At the centre of any GSM system there is the network and switching subsystem (NSS) that connects the GSM network with the public land network (i.e. a PSTN), performs the handovers between BSS’s, comprises functions for worldwide localization of users and supports charging, accounting and roaming of users between different networks and in different countries. The NSS is comprised of the following switches and databases:
Mobile services switching centre (MSC): High-performance digital ISDN switches, that set up the connections between other MSC’s and the BSC’s, using the A interface. Hence the MSC’s are the backbone of any GSM network. Normally one MSC will manage many BSC’s in a geographical area. Some MSC’s are gateway MSC (GMSC) that provide connections to other fixed networks (e.g. PSTN). Using additional functions such as the interworking functions (IWF) an MSC can also connect to public data networks (PDN) such as X.25.
Home Location Register (HLR): The most important database in a GSM network is the HLR as it stores all the relevant information about the users. Information such as the mobile station ISDN number (MSISDN), services subscribed to, and the authentication key Ki. Furthermore the HLR stores dynamic information like the LA (Location Area) of the MS. As the MS moves geographically around the GSM network, the HLR stores the location of the MS from the LA. This information is used to localize the user within the worldwide GSM network. All of these user specific information elements only exist once for each user in a single HLR. The HLR also supports charging and accounting.
Visitor Location Register (VLR): The VLR associated to each MSC is a very dynamic database which stores all important information needed for the MS users currently in the LA that is associated to the MSC. If a new MS comes into the LA then the VLR is responsible for it. The VLR copies all the relevant information for the MS from the HLR. The structure of the VLR and HLR avoids frequent updates and long-distance signalling of user information.