#	Term	Description
1	HSS	Home Subscriber Server is the Central Repository. HLR is a subset of the HSS (3GPP TS 23.002) User information: Subscription Info, Location Info, Security info, User profile, S-CSCF allocation. HSS acts as the central DB for the IMS network it serves. If network is too large, multiple HSS may exist.
2	SLF	Subscriber Location Function is a HSS Finding Database. It's a simple DB required when multiple HSS exist. Maps UA/subscriber address to HSS.
3	User Identities	Public User ID: John may have many IDs but at least two: sip: 19082223333@sip.vzw.com and tel:19082223333 Private User ID: John is assigned just One. Maps to all John's public IDs. Not user for routing. Used for Authentication. NAI (Network Access Identifier RFC 2486) For example, john_private_ID@rocketmail.com Public Service ID: IDs a services, like a chat room. Not associated with a private UserID. SIM, USIM and ISIM in 3GPP: Stored on Universal Integrated Circuit Card (UICC) that contains limited data plus John's ID info. SIM: Subscriber Identity module - Subs's info on the UICC ISIM: Identity Module - a SIM version that works with UMTS networks. USIM: Universal Subscriber Identity Module.
4	CSCF	Call/Session Control Function: 3 Types: Proxy or P-CSCF : The Mobile access edge / The internet access edge. Interrogating or I-CSCF: Finds the appropriate S-CSCF. Located at the edge of the domain (DNS) Serving or S-CSCF: Located in the Core. Registrar Server and does Call Routing. They all are SIP servers and talk SIP amoung themselves. ICSCF and S-CSCF talk diameter with HSS.
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