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BASE STATION SUBSYSTEM (BSS)

The Base Station Subsystem of a GSM network contains the Base Trans

(BTS), the Base Station Controller (BSC), and the Transcoder Controller ( 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

providing radio coverage functions from their antennae. The channel co

the communication between BTS and MS will be described in section 4.2. Base Station Controller (BSC)

The BSC controls a major part of the radio network. Its most important ta

ensure the highest possible utilization of the radio resources. The main fu 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 t (Location Area). The load situation in the BSC is checked before the pa is sent to the BTS.

Signaling set-up: during call set-up, the MS connection is transferred to a (Stand alone Dedicated Control Channel) allocated by the BSC. If the MS i connection, the BSC checks its processor load before the request is furthe Assignment of traffic channel: after SDCCH assignment, the call set

continues with the assignment of a TCH (Traffic Channel) by the BSC.

place, the radio channel supervision functions in the BSC are informed th

been ordered to change channels. If all TCHs in the cell are occupied a 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

output power based on the received measurements of the uplink and do

sent to the BTS and the MS every 480 ms to maintain good connection qua

Locating: This function continuously evaluates the radio connection to t

necessary, suggests a handover to another cell. This suggestion inclu

handover candidate cells. The decision is based on measurement result and BTS. The locating process is being executed in the BSC.

Handover: if the locating function proposes that a handover take place,

decides which cell to handover to and begins the handover process. If th

to another BSC, the MSC/VLR (Mobile Switching Centre / Visitor Loca

must be involved in the handover. However, in a handover, the MSC/VLR

by the BSC. No decision making is performed in the MSC because it ha information about the connection. Transcoder Controller (TRC) The primary functions of a TRC are to perform transcoding and to

adaptation. The function of converting from the PCM (Pulse Code Mod

information to the GSM speech coder information is called transcoding. T present in both the MS and the BSS.

Rate adaptation involves the conversion of information arriving from the rate of 64kbits/s to a rate of 16kbits/s, or transmission to a BSC (for a

This 16kbits/s contains 13kbits/s of traffic and 3kbits/s of inb information. This is an important function. Without rate adaptation the links to BSC’

four times the data rate capabilities. Such transmission capabilities form

part of the network. By reducing the rate to 16kbits/s, it is possible to us of the transmission links and equipment.

In Ericsson’s GSM systems, the TRC contains units, which perform transco

adaptation. These hardware units are called Transcoder and Rate Ada

(TRAUs). All TRAUs are pooled, meaning that any BSC connected to request the use of one of the TRAUs for a particular call. The TRC also supports discontinuous transmission. If pauses in speech 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

operations and maintenance center (OMC) is connected to all equi

switching system and to the BSC. OMC-R is that connected to the B operations and maintenance needs of the Network Operations division the OMC-R, while the OMC-S connected to the switching system is used

the switch operations. The OMC is the functional entity from which operator monitors and controls the system. The purpose of OMC is to

cost-effective support for centralized, regional and local operational and

activities that are required for a GSM network. An important function o

provide a network overview and support the maintenance activities operation and maintenance organizations.