This European Telecommunication Standard (ETS) defines the service offered by the physical layer (GSM 05-series of Technical Specifications) of the MS-BS interface (GSM 05- and 04-series of Technical Specifications). Its main objective is to be a guidance for the interface between the GSM Technical Specifications in the 05-series and the 04-series. It also specifies the format of signalling channels and the order of bit transmission. As far as possible@ this ETS makes use of the layering principles of the Reference Model for Open System Interconnection (OSI) as contained in CCITT Recommendations X.200 and X.210.