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The Base Station Subsystem (BSS) is the section of a GSM network which is responsible for handling 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.

Base Station Transceiver Subsystem

The Base Station Transceiver Subsystem, or BTS, contains the equipment for transmitting and receiving of radio signals (transceivers), antennas, and equipment for encrypting and decrypting communication with 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 and different sectors of the cell (in the case of sectorised base stations).

Even though GSM is a standard, the reality is that the functions of a BTS vary from vendor to vendor. There are vendors in which the BTS is a plain tranceiver which receives an information from the MU through the Um and then converts it to a PCM based interface, the Abis, and sends it towards the BSC. There are vendors which build their BTSs so the information is preprocessed, target cell lists are generated and even intracell HO can be fully handled, the advantage in this case is low load on the expensive Abis interface.

The BTSs are equipped with radios that are able to modulate layer 1 of interface Um; for GSM 2G+ the modulation type is GMSK, while for EDGE-enabled networks it is 8PSK.

Antenna combiners are implemented to use the same antenna for several TRXs (carriers), the more TRXs are combined the greater the combiner loss will be. Up to 8:1 combiners are found in micro and pico cells only.

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, classicaly, 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.

The BSC is undoubtly 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 a SS7 node with conection to the MSC and SGSN, it also provides all the required data to the network management subsystem as well as to the performance measuring centers.

The data bases for all the sites, including information as carrier 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 modeling of the signal propagation as well as traffic projections.

Transcoder

Although the Transcodification(compressing/decompressing) function is standard defined as a BSC function, there are several vendors which have implemented the solution in a stand alone rack using a propietary interface. 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 performs some of the processing 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

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• Um - The air interface between the MS (Mobile Station) and the BTS. This interface uses LAPD 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.
• A - The interface between the BSC and Mobile Switching Center it is used for carrying the BSSAP user part of the SS7 stack. Although there are usually transcoding units between BSC and MSC, the comunication takes place between these two ending points and the transcoder unit doesn't touch the SS7 information only the voice and CS data are compressed.
• The interface between the Base Station Controller and Transcoder. It is a proprietary interface whose name depends on the vendor, it carries out the A interface information from the BSC leaving it untouched.

• GSM Standard