The present document specifies the link layer used for signalling on the A-bis interface between BSC and BTS. The use and general aspects of the A-bis interface are described in TS 48.051
and the interface aspects in TS 48.052
The Link Access Procedure on the D-channel (LAPD) specification used on the A-bis interface in the GSM PLMN is a subset of the EN 300 125 which in turn has less options than the CCITT Recommendation Q.921.
This description contains first the protocol definitions and second the services provided by the layer 2 to the layer 3.
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.
For a specific reference, subsequent revisions do not apply.
For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
: "Vocabulary for 3GPP Specifications".
: "Base Station Controller - Base Transceiver Station (BSC - BTS) interface; General aspects".
: "Base Station Controller - Base Transceiver Station (BSC - BTS) interface; Interface principles".
CCITT Recommendation Q.921: "Integrated services digital network (ISDN) user-network interface - Data link layer specification".
EN 300 125: "Integrated Services Digital Network (ISDN); User-network interface data link layer specification Application of CCITT Recommendations Q.920/I.440 and Q.921/I.441".
Abbreviations used in the present document are listed in TR 21.905