Every telephone network needs a specific structure to route incoming calls to the correct exchange and then on to the subscriber. In a mobile network, this structure is very important because the subscribers are mobile. As subscribers move through the network, these structures are used to monitor their location.
CELL
A cell is the basic unit of a cellular system and is defined as the area of radio coverage given by one BS antenna system. Each cell is assigned a unique number called Cell Global Identity (CGI). In a complete network covering an entire country, the number of cells can be quite high.
LOCATION AREA (LA)
A Location Area (LA) is defined as a group of cells. Within the network, a subscriber’s location is known by the LA, which they are in. The identity of the LA in which an MS is currently located is stored in the VLR. When an MS crosses the boundary between two cells belonging to different LA’s, it must report its new Location Area to the network1. If it crosses a cell boundary within a LA, it does not report its new cell location to the network. When there is a call for an MS, a paging message is broadcast within all the cells belonging to the relevant LA.
MSC SERVICE AREA
An MSC service area is made up of a number of LAs and represents the geographical part of the network controlled by one MSC. In order to be able to route a call to an MS, the subscriber's MSC service area is also recorded and monitored. The subscriber's MSC service area is stored in the HLR.
PLMN SERVICE AREA
A Public Land Mobile Network (PLMN) service area is the entire set of cells served by one network operator and is defined as the area in which an operator offers radio coverage and access to its network. In any one country there may be several PLMN service areas, one for each mobile operator's network.
GSM SERVICE AREA
The GSM service area is the entire geographical area in which a subscriber can gain access to a GSM network. The GSM service area increases as more operators sign contracts agreeing to work together. Currently, the GSM service area spans dozens of countries across the world from Ireland to Australia and South Africa. International roaming is the term applied when an MS moves
from one PLMN to another when abroad.
The figures below show alternative views of the same network:
- The first figure shows the network nodes and their layout across the network. For simplicity, this may be referred to as the hardware view of the network.
- The second figure shows the geographical network configuration. For simplicity, this may be referred to as the software view of the network.
“Hardware” view of a sample network
“Software” view of a sample network
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