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Topic: LAC in GSM

Location Area Code(LAC) :  To locate the location of MS, the whole area covered by each GSM PLMN is divided into different location areas. LAC is used to identify different location areas.

LAI contains LAC, which is composed of two bytes. LAC adopts hexadecimal coding. The available range is from 0001H to FFFEH. The code 0000H and FFFFH cannot be used . One location area can contain one or more cells.

When MS is powered on or LAC of current cell is found to be different from its originally stored contents, MS will inform network of the current location area via location update, and the network uses the LAI for paging. Generally the allocation and coding of LAC is set at the early stage of network construction, and seldom modified during the operation.

The size of location area (LA) is one of key factors in the system. If the LA coverage is too small, the chances for MS to update location increase, and this will increase the signaling load in the system. If the LA coverage is too large, when network conducts paging to the MS, the same paging information will be transmitted in a large number of cells, and this will lead to the heavy load on CCCH.

The adjustment of LA size has no unified standard. Operating departments can decide whether to adjust the size according to the currently running network. If the CCCH signaling load is heavy because of too big LA coverage, then reduce the size of LA, and vice versa.

It is generally recommended to set the LA as large as possible. The calculation of LA is related with the paging strategies of different manufacturers. In the early stage of network construction, the traffic is not heavy, so the TRX number in one LA can be larger than this value. It is necessary to monitor the PCH load and the increase of traffic in a long term. If necessary, PCH capacity can be increased by adding one extended BCCH channel.

While making LA planning, try to make use of the geographical distribution and behaviors of mobile subscribers to allocate the LA, so as to achieve the objective of reducing the times of location update at the boundary between location areas. Please note that LAC in cell parameters must be in consistent with that in MSC. Otherwise, call setup failure will occur.

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