Network ConnectivityThe network must be able to locate a called mobile and route signals between the parties as efficiently as possible and maintain the call.
In derivatives of the GSM, each mobile is registered in a database, called the home location register (HLR); if the mobile migrates outside the home territory, the mobile registers itself with the visiting system's visitor location register (VLR). The VLR conveys the location of each visitor mobile to the mobile's HLR, thereby refreshing the location status of the mobile. Whenever a call is addressed to a mobile, the mobile switching centre interrogates the mobile's HLR for the mobile's location and establishes the call through an appropriate route. Figure 1.7(a) shows a routing scheme in a non geostationary system with satellite ground hops, while Figure 1.7(b) represents an intersatellite link routing scheme. We have already introduced the concept of handover for maintaining RF link connectivity during a call, but Chapter 8 revisits connectivity issues in greater detail.
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