ISO/IEC 8208:1990

INTERNATIONAL ISO/IEC
STANDARD
Second edition
1990-03-1 5
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 8208 : 1990 (E)
.
ISOAEC 8208 : 1990 (E)
Page
CONTENTS
V
Foreword .
1 Scope .
2 Normative references .
3 General considerations .
3.1 Compatibility with versions of CClTï Recommendation X.25 .
3.2 Environments .
33 Differences in DTE/DTE and DTE/DCE operation . 4
3.4 Operation over circuit-switched connections .
3.5 Provision of the OS1 Network Service .
3.6 External Packet Layer interactions .
3.7 Logical channels .
3.8 Packet Layer entity .
3.9 Packet types . 7
3.10 Procedures for initialization .
4 Procedures for restart . 9
4.1 Originating a restart request . 9
4.2 Receiving a restart indication . 11
43 Restart collision . 11
4.4 Restart confirmation . 11
Determining “DTE or “DCE characteristics . 11
5 Procedures for Virtual Call setup and clearing . 12
5.1 Ready state . 12
5.2 Proc+Ves for Vial Call setup . . . : . 12
5.3 . Rejecting a call . : . . 15
5.4 Aborting a call request . 15
Procedures for Virtual Call clearing . 15
6 Procedures for data and interrupt transfer . 16
6.1 States for data and interrupt transfer .
6.2 Maximum User Data Field length of DATA packets . 17
63 Delivery Confirmation bit . 17
6.4 More Data mark .
65 Complete packet sequence .
6.6 Qualifier bit .
6.7 Fragmentation and reassembly of messages . 20
68 Procedures for interrupt .
6.9 Transit delay of DATA packets . 22
*
Q ISO/IEC 1990
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7 Procedures for flow control . 22
7.1 Flow control . 22
7.2 Throughput characteristics and throughput classes . 25
8 Procedures for reset . 26
8.1 Originating a reset request . 26
8.2 Receiving a reset indication . 26
83 Reset collision . 28
8.4 Reset confirmation . 28
9 Effects of clear, reset, and restart procedures on the transfer of packets . 28
10 Effects of Layers 1 and 2 on the Packet Layer . 28
11 Error handling . 29
11.1 The DIAGNOSTIC packet . 29
11.2 Nonreceipt of window-rotation information . 30
113 Receipt of erroneous DATA packets . 31
12 Packet formats . 31
12.1 General . 31
12.2 Call setup and call clearing packets . 33
123 DATA and interrupt packets . 40
12.4 Flow control packets . 41
12.5 Reset packets . 42
12.6 Restart packets . 44
12.7 DIAGNOSTIC packet . 45
12.8 REJECT packet . 46
12.9 Registration packets . 47
13 Procedures for optional user facilities . 48
13.1 On-line Facility Registration . 48
13.2 Extended Packet Sequence Numbering . 56
133 D-bit Modification . 56
13.4 Packet Retransmission . 57
13.5 Incoming Calls Barred . 58
13.6 Outgoing Calls Barred . 58
13.7 One-way Logical Channel Outgoing . 58
13.8 One-way Logical Channel Incoming . 58
13.9 Nonstandard Default Packet Sizes . 58
13.10 Nonstandard Default Window Sizes . 58
13.11 Default Throughput Classes Assignment . 59
13.12 Flow Control Parameter Negotiation . 59
13.13 Throughput Class Negotiation .
13.14 Closed User Group related facilities . 61
13.15 Bilateral Closed User Group related facilities . 66
13.16 Fast Select . 67
13.17 Fast Select Acceptance . 67
13.18 Reverse Charging . 68
13.19 Reverse Charging Acceptance . 68
13.20 Local Charging Prevention . 68
13.21 Network User Identification (NUI) related facilities . 68
13.22 Charging Information . 69
13.23 RPOA related facilities . 69
13.24 Hunt Group . 70
13.25 Cali Redirection and Call Deflection related facilities . 71
13.26 Called Line Address Modified Notification . 72
13.27 Transit Delay Selection and Indication . 73
14 Procedures for optional CCXTT-specified DTE facilities . 73
14.1 Calling Address extension . 73
14.2 Called Address Extension . 73
143 Minimum Throughput Class Negotiation .
iii
ISO/IEC 8208 : 1990 (E)
Foreword
IS0 (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) together form a system for worldwide standardization as a whole. National bodies that are
members of IS0 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. IS0
and IEC technical committees collaborate in fields of mutual interest. Other international organizations,
governmental and non-governmental, in liaison with IS0 and IEC, also take part in the work.
In the field of information technology, IS0 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 approval before their acceptance as International Standards. They an approved in accordance
with procedures requiring at least 75% approval by the national bodies voting.
Intemational Standard ISO/iEC 8208 was prepared by Technical Committee ISOWC JTC 1, Information
technology.
This second edition cancels and replaces the first edition (IS0 8208:1987).
Amex A forms an integral part of ISO/IEC 8208. Annex B is for information only.
V
INTERNATIONAL STANDARD ISOAEC 8208 : 1990 (E)
Information technology ~ - I---_ Data communications -
Protocol) for Data Terminal Equipment
~ "---
i
1 scope 2 Normative references
The following standards and recommendations contain
This International Standard specifies the procedures, formats
and facilities at the Packet Layer for Data Terminal Equipment provisions which, through reference in this text, constitute
(DTE) operating in conformarice with CClTï Recommendation of
provisions of this Intemational Standard. At the time
X.25. Both Virtual Call and Permanent Virtual Circuit modes publication, the editions indicated were valid. All standards
ration are covered. and recommendations are subject to revision, and parties to
i ~~ *A" " - II.
agreements based on this International Standard are encouraged
_erein\can be used in both
to investigate the possibility of applying the most recent
(OSI) and non-OS1
editions of the standards and recommendations listed below.
environments. When used within the context of OSI, the
Members of IEC and IS0 maintain registers of currently valid
Packet Layer protocol is encompassed in the Network Layer of
International Standards. The CClTï secretariat maintains a
the OS1 Reference Model, IS0 7498.
register of currently valid CClTï Recommendations.
This International Standard covers DTE operation at the Packet
Layer when accessing a public or private packet-switched IS0 7498 : 1984, I@ormation processing systems - Open
network conforming to CCITT Recommendation X.25 by Systems Interconnection - Basic Reference Model (see also
means of a dedicated path or a circuit-switched connection. It
CCm Recommendation X.200, CCITï Blue Book, 1988).
also covers the additional Packet Layer procedures necessary
for two DTEs conforming to this International Standard to IS0 7776 : 1986, Information processing systems - Data
communicate directly (i.e., without an intervening packet- communications - High-level data link control procedures -
switched network) over a dedicated path, a circuit-switched
Description of the X.25 LAPB-compatible DTE data link
connection, or a local area network (LAN). procedures.
This International Standard also covers private networks that IS0 8348 : 1987, Information processing systems - Data
use CCITT Recommendation X.25 to connect to packet- communications - Network service definition (see also CCITl
switched public data networks and that may also offer an X.25 Recommendation X.213, CCITT Blue Book, 1988).
interface to a DTE (see annex A).
IS0 8348lAdd. 2 : 1988, Information processing systems -
The first edition of this International Standard was based on the
Data communications - Network service definition -
1984 CCITT Red Book text of Recommendation X.25. It also
Addendum 2: Network layer addressing (see also CCITT
contained the necessary provisions for compatibility with the
Recommendation X.213, CCUT Blue Book, 1988).
earlier 1980 CClTï Yellow Book text of Recommendation
X.25. This second edition is based on the 1488 CCl'lT Blue
IS0 8348lAdd. 3 : 1988, Information processing systerny -
Book text of Recommendation X.25. Retained within this
Data communications - Network service definition -
second edition are the necessary provisions for compatibility
Addendum 3: Additional features of the network service (see
with the 1984 and 1980 versions of X.25. The differences
also CCITï Recommendation X.213, CCITï Blue Book, 1988)
between the first and second editions of this International
Standard are summarized in annex B. IS0 8878 : 1987, Information processing systems - Daid
communications - Use of X.2.5 to provide the OS1 connection-
It should be noted that this International Standard and CClTï
mode network service (see also CC'ITT Recommendation X.223,
Recommendation X.25 as it applies to DTEs are different in
CCITï Blue Book, 1988).
scope. This International Standard contains the specifications
that Recommendation X.25 places on DTEs. In addition, this IS0 8880-2 : 1990, Information technology - Dacl
International Standard contains added specifications to facilitate
communications - Protocol combinations to provide ami
interworking between DTEs and to cover direct DTE-to-DTE
support the OS1 network service - Part 2: Provision and
operation. This broader scope has to be recognized in the support of the connection-mode network service.
application of this International Standard.
ISO/IEC 8881 : 1989, Information processing systems - Duta
communications - Use of the X.25 packet level protocol in
local area networks.
ISO/IEC 8208 : 1990 (E)
Link Layer protocol for transmission to a DXE. On the
ISO/iEC 8886 : 1990, Information technology - Data
receiving side, the Packet Layer in a DTE performs the basic
communications - Data link service definition for Open
functions of receiving packets from the Data Link Layer,
Systems Interconnection (see also CCIlT Recommendation
checking packets for correctness, stripping off packet headers,
X.212, CCIIT Blue Book, 1988).
and formulating messages from the packetized user data and
ISO/reC 9574 : 1989, Information technology -
passing them to a higher layer entity in the DTE.
Telecommunications and information exchange between systems
This International Standard presents a description of the Packet
- Provision of the OSI connection-male network service by
Layer for "Virtual Call" service and "Permanent Virtual
packet mode terminal equipment connected to an Integrated
Service Digital Network (ISDN). Circuit" service.
The following information is presented
ISO/iEC TR 10029 : 1989. Information technology -
Telecommunications and information exchange between systems
3);
general considerations (clause
- Operation of an X.25 intenvorking unit.
procedures for exchanging packets across a DTEDXE
interface (clauses 4 through 11). Clause 5 applies to
CCTï Recommendation D.12, Measurement unit for charging
the setup and clearing procedures for Virtual Call
by volume in the international packet-switched data
service, while the other clauses apply to both Virtual
transmission service, CCLT Blue Book, 1988.
Call service and Permanent Virtual Circuit service.;
CCïïï Recommendation X.2, International data transmission
packet formats (clause 12);
services and optional user facilities in public &ta network and
ISDNs, CClïT Blue Book, 1988.
procedures for optional user facilities that may be
CCïïï Recommendation X.25, Interface between data terminal available on a DTEDXE interface (clauses 13 and 14);
equipment (DTE) and data circuit-terminating equipment
formats for optional user facilities and registration-
(DCE) for terminals operating in the packet mode and
facilities (clauses 15 and 16, respectively);
connected to public data networks by dedicated circuit, CCIlT
coding of the Diagnostic Code Field (clause 17);
Blue Book, 1988.
timers and retransmission counts (clause 18);
CCïïï Recommendation X.29, Procedures for the exchange of
control information and user data between a packet
state diagrams and state tables (clauses 19 and 20,
assemblyldisassembly (PAD) facility and a packet mode DTE or
respectively); and
another PAD, CCIIT Blue Book, 1988.
applications of this International Standard to private
CCïïï Recommendation X.31, Support of packet mode
networks that connect to a packet-switched public data
terminal equipment by an ISDN, CCIlT Blue Book, 1988.
network and that may also offer an X.25 interface to a
CCïïï Recommendation X.32, Interface between data terminal DTE (see annex A).
equipment (DTE) and data circuit-terminating equipment
To facilitate comprehension of this International Standard, a
(DCE) for terminals operating in the packet mode and
number of conventions have been adopted in the presentation of
accessing a packet switched public data network through a
the text:
public switched telephone network or an integrated services
digital network or a circuit switched public data network, the names of states and packets are in full capitals;
a)
CClïT Blue Book, 1988.
the names of the optional user facilities, packet fields,
b)
CCïïï Recommendation X.96, Call progress signals in public causes and diagnostics are in initial capitals;
data networkr, CCIIT Blue Book, 1988.
italicized tat is used to denote differences between
c)
CCI" Recommendation X.244. Procedure for the exchange of Virtual Call and Permanent Virtual Circuit service and
protocol identification during virtual call establishment on between DTEDTE and DTE/DCE interfaces (entire
packet-switched public data networks, CCITT Blue Book, 1988. clauses or subclauses that pertain to one service or to
one interface type are not italicized; the appropriate
3 General considerations environment is denoted at the beginning of the clause
or subclause);
This International Standard defines, from the viewpoint of a
DTE, the Packet Layer, which governs the transfer of packets terms not explicitly defined within this International
at a DTEDCE or DTE/DTE interface.') On the transmitting Standard are taken from the referenced CClTï X-series
side, the Packet Layer in a sending DTE performs the basic recommendations.
function of packetizing messages delivered by a higher layer
The Packet Layer procedures in this International Standard are
entity in the same DTE before giving the information to a Data
based on an underlying service (for example, that provided by
1) The term "DXE" is used in those contcxta whm it wmid not maner whether a DTE or a DCE was being rcfd to. Therefore, this International Standard can hL
viewed as defining the Packs hycr at the iYïEDXE inmfaffi.
ISO/IEC 8208 : 1990 (E)
expanded capabilities for the following optional user
IS0 7776 or, more generally, the provision of the OS1 Data
a)
Link Service defined in ISO/IEC 8886) that provides: facilities
a) a negligible residual-bit-error rate; - Network User Identification (NUI) related facilities
(see 13.21),
b) a negligible out-of-sequence rate; and
- RPOA related facilities (see 13.23), and
c) a negligible packet-loss and duplication rate.
- Call Redirection and Call Deflection related
The Packet Layer provides the following functional capabilities
facilities (see 13.25);
that facilitate reliable and efficient data communications:
for 1984 operation, Call Deflection and NUI Override
a) multiplexing - the ability to support multiple
were not àefined and the NUI and RPOA facilities
communications;
were not explicitly separated into subscription and
negotiation facilities;
b) data transfer - the ability to send and receive data;
the following CCITT-specifred DTE facilities
c) flow control - the ability to control the flow of data;
b)
- Priority (see 14.5). and
d) interrupt transfer - the ability to send and receive a
small amount of information independent of the data
- Protection (see 14.6);
stream;
for 1984 operation the above facilities were not
error control - the ability to detect Packet Layer
e)
defined;
errors;
coding of the following CCR"T-.vpecifred DTE facilities
e)
f) reset and restart - the ability to reinitialize
were modifred
communication paths in the event that Packet Layer
errors are encountered. - Called Address Extension (see 15.3.2.1), ami
A number of design principles were used in the formulation of - Calling Address Extension (see 15.3.2.2);
the Packet Layer procedures for DTEs specified in this
for 1984 operation only BCD encoding of the
Intemational Standard
addresses is permitted.
a) conform fully to CCITT Recommendation X.25 for
the throughput class of 64 O00 bids; for 1984 operation
d)
operation with a packet-switched network;
the largest throughput class is 48 O00 bids.
b) minimize the differences between operating with a
3.1.2 Limitations for compatibility with X.25-1980
packet-switched network and operating directly with
another DTE; For DTEs needing to operate with the 1980 (Yellow Book)
version of Recommendation X.25, the following 1984
c) provide, where possible, the opportunity for recovery
capabilities are not used in addition to those cited in 3.1.1:
from an error condition without incurring data loss at
the Packet Layer; maximum User Data Field lengths in DATA packets of
a)
2 048 and 4 096 octets (see 6.2); for 1980 operation,
align the services provided by the Packet Layer with
d)
the largest maximum User Data Field length allowed is
the Network Layer services defined for ûpen Systems
1 024 octets;
Interconnection; and
b) Facility Fields in CU REQUmT, INCOMING
e) follow the organization of text in CCI7T
CALL, CALL ACCEPTED, and CALL CONNECTED
Recommendation X.25.
packets with a length from 64 to 109 octets (see
3.1 Compatibility with versions of CCITT 12.2.1.1 and 12.2.2.1); for 1980 operation, this field is
limited to 63 octets and bit 7 of the Facility Length
Recommendation X.25
Field shall be set to 0;
The Packet Layer procedures and formats specified in this
International Standard are compatible with the 1988 (Blue c) cause codes with bit 8 set to ono in CLEAR
Book) version of CCIlT Recommendation X.25. REQUESTIINDICATION. RESET
REQUESTANDIC ATION, and RESTART
NOTE - The TOANPI capability introduced into the 1988 version of
REQUEST/INDICATION packets (see 12.2.3.1.1,
CCITï Recommendation X.25 is not included in this Intemationai
12.5.1.1, and 12.6.1.1, respectively); for 1980
Standard since CCITï has designated it for further study.
operation, this bit shall be set to zero;
For DTEs needing to operate with the earlicr versions of
d) rwnzero Address Length and Facilio Length Fields in
Recommendation X.25, the following restrictions apply.
CLEAR REQUEST and CLEAR INDiCA'I'ION packets
3.1.1 Limitations for compatibility with X.25-1984
(see 12.23.2); for 1980 operation, these length frelds
shall indicate zero octets and may only be present
For DTEs needing to operate with the 1984 (Red Book) version
when the packet contains a Clear User Data Field;
of Recommendation X.25, the following 1988 capabilities are
not used:
ISO/IEC 8208 : 1990 (E)
the extended format for CLEAR CONFIRMATION
i) the CCm-specified DTE facilities and the associated
packets (see 122.4.2); for 1980 operation. only the facility marker (see clause 14 and 15.1, respectively);
basic format may be used; for 1980 operation, these facilities and the marker
cannot be used.
Interrupt User Data Fields in INTERRUPT packets
containing from two to 32 octets (see 12.3.2); for 1980 3.2 Environments
operation, this field shall contain exactly one octet;
The DTE aspects of the Packet Layer protocol set forth in this
the following optional user facilities: International Standard are applicable to a number of
environments including:
- On-line Facility Registration (see 13.1),
a) DTE/DCE operation:
- Local Charging Prevention (see 13.20).
- DTE access to a DCE via a dedicated path,
- Network User Identification (see 13.21),
- DTE access to a DCE via a circuit-switched
- Charging IMormation (see 13.22),
connection (circuit-switched data network, circuit-
- Hunt Group (see 13.24), switched capability of an Integrated Services
Digital Network (ISDN), or the switched telephone
- Call Redirection and Call Dejection Notification
network). Additional considerations are given in
(see 1325),
3.4.
- Called Line Address Modified Notification (see
13.26), and NOTES
1 The situation where the "DW is a private network
- Transit Delay Selection and Indication (see 13.27);
accessing a public network DCE is covered in annex A.
for 1980 operation, the above facilities cannot be used,
2 The DCE may be a packet-switched data network operating
expanded capabilities for the following optional user
in accordance with CClTï Recommendation X.25 or a packet
facilities: handler capability in an ISDN operating in accordance with
CCITT Recommendation X.3 1.
- Closed User Groups (CUG): subscription to the
Closed User Group With Outgoing andior
b) DTEDTE operation:
Incoming Access Facilities without a preferential
- DTE-to-DTE operation over a leased line (data
CUG (see 13.14.2 and 13.143, respectively), use of
network, ISDN or telephone network),
the extended format of the CUG Selection Facility
for indicating membership in more than 100 CUGs
-
DTE-to-DTE operation over a circuit-switched
(see 13.14.6), and the use of the Closed User
connection (circuit-switched data network, circuit-
Group With Outgoing Access (CUGIOA) Selection
switched capability of an ISDN, or the switched
Facility (see 13.14.7); for 1980 operation, all CUG
telephone network). Additional considerations are
subscriptions shall specify a preferential CUG, only
given in 3.4,
the basic format of the CUG Selection Facility is
- DTE-to-DTE operation over a Local Area Network
allowed fir indicakg membership in 100 or kss
(LAN). The provisions of ISO/iEC 8881 apply.
CUGs, and the CUGIOA Selection Facility cantwt
be used,
NOTE 3 - The situation where a "DTE" is a gateway
on the LAN to other networks is covered in annex A.
a
- Fast Select and Fast Select Acceptance (see 13.16
and 13.17, respectively): inclusion of a Clear User
Differences between DTE/DCE and DTE/DTE operation are
Data Field in CLEAR REQUEST and CLEAR
enumerated in 3.3.
INDICATION packets afier call setup has been
3.3 Differences in DTEDTE and DTWDCE
completed; for 1980 operation, the above packets
operation
can contain a Clear User Data Field only when
sent or received in direct response to an
For the most part, much of the Packet Layer protocol described
INCOMING CALL or a CALL REQUEST packet,
hcrcin is independent of whether the DTE is connected to a
respectively, and
DCE (e.g., X.25 network environment) or directly to another
DTE. However, there are certain procedures within CCïïï
- RPOA Selection (see 13.23): use of the extended
Recommendation X.25 that are not mandatory of a DTE but are
format of the RPOA Selection Facility to select one
required in a DTE/DTE environment. To minimize the number
or more RPOAs, and agreement for a period of
of differences that arise when considering whether connection
time with the DCE to a set of RPOAs to pertain to
is to a DCE or to another DTE, the following procedures are
all CALL REQUEST packets; for 1980 operation, a
always required of a DTE:
DTE wishing to select an RPOA can only do so in
a CALL REQUEST packet and can only use the
a) the Address Length Fields and the Facility Length
basic format of the RPOA Selection Facility to
Field shall be supplied in CALL ACCEPTED packets
select a single RPOA; and
even if they indicate that no address and facility
information, respectively, are present;

ISO/IEC 8208 : 1990 (E)
f) Use of the optional On-line Facility Registration
b) the Diagnostic Code Field in RESTART REQUEST,
Facility (see 13.1) requires agreement for each
CLEAR REQUEST, and RESET REQUEST packets
direction of registration-procedure initiation. That is,
shall be supplied even if it indicates "No Additional
for a given direction of registration-procedure
Information" (that is, although specific diagnostics are
initiation, agreement to use this facility permits the
defined for particular error situations, a DTE may use
more general codes as discussed in note 2 of table 25); initiating DTE to transmit REGISTRATION
REQUEST packets and requires the responding DTE to
a DATA packet whose User Data Field is less than the
c)
process received REGISTRATION REQUEST packets.
maximum ailowed and which has its D-bit set to O and
(In a DTE/DCE environment, a DTE will not receive a
M-bit set to 1 shall not be transmitted; and
REGISTRATION REQUEST packet.)
d) upon notification that the Data Link Layer has
g) Use of the optional Packet Retransmission Facility (see
completed its initialization procedures or that it has
13.4) requires agreement for each direction of
recovered from a failure in which the Data Link Layer
transmission of DATA packets. That is, for a given
was in the disconnected phase, the DTE shall transmit
direction of transmission of DATA packets, agreement
a RESTART REQUEST packet across the DTE/DXE
to use this facility permits the destination DTE to
interface.
transmit REJECT packets and requires the source DTE
to process received REJECT packets. (In a DTE/DCE
However, for a few of the procedures described in the
environment, a DTE will not receive a REJECT
following clauses, consideration shall be given to whether the
DTE is connected to a DCE or another DTE. For a DTEDTE packet.)
environment, these considerations are listed below.
Use of optional Fast Select Facility (see 13.16) shall
h)
be agreed to by both DTEs prior to transmission of
a) One of the DTEs shall act as a DCE for
any call setup packets which utilize this facility. (In a
- logical channel selection during Virtual Call setup
DTEIDCE environment, such prior agreement is not
(see figure I),
required - a DTE may always use this facility at call
setup .)
- resolution of Virtual Call collision (see 5.25).
(This choice is made independently for each of the i) A called DTE which subscribes to the Flow Conhol
Parameter Negotiation Facility (see 13.12) ador the
DTEs Packet hyer entities; see 3.8.)
Throughput Class Negotiation Facility (see 13.13) will
The restart procedure (see 45) may be used to not receive, in an INCOMING CALL packet, a facility
indication from which to negotiate if the calling DTE
determine which DTE acts as a DCE and which DTE
is satisfied with the default values and, thus, hm not
maintains its role as a DTE with respect to the above
items. (The procedures in 45 may be used in the included the facility request in its CALL REQUEST
general case of a DTEIDXE interface via a dedicated packet. In a similar manner, a calling DTE which
subscribes to these facilities will not receive, in a
path or a circuit-switched connection. Alternatively, if
CALL CONNECTED packet, a facility indication if the
a DTE is to operate only in a DTEIDCE environment
called DTE is satisfied with the values in the
or a DTEIDTE environment where, by administration,
INCOMING CU packet and, thus, has not included a
the roles can be predetermined and fmed, then the
facility request in its CALL ACCEPTED packet. (In a
DTE may be initialized to act appropriately.)
DTEIDCE environment, these facility indications are
b) A DTE shall be able to accept a RESTART
always present if the DTE has subscribed to these
INDICATION packet with a Restarting Cause Field of
facilities.)
"DTE Originated," an event which does not occur in a
DTE/DCE environment. 3.4 Operation over circuit-switched connections
When communications between a DTE and DXE involves a
c) A DTE should not receive a RESTART, CLEAR. or
circuit-switched connection (e.g., through a circuit-switched
RESET INDICATION packet with a Cause Field other
data network, circuit-switched capability of an Integrated
than "DTE Originated" (although this may occur in a
DTE/DCE environment). Therefore, the DTE may Services Digital Network, or through the switched telephone
either handle such a packet as it does in a DTE/DCE network), identification procedures may be required. Such
environment (i.e., process the packet normally) or treat procedures, including those at the Packet Layer, are defined in
it as an error (DTEDTE environment only). CCïïï Recommendation X.32.
A DTE may transmit a DIAGNOSTIC packet in the Most communications over a circuit-switched connection are
d)
appropriate circumstances (see 11.1) only if it can between DTEs and DXEs that have been arranged, by some
suppress its generation when connected to a network. prior administrative procedure, to be compatible. Agreement
must be reached, for example, as to what logical channels will
e) A DTE may ignore or treat as an error the receipt of
be used, the window sizes to be used, and a number of other
facility codes that do not apply to a DTEIDTE
items pertaining to Packet Layer operation. In some cases,
environment.
however, it may be desirable to allow for random
communications, where a DTE accesses a DXE via a circuit-
ISO/IEC 8208 : 1990 (E)
- transfer data and interrupt information (see clause
switched connection without prior agreement (for example, an
electronic mail-order service). To allow for this, the following 6), and
subset of the Packet Layer procedures will be used:
- reinitialize a logical channel (see 8.1).
a) the interface shaii consist of a single two-way Virtual
It is required that sufficient information be made
Cail logical channel using Logical Channel Identifier 1;
available to the protocol to allow it to execute these
procedures. Note that, in certain circumstances, the
b) the procedures described in 4.5 are required,
Packet Layer protocol can, on its own accord,
c) the default values for all applicable parameters listed in
terminate a Virtual Call or reinitialize a logical
clause 18 shall apply; parameters T24, T25, T27, T28,
channel; and
R25, R27. and R28 and the procedures in 11.2, 11.3,
13.1, and 13.4 do not apply;
d) reporting to a higher layer entity the occurrence of
certain Packet Layer protocol events including:
d) the reset procedures shall apply if erroneous DATA
__ -
packets are received (see 11.3); and - (re)initialization of all logical channels (see 4.2),
- receipt of an incoming request to set up a Virtual
e) no optional user facilities shall be allowed.
Call (see 5.2.2),
Extensions beyond this basic set of procedures and capabilities
can be obtained through the use of procedures defined in - termination of a Virtual Call (see 55.2).
CCïïï Recommendation X.32.
- receipt of data and interrupt information (see clause
3.5 Provision of the OS1 Network Service 61, and
The Packet Layer protocol specified in this International - reinitialization of a logical channel (see 8.2).
Standard can be used to support the OS1 connection-mode .
Along with the signal of their occurrence, the Packet
Network Service m a variety of environments (e.g., see IS0
Layer also provides to the higher layer entity any data
8880-2). The Packet Layer protocol supports all the elements
associated with these events. In addition, the Packet
of the OS1 connection-mode Network Service specified in IS0
Layer may also signal the status of the items listed in
8348 and its Addendum 3. Mappings to/from the Packet Layer
(c) above.
protocol elements and the primitives and parameters of the
connection-mode Network Service are described in IS0 8878. 3.7 Logical channels
-
Additional provisions applicable in an ISDN environment are
To enable simultaneous Virtual Calls anaor Permanent Virtual
described in ISO/IEC 9574.
Circuits, logical channels are used. Each Virtual Call and
-
3.6 External Packet Layer interactions Permanent Virtual Circuit is assigned a Logical Channel
Identifier,') which is a number in the range from 1 through
The protocol described here is independent of any external
4 095. For each Virtual Call, a Logical Channel Identifier is
considerations. However, the initiation of certain Packet Layer
awigned during the call setup phase from a range of previously
protocol procedures is directed by elements outside the
agreed-upon Logical Channel Identijîers. For each Permanent
of certain Packet Layer
protocol. Likewise, the occurrence
Virtual Circuit, a Logical Channel identijîer is assigned in
protocol events are to be reported appropriately. These external
agreement with the DXE. (Logical Channel Identifier O shall
interactions include:
not be assigned to a Virtual Call or a Permanent Virtual
Circuit.)
a) requesting, of the Data Link Layer, transmission of
outgoing packets; A DTE's use of logical channels is agreed upon for a period of
time with the DXE. Figure 1 shows the structure for assigning
b) receiving, from the Data Link Layer, incoming packets;
logical channels used for Virtual Calls and Permanent Virtual
c) accepting requests from a higher layer entity to initiate Circuits.
certain Packet Layer protocol procedures including:
3.8 Packet Layer entity
- initialize the Packet Layer (see 4.1),
The concept of communication via logical channels is native to
- originate a Virtual Call (see 5.2.1), Packet Layer terminology. It is conceivable, however, that a
DTE may have one or more connections to one or more packet
- accept a Virtual Call (see 5.2.3),
networks and/or to one or more DTEs without an intervening
- terminate a Virtual Call (see 55.I),
packet network. At this point, therefore, it is necessary to
introduce the concept of a "Packet Layer entity." One such
1) A logical chuinel may be identified as one 12-bit field or two subfielda containing 4 and 8 bits, respectively. When viewed as one field, the term "Logical Channel
Idmtifier" or just 100giai channel" is used, when viewed as two fields, the terms "logical channei group number" (4 bits) and "logical channel number" (8 bits) are used.

ISOAEC 8208 : 1990 (E)
In the case of a single logical channel DTE/DXE interface, logical channel 1 will be used.
In the case of a multiple logical channel DTE/DXE interface, a range of logical channels will be agreed to according to the following
diagram:
Virtual Calls
I
Permanent
Virtual One-way One-way
Circuits incoming Two-way outgoing
r---l* * -
22 -22 22
O1 LIC HIC LTC HTC LOC
HOC
4 095
Logical Channel Identifier
LIC: Lowest Incoming Channel LTC: Lowest Two-way Channel LOC: Lowest Outgoing Channel
HIC: Highest Incoming Channel HTC: Highest Two-way Channel HOC: Highest Outgoing Channel
Logical channels 1 through LIC-1: range of logical channels which may be assigned to Permanent Virtual Circuits
Logical channels LIC through HIC: range of logical channels which are assigned as one-way incoming for Virtual Calls
Logical channels LTC through HTC: range of logical channels which are assigned as two-way for Virtual Calls
Logical channels LOC through HOC: range of logical channels which are assigned as one-way outgoing for Virtual Calls
Logical channels HIC+1 through LTC-1. HTC+1 through LOC-1, and HûC+1 to 4 095 are non-assign4 logical channels
NOTES
1 The reference to the Logical Channel Identifiers is made according to a set of contiguous numbers from O (iowest) to 4 095 (highest) using the 12 bits
made up of bits 4 through 1 of octet 1 and all bits of octet 2 of each packet (see 12.1.2). The numbering is binary-coded using bit positions 4 through 1
of octet 1 followed by bit positions 8 t
...