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The HSS is the master database for the PLMN, and while logically it is viewed as one entity, in practice it will be made up of several physical databases. This depending on the number of subscribers and the extent of the services which need to be supported. The HSS holds variables and identities for the support, establishment and maintenance of calls and sessions made by subscribers. This includes the subscriber’s IMSI, security variables and location information. As well as the basic HLR/AuC functions, the HSS may be enhanced through the use of additional databases and reference points. This enables the network to offer the subscriber advanced services and features by interfacing with service application servers based on CAMEL, OSA (Open Service Access) and SIP.
