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The server is structured in three sub-systems at least:
The CSCF provides session control for subscribers accessing services within the IM (IP Multimedia) CN. In essence the CSCF is a SIP Server. It has responsibility for interacting with network databases such as the HSS for mobility and AAA (Access, Authorization and Accounting) Servers for security.
The Home Subscriber Server (HSS), or User Profile Server Function (UPSF), is a master user database that supports the IMS network entities that actually handle calls. It contains the subscription-related information (user profiles), performs authentication and authorization of the user, and can provide information about the user's physical location. It is similar to the GSM Home Location Register (HLR) and Authentication Centre (AUC).
A Subscriber Location Function (SLF) is needed to map user addresses when multiple HSSs are used. Both the HSS and the SLF communicate through the Diameter protocol. This Diameter is also called as AAA protocol i.e Authentication, Accounting and Authorization.
User identities
Normal 3GPP networks use the following identities:
International Mobile Subscriber Identity (IMSI)
Temporary Mobile Subscriber Identity (TMSI)
International Mobile Equipment Identity (IMEI)
Mobile Subscriber ISDN Number (MSISDN)
IMSI is a unique phone identity that is stored in the SIM. To improve privacy, a TMSI is generated per geographical location. While IMSI/TMSI are used for user identification, the IMEI is a unique device identity and is phone specific. The MSISDN is the telephone number of a user.
IMS also requires IP Multimedia Private Identity (IMPI) and IP Multimedia Public Identity (IMPU). Both are not phone numbers or other series of digits, but Uniform Resource Identifier (URIs), that can be digits (a tel-uri, like tel:+1-555-123-4567) or alphanumeric identifiers (a sip-uri, like sip:john.doe@example.com). There can be multiple IMPU per IMPI (often a tel-uri and a sip-uri). The IMPU can also be shared with another phone, so both can be reached with the same identity (for example, a single phone-number for an entire family).
The HSS user database contains, the IMPU, IMPI, IMSI, and MSISDN and other information.
The platform embeds a SIP Container to manage all applications (UI (Web), Register process & SIP functionality ). The web server application takes the request, processes it and add or remove a user from the IMS network. It also verifies data (format, duplicity, bots).
The users are stored into the usermanager.xml file. This file is part of the IMS system, and it is managed by the SDS environment.
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