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.
Subscriber Location Function is a HSS Finding Database. It's a simple DB required when multiple HSS exist. Maps UA/subscriber address to HSS.
Public User ID: John may have many IDs but at least two:
sip: firstname.lastname@example.org 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.
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.