4 General

3GPP44.006Data Link (DL) layer specificationMobile Station - Base Station System (MS - BSS) interfaceRelease 17TS

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.

NOTE 1: The term Dm channel is used for convenience to designate the collection of all the various signalling channels required in the GSM system. See also 3GPP TS 44.003.

The concepts, terminology, overview description of LAPDm functions and procedures, and the relationship with other Technical Specifications are described in general terms in 3GPP 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 3GPP TS 44.003.

NOTE 2: AGCH, NCH and PCH are sometimes referred to by the collective name CCCH and FACCH, SACCH and SDCCH are, similarly, referred to by the collective name DCCH.

LAPDm may also be used on other types of channel.

NOTE 3: As stated in 3GPP TS 44.005, the term "data link layer" is used in the main text of this Technical Specification. However, mainly in figures and tables, the terms "layer 2" and "L2" are used as abbreviations. Furthermore, in accordance with 3GPP TS 24.007 and 3GPP TS 44.018, the term "layer 3" is used to indicate the layer above the data link layer.

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.

4.1 Options

Support of short L2 header type 1 is an option in both the mobile station and the network; under certain conditions the support is mandatory, as specified in other Specifications. A layer 2 protocol entity not implementing short L2 header type 1 shall diagnose an E/A bit error and proceed as defined in annex G.2.3.