BS ISO IEC 3309:1991 download.Information technology — Telecommunications and information exchange between systems — High-level Data Link Control (HDLC) procedures — Frame structure.
1 Scope
BS ISO IEC 3309 specifies the frame structure for data communication systems using bit-oriented high-level data link control (HDLC) procedures. It defines the relative positions of the vanous 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 arc described.
Control field encodings and formats are defined in other nternational Standards.
2 NormatIve reference
The following standard contains provisions which, through reference in this text, constitute provision of BS ISO IEC 3309. At the time of publication, the edition indicated was valid. All standards are subject to revision, and parties to agreements based on BS ISO IEC 3309 are encouraged to investigate the possibility of applying the most recent edition of the standard indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.
ISO 238219: 1984, Data processing – Vocabulary Part 09: Data comnuu’Jcation.
ISO/IEC 646: 1991, Information technology — ISO 7-bit coded character set for Information Interchange
3 Basic frame structure
In HDLC. all transmissIons are in frames. The basic frame strijctuire does not include bits inserted for bii-synchroniiation (I.e., start or stop elements, see 4.7.2) or bits or octets Inserted for transparency (see 4.5).
4.2 Address field
In command framem, the address shall identify the data station(s) for which the command Is intended. In response frames, the address shall idcntify the data station from which the response originated.
4.3 Control field
The control field indicates the type of commands or responses, and contains sequence numbers, where appropriate. The control field shall be used
a) to convey a command to the addressed data station(s) to perform a particular operation, or
b) to convey a response to such a command from the addressed data station.
4.4 InformatIon field
Information may be any sequence of bits. In most cases it will be linked to a convenient character structure, for example octets, but, it required, it may be an unspecified number of bits and unrelated to a character structure.
For start/stop transmission there shill be eight (8) information bits between the stat element and the stop clement. If the information field is other than a multiple of 8 bits the final remainder less than an octet will require pad bits to complete the ocleL The method of providing and unambiguously identifying the pad bits is not a subject of this International Standard.
4.5 Transparency
4.5.1 Synchronous transndssloa
The transmiuer shall examine the frame content between the two flag sequences including the address, control and FCS fields and shill insert a N0 bit after all sequences of 5 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 fram, content and shall discard any 0N bit which directly follows 5 contIguous “1” bits.
433 , Startlstop transmission — basic transparency
The control escape octet is a transparency identifier that identifies an octet occwTing within a frame to which the following transparency procedure is applied. The encoding of the control escape octet is:
4.8.2 Start/stop transmission
Inter-frame time fill shall be accomplished by transmitting continuous mark-hold condition (logical 1M slate) or continuous flags, or a combination o( both.
4.9 Invalid frame
4.9.1 Synchronous transmission
An invalld frame is defined u one that is not properly bounded by two flags or one which ii too short (that Is, shorter than 32 bits between flags when using the 16-bit FCS and shorter than 48 bits between flap when using the 32-bit FCS). Invalid frames shall be ignored. Thus, a frame which ends with an all Ni0 bit sequence of lengthequal to or greater than seven bits shall be ignored.
As an example. one method of aborting a frame would be to transmit 8 contiguous NIN j.i•
4.9.2 Start/stop transmission
An invalid frame is defined as one that is not properly bounded by two flap or one that is too short (that is, shorter than four Dctets between flags when using the 16-bit FCS and shorter than six octets between flags when using the 32-bit RS, excluding octets inserted for transparency), or one in which octet framing is violated (i.e., a “0” bit occurs where a slop element is expected), r one that ends with a control escape-closing flag sequence. Invalid frames shall be ignored.
5 Extensions
5.1 Extended address field
A single octet address field shall normally be used and iii 256 combinations shall be available.
However, by prior agreement, the address field range can be extended by reserving the first transmitted bit (low-order) of each address octet which would then be set So binary zero to indicate that the following octet is an extension of the address field. The format of the extended octet(s) shall be the same as that of the first octet. Thus, the address field may be recursively extended. The last octet of an address field is indicted by setting the low. order bit to binary one.
When extension is used, the presence of a binaiy “1” in the first transmitted bit of the first address octet Indicates that only one address octet is being used. The use of address extension thus restricts the range of single octet addresses to 128.
5.2 Extended control field
The control field may be extended by one or more octets. The extension methods and the bit patterns for the commands and responses are defined in related International Standard(s).
6 Addressing conventions
6.1 General
The following conventions shall apply in the assignment of
addresses of data stations for which commands are intended.
6.2 All-statIon address
The address field bitpauern 11111111 is defined uthe all-station address.
The all-station address shall only be used with command frames, and it shall instruct all receiving data stations to accqx and action the associated command frame. Arty response to a command with the all-station address shall contain the assigned individual address of the data station transmiuing the response.
The all-station address may be used for all-station polling. When there is more than one receiving data station for which a command with the all-station is intended, any responses from these data stations shall not interfere with one another.
N(YTE — The mechanism used to avoid overlapping responics to a poll using the aU-station address Is net specified in the International Standaid.
The all-station address may be used to determine the data link level identification (assigned address) of data station(s) when tmbiown for example, in switched or reconflgtrcd siwations.
6.3 No-station address
The bit pattern 00000000 in the first octet of the extended or non-extended address field is defined as the no-station address.
The no-station address shall never be assigned to a data station.