Information technology — Telecommunications and information exchange between systems — High-level data link control (HDLC) procedures — Frame structure

Specifies the frame structure for data communication systems using bit-oriented high-level data link control (HDLC) procedures. Defines the relative positions of the various components of the basic frame and the bit combination for the frame delimiting sequence (flag). The mechanisms used to achieve bit pattern independence (transparency) within the frame are also defined. In addition, two frame checking sequences (FCS) are specified, the rules for address field extension are defined, and the addressing conventions available are described.

Technologies de l'information — Télécommunications et échange d'informations entre systèmes — Procédures de commande de liaison de données à haut niveau (HDLC) — Structure de trame

General Information

Status
Withdrawn
Publication Date
22-Dec-1993
Withdrawal Date
22-Dec-1993
Current Stage
9599 - Withdrawal of International Standard
Start Date
22-Aug-2002
Completion Date
22-Aug-2002
Ref Project

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ISO/IEC 3309:1993 - Information technology -- Telecommunications and information exchange between systems -- High-level data link control (HDLC) procedures -- Frame structure
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INTERNATIONAL
STANDARD
Fifth edition
1993-12-15
Information technology -
Telecommunications and information
exchange between systems - High-level
data link control (HDLC) procedures -
Frame structure
Technologies de / ‘information - Te ’le ’communications et bchange
d ’informations entre syst&mes - Proc6dures de commande de
liaison de don&es B haut niveau (HDLC) - Structure de trame
Reference number
ISO/IEC 3309:1993(E)
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ISO/IEC 3309:1993(E)
Paee
Contents v
. . .
111

Foreword ...............................................................................................................

Introduction .............................................................................................................

1 scope ..............................................................................................................

2 Normative references .....................................................................................

3 Basic frame structure ......................................................................................

4 Elements of the frame ....................................................................................

5 Extensions ......................................................................................................

6 Addressing conventions ..................................................................................

Annex
A Explanatory notes on the implementation of the frame checking sequence..
0 ISO/IEC 1993

All rights reserved. Unless otherwise specified, 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 l Case postale 56 l CH-1211 Genkve 20 . Switzerland
Printed in Switzerland
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Q ISO/IEC
ISO/IEC 3309:1993(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 3309 was prepared by Joint Technical Committee
ISO/IEC JTC 1, Information technology, Subcommittee SC 6, Telecommuni-
cations and information exchange between systems.
This fifth edition cancels and replaces the fourth edition (ISO/IEC 3309: 1991),
and incorporates ISO/IEC 3309 amendment 2.
Annex A of this International Standard is for information only.
l . .
111
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-a ISO /IEC
ISO /IEC 3309 :1993 (E)
In troduc ti on
o f

This In te rna tional S tanda rd is one o f a se ri es to be used in the implemen ta tion

transmission facili ties .
va ri ous applica tions which u tilize synch ronous o r s ta rt/s top
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INTERNATIONAL STANDARD @ IsobEc ISO/IEC 3309:1993(E)
Information technology - Telecommunications and
information exchange between systems - High-level data
link control (HDLC) procedures - Frame structure
(i.e., start or stop elements, see 4.7.2) or bits or octets inserted
1 scope
for transparency (see 4.5).
This International Standard specifies the frame structure for
Each frame consists of the following fields (transmission
data communication systems using bit-oriented high-level
sequence left to right):
data link control (HDLC) procedures. It defines the relative
positions of the various components of the basic frame and
the bit combination for the frame delimiting sequence (flag).
Address Control Infmnatio~ FCS
FM! nag
The mechanisms used to achieve bit pattern independence
01111110 8 bits 8 bits 16or32bits 01111110
(transparency) within the frame are also defined. In addition,
two fhme checking sequences (FCS) are specified; the rules
* An unspecified number of bits which in some cases may be
for address field extension are defined; and the addressing
a multiple of a particular character size; for example, an octet.
conventions available are described.
where
Control field encodings and formats are defined in other
International Standards.
Flag = flag sequence
Address = data station address field
2 Normative references
Control = control field
The following standards contain provisions which, through
reference in this text, constitute provisions of this
Information = information field
International Standard. At the time of publication, the
editions indicated were valid. All standards are subject to
FCS = frame checking sequence field
revision, and parties to agreements based on this
International Standard are encouraged to investigate the
Frames containing only control sequences form a special case
possibility of applying the most recent editions of the
where there is no information field. The format for these
standards indicated below. Members of IEC and IS0
hmes shall be
maintain registers of currently valid International Standards.
1 Flag lAddresslControll FCS I Flag 1
01111110~ 8bits 1 Wits ~16or32bits~01111110
ISO/IEc 646:1991, IrIformution technology - IS0 7-bit co&d
character set for information interchange.
IS0 2382-9 : 1984, Data processing - Vocabulary - Part
09: Data conmunkation.
4 Elements of the frame
4.1 Flag sequence
3 Basic frame structure
All frames shall start and end with the flag sequence. All data

In HDLC, all transmissions are in frames. The basic frame stations which are attached to the data link shall continuously

Thus, the flag is used for frame

structure does not include bits inserted for bit-synchronization hunt for this sequence.

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o ISO/IEC
ISO/IEC 3369:1993(E)

synchronization. A single flag may be used as both the the octet. The method of providing and unambiguously

closing flag for one frame and the opening flag for the next identifying the pad bits is not a subject of this International

frame. Standard.
4.5 Transparency
4.2 Address field
45.1 Synchronous transmission
In command frames, the address shall identify the data
The transmitter shall examine the frame content between the
station(s) for which the command is intended. In response
two flag sequences including the address, control and FCS
frames, the address shall identify the data station from which
fields and shall insert a “0” bit after all sequences of 5
the response originated.
contiguous “1” bits (including the last 5 bits of the FCS) to
ensure that a flag sequence is not simulated. The receiver
shall examine the frame content and shall discard any “0” bit
4.3 Control field
which directly follows 5 contiguous “1” bits.
The control field indicates the type of commands or
responses, and contains sequence numbers, where
4.5.2 Start/stop transmjssion - basic transparency
appropriate. The control field shall be used
Two levels of transparency processing are specified for use
a) to convey a command to the addressed data station(s)
with start/stop mode transmission. These are seven-bit data
to perform a particular operation, or
path transparency (SBDPT), specified in subclause 4.5.2.1;
b) to convey a response to such a command from the
and control-octet transparency, specified in subclause 4.5.2.2.
addressed data station.
Control-octet transparency shall always be performed.
SBDPT is an option, use or non-use of which is selected for a
given data link by means outside the scope of this
4.4 Information field
International Standard (e.g., a priori knowledge, bilateral
Information may be any sequence of bits. In most cases it
agreement, heuristic implementation techniques).
will be linked to a convenient character structure, for example
octets, but, if required, it may be an unspecified number of
452.1 Seven-bit data path transparency
bits and unrelated to a character structure.
When SBDPT is selected, the content of each frame, from
For start/stop transmission there shall be eight (8) information
address field to FCS field inclusive, shall be transferred
bits between the start element and the stop element. If the
between sender and receiver as a frame-image derived from
information field is other than a multiple of 8 bits, the final
the original frame as follows, and as indicated in figure 1.
remainder less than an octet will require pad bits to complete
Original
FCS
Address
Frame
n <= 7 octets
. .
. .
. .
Original
Segment
. . . . . . . . .
. . . . . . . . .
Image
i() i() i() io . . . . . . . . . 0
i() i() i()
. . . . . . . . . . . . . . . .
Segment -I
*.....a-
. . :
. l . ...***
. .*
*....*” .
. .
. *....a-* .’
*..- . :
,..***
. .-
. . . ...**=. .
. l ..*- ••9 :
I 8 octets 1 8 octets n+l octets
1 I
Frame
Image
Figure 1 - Original Frame to Frame-Image Transition
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ISODEC 3309:1993(E)
0 ISO/IEC

The sequence of octets making up the frame content is control, and FCS fields and, following completion of the FCS

considered as divided into a sequence of contiguous seven-
calculation, shall:
octet segments, with possibly a final segment having length
a) Upon the occmence of the flag or a control escape
between one and six octets inclusive. These segments are
and
octet, complement the 6th bit of the octet,
referred to as “original segments.”
Insert a control escape octet immediately preceding
The fkne-image consists of a sequence of image segments b)
the octet resulting from the above prior to transmission.
defined, in one-to-one correspondence with the original
segments, as follows:
The receiver shall examine the frame-image between the two
flag octets and shall, upon receipt of a control escape octet
a) image segments occur in the
same order as the
and prior to FCS calculation:
corresponding original segments;
a) Discard the control escape octet’ and
b) each image segment is one octet longer than its
original segment;
Restore the immediately following octet
b) bY
complementing its 6th bit.
c) the first part of each image segment is a copy of its
original segment’ but with the most significant bit
NOTE - Other octet values may optionally be included in the
(MSB) of each octet set to zero;
transparency procedure by the transmitter. Such inclusion shah
d) the remaining, final, octet of each image segment has
be subject to prior system/application agreement.
its least significant bit (LSB) set to the value of the MSB
of the last octet of the original segment’ its next to least
4.53 Start/stop transmission - extended transparency
significant bit set to the value of the MSB of the next to
last octet (if any) of the original segment, and so on;
When necessary and by prior agreement between the stations,
the transmitter may apply the above transparency procedure
e) in the final octet of each image segment, all higher
(4.5.2) to octets in the groups defined below, in addition to
order bits for which no corresponding octet exists in the
the flag and control escape octets.
original segment are set to zero.
4.53.1 Flow-control transparency
NOTE 1 - At the transmitter, the final octet of each image
segment can be generated by shifting left the MSB of each octet
The flow-control transparency option provides transparency
in the original segment, in sequence, into an initially zero octet:
processing for the DCl/XON and DC3/XOFF control
this achieves the correct bit-positioning both for complete
characters defined in IS0 646 (i.e., 1000100x and 1100100x,
seven-octet segments and any short segment at the end of the
respectively, where “x” may be either “0” or “1 ”). This has
frame.
the effect of assuring that the octet stream does not contain
values which could be interpreted by intermediate equipment
NOTE 2 - The MSB of each image-segment octet is defined
as flow control characters (regardless of parity).
as zero only for uniqueness of the mapping: because its value is
known and plays no part in the reconstruction of the original
4.53.2 Control-character octet transparency
segment at the receiver, it need not actually be transferred across
The control-character octet transparency option provides
data paths that, far example, force parity setting of the MSB of
transparency processing for all octets in which both the 6th
eachoctet.
and 7th bits are “0” (i.e., xxxxxOOx, where “x” may be either
“0” or “1 ”) as well as for the DELETE character octet (i.e.,
4.5.2.2 Control-octet transparency
111111 lx, where “x” may be either “0” or “1 ”). This has the
effect of assuring that the octet stream does not contain
The following transparency mechanism shall be applied to
values which could be interpreted by intermediate
each frame-image: a frame-image is as defined in subclause
equipment as the control characters or DELETE character
4.5.2.1 when SBDVT is selected, and otherwise is identical to
defined by IS0 646 (regardless of parity).
the frame content from address field to FCS field inclusive.
4.6 Frame checking sequencing (FCS) field
The control escape octet is a transparency identifier that
identifies an octet occurring within a frame to which the
4.6.1 General
following transparency procedure is applied. The encoding
Two frame checking sequences are specified; a 16-bit frame
of the control escape octet is:
checking sequence and a 32-bit frame checking sequence.
The 16-bit frame checking sequence is normally used. The
12345678 Bit position in octet
32-bit frame checking sequence is for use by prior agreement
10111110
in those cases that need a higher degree of protection than
d can be provided by the 16-bit frame checking sequence.
Low or&r bit, first bit
lransmitted&eived
NOTES
The transmitter shall examine the frame-image between the
degrees
1. If future a@ications show that other of protection
opening and closing flag sequences including the address,
in the I
are needed, different numbers of bits FCS wiJl be
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o ISO/IEC
ISO/IEC 3309:1-993(E)
specified, but they will be an integral number of octets.
xk(x31 + x30 + xB + P + xn + P + P + xzA +
2. Explanatory notes on the implementation of the frame
x= + xn + x21 + xm + xl9 + x1* + x1’ + xl6 +
checking sequence are given in Annex A.
xl5 + xl4 + xl3 + xl2 + xl1 + xl0 + x9 + x8 +
4.6.2 li-bit frame checking sequence
.x7+x6 +x5 +x4 +x3 +x2+x+ 1)
The 16-bit FCS shall be the ones complement of the sum
(modulo 2) of divided (modulo 2) by the generator polynomial
the remainder of x32 + x” + xw + x= + xl6 + xl2 + xl1 + xl0 +
Xk(P + xl4 + xl3 + xl2 + xl1 + xl0
+ x9 +
xa+x7+xs+x4+x2+x+1,
where k is the number of bits in the frame existing
x8+x7+x6+x5+x4+x3+x2+x+1)
between, but not inclu
...

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