A virtual device driver (VxD) is a software device driver that emulates hardware and other devices so that multiple applications running in protected mode can access hardware interrupt channels, hardware resources and memory without causing conflicts. The Vxd was superseded by the Windows Driver Model (WDM) and is now obsolete.
A visitor location register (VLR) is a database that contains information about the subscribers roaming within a mobile switching center’s (MSC) location area. The primary role of the VLR is to minimize the number of queries that MSCs have to make to the home location register (HLR), which holds permanent data regarding the cellular network’s subscribers. Ideally, there should be only one visitor location register per MSC, but it is also possible for a single VLR to serve multiple MSCs.
The kind of data (referred to as "fields" in database terms) stored in a VLR is similar to that stored in the HLR. That is, the VLR also holds the international mobile subscriber identity (IMSI) and the mobile subscriber integrated services digital network (MSISDN), the services allowed for a particular IMSI/MSISDN pair, and authentication data, all of which correspond to a particular subscription. The main difference between data stored in the VLR and that found in the HLR is that some of the data in the latter is more permanent, while the data in the former changes more frequently. Also, if a network is designed such that one VLR serves each MSC’s location area, there will be fewer records in the VLR than the HLR. As soon as a subscriber moves into an MSC’s location area, the corresponding record is updated in the VLR. Subsequently, the subscriber’s HLR is automatically informed of the change. A visitor location register may also perform the following functions:
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