Scope
This International Standard defines an application of the following HDLC standards: ISO/IEC 3309, ISO/IEC 4335, ISO 7478, and ISO/IEC 7809. When there is difficulty in the interpretation of a reworded requirement from one of the other International Standards, the original requirement of ISO/IEC 3309, ISO/IEC 4 335, ISO 7478 or ISO/IEC 7809 is definitive. It also defines the structure, elements and procedures for the operation of a DTE using the X.25 LAPB protocol as specified in ITU-T Recommendation X. 25. (Future revisions of this International Standard will be made in accordance with revisions of ITU-T Recommendation X.25. The present version is based on the 1993 ITU-T Recommendation X.25.) The procedures are applicable to data interchange between a DTE and a DCE, or between two DTEs. The procedures are defined for use on duplex links, using synchronous transmission or start/stop transmission.
Clause 3 describes two frame structures: one for basic (modulo 8) operation and one for extended (modulo 128) operation. Basic (modulo 8) operation is the ISO/IEC balanced asynchronous class of procedure with optional functions 2 and 8 (BAC, 2, 8). Extended (modulo 128) operation is the ISO/IEC balanced asynchronous class of procedure with optional functions 2, 8 and 10 (BAC, 2, 8, 10). For those DTE/DCE connections that support both basic (modulo 8) operation and extended (modulo 128) operation, the choice is made at subscription-time only. For those DTE/remote DTE connections that support both basic (modulo 8) operation and extended (modulo 128) operation, the choice is made by bilateral agreement.
NOTE - The procedure herein described as basic (modulo 8) operation is the only one available in all public data networks.
Clause 3 also describes two methods for encoding the frames, as sequences of bits when synchronous transmission is used, and as sequences of octets when start/stop transmission is used. The start/stop encoding specifies optional mechanisms, for use in environments that are sensitive to transmission of octets with values that could be interpreted as ISO/IEC 646 control characters, a nd/or in environments that support transfer of only seven data bits per start/stop character. The choice of encoding is made by bilateral agreement, or other suitable means, to suit the data transmission characteristics of the environment.
Clause 4 describes the elements of procedures. Some aspects are only operable for the basic (modulo 8) operation and some for the extended (modulo 128) operation.
Clauses 5 and 6 describe the single link procedure (SLP) which is derived from the frame structure and the elements of procedures, and an optional multilink procedure (MLP), respectively. The SLP is used for data interchange over a single data link and the MLP is used for data interchange over a multiple of parallel SLPs. An MLP is required if the effects of individual SLP failures are not to disrupt the higher layer operation. An MLP can also be used over a single SLP by prior bilateral agreement. For DTE/DCE connections the choice of an MLP operation or not is made at subscription-time only. For DTE/remote DTE connections, the choice is made by bilateral agreement.
Where choices among alternative actions are indicated in the procedures, a recommended choice is usually indicated. Unless specifically stated otherwise, the choice of action does not affect interoperability with other implementations of this International Standard although efficiency of operation may be affected. Where such choices do affect interoperability, the procedures explicitly state that prior bilateral agreement on the choice of procedure with the remote end is needed. An attempt has been made to minimize such choices consistent with the need to satisfy a broad range of applications. A basic requirement for all implementations of this International Standard is that they be capable of responding, as specified, to any actions taken at the remote end that are permitted by this International Standard (except possibly for those procedures whose use involves prior bilateral agreement.)
Clause 7 covers the Static Conformance requirements, the Dynamic Conf ormance requirements, and the Protocol Implementation Conformance Statement (PICS).
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