Base Station Subsystem
From Wikipedia, the free encyclopedia.
The Base Station Subsystem (BSS) is the section of a GSM network which is responsible for handeling traffic between a mobile phone and the Network Switching Subsystem. The BSS carries out transcoding of speech channels, allocation of radio channels to mobile phones, paging and many other tasks related to the radio network.
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Base Transceiver Station
The Base Transceiver Station, or BTS, contains the equipment for transmitting and receiving of radio signals (transcievers), antennas, and equipment for encrypting and decrypting communcation between the Base Station Controller. Typically a BTS for anything other than a picocell will have several different transceivers (TRXs) which allow it to serve several different frequencies or even several different cells (in the case of sectorised base station).
Sectorisation
By using directional antennae on a base station, each pointing in different directions, it is possible to sectorise the base station so that several different cells are served from the same location. This increases the traffic capacity of the base station (each frequency can carry eight voice channels) whilst not greatly increasing the interference caused to neighboring cells (in any given direction, only a small number of frequencies are being broadcast).
Base Station Controller
The Base Station Controller (BSC) provides the intelligence behind the BTSs. Typically a BSC has 10s or even 100s of BTSs under its control. The BSC handles allocation of radio channels, receives measurements from the mobile phones, controls handovers from BTS to BTS (except in the case of an inter-MSC 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). Overall, 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.
Transcoder
The Transcoder (TCU) is not fully specified as part of the standard, being closely linked to the BSC. This subsystem is also referred to as the TRAU (Transcoder and Rate Adaptation Unit). The transcoding function converts the voice channel coding between the GSM (Regular Pulse Excited-Linear Predictive a.k.a RPE-LPC) coder and the CCITT standard PCM (G.711 A-law or u-law). Since the PCM coding is 64 kbit/s and the GSM coding is 13 kbit/s, this also involves a rate adaption function to compress voice channels from the 64 kbit/s PCM standard to the 13 kbit/s rate used on the air interface. 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.
However, at least in Siemens' and Nokia's architecture, this is an identifiable separate 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, transmission costs can be reduced.
Packet Control Unit
The Packet Control Unit (PCU) is a late addition to the GSM standard. It does some of the equivalent tasks of the BSC, but for packet data. The allocation of channels between voice and data is controlled by the base station, but once a channel is allocated to the PCU, the PCU takes full control over that channel.
The PCU can be built into the base station, built into the BSC or even, in some proposed architectures, it can be at the SGSN site.
BSS Interfaces
- Um - The air interface between the MS (Mobile Station) and the BSC. This interface uses BSSAP protocol, which is used to conduct Handover, Authentication, Authorization, Location Update and so on.
- Abis - The interface between the Base Tranciever Station and Base Station Controller. Generally consists of DS-1 or ES-1 circuits.
- Ater - The interface between the Base Station Controller and Transcoder.
- A -The interface between the Transcoder and Mobile Switching Center.
