The use and general aspects of the A-bis interface are given in Technical Specification GSM 08.51. This specification defines the structure of the physical layer (layer 1) of the BSC - BTS/TRX interface for supporting traffic channels and control channels. Use of the physical layer for supporting link protocol is covered in Technical Specification GSM 08.56. The physical layer is the lowest layer in the OSI Reference Model and it supports all functions required for transmission of bit streams on the physical medium. For this specification only digital transmission will be considered, the use of analogue transmission is a national concern.