Components of IMS Architecture

HSS (Home Subscriber Server) - or USPF (user profile server function) is a master user database that supports the IMS network entities that actually handles the calls. It contains subscriber information, performs authentication and authorization of the user and can provide information about the subscribers location and IP information. It is similar to GSM HLR (Home location register) and Authentication Centre (AuC) A Subscriber Location Function (SLF) is needed to map user addresses when multiple HSS are used. 

User Identities 

Various identities may be associated with IMS, IP Multimedia Private Identity (IMPI), IP Multimedia Public Identity (IMPU), Globally Routable User Agent URI (GRUU), Wildcard Public User Identity. Both IMPU and IMPI are not phone numbers or other series of digits, but they are URIs that can be digits ( a tel url such as tel:+1-555-123-4567) or alphanumeric identifiers (a sIP URI such as sip:john.doe@example.com)

IP Multimedia Private Identity (IMPI)

IMPI is IP Multimedia Private Identity which is a permanently allocated global identity assigned by the home network operator and is used for example for Registration, Authorisation, Administratio and Accounting purposes. Every IMS user will have one IMPI 

IP Multimedia Public Identity (IMPU)

IMPU is used by any user for requesting communication to other users (e.g. this might be included on a business card). There can be multiple IMPUs per IMPI. The IMPU can be shared with another phone, so that both can be reached with same identity (for e.g. single phone number for entire family) 

Globally Routable User Agent URI 

Globally routable User Agent URI is an identify that identifies a unique combination of IMPU and UE instance. There are two types of GRUU, Public-GRUU and temporary GRUU/ 

P-GRUU - lives longer and reveals the IMPU 

T-GRUU - do not reveal IMPU and are valid only until the contact is explicitly de-registered or the current registration expires. 

Wild Card Public User Identity 

This expresses a set of IMPUs grouped together. The HSS subscriber database contains the IMPU, IMPI, IMSI, MISDN, subscriber service profiles, service triggers, and other information. 

References: 

http://en.wikipedia.org/wiki/IP_Multimedia_Subsystem