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The Visitor Location Register (VLR) comes as a pair with the Mobile Switching Centre (MSC). This stores the revevant details of all the MS that are connected to that MSC. The MS connected to the VLR uses a Mobile Station Roaming Number (MSRN), generated from the MS IMSI, to identify all the MS connected the the MSC. The VLR stores the data needed to allow the subscriber access to the services they require without going to the HLR. Initially, on switch on, the data for the VLR is generated from the subscriber record in the Home Location Register (HLR). This is used to make calls, send SMS messages and data. When a subscriber moves from one MSC to another, the data within the VLR moves with them allowing the subscriber to remain connected to the network.
When a subscriber is roaming (from another country or network), the system is the same. A unique MSRN is allocated and the HLR details copied into the VLR, and stored as long as the connection is live. To get a MSRN a MS needs a "home" IMSI, which a roaming customer does not have. When a MS is roaming a negotiation between the two networks gives the MS permission to use the host network and the HLR of the host network allocates a TIMSI which can be used to gain access to the VLE / MSC. This allows a roaming MS to connect to a foreign network as if it were their home network without the network having to go to the HLR each time a service is requested. Additionally it allows the host network to charge for the services either paid for by the subscriber or network.
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