The Home Subscriber Server (HSS) is a database that stores the profile of all the users of a domain. A user's profile includes security-related information such as cryptographic keys, service-related information such as to which services the user is subscribed, and other information such as the S-CSCF that has been allocated to the user or the user's location.
An HSS is supposed to be a central database for a domain. However, some networks have more users than a single HSS can handle. These networks contain more than one HSS.
Networks with more than one HSS also contain a Subscription Location Function (SLF). Nodes that need to query an HSS about a particular user first query the SLF, which returns the address of the HSS handling the user.
Both the HSS and the SLF support interfaces based on Diameter . The HSS implements the Cx interface and the SLF implements the Dx interface. The SLF is, effectively, a Diameter redirection agent.
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