This Global System for Mobile communications Technical Specification ( GTS) gives the principles on which the detailed interface specifications in the rest of the GSM 08.0X series of Technical Specifications are based. The set of fixed equipment accessed from the MSC through one particular instance of the interface will be later referred to as a Base Station System (BSS). A BSS ensures the coverage of n cells, where n can be 1 or more. The function of a BSS may be further subdivided into a control functio n, performed by one Base Station Controller (BSC) and a transceiving f unction, performed by n Base Transceiver Station equipments (BTS), o ne for each cell. However, the study of such a split is outside the sc ope of the 08.0x series of Technical Specifications, where the BSS wil l be considered as a whole. The BSS-MSC interface defined in the GSM 08 series of Technical Specif ications is designed to support a wide range of possible architectures on both sides. Characteristics like the physical location of the tran scoders/rate adaptation inside the BSS (either integrated into the tra nsceivers or very near to the MSC) or the use of traffic or signalling concentration at either side are left to the operators choice. Annex A to this Technical Specification contains guidance information concerning the use of remote mobile switching units, which for the purposes of this specification are considered as part of the MSC. Direct connection between two BSSs is not supported by this interface. This interface is based on the use of 1 or more 2 048 kbit/s digital t ransmission system interfaces. Each 2 048 kbit/s interface provides 31 *64 kbit/s channels which can be used for traffic or signalling as the operator requires. The signalling is layered, terminology similar to that in the OSI refe rence model is used in this series, however the layers referred to are not identical to the equivalently named layer in the OSI model. This interface is defined at the boundary of the MSC and has a per cha nnel bit rate of 64 kbit/s, but the net radio path traffic channel is at a rate of less than 16 kbit/s. A transcoder or rate adapter function is thus needed for the rate conversion. The interface is designed such that the transcoding or rate adaptation function may be geographically situated at either the MSC site or the BSS site, however the transcoder is considered to be part of the BSS. The interface has been designed around the aims of GSM 08.01 allowing each component and the system as a whole to evolve.