A GSM Public Land Mobile Network (PLMN) consists of at least one Service Area controlled by a Mobile Switching Center (MSC) connected to the Public Switched Telephone Network (PSTN). 

A Base Station Subsystem (BSS) consists of

a Base Station Controller (BSC)

• at least one radio saccess point or Base Transceiver Station (BTS) for Mobile Stations (MS),
• which are mobile phones or other handheld devices (for example PDA computers) with phone interface. 

A BTS, with its aerial and associated radio frequency components, is the actual transmission and reception component. A Network Cell is the area of radio coverage by one BTS. One or more BTSs are in turn managed by a BSC. A network cell cluster covered by one or several BSSs can be managed as a Location Area (LA). All these BSSs must however be controlled by a single MSC. In Figure gsm2 is shown three LAs of 3, 4 and 4 cells respectively with a MS moving across cell and LA boundaries.
 

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