Scope
This International Standard defines an application of the following HOLC Standards: ISO 3309, ISO 4335, ISO 7478, and ISO 7809. When there is difficulty in the interpretation of a reworded requirement fro1 one of the other International Stan¬dards, the original requirement of ISO 3309, ISO 4335, ISO 7478 or ISO 7809 is definitive. It also defines the structure, elements and procedures for the operation of a DTE using the X.25 Level 2 LAPS protocol as specified in CCITT Recommen¬dation X.25.1) The procedure is applicable to data interchange between a OTE and a DCE, or between two DTEs. The pro¬cedures are defined for use on synchronous, duplex links.
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 balanced asynchronous class of procedure with optional functions 2 and 8 (BAC, 2, 8). Extended (modulo 128) operation is the ISO balanced asynchronous class of procedure with optional func¬tions 2, 8, and 10 (SAC, 2, 8, 10). For those DTE/DCE connec¬tions 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 connec¬tions that support both basic (modulo 8) operation and extended (modulo 128) operation, the choice is made by bilateralagreement.
NOTE - The procedure herein described as basic {modulo 81 operation is the only one available in all public data networks.
Clause 4 describes the elements of procedures. Some aspects are only operable for the basic (modulo 8) operation and some for the extended (modulo 1281 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 in¬dividual SLP failures are not to disrupt the higher level opera¬tion. 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.
