This Global System for Mobile communications Technical Specification ( GTS) specifies the link layer used for signalling on the A-bis interfa ce between Base Station Controller (BSC) and Base Transceiver Station (BTS). The use and general aspects of the A-bis interface are describe d in GSM 08.51 and the interface aspects in GSM 08.52. The Link Access Procedure on the D-channel (LAPD) specification used o n the A-bis interface in the GSM PLMN is a subset of the ETS 300125 w hich in turn has less options than the CCITT Recommendation Q.921. This description contains first the protocol definitions and second th e services provided by the layer 2 to the layer 3.