit Delay Negotiation . 81
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ISO/IEC 8208 : 2000(E)
14.5 Priority.81
14.6 Protection.81
14.7 Expedited Data Negotiation.81
15 Format for Facility Field in call setup/clearing packets .82
15.1 General.82
15.2 Coding of the Facility Field for optional user facilities .83
15.3 Coding of the Facility Field for ITU-T specified DTE facilities.89
16 Format for Registration Field in registration packets.92
16.1 General.92
16.2 Coding of the Registration Field for registration-facilities .93
17 Diagnostic codes .95
18 Timers and retransmission counts .101
19 State diagrams.105
20 State tables.111
21 Conformance .120
21.1 Static conformance.120
21.2 Protocol Implementation Conformance Statement .120
21.3 Dynamic conformance .120
Annexes
A  Private networks.123
B  PICS Proforma .131
C  Differences between various editions of ISO/IEC 8208.159
D  Abbreviations.169
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ISO/IEC 8208 : 2000(E)
Foreword
ISO (the International Organization for Standardization) and IEC (the International
Electrotechnical Commission) form the specialized system for worldwide standardization.
National bodies that are members of ISO or IEC participate in the development of
International Standards through technical committees established by the respective
organization to deal with particular fields of technical activity. ISO and IEC technical
committees collaborate in fields of mutual interest. Other international organizations,
governmental and non-governmental, in liaison with ISO and IEC, also take part in the
work.
International Standards are drafted in accordance with the rules given in the ISO/IEC
Directives, Part 3.
In the field of information technology, ISO and IEC have established a joint technical
committee, ISO/IEC JTC 1. Draft International Standards adopted by the joint technical
committee are circulated to national bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the national bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International
Standard may be the subject of patent rights. ISO and IEC shall not be held responsible
for identifying any or all such patent rights.
International Standard ISO/IEC 8208 was prepared by Joint Technical Committee
ISO/IEC JTC 1, Information technology, Subcommittee SC 6, Telecommunications and
information exchange between systems.
This fourth edition cancels and replaces the third edition (ISO/IEC 8208:1995), which
has been technically revised.
Annexes A and B form a normative part of this International Standard. Annexes C and D
are for information only.
 ISO/IEC 2000 – All rights reserved
vi
INTERNATIONAL STANDARD ISO/IEC 8208 : 2000(E)
Information technology  Data communications 
X.25 Packet Layer Protocol for Data Terminal Equipment
It should be noted that this International Standard and ITU-T
1 Scope
Recommendation X.25 as it applies to DTEs are different in
This International Standard specifies the procedures, formats and
scope. This International Standard contains the specifications that
facilities at the Packet Layer for Data Terminal Equipment (DTE)
ITU-T Recommendation X.25 places on DTEs. In addition, this
operating in conformance with ITU-T Recommendation X.25.
International Standard contains added specifications to facilitate
Both Virtual Call and Permanent Virtual Circuit modes of
interworking between DTEs and to cover direct DTE-to-DTE
operation are covered.
operation. This broader scope has to be recognized in the
application of this International Standard.
The Packet Layer protocol specified herein can be used in both
Open Systems Interconnection (OSI) and non-OSI environments.
When used within the context of OSI, the Packet Layer protocol 2 Normative references
is encompassed in the Network Layer of the OSI Reference
The following normative documents contain provisions which,
Model, ITU-T Rec. X.200 | ISO/IEC 7498-1.
through reference in this text, constitute provisions of this
International Standard. For dated references, subsequent
This International Standard covers DTE operation at the Packet
amendments to, or revisions of, any of these publications do not
Layer when accessing a public or private packet-switched
apply. However, parties to agreements based on this International
network conforming to ITU-T Recommendation X.25 by means
Standard are encouraged to investigate the possibility of applying
of a dedicated path or a circuit-switched connection. It also
the most recent editions of the normative documents indicated
covers the additional Packet Layer procedures necessary for two
below. For undated references, the latest edition of the normative
DTEs conforming to this International Standard to communicate
document referred to applies. Members of ISO and IEC maintain
directly (i.e., without an intervening packet-switched network)
registers of currently valid International Standards. The
over a dedicated path, a circuit-switched connection, or a local
Telecommunication Standardization Bureau of the ITU maintains
area network (LAN).
a register of currently valid ITU-T Recommendations.
This International Standard also covers private networks that use
2.1 Identical Recommendations | International
ITU-T Recommendation X.25 to connect to packet-switched
Standards
public data networks and that may also offer an X.25 interface to
a DTE (see annex A).
ITU-T Recommendation X.200 (1994) | ISO/IEC 7498-1 : 1994,
Information technology  Open Systems Interconnection 
To evaluate conformance of a particular implementation, it is
Basic Reference Model: The Basic Model
necessary to have a statement of which capabilities and options
have been implemented. Such a statement is called a Protocol
ITU-T Recommendation X.212 (1995) | ISO/IEC 8886 : 1996,
Implementation Conformance Statement (PICS), as defined in
Information technology  Open Systems Interconnection 
ITU-T Rec. X.290 | ISO/IEC 9646-1. Annex B provides the PICS
Data link service definition
proforma in accordance with the relevant guidance given in ITU-
ITU-T Recommendation X.213 (1995) | ISO/IEC 8348 : 1996,
T Rec. X.296 | ISO/IEC 9646-7.
Information technology  Open Systems Interconnection 
The first edition of this International Standard was based on the
Network service definition
1984 CCITT Red Book text of Recommendation X.25. It also
ITU-T Recommendation X.263 (1998) | ISO/IEC TR 9577:1999,
contained the necessary provisions for compatibility with the
earlier 1980 CCITT Yellow Book text of Recommendation X.25.
Information technology  Protocol identification in the network
The second edition was based on the 1988 CCITT Blue Book layer
text of Recommendation X.25. The third edition is based upon
ITU-T Recommendation X.273 (1994) | ISO/IEC 11577 : 1995,
the 1993 version of X.25. This fourth edition is based on the
Information technology  Open Systems Interconnection 
1996 version of X.25. Retained within this fourth edition are the
Network layer security protocol
necessary provisions for compatibility with the 1993, 1988, 1984
and 1980 versions of X.25. The differences between various
CCITT Recommendation X.612 (1992) | ISO/IEC 9574 : 1992,
editions of this International Standard are summarized in annex
Information technology  Provision of the OSI connection-mode
C.
network service by packet-mode terminal equipment connected to
an integrated services digital network (ISDN)
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ISO/IEC 8208 : 2000(E)
CCITT Recommendation X.613 (1992) | ISO/IEC 10588 : 1993, ITU-T Recommendation X.32 (1996), Interface between data
terminal equipment (DTE) and data circuit-terminating
Information technology  Use of X.25 Packet Layer Protocol in
conjunction with X.21/X.21bis to provide the OSI connection- equipment (DCE) for terminals operating in the packet mode and
accessing a packet switched public data network through a
mode Network Service
public switched telephone network or an integrated services
CCITT Recommendation X.614 (1992) | ISO/IEC 10732 : 1993,
digital network or a circuit switched public data network
Information technology  Use of X.25 Packet Layer Protocol to
ITU-T Recommendation X.75 (1996), Packet-switched
provide the OSI connection-mode Network Service over the
signalling system between public networks providing data
telephone network
transmission services
2.2 Paired Recommendations | International
ITU-T Recommendation X.96 (1993), Call progress signals in
Standards equivalent in technical content
public data networks
− CCITT Recommendation T.50 (1992), International
ITU-T Recommendation X.121 (1996), International numbering
Reference Alphabet (IRA)
plan for public data networks
ISO/IEC 646 : 1991, Information technology  ISO 7-bit
ITU-T Recommendation X.301 (1996), Description of the
coded character set for information interchange
general arrangements for call control within a subnetwork and
− ITU-T Recommendation X.223 (1993), Use of X.25 to
between subnetworks for the provision of data transmission
provide the OSI connection-mode network service for ITU-T
services
applications
CCITT Recommendation X.610 (1992), Provision and support
ISO/IEC 8878 : 1992, Information technology 
of the OSI connection-mode network service
Telecommunications and information exchange between
ISO/IEC 7776 : 1995, Information technology 
systems  Use of X.25 to provide the OSI Connection-mode
Telecommunications and information exchange between systems
Network Service
 High-level data link control procedures — Description of the
− ITU-T Recommendation X.290 (1995), OSI conformance
X.25 LAPB-compatible DTE data link procedures
testing methodology and framework for protocol
ISO/IEC 8881 : 1989, Information processing systems  Data
Recommendations for ITU-T applications  General
communications  Use of the X.25 packet level protocol in local
concepts
area networks
ISO/IEC 9646-1 : 1994, Information technology  Open
ISO/IEC TR 10029 : 1989, Information technology 
Systems Interconnection  Conformance testing
Telecommunications and information exchange between systems
methodology and framework  Part 1: General concepts
 Operation of an X.25 interworking unit
− ITU-T Recommendation X.296 (1995), OSI conformance
ISO/IEC 10039 : 1991, Information technology  Open Systems
testing methodology and framework for protocol
Interconnection  Local area networks  Medium Access
Recommendations for ITU-T applications  Implementation
Control (MAC) service defintion
Conformance Statements
ISO/IEC TR 13532 : 1995, Information technology 
ISO/IEC 9646-7 : 1995, Information technology  Open
Telecommunications and information exchange between systems
Systems Interconnection — Conformance testing
 Protocol combinations to provide and support the OSI
methodology and framework  Part 7: Implementation
network service
Conformance Statements
RFC 1166, Internet numbers, July 1990.
2.3 Additional references
CCITT Recommendation D.12 (1988), Measurement unit for
3 General considerations
charging by volume in the international packet-switched data
This International Standard defines, from the viewpoint of a
communication service
DTE, the Packet Layer, which governs the transfer of packets at a
ITU-T Recommendation X.2 (1996), International data
DTE/DCE or DTE/DTE interface. On the transmitting side, the
transmission services and optional user facilities in public data
Packet Layer in a sending DTE performs the basic function of
networks and ISDNs
packetizing messages delivered by a higher layer entity in the
same DTE before giving the information to the Data Link Layer
ITU-T Recommendation X.25 (1996), Interface between data
for transmission to a DXE. On the receiving side, the Packet
terminal equipment (DTE) and data circuit-terminating
Layer in a DTE performs the basic functions of receiving packets
equipment (DCE) for terminals operating in the packet mode and
from the Data Link Layer, checking packets for correctness,
connected to public data networks by dedicated circuit
stripping off packet headers, and formulating messages from the
ITU-T Recommendation X.29 (1997), Procedures for the
exchange of control information and user data between a packet
assembly/disassembly (PAD) facility and a packet mode DTE or
The term “DXE” is used in those contexts where it would not matter
another PAD
whether a DTE or a DCE was being referred to. Therefore, this
International Standard can be viewed as defining the Packet Layer at the
ITU-T Recommendation X.31 (1995), Support of packet mode
DTE/DXE interface.
terminal equipment by an ISDN
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ISO/IEC 8208 : 2000(E)
packetized user data and passing them to a higher layer entity in c) a negligible packet-loss and duplication rate.
the DTE.
The Packet Layer provides the following functional capabilities
that facilitate reliable and efficient data communications:
This International Standard presents a description of the Packet
Layer for “Virtual Call” service and “Permanent Virtual Circuit”
a) multiplexing — the ability to support multiple
service.
communications;
The following information is presented:
b) data transfer — the ability to send and receive data;
a) general considerations (clause 3);
c) flow control — the ability to control the flow of data;
b) procedures for exchanging packets across a DTE/DXE
d) interrupt transfer — the ability to send and receive a small
interface (clauses 4 through 11). Clause 5 applies to the
amount of information independent of the data stream;
setup and clearing procedures for Virtual Call service,
while the other clauses apply to both Virtual Call service
e) error control — the ability to detect Packet Layer errors;
and Permanent Virtual Circuit service;
f) reset and restart — the ability to reinitialize
c) packet formats (clause 12);
communication paths in the event that Packet Layer errors
are encountered.
d) procedures for optional user facilities that may be available
on a DTE/DXE interface (clauses 13 and 14);
A number of design principles were used in the formulation of
the Packet Layer procedures for DTEs specified in this
e) formats for optional user facilities and registration-facilities
International Standard:
(clauses 15 and 16, respectively);
a) conform fully to Recommendation X.25 for operation with
f) coding of the Diagnostic Code Field (clause 17);
a packet-switched network;
g) timers and retransmission counts (clause 18);
b) minimize the differences between operating with a packet-
h) state diagrams and state tables (clauses 19 and 20, switched network and operating directly with another DTE;
respectively);
c) provide, where possible, the opportunity for recovery from
i) conformance requirements (clause 21); an error condition without incurring data loss at the Packet
Layer;
j) applications of this International Standard to private
networks that connect to a packet-switched public data d) align the services provided by the Packet Layer with the
network and that may also offer an X.25 interface to a DTE Network Layer services defined for Open Systems
(annex A); and Interconnection; and
k) the PICS proforma (annex B). e) generally follow the organization of text in
Recommendation X.25.
To facilitate comprehension of this International Standard, a
number of conventions have been adopted in the presentation of
3.1 Compatibility with versions of Recommendation
the text:
X.25
The Packet Layer procedures and formats specified in this
a) the names of states and packets are in full capitals;
International Standard are compatible with the 1996 version of
b) the names of the optional user facilities, packet fields,
ITU-T Recommendation X.25.
causes and diagnostics are in initial capitals;
NOTES
c) italicized text is used to denote differences between Virtual
1 Although the On-line Facility Registration optional user facility has
Call and Permanent Virtual Circuit service and between
been deleted in the 1996 version of Recommendation X.25, it is retained
DTE/DTE and DTE/DCE interfaces (entire clauses or
in this International Standard for backward compatibility with the 1993,
subclasses that pertain to one service or to one interface
1988 and 1984 versions of Recommendation X.25 and for DTE/DTE
type are not italicized; the appropriate environment is
operation.
denoted at the beginning of the clause or subclause);
2 Annex C summarizes the differences between the various editions of
d) terms not explicitly defined within this International
this International Standard.
Standard are taken from the referenced ITU-T X-series
For DTEs needing to operate with the earlier versions of
recommendations.
Recommendation X.25, the following restrictions apply.
e) abbreviations are listed in annex D.
3.1.1 Limitations for compatibility with X.25-1993
The Packet Layer procedures in this International Standard are
For DTEs needing to operate with the 1993 version of
based on an underlying service (for example, that provided by
Recommendation X.25, the following 1996 capabilities are not
ISO/IEC 7776 or, more generally, the provision of the OSI Data
used:
Link Service defined in ITU-T Rec. X.212 | ISO/IEC 8886) that
provides: a) super extended (modulo 32 768) packet sequence
numbering; for 1993 operation only normal (modulo 8)
a) a negligible residual-bit-error rate;
and extended (modulo 128) packet sequence numbering is
supported (see 7.1.3);
b) a negligible out-of-sequence rate; and
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ISO/IEC 8208 : 2000(E)
b) expanded addressing capabilities with the A-bit = 1
3.1.3 Limitations for compatibility with X.25-1984
(TOA/NPI format); for 1993 operation only the Alternative
For DTEs needing to operate with the 1984 (Red Book) version
address capability is supported (see 12.2);
of Recommendation X.25, the following 1988 capabilities are not
used in addition to those cited in 3.1.1:
c) the following optional user facilities
a) expanded capabilities for the following optional user
 Super Extended Packet Sequence Numbering (see
facilities
13.2), and
 Network User Identification (NUI) related facilities
 TOA/NPI address Subscription (see 13.29);
(see 13.21),
for 1993 operation, these facilities were not defined.
 RPOA related facilities (see 13.23), and
3.1.2 Limitations for compatibility with X.25-1988
 Call Redirection and Call Deflection related facilities
For DTEs needing to operate with the 1988 (Blue Book) version
(see 13.25);
of Recommendation X.25, the following 1993 capabilities are not
used: for 1984 operation, Call Deflection and NUI Override
were not defined and the NUI and RPOA facilities were not
a) expanded format of the Address Fields; for 1988
explicitly separated into subscription and negotiation
operation, only the Address Block with the A-bit = 0 is
facilities;
permitted (see 12.2);
b) the following CCITT-specified DTE facilities
b) Facility Fields in CALL REQUEST, INCOMING CALL,
CALL ACCEPTED, CALL CONNECTED, CLEAR
 Priority (see 14.5), and
REQUEST, CLEAR INDICATION, and CLEAR
 Protection (see 14.6);
CONFIRMATION, packets with a length from 109 to 255
octets (see 12.2.2, 12.2.3.1, 12.2.4.1, 12.2.5.2 and
for 1984 operation, the above facilities were not defined;
12.2.6.2); for 1988 operation, this field is limited to 109
c) coding of the following CCITT-specified DTE facilities
octets.
were modified
c) the following optional user facilities
 Calling Address Extension (see 15.3.2.1), and
 Alternative Addressing related facilities (see 13.28);
 Called Address Extension (see 15.3.2.2);
for 1988 operation, these facilities were not defined;
for 1984 operation, only BCD encoding of the addresses is
d) expanded capabilities for the following optional user
permitted; and
facilities
d) the throughput class of 64 000 bit/s; for 1984 operation the
 Throughput Class Negotiation facilities (see 13.13),
largest throughput class is 48 000 bit/s.
and
3.1.4 Limitations for compatibility with X.25-1980
 Call Redirection and Call Deflection related facilities
For DTEs needing to operate with the 1980 (Yellow Book)
(see 13.25);
version of Recommendation X.25, the following 1984
capabilities are not used in addition to those cited in 3.1.2:
for 1988 operation, the Extended Throughput Class
Negotiation Facility and the Inter-network Call
a) maximum User Data Field lengths in DATA packets of
Redirection and Deflection (ICRD) control facilities were
2 048 and 4 096 octets (see 6.2); for 1980 operation, the
not defined;
largest maximum User Data Field length allowed is 1 024
e) coding of the following CCITT-specified DTE facilities octets;
(renamed “ITU-T specified DTE facilities” in the 1993
b) Facility Fields in CALL REQUEST, INCOMING CALL,
version of Recommendation X.25) were modified
CALL ACCEPTED, and CALL CONNECTED packets with
a length from 64 to 109 octets (see 12.2.3.1 and 12.2.4.1);
 Minimum Throughput Class Negotiation (see
for 1980 operation, this field is limited to 63 octets and bit
15.3.2.3);
7 of the Facility Length Field shall be set to 0;
for 1988 operation, only the basic format existed; and
c) cause codes with bit 8 set to one in CLEAR
f) the throughput classes of 128 000 bit/s and 192 000 bit/s
REQUEST/INDICATION, RESET
(in the basic format) and throughput classes from 256 000
REQUEST/INDICATION, and RESTART
bit/s up to, and including, 2 048 000 bit/s (in the extended
REQUEST/INDICATION packets (see 12.2.5.1.1,
format); for 1988 operation, the largest throughput class is
12.5.1.1, and 12.6.1.1, respectively); for 1980 operation,
64 000 bit/s.
this bit shall be set to zero;
It should also be noted that the term “RPOA” meaning
d) nonzero Address Length and Facility Length Fields in
Recognized Private Operating Agency used in 1988 and earlier
CLEAR REQUEST and CLEAR INDICATION packets (see
versions of Recommendation X.25 has been replaced by the term
12.2.5.2); for 1980 operation, these length fields shall
“ROA” meaning Recognized Operating Agency in the 1993
indicate zero octets and may only be present when the
version of Recommendation X.25.
packet contains a Clear User Data Field;
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ISO/IEC 8208 : 2000(E)
e) the extended format for CLEAR CONFIRMATION packets 1980 operation, these facilities and the marker cannot be
(see 12.2.6.2); for 1980 operation, only the basic format used.
may be used;
3.2 Environments
f) Interrupt User Data Fields in INTERRUPT packets
The DTE aspects of the Packet Layer protocol set forth in this
containing from two to 32 octets (see 12.3.2); for 1980
International Standard are applicable to a number of
operation, this field shall contain exactly one octet;
environments including:
g) the following optional user facilities:
a) DTE/DCE operation:
 On-line Facility Registration (see 13.1),
 DTE access to a DCE via a dedicated path,
 Local Charging Prevention (see 13.20),
 DTE access to a DCE via a circuit-switched connection
(circuit-switched data network, circuit-switched
 Network User Identification (see 13.21),
capability of an Integrated Services Digital Network
 Charging Information (see 13.22),
(ISDN), or the switched telephone network).
Additional considerations are given in 3.4.
 Hunt Group (see 13.24),
NOTES
 Call Redirection and Call Deflection Notification (see
13.25), 1 The situation where the “DTE” is a private network
accessing a public network DCE is covered in annex A.
 Called Line Address Modified Notification (see 13.26),
2 The DCE may be a packet-switched data network operating
and
in accordance with Recommendation X.25 or a packet
handler capability in an ISDN operating in accordance with
 Transit Delay Selection and Indication (see 13.27);
Recommendation X.31.
for 1980 operation, the above facilities cannot be used;
b) DTE/DTE operation:
h) expanded capabilities for the following optional user
 DTE-to-DTE operation over a leased line (data
facilities:
network, ISDN or telephone network),
 Closed User Groups (CUG): subscription to the Closed
 DTE-to-DTE operation over a circuit-switched
User Group With Outgoing and/or Incoming Access
connection (circuit-switched data network, circuit-
Facilities without a preferential CUG (see 13.14.2 and
switched capability of an ISDN, or the switched
13.14.3, respectively), use of the extended format of the
telephone network). Additional considerations are
CUG Selection Facility for indicating membership in
given in 3.4,
more than 100 CUGs (see 13.14.6), and the use of the
Closed User Group With Outgoing Access (CUG/OA)
 DTE-to-DTE operation over a Local Area Network
Selection Facility (see 13.14.7); for 1980 operation, all
(LAN). The provisions of ISO/IEC 8881 apply.
CUG subscriptions shall specify a preferential CUG,
only the basic format of the CUG Selection Facility is NOTE 3 — The situation where a “DTE” is a gateway on
the LAN to other networks is covered in annex A.
allowed for indicating membership in 100 or less
CUGs, and the CUG/OA Selection Facility cannot be
Differences between DTE/DCE and DTE/DTE operation are
used,
enumerated in 3.3.
 Fast Select and Fast Select Acceptance (see 13.16 and
3.3 Differences in DTE/DTE and DTE/DCE operation
13.17, respectively): inclusion of a Clear User Data
For the most part, much of the Packet Layer protocol described
Field in CLEAR REQUEST and CLEAR INDICATION
herein is independent of whether the DTE is connected to a DCE
packets after call setup has been completed; for 1980
(e.g., X.25 network environment) or directly to another DTE.
operation, the above packets can contain a Clear User
However, there are certain procedures within Recommendation
Data Field only when sent or received in direct
X.25 that are not mandatory of a DTE but are required in a
response to an INCOMING CALL or a CALL
DTE/DTE environment. To minimize the number of differences
REQUEST packet, respectively, and
that arise when considering whether connection is to a DCE or to
 RPOA Selection (see 13.23): use of the extended format
another DTE, the following procedures are always required of a
of the RPOA Selection Facility to select one or more
DTE:
RPOAs, and agreement for a period of time with the
a) the Address Length Fields and the Facility Length Field
DCE to a set of RPOAs to pertain to all CALL
shall be supplied in CALL ACCEPTED packets even if
REQUEST packets; for 1980 operation, a DTE wishing
they indicate that no address and facility information,
to select an RPOA can only do so in a CALL
respectively, are present;
REQUEST packet and can only use the basic format of
the RPOA Selection Facility to select a single RPOA;
b) the Diagnostic Code Field in RESTART REQUEST,
and
CLEAR REQUEST, and RESET REQUEST packets shall
be supplied even if it indicates “No Additional
i) the CCITT-specified DTE facilities and the associated
Information” (that is, although specific diagnostics are
facility marker (see clause 14 and 15.1, respectively); for
 ISO/IEC 2000 – All rights reserved 5

ISO/IEC 8208 : 2000(E)
defined for particular error situations, a DTE may use more REGISTRATION REQUEST packets and requires the
general codes as discussed in note 1 of table 25); responding DTE to process received REGISTRATION
REQUEST packets. (In a DTE/DCE environment, a DTE
c) a DATA packet whose User Data Field is less than the
will not receive a REGISTRATION REQUEST packet.)
maximum allowed and which has its D-bit set to 0 and M-
bit set to 1 shall not be transmitted; and g) Use of the optional Packet Retransmission Facility (see
13.4) requires agreement for each direction of transmission
d) upon notification that the Data Link Layer has completed
of DATA packets. That is, for a given direction of
its initialization procedures or that it has recovered from a
transmission of DATA packets, agreement to use this
failure in which the Data Link Layer was in the
facility permits the destination DTE to transmit REJECT
disconnected phase, the DTE shall transmit a RESTART
packets and requires the source DTE to process received
REQU
...