The base station controller (BSC) provides, classically, the
intelligence behind the BTSs. Typically a BSC has tens or even hundreds of BTSs under its control. The BSC handles allocation of radio channels, receives measurements from the mobile phones, and controls handovers from BTS to BTS (except in the case of an inter-BSC handover in which case control is in part the responsibility of the
anchor MSC). A key function of the BSC is to act as a
concentrator where many different low capacity connections to BTSs (with relatively low utilisation) become reduced to a smaller number of connections towards the
mobile switching center (MSC) (with a high level of utilisation). This means that networks are often structured to have many BSCs distributed into regions near their BTSs which are then connected to large centralised MSC sites. The BSC is undoubtedly the most robust element in the BSS as it is not only a BTS controller but, for some vendors, a full switching center, as well as an
SS7 node with connections to the MSC and
serving GPRS support node (SGSN) (when using
GPRS). It also provides all the required data to the operation support subsystem (OSS) as well as to the performance measuring centers. A BSC is often based on a distributed computing architecture, with redundancy applied to critical functional units to ensure availability in the event of fault conditions. Redundancy often extends beyond the BSC equipment itself and is commonly used in the power supplies and in the transmission equipment providing the A-ter interface to PCU. The databases for all the sites, including information such as
carrier frequencies, frequency hopping lists, power reduction levels, receiving levels for cell border calculation, are stored in the BSC. This data is obtained directly from radio planning engineering which involves modelling of the
signal propagation as well as traffic projections.
Transcoder The transcoder is responsible for
transcoding the voice channel coding between the coding used in the mobile network, and the coding used by the world's terrestrial circuit-switched network, the
Public Switched Telephone Network. Specifically, GSM uses a
regular pulse excited-long term prediction (RPE-LTP) coder for voice data between the mobile device and the BSS, but
pulse-code modulation (
A-law or
μ-law standardized in
ITU G.711) upstream of the BSS. RPE-LPC coding results in a data rate for voice of 13 kbit/s where standard PCM coding results in 64 kbit/s. Because of this change in data rate
for the same voice call, the transcoder also has a buffering function so that PCM 8-bit words can be recoded to construct GSM 20 ms traffic blocks. Although transcoding (compressing/decompressing) functionality is defined as a base station function by the relevant standards, there are several vendors which have implemented the solution outside of the BSC. Some vendors have implemented it in a stand-alone rack using a proprietary interface. In
Siemens' and
Nokia's architecture, the transcoder is an identifiable separate sub-system which will normally be co-located with the MSC. In some of
Ericsson's systems it is integrated to the MSC rather than the BSC. The reason for these designs is that if the compression of voice channels is done at the site of the MSC, the number of fixed transmission links between the BSS and MSC can be reduced, decreasing network infrastructure costs. This subsystem is also referred to as the
transcoder and rate adaptation unit (
TRAU). Some networks use 32 kbit/s
ADPCM on the terrestrial side of the network instead of 64 kbit/s
PCM and the TRAU converts accordingly. When the traffic is not voice but data such as fax or email, the TRAU enables its rate adaptation unit function to give compatibility between the BSS and MSC data rates. ==Packet control unit==