ISO/IEC 4335:1991

I NTER NATIONAL ISO/IEC
STANDARD
Fourth edition
1991 -09-1 5
I n f o r m a t i o n tech no log y - Te le co m m u n ica t io ns
and information exchange between systems -
High-level data link control (HDLC) procedures -
Elements of procedures
Technologies de /'information - Télécommunications et échanges d'information
entre systèmes - Procédures de commande de liaison de données à haut niveau
(HDLC) - Eléments de procédures
Reference number
iSO/IEC 4335 : 1991 (E)
ISODEC 4335 : 1991 (E)
Contents Page
...
Foreword . . . . . . . . . . . u1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . , . . . .iv
1 scope . 1
2 Nonnative references . . . . . . . . . . .1
3 Definitions . 1
4 Data link channel states . 3
5 Modes . 4
6 Control field and parameters . 6
7 Commands and responses . 9
8 Exception condition reporting and recovery . 18
Annexes
A Time-out function consideratio . 23
B Examples of the use of commands and responses . 26
C Assigned XID information field format identifiers . 45
Q Iso/IEC 1991
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Printed in Switzerland.
LI
ISOAEC 4335 : 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 @io of the national bodies
casting a vote.
International Standard ISO/IEC 4335 was prepared by Joint Technical Committee
ISOAEC JTCl, Information technology.
This fourth edition cancels and replaces the third edition (IS0 4335 : 1987), which has
been technically revised.
Annexes A, B and C of this International Standard are for information only.
iii
ISODEC 4335: 1991 (E)
Introduction
General
High-level data link control (HDLC) procedures are designed to permit synchronous
or start/stop, code-transparent data transmission. This International Standard describes
the HDLC elements of procedures. Further studies are in progress to iden@ and
define additional elements of procedures which may be included at a later date.
in HDLC procedures, the normal cycle of the code-transparent data communication
between two data stations consists of the transfer of frames containing information
from the data source to the data sink acknowledged by a frame in the opposite
direction. Until the data station comprising the data source receives an
acknowledgement, it holds the original information in memory in case the need should
arise for retransmissions.
Data sequence integrity between the data source and the data sink is effected by means
of a numbering scheme, which is cyclic withiin a modulus specified in this
International Standard and measured Li tern of frames, An independent numbering
scheme is used for each data source/data sink combination on the data link.
The acknowledgement function is accomplished by the data sink informing the data
source of the next expected sequence number. This can be done in a separate frame,
not containing information, or withiin the control field of a frame containing
information.
HDLC procedures are applicable to unbalanced data links and to balanced data links.
Unbalanced data links
An unbalanced data link involves two or more participating data stations. For control
pwposes, one data station on the data link assumes responsibility for the organization
of data flow and for unrecoverable data link level error conditions. The data station
assuming these responsibilities is known as the primary station, and the frames it
transmits are referred to as command frames. The other data stations on the data link
are known as the secondary stations, and the frames they transmit are referred to as
response frames.
For the transfer of data between the primary station and the secondary stations, two
cases of data link control are considered (see figures 1 and 2). In the first case, the
data station comprising the data source performs a primary station data link control
function and mtrols the data station comprising the data sink that is associated with a
secondary station data link control function, by select-type commands.
iv
ISOAEC 4335 : 1991 (E)
In the second case, the data station cornpising the data sink performs a primary
station data link control function and controls the data station comprising the data
source that is associated with a secondary station data link control function, by poll-
type commands.
The information flows from the data source to the data sink, and the
acknowledgements are always transmitted in the opposite direction.
These two cases of control may be combined so that the data link becomes capable of
two-way alternate communication, or two-way simultaneous communication.
Balanced data links
A balanced data link involves only two participating data stations. For control
purposes, each data station assumes responsibility for the organization of its data flow
and for unrecoverable data link level emr conditions associated with the
transmissions that it originates. Each data station is known as a combined station and
is capable of transmitting and receiving both command and response frames.
For the transfer of data between combined stations, the data link control functions
illustrated in figure 3 are utilized. The data source in each combined station controls
the data sink in the other combined station by the use of select-type commands. The
information flows from the data source to the data sink, and the acknowledgements are
always transmitted in the opposite direction. The poll-type commands may be used by
each combined station to solicit acknowledgements and status responses from the
other combined station.
Select/infonnation
Primary Secondary
Station station
Acknowledgement
Data swrce Data sink
Figure 1 - Unbalanced data link configuration (case 1)
Polvacknowledgement
Primary Semdary
Station station
I
Data sink Data source
Figure 2 - Unbalanced data iink configuration (case 2)
Seleci/infomation/acknowledgement/poil
Combined Combined
Station station
I
Data sink/data source
Figure 3 - Balanced data link configuration
V
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INTERNATIONAL STANDARD ISOAEC 4335 : 1991 (E)
Information technology - Telecommunications and
information exchange between systems - High-level data
link control (HDLC) procedures - Elements of procedures
1 scope currently valid International Standards.
This International Standard specifies elements of data link
ISO/lEC 2382-9 : 1984. Igormatwn technobgy - Vocabulary
control procedures for synchronous or start/stop, code-
- Part 9 : Data communication.
transparent data transmission using the HDLC frame structure
specified in IS0 3309 and independent frame numbering in
ISO/IEC 3309 : 1991, Information technology -
both directions.
Telecommunications and urformation exchange between
system - High-level data link control (HDLC) procedures -
These HDLC elements of procedures are defined specifically
Frame structure.
in terms of the actions that occur on receipt of commands at a
secondary station or a combined station.
3 Definitions
This Intemational Standard is intended to cover a wide range
of applications; for example one-way, two-way alternate or For the purposes of this International Standard, the following
two-way simultaneous data communication between data definitions apply (see also ISO/IEC 2382-9).
stations which are usually buffered, includmg operations on
different types of data circuits; for example multipoinSpoint-
3.1 abort: A function invoked by a sending primary,
to-point, duplex/half-duplex, switched/non-switched,
secondary or combined station causing the recipient to
synchronous/start-stp, etc.
discard (and ignore) ail bit sequences transmitted by the
sender since the precediig flag sequence.
The defined HDLC elements of procedures are to be
considered as a common basis for establishing different types
of control procedures. This International Standard does not 3.2 accept: The condition assumed by a data station
define any single system and should not be regarded as a (primary, secondary or combined station) upon accepting a
specification for a data communication system. Not all of the correctly received frame for processing.
commands or responses are required for a particular system
implementation.
33 address field (A): The sequence of eight (or any
multiple of eight, if extended) bits immediately following the
opening flag sequence of a frame identifying the
2 Normative references
secondary/combied station sending (or designated to
The following standards contain provisions which, through receive) the frame.
reference in this text, constitute provisions of this
Intemational Standard. At the time of publication, the
3.4 address field extension : Enlarging the address field to
editions indicated were valid. All standards are subject to
include more addressing information.
revision, and parties to agreements based on this International
Standard are encouraged to investigate the possibility of
applying the most recent editions of the standards indicated
below. Members of IEC and IS0 maintain registers of
ISODEC 4335 : 1991 (E)
3.5 basic status : A sewndary/combined station's 3.13 control field extension : Enlarging the control field to
capability to send or receive a frame containing an include additional control information.
information field.
3.14 data communication: See ISO/IEC 2382-9, term
09.01.03.
3.6 centraiized control : A control in which all the primary
station functions of the data link are centralized in one data
station.
3.15 data link: An assembly of two or more terminal
installations and the interconnecting line operating according
to a particular method that permits information to be
3.7 combined : That part of a data station that supports the
exchanged.
combmed station control functions of the data link.
NOTE - The canbined station generates commands and NOTE - in this context, the term "texminal installation" does
not include the data source and the data sink.
responses for transmission and interprets received commands
and responses. Specific respsibjiities assigned to a combined
station include:
3.16 data link level: The conceptual level of control or
a) initiaiization of control signal interchange; processing logic existing in the hierarchical sîructure of a data
station (primary, secondary or combined station) that is
b) organization of data flow;
responsible for maintaining control of the data link.
c) interpretation of received commands and generation of
appropriate responses; and
NOTE - The data link level functions provide an interface
between the data station high level logic and the data link.
d) actions regarding error control and error recovery
These functions include:
functions at the data link level.
a) transparency;
3.8 command: In data communication, an instruction
b) address/control field interpretation;
represented in the control field of a frame and transmitted by
c) command/response generation. transmission and
the primary/combmed station. It causes the addressed
interpretation; and
secondary/combined station to execute a specific data link
control function. d) frame check sequence computation and interpretation.
3.17 data transmission: See ISO/IEC 2382-9, term
3.9 command frame :
09.01.02.
a) All frames transmitted by a primary station.
b) Those frames transmitted by a combined station that
3.18 duplex transmission: See ISO/IEC 2382-9, term
contain the address of the other combined station.
09.03.01.
3.10 contention mode : A mode of transmission in which a
3.19 exception condition: The condition assumed by a
transmitter can send on its own initiative.
secondary/combined station upon receipt of a frame which it
cannot execute due either to a transmission error or to an
internal processing malfunction.
3.11 control escape (CE) : The unique sequence of eight
bits (101 11 110) employed to indicate the following octet has
been modified according to the transparency algorithm for
3.20 flag sequence (F) : the unique sequence of eight bits
start/stop transmission environments.
(O1 11 11 10) employed to delimit the opening and closing of a
frame.
3.12 control field (C) : The sequence of eight (or sixteen, if
extended) bits immediately following the address field of a
3.21 frame : The sequence of address, control. information,
frame.
FCS fields, bracketed by opening and closing flag
and
sequences.
NOTE -The matent of the control field is interpreted by:
NOTE - A valid frame is at least 32 bits in length and contains
a) the receiving secondary/combined station, designated by
an address field, a control field and a frame check sequence. A
the address field, as a command instructing the performance
frame may or may not include an information field.
of sane specific function; and
b) the receiving primary/combined station as a response
3.22 frame check sequence (FCS) : The field immediately
secondary/combined station, designated by the
fmn the
preceding the closing flag sequence of a frame, containing the
address field, to one or more commands.
bit sequence that provides for the detection of transmission
I
ISO/IEC 4335 : 1991 (E)
exrors by the receiver.
the secondary/combined station to one or more commands.
3.23 half-duplex transmission : See ISO/IEC 2382-9, term
332 response frame:
09.03.02.
a) All frames transmitted by a secondary station.
b) Those frames transmitted by a combined station that
3.24 higher level: The conceptual level of control or
contain the adàress of the transmitting combined station.
processing logic existing in the hierarchical structure of a data
station (primary, secondary or combined station) that is above
the data link level and upon which the performance of data
333 secondary station : The data station that executes data
link level functions are dependent; for example device
link control functions as instnicted by the primary station.
control, buffer allocation, station management, etc.
NOTE - A secondary station inteqmts received commands
and generates responses for transmission.
3.25 information field (INFO): The sequence of bits,
occurring between the last bit of the control field and the first
bit of the frame check sequence.
334 secondary staüon status : The current condition of a
secondary station with respect to processing the series of
NOTE - The information field contents of I and UI frames are
commands received from the primary station.
not interpreted at the data link level.
335 two-way alternate data communication: See
3.26 interframe time fiii: The sequence or condition
ISO/IEC 2382-9. term 09.05.03.
transmitted between frames.
336 two-way simultaneous data communication : See
3.27 intraframe time fill: in starvstop transmission, the
ISO/IEC 2382-9, term 09.05.02.
sequence or condition transmitted within a frame when the
next octet is not available for contiguous transmission
337 unnumbered commands : The commands that do not
immediately following the preceding octet. For synchronous
contain sequence numbers in the control field.
transmission, there is no provision for intraframe time fill.
338 unnumbered responses: The responses that do not
3.28 invalid frame: A sequence of bits, following the
contain sequence numbers in the control field.
receipt of an apparent opening flag sequence, that either
a) is terminated by an abort sequence; or
4 Data link channel states
b) contains less than 32 bits before an apparent closing
flag sequence is detected. 4.1 Active data link channel state
4.1.1 General
3.29 primary station : The data station that supports the
A data link channel is in an active state when the primary
primary station control functions of the data link, i.e. that
station, a secondary station or a combined station is actively
generates commands for transmission and interprets received
transmitting a frame, a single abort sequence, or interframe
responses.
time fill. In the active state, the right to continue transmission
shall be reserved.
NOTE - Specific responsibilities assigned to the primary
statim include:
4.1.2 Abort
a) initiaikation of cmtrol signal interchange;
4.1.2.1 Synchronous transmission
b) organization of data flow; and
Aborting a frame shall be accomplished by transmitting at
c) actions regarding error control and error recovery
least seven contiguous "1" bits (with no inserted "O bits) to
functions at the data iink level.
end the frame. Receipt of seven contiguous "1" bits shail be
interpreted as an abort and the receiving data station shall
330 primaryhecondary station : The general case where
ignore the kame.
the station may be either a primary station or a secondary
station.
NOTE - If mm than seven "1" bits are sent to aboit, care
should be taken because if 15 or more "1" bits are sent,
including those already transmitted at the time of the decision m
331 response : In data communication, a reply represented
abort, an idle data link channel state wiii resuit.
in the control field of a response frame that advises the
primary/cmmbmed station with respect to the action taken by
ISODEC 4335 : 1991 (E)
a) normal response mode (NRM);
4.1.23 Starflstop transmission
Aborting a frame shall be accomplished by transmitting the b) asynchronous response mode (ARM); and
two-octet sequence "control escapeclosing flag". Receipt of
c) asynchronous balanced mode (ABM).
this sequence shall be interpreted as an abort and the
receiving data station shall ignore the frame.
5.1.1 Normal response mode (NRM)
in NRM, which is an unbalanced data link operation mode,
4.13 Interframe time fill
as the
the secondary station shail initiate transmission only
4.13.1 Synchronous transmission
result of receiving explicit permission to do so from the
primary station. After receiving permission, the secondary
Interframe time fill shall be accomplished by transmitting
station shall initiate a response transmission. The response
contiguous flags between frames.
transmission shall consist of one or more frames while
maintaining an active data link channel state. The last frame
4.13.2 Starflstop transmission
of the response transmission shall be explicitly indicated by
the secondary station. Following indication of the last frame.
Interframe time fill shall be accomplished by the transmission
the secondaxy station shall stop transmitting until explicit
of either continuous flags or marking condition (logical "1"
permission is again received from the primary station.
state) between frames. Selection of the interfiame time fill
method depends on systems requirements.
5.1.2 Asynchronous response mode (ARM)
4.1.4 Intraframe time fill
In ARM, which is an unbalanced data link operational mode,
the secondary station may initiate transmission without
4.1.4.1 Synchronous transmission
receiving explicit permission to do so from the primary
There is no provision for time fill within a frame.
station. Such an asynchronous transmission may contain
single or multiple frames and shall be used for information
field transfer and/or to indicate status changes in the
4.1.4.2 Starflstop transmission
secondary station (for example. the number of the next
Inter-octet time fill within a frame shall be accomplished by
expected information frame, transition from a ready to a busy
transmitting continuous mark-hold condition (logical "1"
condition or vice versa, occurrence of an exception
state). There is no provision for time fill within an octet (i.e.
condition).
between the start element and stop element).
5.13 Asynchronous balanced mode (ABM)
4.2 Idle data link channel state
In ABM, which is a balanced data link operation mode, either
4.2.1 Synchronous transmission
combined station may send commands at any time and may
initiate response frame transmission without receiving
A data link channel is in an idle state when a continuous "1"
explicit permission to do so from the other combined station.
state is detected that has persisted for at least 15 bit times;
Such an asynchronous transmission may contain single or
detection of the idle state at the data link level shall be
multiple frames and shall be used for information field
considered to indicate that the remote data station has
transfer and/or to indicate stahis changes in the combined
relinquished its right to continue transmission.
station (for example, the number of the next expected
to a busy condition
information frame, transition from a ready
4.2.2 Stadstop transmission
or vice vers4 occurrence of an exception condition).
A data link channel is in the idle state when a coniinuous
mark-hold condition persists for the period of time
5.2 Non-operational modes
determined by a system-specified timeout function. The
The three non-operational modes are:
duration of this timer is not a subject of this International
Standard.
a) normal disconnected mode (NDM);
b) asynchronous disconnected mode (ADM); and
5 Modes
c) initialization mode (IM).
Three operational modes and three non-operational modes are
The disconnected modes (NDM and ADM) differ from the
defined.
operational modes in that the secondary/combined station is
logicaliy disconnected from the data link; i.e. no information
(I) or supervisory frames are transmitted or accepted. The
5.1 Operational modes
initialization mode (IM) differs from the operational modes in
"fie three operational modes are:
that the secondary/combined station data link control program
is either in need of regeneration or is in need of an exchange
ISO/IEC 4335 : 1991 (E)
of the parameters to be used in an operational mode. A secondary/unnbind station in a disconnected mode (NDM
or ADM) shall as a minimum capability, be capable of
These two disconnected modes (NDM and ADM) are
generating the DM response with the F bit set to "1" in
provided to prevent a secondary/combined station from
response to a command frame received with the P bit set to
appearing on the data link in a fully operational mode during
"1".
unusual situations or exception conditions since such
operation could cause:
A secondary/combined station in a disconnected mode (NDM
or ADM) receiving a DISC command shall respond with the
a) unintended contention in ARM;
DM responses. A secondary/combmed station in the
initialization mode receiving a DISC command shall respond
b) sequence number mismatch between the primary
with the unnumbered acknowledgement (UA) response if it is
station and the secondary station, or between combined
capable of actioning the command A secondary/command
station; or
station in an operational mode receiving a DISC command
c) ambiguity in the primary/combined station as to the
shall respond with the UA response.
status of the secondary/other combined station.
to receiving a
Examples of possible conditions (in addition
A secondary station shall be system predehed as to the
DISC command) which shall cause a secondary/combined
condition(s) that causes it to assume a disconnected mode.
station to enter a disconnected mode are:
The disconnected mode (NDM or ADM) shall also be system
predefined. A combined station shall be system predefined as
a) the seumdary/combined station power is turned on,
to the condition(s) that causes it to assume the asynchronous
or restored following a temporary loss of power;
* disconnected mode (ADM).
b) the secondary/combmed station data link level logic
The secondary station capability is a disconnected mode shall
is manually reset; and
be limited to:
c) the secondary/combined station terminal is manuaily
switched from a local (home) condition to a connected-
a) accepting and responding to one of several
on-thedata-link (on-line) condition.
appropriate mode-setting commands [set normal
response mode (SNRM), set asynchronous response
A secondary/combined station in a non-operational mode
mode (SARM), set normal response mode extended
shall not establish a frame reject exception condition.
(SNRME). set asynchronous response mode extended
(SNE), set initialization mode (SM), and disconnect
5.2.1 Normal disconnected mode (NDM)
(DISC)l;
In NDM, which is an unbalanced data link non-operational
b) accepting and responding to an exchange
identification @ID) command; mode. the secondary station shall be logically disconnected
from the data link and shall, therefore. not be permitted to
c) accepting and responding to a test (TEST) command;
accept information in I command frames or to transmit
information in I response frames. The secondary station
d) accepting and responding to an unnumbered poll
(UP) command; shall, however, be permitted to accept information in üI
command frames and to transmit information in üI response
e) transmitting a disconnected mode (DM), request
frames. The secondary station has normal mode respond
initialization mode (RIM), exchange identification
opportunity and shall initiate a single frame response
@ID), or request disconnect (RD) response frame at a
transmission, indicating its status, as a result of receiving a
respond opportunity to solicit a specific action on the
command frame with the P bit set to "1"; optionally, it may
part of the primary station;
initiate such a response as the result of receiving a UP
command with the P bit set to "O.
f) accepting an unnumbered information (UI) command;
and
In this mode, a secondary station shall action oniy mode
g) transmitting a UI response at a respond opportunity.
setting, XID. Ui and TEST commands. Mode setting
commands, except the DISC command, that can be actioned
The capability of a combined station, as a receiver of
shall be responded to with the UA response at the earliest
commands, in the asynchronous disconnected mode, shall be
respond opportunity. A XID or TEST command that can be
the same as that stated above for a secondary station
actioned shali be responded to with the XID and TEST
(appropriate mode setting commands for a Combined station
response, respectively, at the eariiest respond opportunity.
include the set asynchronous balanced mode (SABM), set
Receipt of an implemented mode setting, XID or TEST
asynchronous balanced mode extended (SABME), SIM and
command that cannot be actioned. or receipt of any other
DISC commands]. In addition, since the combined station
command (except a UI command) with the P bit set to "1".
has the ability to transmit commands at any time, the
shall cause a secondary station in NDM to respond at the
combined station may transmit an appropriate mode setting,
earliest respond opportunity with the DM response, or, if the
XID, Ui or TEST command.
secondary station determines it is unable to function, with the
ISO/IEC 4335 : 1991 (E)
RIM response. Receipt of a UI command with the P bit set to
secondary/combined station in ADM.
"1" shall cause a secondary station in NDM to respond at the
earliest respond opportunity with a üI response. with a DM Because a combmed station is also a generator of commands,
response, or with a RIM response. In the case where an it can terminate a disconnected mode at any time by
implemented mode setting, XID or TEST command has been transmitting an appropriate mode setting command (SABM,
received but cannot be actioned or a status condition is to be SABME, or SIM). Such action can be taken spontaneously or
reported, a UP command with the P bit set to "O' shall cause a as a result of transmission received from the other combined
secondary station in NDM to respond with a DM or RIM
station (for example, a DM or RIM response).
response, as approPriate. Any command with the P bit set to
"O", other than the implemented mode setting, XID, UI,
5.23 Initialization mode (IM)
TEST or UP commands as described above, may be ignored
by the secondary station in NDM. In IM. which is an unbalanced data link or balanced data link
non-operational mode, a secondary/one combined station data
link control program may be initialized or regenerated by the
5.23 Asynchronous disconnected mode (ADM)
primary/other combmed station action, or other parameters to
be used in the operational mode may be exchanged. IM is
In ADM. which is an unbalanced data link or balanced data
invoked when the primary/one combined station concludes
link non-operational mode, the secondary/combined station
that a secondary/other combined station is operating
shall be logically disconnected from the data link and shall,
abnormally and needs its data link control program corrected,
therefore, not be permitted to accept information in I
and for upgrading a secondary/other combmed station data
command frames/I command or response frames,
link control program. Similarly, a secondary/one combined
respectively. or to transmit information in I response frames/I
station may determine it is unable to function due to program
command or response frames, respectively. The
checks and request IM to obtain a good program from the
secondary/combined station shall, however, be permitted to
primary/other combined station.
accept information in UI command frames/UI command or
response frames, respectively, or to transmit information in
A secondary/combined station shall enter IM upon sending a
UI response frames/UI command or response frames,
UA response, at its system predefined respond opportunity, in
respectively. The secondary station, or combined station as a
response to the receipt of a SIM command. A
receiver of commands. has asynchronous mode respond
secondary/combined station may request a SIM command by
opportunity and may initiate a response transmission in two-
sending a RIM response. In IM, the primary/one combmed
way alternate exchange upon detection of an idle data link
station and a secondary/other combined station may exchange
channel state, and in two-way simultaneous exchange at any
information in the predetermined manner specified for that
time. Such a response transmission shall only consist of a U1
secondary/each combmed station (for example, UI or I
response frame. a request for a mode setting command (DM),
frames).
a request for exchange of identification (XID), or a request
for initialization (RIM) if the secondary station, or combined
IM shall be terminated when a secondary/combined station
station as a receiver of commands, determines it is unable to
receives and acknowledges (via a UA response) one of the
function. The combined station, as a transmitter of command
other mode setting commands, or when entering the
frames, is also permitted to send a UI command frame at any
disconnected mode caused by internal constraints such as loss
asynchronous mode respond opportunity.
of power.
In this mode a secondary station, or combined station as a
receiver of commands. shall action only mode setting, XID,
6 Control field and parameters
UI and TEST commands. Mode setting commands, except
the DISC command, that can be actioned shail be responded
6.1 Control field formats
to with the UA response at the earliest respond opportunity.
6.1.1 General
A XID or TEST command that can be actioned shall be
responded to with the XID or TEST response, respectively, at
The three formats defined for the control field (see table 1)
the earliest respond opportunity. Receipt of an implemented
are used to perform numbered information transfer, numbered
mode setting, XiD or TEST command that cannot be
supervisory functions and unnumbered control functions and
actioned, or receipt of any other command (except a U1
unnumbered information transfer.
command) with the P bit set to "1". shall be responded to with
a DM response, or, if the secondary station, or combined
6.1.2 information transfer (I) format
station as a receiver or commands. determines it is unable to
function, with the RIM response. Receipt of a U1 command
The I format is used to perform an information transfer. The
with the P bit set to "1" shall cause a secondary/combined
functions of N(S). N(R) and P/F are independent; i.e. each I
station in ADM to respond at the earliest respond opportunity
fiame shall have an N(S) sequence number, an N(R) sequence
with a UI response, with a DM response. or with a RIM
number which may or may not acknowledge additional I
response. Any command with the P bit set to "O", other than
frames at the receiving data station, and a P/F bit that may be
the implemented mode setting, XID, UI, TEST or UP
set to "1" or "O.
commands as described above. may be ignored by the
ISODEC 4335 : 1991 (E)
Table 1 - Control field formats modulo 128 are shown in table 4 (see 7.4).
The maximum numbex of sequentiaily numbered I frames that
a primary, secondary or combined station may have
format for
outstanding (i.e. unacknowledged) at any given time shaü
never exceed one less than the modulus of the sequence
information transfer command
numbem. This restriction is to prevent any ambiguity in the
respnse (I format)
association of transmitted I frames with sequence numbers
supervisory commands/
during normal operation and/or error recovery action.
responses (S format) 1 O S S P/F N(R)
Unnumbered commands/
NOTE - The number of outstanding I fms may be further
responses (U format) 11 M M P/F M M M
restricteù by the data station frame storage capabilitr, for
example. the number of I frames that cllll be stored for
* N(S) = transmitting send sequence number
transmission andor retransmission in the event of a transmission
(bit 2 = low-order bit)
error. ûptimum data link efficiency can oniy be obtained,
however, if the minimum data station frame storage capacity is
sufficient for the maximum anticipated mmd trip transmission
N(R) = transmitting receive sequence number
delay.
(bit 6 = low-order bit)
S = supervisory function bit 62.2 Frame state variables and sequence numbers
62.2.1 General
M = modifier function bit
In HDLC operations, each data station shall maintain an
= poll bit - primary station or combined independent send state variable V(S) and an independent
P/F
station command frame transmissions/final receive state variable V(R) for the I frames it sends to and
bit - secondary station or combined station
receives from another data station. Each secondary station
response frame transmissions
shall maintain a V(S) for the I frames it transmits to the
(I = powfinal) primary station and an V(R) for the I frames it correctly
receives from the primary station. In the same manna, the
primary station shall maintain an independent V(S) and V(R)
for I frames sent to and received from, respectively. each
6.13 Supervisory (S) format
secondary station on the data link. Each combined station
shall maintain a V(S) for the I frames it transmiis to the other
The S format is used to perform data lii supervisory control
combined station, and a V(R) for the I frames it correctly
functions such as acknowledging I frames, requesting
receives from the other combined station.
retransmission of I frames, and requesting a temporary
suspension of transmission of I frames. The functions of
N(R) and P/F are independent; i.e. each S format frame shall
6.2.2.2 Send state variable V(S)
have an N(R) sequence number which may or may not
The send state variable denotes the sequence number of the
acknowledge additional I frames at the receiving data station,
next in-sequence I frame to be transmitted. The send state
and a P/F bit that may be set to "1" or "O.
variable can take the value O to modulus minus one inclusive
(where modulus is the modulus of the sequence numbering
6.1.4 Unnumbered (U) format
scheme and the numbers cycle through the entire range). The
value of the send state variable shall be incremented by one
The U format is used to provide additional data link control
with each successive I frame transmission, but shall not
functions and unnumbered information transfer. This format
exceed N(R) of the last received frame by more than modulus
shall contain no sequence numbers, but shall include a P/F bit
minus one.
that may be set to "1" or "O. Five "modifier" bit positions
are available, this allowing definition of up to 32 additional
command functions and 32 additional response functions.
6.2.23 Send sequence number N(S)
Only I frames shall contain N(S), the send sequence number
6.2 Parameters
of transmitted frames. Prior to transmission of an in-
sequence I frame. N(S) shall be set equal to the value of the
6.2.1 Modulus
send state variable.
Each I frame shall be sequentially numbered with a number
which may have the value O to modulus minus one inclusive
6.2.2.4 Receive state variable V(R)
(where modulus is the modulus of the sequence number).
The modulus equals 8 or 128. The sequence numbers cycle
The receive state variable denotes the sequence number of the
through the entire range. The control field formats for
next in-sequence I frame expected to be received. The
modulo 8 are shown in table 1. The control field formats for
receive state variable can take the value O to modulus minus
ISO/IEC 4335 : 1991 (E)
63.1.1 General
one inclusive (where modulus is the modulus of the sequence
numbering scheme and the numbers cycle through the entire
The P bit set to '1" shall be used to solicit a response frame
rate). The value of the receive state variable shall be
with the P bit set to "1" from the secondary/combined station.
incremented by one on receipt of an error-free, in-sequence I
frame whose send sequence number N(S) equals the receive
On a data lii. oniy one frame with a P bit set to "1" shall be
state variable.
outstanding in a given direction at a given time. Before a
primary/unnbined station issues another frame with the P bit
set to "I", it shall have received a response frame from the
63.25 Receive sequence number N(R)
secondary/combmed station with the F bit set to "1". If no
All I frames and S format frames shall contain N(R) which
valid response frame is obtained within a systemdehed
with the exception of the selective reject (SREJ) supervisory
time-out period, the retransmission of a command with the P
frame with the P/F bit set to "O shall indicate the N(S)
bit set to "1" for error recovery purposes shall be permitted.
sequence number of the next expected I frame.
63.13 Functions of the poll bit in NRh4
With this exception, prior to transmission of an I frame or S
format frame, the N(R) shall be set equal to the current value
In NRM, the P bit shall be set to '1" to solicit response frames
of the receive state variable. The N(R) indicates that the
from the secondary station. The secondary station shall not
station transmitting the N(R) has correctly received ail I
transmit until it receives either a command frame with the P
frames numbered up to N(R) - 1 inclusive.
bit set to "1" or a UP command.
In the case of the SREJ frame with the P/F bit set to "O". the
The secondary station may send I frames upon receipt of an I
N(R) indicates only that the I frame with N(S) equal to N(R)
frame with the P bit set to "l", certain S frames (RR, REJ or
has not been received.
SREJ) with the P bit set to "1". a UI command with the P bit
set to "1". or a UP command with the P bit set to "1" or "O.
As more than one SREJ frame with the P/F bit set to "O" may
be outstanding at any one time, it is necessary to ensure that
63.13 Functions of the poll bit in ARM and ABM
all non-received I frames are ultimately correctly received.
This may be achieved by multiple variable counters or by
In ARM and ABM. the P bit set to "1" shall be used to solicit
other means.
a response, at the earliest respond opportunity, with the F bit
set to "1".
(See 7.2.4 for definition of the use of the SREJ command and
response.) NOTE - For example, if the primary/oombined station requYes
positive acknowledgement that a particular command has been
received, it sets the P bit in the command to "1". This forces a
633 Poll/final (PIF) bit
response from the secondary/wmbined station as described in
6.3.2.2.
The poll (P) bit set to "1" shall be used by the
primary/combmed station to solicit (poll) a response or
sequence of responses from the secondq
63.2 Functions of the final bit
station(s)/combmed station.
A response frame with the F bit set to "1" shall be used by the
secondary/combmed station to acknowledge the receipt of a
The final (F) bit set to "1" shall be used:
command frame with the P bit set to "1".
a) by a secondary station in NRM to indicate the final
frame transmitted as the result of a previous soliciting
63.2.1 Functions of the final bit in NRM
(poll) comma114 and
in NRM, if the right to transmit was acquired by the receipt
b) by a secondary station in ARM and by a combined
of a P bit set to "1". then the secondary station shall set the F
station in ABM to indicate the response frame
bit to "1" in the last frame of its response transmission. If the
transmitted as the result of a soliciting (poll) command.
right to transmit was acquired by the receipt of a UP
command with the P bit set to "O, then the secondary station
63 Functions of the polYfina1 (P/F) bit
shali set the F bit to "O in each frame (including the last
The poll/final (P/F) bit shall serve a function in both
frame) of its response transmission.
command frames and response frames. (In command frames,
the P/F bit is referred to as the P bit. in response frames. it is
Following transmission of the last frame of its response
referred to as the F bit.)
transmission, the secondary station shall stop transmitting
until either a subsequent command frame with a P bit set to
,e ,, .
1 1s received, or a UP command is received.
63.1 Functions of the poll bit
ISO/IEC 4335 : 1991 (E)
63.22 Functions of the ûnal bit in ARM and ABM secondary/primary stati
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