The present document defines a data link layer protocol to be used for signalling, and possibly also for other applications, on the MS BS interface.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
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Abbreviations used in the present document are listed in TS 21.905.

The present document describes the frame structure, elements of procedure, format of fields and procedures for the proper operation of the Link Access Procedure on the Dm channel, LAPDm. The concepts, terminology, overview description of LAPDm functions and procedures, and the relationship with other Technical Specifications are described in general terms in TS 44.005. The frame formats defined for LAPDm are based on those defined for LAPD. However, there are important differences between LAPDm and LAPD, in particular with regard to frame delimitation methods and transparency mechanisms. These differences are necessary for operation within the constraints set by the radio path. LAPDm supports two modes of operation:

• unacknowledged operation using UI frames;
• acknowledged operation using the multiple frame procedure.

As a choice of implementation, the two modes of operation may be implemented independently of each other. This is possible since there is no interactions between the two modes, other than queuing at the transmitter, even when they coexist on the same physical channel. For BCCHs and CCCHs only the unacknowledged mode of operation needs to be implemented. LAPDm is used for information sent on the control channels BCCH, AGCH, NCH, PCH, FACCH, SACCH and SDCCH as defined in TS 44.003. LAPDm may also be used on other types of channel. This Technical Specification is organized as follows: The frame structure for peer to peer communication is given in clause 5. The elements of procedure and formats of fields are given in clause 6. The elements of layer to layer communication are contained in clause 7. The details of the peer to peer procedures are given in clause 8. clause 6 summarizes the special protocol operations used mandatorily with SAPI=0 and SAPI = 3. The specification for the random access channel is contained in Annex A, even though it is not a LAPDm function. The present document is descriptive and does not constrain the implementation of the random access function. The procedure is used for CHANNEL REQUEST on the RACH and HANDOVER ACCESS on the main DCCH. (Annexes B to F are deleted). Annex G gives an overview of actions taken on frames containing parameter errors.