This TS gives the principles on which the detailed interface specifications in the rest of the 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
Direct connection between two BSSs is not supported by this A-Interface.
This A-Interface may be based on
TDM using 1 or more 2 048 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
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.
full Table of Contents for TS 48.002 Word version: 15.0.0