The present document gives the principles on which the detailed interface specifications in the rest of the 3GPP TS 48.0xx 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 function, performed by one Base Station Controller (BSC) and a transceiving function, performed by "n" Base Transceiver Station equipments (BTS), one for each cell. However, the study of such a split is outside the scope of the 48.0xx series of Technical Specifications, where the BSS will be considered as a whole. The BSS-MSC interface defined in the 3GPP TS 48 series of Technical Specifications is designed to support a wide range of possible architectures on both sides. Characteristics like location of the transcoders/rate adaptation to the MSS or inside the BSS (either physically integrated into the transceivers 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 the present document contains guidance information concerning the use of remote mobile switching units, which for the purposes of the present document are considered as part of the MSC. The BSS-MSC interface is commonly called "A-Interface". It is subdivided into the Control Plane (signalling) and the User Plane (traffic). Throughout this document the term "MSC" is used to reflect both planes of the Mobile Core Network, although in some architecture the Mobile Core Network is split into MSC-Server (MSC-S) and Media Gateway (MGW). Direct connection between two BSSs is not supported by this A-Interface. This A-Interface may be based on

• TDM using 1 or more 2048 kbit/s digital transmission system interfaces. Each 2 048 kbit/s interface provides 31*64 kbit/s channels which can be used for traffic (User Plane) or signalling (Control Plane) as the operator requires, and/or
• IP supporting User Plane and/or Control Plane (SIGTRAN).The signalling is layered, terminology similar to that in the OSI reference model is used in this series, however the layers referred to are not identical to the equivalently named layer in the OSI model.

This A-Interface User Plane is defined at the boundary of the MSC and

• in case of TDM, it has a per channel bit rate of 64 kbit/s, but the net radio path traffic channel is at a rate of less than 16 kbit/s. A speech transcoder or data 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
• in case of IP, it has a flexible channel bit rate, adapted to the payload size. The speech transcoding function can be part of either the BSS or the Core Network or can be omitted (transcoding free operation), allowing e.g. for use of both, PCM encoded speech and compressed speech, over the A-Interface. The data rate adapter function is always located inside the BSS, using a 64kbit/s unrestricted digital interface (Clearmode) over IP.

The A-Interface has been designed around the aims of TS 48.001 allowing each component and the system as a whole to evolve.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

Abbreviations used in the present document are listed in TR 21.905.