This TS 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.