ISO/IEC 7809:1993
(Main)Information technology — Telecommunications and information exchange between systems — High-level data link control (HDLC) procedures — Classes of procedures
Information technology — Telecommunications and information exchange between systems — High-level data link control (HDLC) procedures — Classes of procedures
Describes the HDLC unbalanced classes of procedures, the HDLC balanced class of procedures, and the HDLC connectionless classes of procedures for synchronous or start/stop data transmission. The procedures use the HDLC frame structure defined in ISO/IEC 3309 and the HDLC elements of procedures described in ISO/IEC 4335.
Technologies de l'information — Télécommunications et échange d'informations entre systèmes — Procédures de commande à haut niveau (HDLC) — Classes de procédures
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Standards Content (Sample)
INTERNATIONAL ISO/IEC
7809
STANDARD
Third edition
1993-12-15
Information technology -
Telecommunications and information
exchange between systems - High-level
data link control (HDLC) procedures -
Classes of procedures
Technologies de I’information - TMcommunications et &change
d’informations entre systkmes - Procbdures de commande 9 haut
niveau (HDL C) - Classes de procgdures
Reference number
lSO/lEC 7809: 1993(E)
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ISO/IEC 7809:1993(E)
Page
Contents
. . .
111
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Foreword
iv
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction
.............................................................................................................. 1
1 scope
..................................................................................... 1
2 Normative references
1
3 General description .
4 Unbalanced operation (point-to-point and multipoint) . 7
............................................................... 10
5 Balanced operation (point-to-point)
.........
6 Unbalanced connectionless operation (point-to-point and multipoint). 13
Annex
A Examples of typical HDLC procedural subsets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
0 ISO/IEC 1993
All rights reserved. Unless otherwise specified, no part of this publication may be
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photocopying and microfilm, without permission in writing from the publisher.
ISO/IEC Copyright Office l Case postale 56 l CH-1211 Genkve 20 l Switzerland
Printed in Switzerland
ii
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0 ISO/IEC
ISO/IEC 7809:1993(E)
Foreword
IS0 (the International Organization for Standardization) and IEC (the Inter-
national Electrotechnical Commission) form the specialized system for worldwide
standardization. National bodies that are members of IS0 or IEC participate in
the development of International Standards through technical committees estab-
lished by the respective organization to deal with particular fields of technical
activity. IS0 and IEC technical committees collaborate in fields of mutual inter-
est. 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 voting. Publi-
cation as an International Standard requires approval by at least 75 % of the na-
tional bodies casting a vote.
International Standard ISO/IEC 7809 was prepared by Joint Technical Committee
ISO/IEC JTC 1, Information technology, Subcommittee SC 6, Telecommuni-
cations and inform&on exchange between systems.
This third edition cancels and replaces the second edition (ISOLIEC 7809: 1991),
and incorporates ISO/IEC 7809 amendments 5, 6 and 7.
Annex A of this International Standard is for information only.
. . .
111
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- ISO/IEC 7809 : 1993 (E) 0 ISO/IEC
Introduction
High-level data link control (HDLC) classes of procedures describe methods of data
link operation which permit synchronous or start/stop, code-transparent data
transmission between data stations in a variety of logical and physical configurations.
The classes are defined in a consistent manner within the framework of an overall
HDLC architecture. One of the purposes of this International Standard is to maintain
maximum compatibility between the basic types of procedures, unbalanced, balanced
and connectionless, as this is particularly desirable for data stations with configurable
capability, which may have the characteristics of a primary, secondary, combined,
control, tributary, or peer station, as required for a specific connection.
This International Standard defines five fundamental classes of procedures (two
unbalanced, one balanced, and two connectionless). The unbalanced classes apply to
both point-to-point and multipoint configurations (as illustrated in figure 1) over either
dedicated or switched data transmission facilities. A characteristic of the unbalanced
classes is the existence of a single primary station at one end of the data link plus one
or more secondary stations at the other end(s) of the data link. The primary station
alone is responsible for data link management, hence the designation “unbalanced”
classes of procedures.
Primary -
station
r-l
Secondary
station
N
Figure 1 - Unbalanced data link configuration
The balanced class applies to point-to-point configurations (as illustrated in figure 2)
over either dedicated or switched data transmission facilities. A characteristic of the
balanced class is the existence of two data stations, called combined stations, on a
logical data link, that may share equally in the responsibility for data link
management, hence the designation “balanced” class of procedures.
iv
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ISO/IEC 7809 : 1993 (E)
0 ISO/IEC
I
Combined
Combined
StatiOIl
station
B
A
Figure 2 - Balanced data link configuration
Secondary Primary
station
2
7
Figure 3 - Symmetrical data link configuration
The unbalanced connectionless class applies to point-to-point configurations over either
dedicated or switched data transmission facilities, or to multipoint cotigurations over
dedicated data transmission facilities. A characteristic of the unbalanced connectionless
class is the existence of a single control station at one end of the data link plus one or
more tributary stations at the other end(s) of the data link. The control station is
responsible for determining when a tributary station is permitted to send. Neither the
control station nor the tributary station(s) support any form of connection
establishment/termination procedures, flow control procedures, data transfer
aclulowledgement procedures, or error recovery procedures, hence the designation
“connectionless” class of procedures.
The balanced connectionless class applies to point-to-point configurations over either
dedicated or switched data transmission facilities. A characteristic of the balanced
connectionless class is the existence of two data stations, called peer stations, on a data
link, that are each independently in control of when they can send. Neither peer station
supports any form of connection establishment/termination procedures, flow control
procedures, data transfer acknowledgement procedures, or error recovery procedures,
hence the designation “connectionless” class of procedures.
For each class of procedures, a method of operation is specified in terms of the
capabilities of the basic repertoire of commands and responses that are found in that
class. A variety of optional functions are also listed. Procedural descriptions for the use
of the optional functions are found in clause 6.
It is recognized that it is possible to construct symmetrical cdgurations for operation
on a single data circuit from the unbalanced classes of procedures which are defined in
this International Standard. For example, the combination of two unbalanced procedures
(with I frame flow as commands only) in opposite directions would create a symmetrical
point-to-point configuration (as illustrated in figure 3).
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ISO/IEC 7809 : 1993 (E)
INTERNATIONAL STANDARD@ ISO/IEC
Information technology - Telecommunications and
- High-level
information exchange between systems
data link control (HDLC) procedures - Classes of
procedures
were valid. All standards are subject to revision, and parties
1 scope
to agreements based on this International Standard are
This International Standard describes the HDIC unbalanced
encouraged to investigate the possibility of applying the most
the HDLC balanced class of
classes of procedures,
recent editions of the standards indicated below. Members of
procedures, and the HDLC connectionless classes of
IEC and IS0 maintain registers of currently valid
procedures for synchronous or start/stop data transmission.
International Standards.
Balanced operation is intended for use in circumstances
ISO/IEC 646 : 1991, h$omution technology - IS0 7-bit co&d
which require equal control at either end of the data link.
character set for irgfomution interchange.
Operational requirements are covered in accordance with the
overall HDLC architecture. The procedures use the HDLC
IS0 2382-9 : 1984, Data processing - Vocabulary - Part
frame structure defined in ISO/IEC 3309 and the HDLC
09: Data communication.
elements of procedures described in ISO/IEC 4335.
ISO/IEC 3309 : 1993, Information technology -
For the unbalanced classes, the data link consists of a primary
Telecommunications and inforrnution exchange between
station plus one or more secondary stations and operates in
system - High-level data link control (HDLC)
either the normal response mode or the asynchronous
procedures - Frame structure.
response mode in a point-to-point or multipoint configuration.
For the balanced class, the data link consists of two combined
ISO/IEC 4335 : 1993, Z@k2ation technology -
stations and operates in the asynchronous balanced mode in a
Telecommunications and information exchange between
For the unbalanced
point-to-point configuration.
systems - High-level data link control (HDLL)
connectionless class, the data link consists of a control station
procedures - Elements of procedures.
plus one or more tributary stations and operates in the
unbalanced connectionless-mode in a point-to-point or
technology -
ISO/IEC 8885 : 1993, Information
For the balanced connectionless
multipoint configuration.
Telecommunications and information exchcutge between
class, the data link consists of two peer stations and operates
systems - High-level data link control (HDLC)
in the balanced connectionless-mode in a point-to-point
procedures - General purpose XID frame in$ormation field
configuration. In each class, a basic repertoire of commands
content and format.
and responses is defined, but the capability of the data link
may be modified by the use of optional functions.
3 General description
3.1 Principles
2 Normative references
3.1.1 Types of data station
The following standards contain provisions which, through
3.1.1.1 Two types of data station are defined for the
reference in this text, constitute provision of this International
unbalanced classes of procedures (see figure 4):
Standard. At the time of publication, the editions indicated
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ISO/IEC 7809 : 1993 (E)
a) primary station, which sends commands, receives 3.13 Operational modes
responses and is ultimately responsible for data link
In an unbalanced class, any coupling of a primary station with
layer error recovery;
secondary station(s) shall be operated in either the normal
b) secondary stations, which receive commands, send response mode (NRM) or the asynchronous response mode
responses and may initiate data link layer error (ARM), two-way alternate or two-way simultaneous, in
recovery. accordance with the capability of the configuration being
employed. In the balanced class, two combined stations shall
3.1.1.2 One type of data station is defined for the balanced
be operated in the asynchronous balanced mode (ABM),
class of procedures (see figure 4), i.e. combined stations,
two-way alternate or two-way simultaneous, in accordance
which send both comnknds and responses, receive both
with the capability of the configuration being employed.
commands and responses, and are responsible for data link
layer error recovery. In the unbalanced connectionless class, any coupling of a
control station with tributary station(s) shall be operated in
3.1.13 Three types of data stations are defined for the
the unbalanced co~ectionless mode (UCM), two-way
connectionless classes of procedures (see figure 4):
alternate or two-way simultaneous, in accordance with the
capability of the configuration being employed.
control station in unbalanced connectionless class of
a>
procedure, which sends commands, receives responses,
In the balanced connectionless class, two peer stations shall
but does not support any form of data link layer
be operated in the balanced co~ectionless mode (BCM),
connection establishment/termination, flow control,
two-way alternate or two-way simultaneous, in accordance
acknowledgement, or error recovery;
with the capability of the configuration being employed.
tributary stations in unbalanced connectionless class of
b)
3.1.4 Addressing scheme
procedure, which receives commands, sends responses,
but does not support any form of data link layer In all classes (unbalanced, balanced, and connectionless),
connection establishment/termination, flow control, commands shall always be sent containing a destination data
station address, and responses shall always be sent containing
acknowledgement, or error recovery;
the assigned transmitting data station address.
peer stations in balanced connectionless class of
d
procedures, which send both commands and responses, The “all-station” address or a “group” address may be used to
receive both commands and responses, but are not transmit a command frame simultaneously to all the
responsible for any form of data link layer connection secondary stations on a multipoint configuration or to the
establishment/termination, flow control, defined group of secondary stations. The addressing
convention is specified in ISO/IEC 3309, clause 5. The
acknowledgmen& or error recovery.
mechanism to avoid overlapping responses to multiple station
NOTE - The above terms are introduced in order to avoid
addressing is system dependent and is not specified in either
having to use compound terns such as “connectionless
ISO/IEC 3309 or this International Standard.
secondary station”, etc., throughout the subclauses that &al with
connectionless classes of procedures. 3.15 Send and receive state variables
For each primary-to-secondary or combined-to-combined
3.1.2 Configurations
pairing, a separate pair of send and receive state variables
For the unbalanced classes of procedures, a single primary
shall be used for each direction of transmission of
station plus one or more secondary station(s) shall be
information (I) frames. Upon receipt and acceptance of a
connected together over various types of transmission
mode setting command, both the send and receive state
facilities to build point-to-point or multipoint, half-duplex or
variables of the receiving station, shall be set to zero. Upon
duplex, switched or non-switched configurations.
receipt and acceptance of an acknowledgement response to a
mode setting command, both the send and receive state
For the balanced class of procedures, two combined stations
variables of the originating station shall be set to zero.
shall be connected together over various types of transmission
facilities to build point-to-point, half-duplex or duplex, For each control-to-tributary or peer-to-peer pairing, there are
no send and receive state variables used for each direction of
switched or non-switched configurations.
transmission of data.
For the unbalanced connectionless class of procedures, a
3.2 Fundamental classes of procedures
single control station plus one or more tributary station(s)
shall be connected together over various types of transmission
3.2.1 Designations
facilities to build point-to-point or multipoint, half-duplex or
duplex, switched or non-switched configurations. Five fundamental classes of procedures are defined. They are
designated:
For the balanced connectionless class of procedures, two peer
UNC - Unbalanced operation Normal response mode
stations shall be connected together over various types of
Class;
transmission facilities to build point-to-point, half-duplex or
duplex, switched or non-switched configurations.
UAC Unbalanced operation Asynchronous response
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0 ISO/IEC ISO/IEC 7809 : 1993 (E)
mode Class; 3.2.23 BAC
of commands and
The basic repertoire responses for BAC
BAC - Balanced operation Asynchronous balanced
Sh3llbea.S follows:
mode Class;
commands
Responses
I I
UCC - Unbalanced operation Connectionless - mode
RR RR
Class; and
SABM UA
BCC - Balanced operation Connectionless - mode Class.
DISC DM
In these designations
FRMR
- the first letter, U or B, indicates unbalanced or
balanced operation;
3.2.2.4 ucc
- the second letter, A, N, or C, indicates asynchronous,
The basic repertoire of commands and responses for UCC
normal, or connectionless response mode; and
shall be as follows:
- the third letter, C, stands for class.
Commands Responses
UI
UI
3.2.2 Basic repertoires
3.2.2.5 BCC
The following basic repertoires utilize single octet addressing,
unextended control field format, a 16-bit FCS, and
The basic repertoire of commands and responses for BCC
synchronous transmission.
shall be as follows:
3.2.2.1 UNC
Commands Responses
repertoire of commands and responses for UNC
The basic
UI
follows:
shall be as
commands Responses
3.3 Optional functions
I I
Sixteen optional functions are available (see table 1) to
RR RR
modify the fundamental classes of procedures defined in 3.2.
RNR RNR
These optional functions are obtained by the additions or
SNRM UA
deletions of commands and responses to or from the basic
DISC DM
repertoires, or by the use of alternate address or control field
FRMR
formats or alternate frame checking sequences or alternate
form of transmission (see figure 5). Option 11 is applicable
to the balanced class of procedures only. Options 2,3,4, 8,
3.2.2.2 UAC
11 and 13 are not applicable to the connectionless
9, 10,
The basic repertoire of commands and responses for UAC
classes of procedures.
shall be as follows:
3.4 Consistency of classes of procedures
Commands Responses
I I The consistency in the five classes of procedures, obtained
RR RR through the use of the concepts of modes of operation, basic
RNR
command/response repertoires, and hierarchical structuring, is
SARM UA
shown in figure 5. This consistency in repertoire facilitates
DISC DM
the inclusion of multiple versions of the classes of procedures
FRMR
in a data station that is configurable.
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ISO/IEC 7809 : 1993 (E)
Data Source and
sink *
Command Response Cammand and
Control
control control
response control
:
-T--r
Combined Peer
Primary Chtrol Secondary Tributary
station station station
station station
StZitiOfl
* For send-only I frame stations or receive-only I Erame stations, remove source or sink capability,
as appropriate.
Figure 4 - HDLC stations - Building blocks
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ISO/IEC 7809 : 1993 (E)
0 ISO/IEC
Table 1 - Optional functions
Functional description Required change
Option
1
Add command: XID
Provides the ability to exchange identification and/or characteristics of data stations
Add response: XID
2
Provides the ability for more timely reporting of I frame sequence errors (not for UCC or Add cornman& REJ
Add response: REJ
BCC)
3.1 Add commandz SRET
Provides the ability for more efficient recovery from I frame sequence errors by
requesting retransmission of a single frame (not for UCC or BCC) Add response: SREJ
3.2
Provides the ability for more efficient recovery from I frame sequence enors by Add command: SREJ
requesting retransmission of one or more individual frames with a single request (not for Add response: SREJ
Support multi-selective reject
UCC or BCC)
option
4
Add cornman& UI
Provides the ability to exchange information fields independent of the mode (operational
Add response: UI
or nonoperational) without impacting the I frame sequence numbers (not for UCC or
BCC)
5
Add command: SIM
Provides the ability to initialize a remote data station, and the ability to request
Add response: RIM
initialization
6
Add command: UP
Provides the ability to perform unnumbered group and all-station polling. as well as
unnumbered individual polling
7
Use extended addressing format
Provides for greater than single octet addressing
instead of basic addressing
format
8
Delete response: I
Limits the procedures to allow I frames to be commands only (not for UCC or BCC)
9
Limits the procedures to allow I frames to be responses only (not for UCC or BCC) Delete command: I
10
Use extended control field
Provides the ability to use extended sequence numbering (modulo 128) (not for UCC or
format instead of basic control
BCC)
field format; use SXXME
instead of SXXM
Add command: RSET
Provides the ability to reset the state variables associated with only one direction of
information flow (for BAC only) (not for UCC or BCC)
Add wmman& TEST
Provides the ability to perform a basic data link test
Add response: TEST
Add response: RD
Provides the ability to request logical disconnection (not for UCC or BCC)
Use the 32-bit FCS instead of
Provides for 32-bit frame checking sequence (FCS)
the 16-bit FCS
15.1 Use start/stop transmission with
Provides for start/stop transmission with basic transparency
basic transparency instead of
synchronous transmission
15.2
with basic transparency and flow-control Use start/stop transmission with
Provides for start/stop transmission
basic transparency and flow-
transparency
control transparency instead of
synchronous transmission
15.3
Use start/stop transmission with
Provides for start/stop transmission with basic transparency and control-character octet
basic transparency and wntrol-
transparency
character octet transparency
instead synchronous
of
transmission
16
Use the seven-bit data path
Provides for operation in a start/stop transmission environment that only permits transfer
transparency function, in
of seven data bits per character
conjunction with one of the
Option 15 functions
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- ISO/IEC 7809 : 1993 (E)
0 ISO/IEC
UAC UCC
BAC
COntd
TribUtuy
==Y SCCOIld~ primary Secondary
combined station
station station StptioIl StatiOCl station station
commands Canmands
C-dS COmmrndS COmmUrdS
R- Rcspo- R=P-- Rtsponsts R-
I I UI U-I
I I I I UI
Basic
RR RR RR RR RR
RR
rcptItoi-=
UA UA UA
SNRM SARM SABM
DM DM DM
DISC DISC DISC
synchralous transmission syndlronous transmission
Synchrmous txansmission
synchmnalstr8nsmission synclmnlous-al
Basic addressing format Basic addresshg format
Basic addressing format Basic addressing fomrrt
Basic rdchssing format
16-bit FCS
16-bit FCS
1 &bit FCS l&bit FCS leit FCS
Module 8 Module 8 Module 8
RCSponSC
Canmand CtXlUIlurd
10. For cxtardcd sapaxe numb (not for UCC a BCC)
1 For ida&icatioH
XID+Add+XID Use cxtcndcd control field format instead of
basic cmtrd field fomrrt; use SXXME instead
2 For REJ rccavcry (not fa UCC or BCC) of S?CXM
REJ+Add+REJ
) 11 For on-wry reset (BAC only) (not for UCC or BCC)
RESET c Add
3 For individual frame xtranhkon
rctmwkim (not for UCC or BCC)
3.1 For single individual ihme
SREJcAdd+SREJ
12 For data link test
3.2 For one or more individual frame(s) xetransmission (not for UCC
TESTcAdd+TEST
or BCC)
SREJtAdd+SREJ
Use multi-selective reject option
I-
13 Far lrbqucst disconnect (not far UCC or BCC)
w Add+RD
4 For unnumbered inform&an (not far
UIcAdd+UI
) 14For32bitFCS
5 For initializatiar
Use the 32&t FCS instead of the l&bit FCS
SIM c Add -+ RIM
6FaruMm~*
15 For start/stop transmksion
UP-Add
15.1 Use start/stop truumhiar with basic tmqucncy
I-
instcd ofsynchxmoustransmhiar
15.2 use start/stop trumnkior! with buic tnnspIlarcy
and flow-cumol
tnnqutxyinsteadof
syn- txansmkion
15.3 use start/q -al withbuic~cy
andcuntrol~uactcroctctwspanxzyinstead
S For canmand I i&n= only (not for UCC or BCC)
D&XC+1 of synchImous tr8Mrnis8ion
9 For -se I fkames dy (not for UCC or BCC)
16 For Wstop arvinnunents that permit only seven bits
ICD&C
pcrchurctcr.
Use seven-bit data traqwcncy (in
conjunctial with OIlt of the option 1s filnctials).
i
Figure 5 - HDLC classes of procedures
6
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0 ISO/IEC
ISO/IEC 7809 : 1993 (E)
procedures with the optional functions for REJ recovery
3.5 Conformance to the HDLC classes of
(REJ) and the ability to send I frames as commands
procedures
only.
A data station shall be described as conforming to a given
class of procedures, with optional functions, if it implements
ExumpZe 4: Class UCC 1,12 indicates the unbalanced ’
all wmmands and responses in the basic repertoire for the
connectionless operation wnnectionkss-mode class of
class of procedures as modified by the selected optional
procedures with the optional functions for identifkation
functions, i.e.
@ID) and data link test (TEST).
a) a primary station shall have the ability to receive all of
Example 5: Class BCC 1,14 indicates the balanced
the responses in the basic repertoire for the unbalanced
connectionless operation wnnectionless-mode class of
class of procedures as modified by the selected optional
procedures with the optional functions for identification
functions;
(XID) and 32-bit FCS.
b) a secondary station shall have the ability to receive all
of the commands in the basic repertoire for the
unbalanced class of procedures as modified by the 4 Unbalanced operation (point-to-point and
selected optional functions; multipoint)
c) a combined station shall have the ability to receive all
4.1 General
of the commands and responses in the basic repertoire
The following requirements apply to the procedure for
for the balanced class of procedures as modified by the
unbalanced operation of synchronous or start/stop data
selected optional functions.
transmission over point-to-point or multipoint data links with
d) a control station shall have the ability to receive all of
two-way alternate or two-way simultaneous data transfer.
the responses in the basic repertoire for the unbalanced
The procedure uses the HDLC frame structure defined in
connectionless class of procedures as modified by the ISO/IEC 3309 and the HDLC elements of procedures
selected optional functions;
described in ISO/IEC 4335. It uses the basic
command/response repertoire (see figure 5) designated UNC
e) a tributary station shall have the ability to receive all
(or UAC). Although only the basic commands and responses
of the commands in the basic repertoire for the
are described, there are several optional functions available
unbalanced wnnectionless class of procedures as
for enhanced operation. These are listed in 3.3 and shown in
modified by the selected optional functions;
figure 5.
f) a peer station shall have the ability to receive all of the
commands and responses in the basic repertoire for the NOTE - The HDLC unbalanced classes of prwedures
balanced connectionless class of procedures as modified operate as illustrated in the examples given in ISO/IEC 4335,
by the selected optional functions. Annex B. (See clause 1.)
36 . Method of indicating classes and optional 4.2 Description of the data link
functions
4.2.1 Configuration (see figure 1)
The classes of procedures and the optional functions shall be
The unbalanced operation data link configuration shall consist
indicated by specifying the designation of the class (see 3.2.1)
of one primary station and one or more secondary stations
plus the number(s) of the accompanying optional functions
interconnected by physical layer transmission facilities.
(see 3.3).
Example 1: Class UNC1,2,6,9 indicates the unbalanced 4.2.2 Physical layer transmission facilities
operation normal response mode class of procedures
The physical layer transmission facilities may provide either
with the optional functions for identification (XID), REJ
half-duplex or duplex transmission over switched or non-
recovery (REJ), unnumbered polling (UP), and one-way
switched data circuits.
data flow from the secondary station(s) to the primary
station.
NOTE - In the case of a switched data circuit, the
procedures described assume that the switched data circuit has
Example 2: Class UAC1,5,10,13 indicates the
been established.
unbalanced operation asynchronous response mode class
of procedures with the optional functions for
The data link layer shall not initiate data transmission until an
identification (XID), initialization (SIM, RIM), extended
indication of circuit availability is provided by the physical
sequence numbering (modulo 128), and request
layer. (In some systems providing two-way alternate data
disconnect (RD).
exchange on physical layer data circuits using half-duplex
transmission, this indication of physical layer circuit
Example 3: Class BAC2,8 indicates the balanced
availability is indicated by an idle data link channel state.)
operation
...
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