GSM Network Basics

An Introduction to GSM Networks

 

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BASE STATION SUBSYSTEM (BSS)

 


The Base Station Subsystem of a GSM network contains the Base Transceiver Station (BTS), the Base Station Controller (BSC), and the Transcoder Controller (TRC). A figure of GSM network components is given bellow.

 

GSM Network architecture

 Base Transceiver Station (BTS)

A BTS acts as the interface between MS’s (Mobile Station) and the network, by providing radio coverage functions from their antennae. The channel concept used in the communication between BTS and MS will be described in section 4.2.2.

 

Base Station Controller (BSC)

The BSC controls a major part of the radio network. Its most important task is to ensure the highest possible utilization of the radio resources. The main functional areas of the BSC are:

  • Radio Network Management

  • BTS Management

  • TRC Handling

  • Transmission Network Management

  • Internal BSC Operation and Maintenance

  • Handling of MS connections

BSC handles MS connections during a call setup and during a call.

 

Call set up involves the following processes:

  Paging: the BSC sends paging messages to the BTS’s defined within the desired LA (Location Area). The load situation in the BSC is checked before the paging command is sent to the BTS.

Signaling set-up: during call set-up, the MS connection is transferred to an SDCCH (Stand alone Dedicated Control Channel) allocated by the BSC. If the MS initiated the connection, the BSC checks its processor load before the request is further processed.

Assignment of traffic channel: after SDCCH assignment, the call set-up procedure continues with the assignment of a TCH (Traffic Channel) by the BSC. As this takes place, the radio channel supervision functions in the BSC are informed that the MS has been ordered to change channels. If all TCHs in the cell are occupied an attempt can be made to utilize a TCH in a neighboring cell.

 

The main BSC functions during a call are:

Dynamic power control in MS and BTS: the BSC calculates adequate MS and BTS output power based on the received measurements of the uplink and downlink. This is sent to the BTS and the MS every 480 ms to maintain good connection quality.

 

Locating: This function continuously evaluates the radio connection to the MS, and, if necessary, suggests a handover to another cell. This suggestion includes a list of handover candidate cells. The decision is based on measurement results from the MS and BTS. The locating process is being executed in the BSC.

 

Handover: if the locating function proposes that a handover take place, the BSC then decides which cell to handover to and begins the handover process. If the cell belongs to another BSC, the MSC/VLR (Mobile Switching Centre / Visitor Location Register) must be involved in the handover. However, in a handover, the MSC/VLR is controlled by the BSC. No decision making is performed in the MSC because it has no real time information about the connection.

 

Transcoder Controller (TRC) 

The primary functions of a TRC are to perform transcoding and to perform rate adaptation. The function of converting from the PCM (Pulse Code Modulation) coder information to the GSM speech coder information is called transcoding. This function is present in both the MS and the BSS.

Rate adaptation involves the conversion of information arriving from the MSC/VLR at a rate of 64kbits/s to a rate of 16kbits/s, or transmission to a BSC (for a full rate call). This 16kbits/s contains 13kbits/s of traffic and 3kbits/s of inband signaling information.

This is an important function. Without rate adaptation the links to BSC’ would require four times the data rate capabilities. Such transmission capabilities form an expensive part of the network. By reducing the rate to 16kbits/s, it is possible to use one quarter of the transmission links and equipment.

In Ericsson’s GSM systems, the TRC contains units, which perform transcoding and rate adaptation. These hardware units are called Transcoder and Rate Adaptation Units (TRAUs). All TRAUs are pooled, meaning that any BSC connected to the TRC can request the use of one of the TRAUs for a particular call.

The TRC also supports discontinuous transmission. If pauses in speech are detected, comfort noise is generated by the TRAU in the direction of the MSC/VLR.

 

Data rates for a single call on GSM links

 

 

OMC-R (Operations & Maintenance Center-Radio)

OMC-R is the interface between the BSS and the human working in the system. The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. OMC-R is that connected to the BSC. Thus the operations and maintenance needs of the Network Operations division is provided by the OMC-R, while the OMC-S connected to the switching system is used for controlling the switch operations. The OMC is the functional entity from which the network operator monitors and controls the system. The purpose of OMC is to offer the user cost-effective support for centralized, regional and local operational and maintenance activities that are required for a GSM network. An important function of OMC-R is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.

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Chathuranga Perera,

Electrical Engineer

Ceylon Electricity Board