The present document deals with the functional split between the BSS and the MSC. This functional split is then supported by the other Technical Specifications in the 3GPP TS 48.0xx series. 3GPP TS 48.002 also contains some information on the placement of transcoders/rate adapters, these being functionally part of the BSS though a degree of freedom is allowed in their geographical location. In the case of A interface user plane over IP, transcoders in the BSS may not be needed. Lastly TS 48.002 explains the use of transparent and non transparent signalling information across the interface. The key point is that the majority of call related signalling from the MS is passed in a fairly transparent way through the BSS.

The present document defines the physical layer at the BSS-MSC interface point. The physical interface chosen is either a 2 Mbits/s (32 x 64kbits/s) interface according to the standard ITU-T recommendations or an IP transport based interface according to the standard IETF RFCs. The speech coding called up in the present document is either according to ITU-T G.711 or according to Codecs listed in TS 26.103. The coding of the traffic bit streams for data calls is dealt with in TS 44.021 and TS 48.020.

In order to pass the signalling information between BSS and MSC some reliable transport mechanism has to be used. The basis of the transport mechanism is an internationally agreed protocol known as signalling system No.7. Several services are required from this protocol but two key requirements are that messages can be transferred between the BSS and MSC without corruption, and secondly that a transaction with a particular mobile can be identified. The correct transfer of messages without corruption is handled by the "Message Transfer Part", or in the case of IP-based signalling transport - M3UA and SCTP, of SS7 and this is documented in TS 48.006 which is an exceptions document to the ITU-T, ANSI and IETF specifications. The subset so formed is designed so that it is compatible with a "full" MTP (M3UA/SCTP) such as might be provided at an MSC. For IP-based signalling transport, the involved nodes may take the role of a client or a server with respect to establishing the end-to-end communication. The particular role of the nodes is either determined by configuration or is depending on which peer is acting first in establishing the communication, by that acting as the client. The identification of the transaction involved implies some form of logical connection. This is achieved by using the Signalling Connection Control Part (SCCP) of SS7. Again a minimum subset is formed in order to ease implementation.

In the present document the application parts are described. There are two currently identified in the BSS to MSC interface protocol, these are the:

• BSSOMAP;
• BSSAP.

The BSSAP is further subdivided into two subprotocols, the BSSMAP and the DTAP. The BSSMAP and DTAP are fully defined, the BSSOMAP is only supported in terms of a signalling transport ability. The DTAP text is split between TS 48.006 and TS 48.008 but the text in TS 48.008 defines which layer 3 air interface messages are passed transparently through the BSS and which are analysed at the BSS. The BSSMAP (base station system management application part) is that part of the protocol responsible for all aspects of the radio resource handling at the BSS. The text is structured as a set of procedures which are defined separately and can be employed as felt appropriate by the operator/manufacturer to meet the requirements of the application in which it is being used. The procedures themselves can be driven in different modes depending upon the input parameters received from the MSC or sent from the OMC. The BSSOMAP (base station system operation and maintenance application part) supports all of the O and M communications for the BSS with either the MSC or the BSS. The actual detailed protocol at layer 3 is defined in the 3GPP TS 52.xxx-series.

The present document describes the means by which the radio interface data rates are adapted to the 64 kbits/s needed at the MSC and vice versa, down to the bit level.