ISO/IEC 7809:1991
(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
Technologies de l'information — Télécommunications et échange d'informations entre systèmes — Procédures de commande de liaison de données à haut niveau (HDLC) — Classes de procédures
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Standards Content (Sample)
I NTER NATIONAL ISOIIEC
x I---.\
STANDARD 7809
Second edition
1991 -09-15
I n f o r m a t io n tech no log y - Te leco m m u n ica t io ns
and information exchange between systems -
High-level data link control (HDLC) procedures -
Classes of procedures -
Technologies de /'information - Télécommunications et échange d'informations entre
systèmes - Procédures de commande de liaison de données à haut niveau
(HDLCJ - Classes de procédures
Reference number
ISO/IEC 7809 : 1991 (E)
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ISO/IEC 7809 : 1991 (E)
Contents Page
Introduction . .iv
1 scope . 1
2 Normative references .
3 General description . .1
4 Unbalanced operation (point-to-point and multipoint) . 6
5 Balanced operation (point-to-point) . 9
6 Uses of the optional functions . 11
Annex
A Examples of typical HDUJ procedural subsets . 16
0 ISOAEC 1991
All rights reserved. No part of this publication may be reproduced or utilized in any form or by
any means, electronic or mechanical, including photocopying and microfilm, without permission
in writing from the publisher.
ISO/IEC Copyright Office O Case postale 56 O CH-121 1 Genève 20 O Switzerland
Printed in Switzerland
ii
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ISO/IEC 7809 : 1991 (E)
Foreword
IS0 (the International Organization for Standardization) and IEC (the International
Electrotechnical Commission) form the specialized system for worldwide standardiz-
ation. 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, ISOAEC 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 9'0 of the national bodies
casting a vote.
ISOAEC 7809 was prepared by Joint Technical Committee ISOAEC JTC 1, Znfor-
mation technology.
This second edition cancels and replaces the first edition (IS0 7809 : 1984), whtch has
been technically revised.
Annex A is for information only.
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ISOnEC 7809 : 1991 (E)
Introduction
High-level data link control (HDLC) classes of procedures describe methods of data
link operation which permit synchronous or start/stop, code-wansparent data
transmission between data stations in a variety of logical and physical configurations.
The classes are dehed in a consistent mmer within the framework of an overd
HDLC architecture. One of the purposes of this International Standard is to maintain
maximum compatibdity between the basic types of procedures, unbalanced and
balanced, as this is particulariy desirable for data stations with configurable
capability, which may have the characteristics of a primary. secondary. or combined
station, as required for a specific connection.
Primary -
StetiOXl I
15
Figure 1 - Unbalanced data link configuration
This Intemationai Standard dehes three fundamental ciasses of procedures (two
unbalanced and one balanced). The unbalanced classes apply to both point-to-point
and multipoint conûgurations (as illustrated in figurel) 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 aione is
responsible for data link management, hence the designation "unbalanced" classes of
procedures.
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ISODEC 7809 : 1991 (E)
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.
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 optionai 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 symmehical configurations for operation
on a single data circuit from the unbaland classes of procedures which are de.6ned in
this International Standard. For example, the combmation 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).
Combined
station statim
Figure 2 - Balanced data Unk configuration
Primary
station
2 I
Figure 3 - Symmetrical data link configuration
V
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ISOAEC 7809 : 1991 (E)
INTERNATIONAL STANDARD
Information technology - Telecommunications and
information exchange between systems - High-level
data link control (HDLC) procedures - Classes of
procedures
ISO/iEC 2382-9 : 1984, Data processing - Vocabulary - Part
1 Scope
09: Data communicatwn.
This International Standard describes the HDLC unbalanced
classes of procedures and the HDLC balanced class of
ISO/EC 3309 : 1991, Injiormation technology -
procedures for synchronous or start/stop data transmission.
Telecommunications and injiormation exchange between
systems - High-level data link control (HDLC) procedures -
Balanced operation is intended for use in circumstances
Frame structure.
which require equal control at either end of the data link.
Operational requirements are covered in accordance with the
ISO/IEC 4335 : 1991, Information technoiogy -
overall HDLC architecture. The procedures use the HDLC
Telecommunications and information exchange between
frame structure defined in ISO/IEC3309 and the HDLC
systems - High level data link conirol (HDLC) procedures -
elements of procedures described in ISO/IEC 4335.
Elements of procedures.
For the unbalanced classes, the data link consists of a primary
ISO/IEC 8885 : 1991, Information technoiogy -
station plus one or more secondary stations and operates in
Telecommunications anci information exchange between
either the normal response mode or the asynchronous
systems - High-level data link control (HDLC) procedures -
response mode in a point-to-point or multipoint configuration.
General purpose XID frame illformation field content and
For the balanced class, the data link consists of two combined
format.
stations and operates in the asynchronous balanced mode in a
point-to-point configuration. In each class, a basic repertoire
of commands and responses is defined, but the capability of
3 General description
the data link may be modified by the use of optional
3.1 Principles
functions.
3.1.1 Types of data station
3.1.1.1 Two types of data station are defined for the
2 Normative references
unbalanced classes of procedures (see figure 4):
The following standards contain provisions which, through
a) primary station, which sends commands, receives
reference in this text, constitute provision of this International
responses and is ultimately responsible for data link
Standard. At the time of publication, the editions indicated
layer error recovery;
were valid. All standards are subject to revision, and parties
b) secondary stations, which receive commands, send
to agreements based on this International Standard are
responses and may initiate data link layer error recovery.
encouraged to investigate the possibility of applying the most
recent editions of the standards indicated below. Members of
3.1.1.2 One type of data station is defined for the balanced
IEC and IS0 maintain registers of currently valid
class of procedures (see figure4). i.e. combmed stations,
International Standards.
which send both commands and responses, receive both
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ISO/IEC 7809 : 1991 (E)
commands and responses, and are responsible for data link
layer error recovery.
33.1 Designations
3.1.2 Configurations
For the unbalanced classes of procedures, a single primary
station plus one or more secondary station(s) shall be
Class;
connected together over various types of transmission
facilities to build point-to-point or multipoint. half-duplex or
UAC - Unbaiand operation Asynchronous response
duplex, switched or non-switched configurations.
mode Class; and
For the balanced class of procedures. two combined stations
shall be connected together over various types of transmission
mode Class.
facilities to build point-to-point, halfduplex or duplex,
In these designations
switched or non-switched configurations.
3.13 Operational modes
balanced operation;
In an unbalanced class, any coupling of a primary station with
secondary station@) shall be operated in either the normal
response mode (NRM) or the asynchronous response mode
(ARM). two-way alternate or two-way simultaneous, in
accordance with the capability of the configuration being
employed. In the balanced class, two combined stations shall 3.23 Basic repertoires
be operated in the asynchronous balanced mode (ABM),
two-way alternate or two-way simultanwus, in accordance
with the capability of the Configuration being employed.
3.2.2.1 UNC
3.1.4 Addressing scheme
In aii classes (unbalanced and balanced), commands shall
shaii be as follows:
always be sent containing a destination data station address,
Responses
and responses shall always be sent containing the assigned
I I
transmitting data station address.
RR RR
RNR RNR
The "ail-station'' address or a "group" address may be used to
SNRM UA
transmit a command frame simultaneously to all the
DISC DM
secondary stations on a multipoint configuration or to the
FRMR
defined group of secondary stations. The addressing
convention is specified in ISO/IEC3309, clause5. The
mechanism to avoid overlapping responses to multiple station
3.2.2.2 UAC
addressing is system dependent and is not specified in either
ISO/IEC 3309 or this International Standard.
The basic repertoire of .nmands and responses for UAC
shall be as follows:
3.15 Send and receive state variables comm J ds Responses
I I
For each primary-to-secondary or combined-to-combined
RR RR
pairing, a separate pair of send and receive state variables
RNR RNR
shall be used for each direction of transmission of
SARM UA
information (I) frames. Upon receipt and acceptance of a
DISC DM
mode setting command, both the send and receive state
FRMR
variables of the receiving station, shall be set to zero. Upon
receipt and acceptance of an acknowledgement response to a
mode setting command, both the send and receive state
variables of the originating station shall be set to zero.
2
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ISODEC 7809 : 1991 (E)
flow
Data i:
a
Contd
Primary Combined
station station
* For send-only I frame stations or receive-only I frame stations, remove source or sink capability,
as appropriate.
Figure 4 - HDLC stations - Building blocks
deletions of commands and responses to OT from the basic
3.2.23 BAC
repertoires, or by the use of alternate address or control field
formats or alternate frame checking sequences or alternate
The basic repertoire of commands and responses for BAC
form of transmission (see figure 5). Option 11 is applicable
shall be as follows:
to the balanced class of procedures only.
Commands Responses
I
I
RR RR
3.4 Consistency of classes of procedures
RNR RNR
The consistency in the three classes of procedures, obtained
SABM UA
through the use of the concepts of modes of operation, basic
DISC DM
command/response repertoires, and hierarchical structuring, is
FRh4R
shown in figure 5. This consistency in repertoire facilitates
the inclusion of multiple versions of the classes of procedures
in a data station that is configurable.
3.3 Optional functions
Fifteen optional functions are available (see table I) to
modify the fundamental classes of procedures defined in 3.2.
These optional functions are obtained by the additions or
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ISODEC 7809 : 1991 (E)
Table 1 - Optional functions
Functional description
3ption
1
Provides the ability to exchange identification and/or characteristics
of data stations
2
Provides the ability for more timely reporting of I frame sequence Add ommandRE3
Add f esponse: REJ
errors
3
Provides the ability for more efficient recovery from I katne
sequence errors by requesting retransmission of a single frame
4 Add ommandUI
Provides the ability to exchange information fields independent of the
Add isponse: UI
mode (operational or non-operational) without impacting the I frame
sequence numbers
5 Provides the ability to initialize a remote data station, and the ability Add 0mmand:SIM
to request initialization Addr P sponse: RIM
6 Add &and: UP
Provides the ability to perform unnumbered group and all-station
poliing as well as unnumbered individual polling
7
Provides for greater than single octet addressing Use e tended addressing format
instea) of basic addressing
form
8 Limits the procedures to allow I frames to be commands only
9 Limits the procedures to allow I frames to be responses only
10 Provides the ability to use extended sequence numbering (modulo
128)
11 Add <)o/nmBnd: RSET
Provides the ability to reset the state variables associated with only
one direction of information flow (for BAC only)
12 Provides the ability to perform a basic data link test and TEST
E :ense: TEST
13
Provides the ability to request logical disconnection
14
Provides for 32-bit frame checking sequence (FCS)
15 Use start/stop transmission
Provides fm starthtop transmission
instei of syrichronous
trans sion
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ISOAEC 7809 : 1991 (E)
UNC
UAC
BAC
Secondary
primary Secondary p"mary
canbined station
Station
station station Station
Commands Responses Commands Responses Commands Responses
I I I I
I I
Basic
RR RR RR RR
RR RR
repertoire
RNR RNR RNR RNR
RNR RNR
SNRM UA SABM UA
SARM UA
DISC DM
DISC DM DISC DM
FRMR FRMR FRMR
Synchronous transmission
Synchronous transmission Synchmous transmission
Basic addressing format
Basic addressing format Basic addressing format
16-bit FCS
1 &bit FCS 16-bit FCS
Modulo 8 Modulo 8 8
Moduio
I
1
Command Response Command Response
For identificatisn
For command I frames dy
1
-€ -8 Delete --c I
XID c Add + XID
For RET recovery
For response I frames only
2 4 z
RET c Add -+ REJ I f- Delete
L I I I
l
For single frame retransmission For extended sequence numbenng
3 < b 10 Use extended.umtro1 field format
SRET c Add SRET instead of basic control field fonnat;
use SXXME instead of SXXM
For one-way reset (BAC oniy)
For unnumbered information
<
UI- Add-, UI l1 RSET f- Add
Optional
functions
For data Iuik test
For initiaiization
12
c >
TEST e Add + "EST
SIM t Add -.) RIM
For request disconnect
For unnumbered polling
* 13
<
UP t Add
Add - RD
For multi-octet addressing
For 32-bit FCS
7 Use extended instead of c * 14 Use the 32-bit FCS instead
of the 16-bit FCS
basic addressing fonnat
For stadstop transmission
I * 15 Use stadstop transmission instead
of synchronous transmission
Figure 5 - HDLC classes of procedures
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ISODEC 7809 : 1991 (E)
two-way alternate or tw>-way simultaneous data transfer.
3.5 Conformance to the HDLC classes of
The procedure uses the 3DK frame structure defined m
procedures
ISO/IEC33û9 and the HDLC elements of procedures
A data station shall be described as conforming to a given
described in ISOT4335. It uses the basic
class of procedures. with optional functions, if it implements
commandCresponse repertoire (see figure 5) designated üNC
all commands and responses in the basic repertoire for the
(or UAC). Although only the basic commands and responses
class of procedures as modified by the selected optional
are described, there are several optionai functions available
functions, i.e.
for enhanced operation. Tliese are listed in 3.3 and shown in
figure 5.
a) a primary station shall have the ability to receive all of
the responses in the basic repertoire for the unbalanced
unbalanced ciasses of procedures
class of procedures as modified by the selected optional
examples given in ISû/iEC 4335,
functions;
b) a secondary station shall have the ability to receive all
of the commands in the basic repertoire for the
4.2 Description of the
unbalanced class of procedures as modified by the
selected optional functions;
c) a combied station shall have the ability to receive all
of the commands and responses in the basic repertoire
for the balanced class of procedures as modified by the interconnected by
selected optional functions.
3.6 Method of indicating classes and optional
functions
The classes of procedures and the optional functions shall be
switched data circuits.
indicated by specifying the designation of the class (see 3.2.1)
plus the number@) of the accompanying optional functions NOTE - In the of a switched data circuit, the
(see 3.3).
procedures described that the switched data circuit has
been established.
Exmple I: Class UNC1,2,6,9 indicates the unbalanced
operation normal response mode class of procedures The data link layer shall no initiate data transmission until an
with the optional function for identification @ID). REJ indication of circuit availa ility is provided by the physical
recovery (REJ), unnumbered polliig (UP), and one-way layer. (In some systems rovidig two-way alternate data
data flow from the secondary station($ to the primary exchange on physical lay r data circuits using half-duplex
Station. transmission, this indic ion of physical layer circuit
availability is indicated by i idle data link channel state.)
Exmple 2: Class UAC1,5,10,13 indicates the
unbalanced operation asynchronous response mode class
43 Description of
of procedures with the optional functions for
identification (XiD), initialization (SIM, RIM), extended 43.1 General
sequence numbering (modulo 128). and request
disconnect (RD).
ExMipie 3: Class BAC2.8 indicates the balanced
operation asynchronous balanced mode class of
procedures with the optional functions for REJ recovery
(REJ) and the ability to send I frames as commands
only.
4 Unbalanced operation (point-to-point and
multipoint)
4.1 General
The following requirements apply to the procedure for
unbalanced operation of synchronous or start/stop data
iink,
transmission over point-to-point or multipoint data links with
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ISO/IEC 7809 : 1991 (E)
addressed secondary station(s), upon receiving the DISC
b) sending information transfer, supervisory and
command carrectiy, shall send a UA response at its first
unnumbered commands; and
opportunity and shall enter the normal disconnect mode
c) checking received responses.
(NDM), or the asynchromus disconnected mode (ADM), as
predehed for that secondary station. If, upon receipt of the
Each secondary station shall be responsible for:
DISC command, the addressed secondary station is already in
the disconnected mode, it shall send the DM response. The
a) checking received commands; and
primary station, upon receiving a UA or DM response to a
b) sending information transfer, supervisory and
sent DISC command, shall stop the response time-out
unnumbered responses as required by the received
function (or equivalent).
commands.
When it is a multipoint configuration. the UA response fiom
secondary stations shall not interfere with one another. The
4.4 Detailed definition of the procedures
mechanism used to avoid overlapping responses to the
The procedures for a permanently connected data link or an disconnect (DISC) command using a pup address or the all
established switched connection are defined in 4.4.1 to 4.4.6. station address is systemdefined.
The protocol for establishing and disconnecting a switched if the DISC command, UA response or DM response is not
data circuit is not withiin the scope of this International received correctly, it shall be ignored by the receiving station.
Standard. However, the ability to exchange identification This will result in the expiry of the primary station's response
anaor characteristics after the switched connection is
time-out function (or equivalent). and the primary station may
established is available as an optional function.
resend the DISC command and restart the response time-out
function (or equivalent) (see 4.4.3).
4.4.1 Setting up and disconnecting the data link
This action may continue until either the UA response or a
4.4.1.1 Setting up the data link
DM response has been received correctly or until recovery
action takes place at a higher level.
The primary station shall initialize the data link with a
secondary station by sending a SNRM (or SARM) command
and shall start a response time-out function (or equivalent).
4.4.13 Procedure in a disconnected mode
The addressed secondary station, upon receiving the SNRM
(or ADM) shall monitor
A secondary station in NDM
(or SARM) command correctly, shall send the UA response
commands, shall react, at the earliest respond opportunity, to
at its first opportunity and shall set its send and receive state
a SNRM (or SARM) command as outlined in 4.4.1.1, and
variables to zero. If the UA response is received correctly,
shali respond with a DM response to a received DISC
the data link set up to the addressed secondary station is
command. The secondary station shaii respd to other
complete, and the primary station shall set its send and
commands received with the P bit set to "1" with a
receive state variables relative to that secondary station to
disconnected mode (DM) response with the F bit set to "1".
zero and shall stop the response time-out function (or
Other commands received with the P bit set to "O" shall be
equivalent). If, upon receipt of the SNRM (or SARM)
ignored. The DM response shall be used to report the
command, the secondary station determines that it cannot
secondary station status asynchronously in ADM.
enter the indicated mode, it shall send the DM response. If
the DM response is received correctly, the primary station
shall stop the response time-out function (or equivalent).
4.4.2 Exchange OP information (I) Prames
4.4.2.1 Sending I frames
If the SNRM (or SARM) command, UA response or DM
response is not received correctly, it shall be ignored. The The control field format shall be as defined in ISO/IEC 4335
result will be that the primary station's response time-out (see clause 1) for an I frame with N(S) set to the value of the
function (or equivalent) will run out, and the primary station send state variable V(S) and with N(R) set to the value of the
may resend the SNRM (or SARM) command and restart the
receive state variable V(R). Following data link set-up, both
response time-out function (or equivalent) (see 4.4.3).
V(S) and V(R) shail be set to zero. The maximum length of I
frames shall be a systemdefined parameter.
This action may continue until a UA response has been
received correctly or until recoveIy action takes place at a If the data station is ready to send an I frame numbered N(S),
higher level. where N(S) is equal to the last received acknowledgement
plus the modulo - 1, the data station shall not send the I frame
but shall follow the procedures described in 4.4.3.
4.4.1.2 Disconnecting the data link
The primary station shall disconnect the data link(s) with
secondary station@) by sending a DISC command and shall
start a response time-out function (or equivalent). The
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ISO/IEC 7809 : 1991 (E)
4.4.2.2 Receiving I frames In NRM. a secondary sta on shall depend on the primary
station to initiate time-out rLvery .
After a data station receives correctly an in-sequence I frame
[i.e., N(S) equals îhe value of the receive state variable V(R)]
The duration of time-out ctions (or equivalent) shall be
that it can accept, it shall increment its receive state variable
system-dependent and su 'ect to bilaterai agreement. To
V(R), and, at its next oppomniity to send, take one of the
resolve possible contention situations in ARM, the duration
following actions:
of the secondary station's * e-out function shall be different
from that of the primary sta 'on.
a) If information is available for transmission and the
Y
remote data station is ready to receive, it shall act as
described in 4.4.2.1 and acknowledge the received I
4.4.4 PIF bit usage
frame(s) by setting N(R) in the control field of the next
PP bit usage in the
transmitted I frame to the value of V(R).
and UAC, shall be as
b) If information is not available for transmission but the
data station is ready to receive I frames, the data station
4.43 Two-way alternate c nsiderations
shaU send an RR frame and acknowledge the received I
framds) by setting N(R) to the value of V(R).
In the case of normal respo d oppommity, two-way alteniate,
data link operation
c) If the data station is not ready to receive further I
frames, the data station may send an RNR frame and
a) transmission from e primary station shall not be
acknowledge the received I frame(s) by setting the N(R)
allowed until either
to the value of V(R).
e with the F bit set to "1". M
1) receipt of a fr
If the data station is unable to accept the correctly received I
2) expiry of a no-r I pome time-out function; and
frame+). V(R) shall not be incremented. The data station
may send an RNR frame with the N(R) set to the value of
WR).
4.4.23 Reception of incorrect frames
If a frame is received with an incorrect FCS. it shall be
discarded.
mentioned period.
If an I frame is received with a correct FCS but with an
In the case of normal respon oppommity, two-way alternate,
incorrect N(S). the receiving data station shall ignore the N(S)
This data link operation, a data station shall not accept further
field and discard the information field in that frame.
frames after a frame with th P/F bit set to "1" was accepted
shali continue until the expected I frame is received correctly.
and before it sends a frame ith the F/P bit, respectively, set
The data station shall, however, use the P/F and N(R)
to "1".
indications in the discarded I frames. The data station shall
then acknowledge the expected I frame, when received
In the case of asynchronou respond opportunity, two-way
correctly, as described in 4.4.2.2.
I
alternate, data link operation, transmission from a data station
shall not be allowed until ei er
The P/F recovery (checkpointing) shall cause retransmission
of the I frames received incomectly, as described in 4.4.4.
a) detection of an idl data link channel state after
receipt of a frame or a fl g; or
4.4.2.4 Data station receiving acknowledgments
period of inactivity (idle data
b) the end of an extend
A data station receiving an I, RR or RNR frame with a valid
link channel state). i
N(R)=x shall treat as acknowledged all previously
transmitted I frames up to and including the I frame
transmitted with N(S) equal to x - 1.
4.43 Time-out considerations
physical layer.
In or& to detect a non-reply or lost-reply condition, each
primary station shall provide a response time-out function (or
If no frames are transmitted om either data station while in
equivalent). Also. in ARM. each secondary station shall
ARM and information is aiting for transmission, it is
provide a command time-out function (or equivalent). In
advisable that the data statio transmits at first a supervisory
I
each case, the expiry of the time-out function (or equivalent)
frame only in order to avoid 1 ng time -very action, which
shall be used to initiate appropriate error recovery procedures.
would occur in the case of I J me contention.
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ISO/IEC 7809 : 1991 (E)
layer. (In some systems providing two-way alternate data
If a data station has transmitted frames and no further frames
exchange on physical layer data circuits using half-duplex
are pending for transmission, it shall give the right to transmit
transmission, this indication of physical layer circuit
to the remote data station.
availability is indicated by an idle data link channel s
...
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