Core network

Home subscriber server

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).

An SLF (Subscriber Location Function) is needed to map user addresses when multiple HSSs are used. Both the HSS and the SLF communicate through the Diameter protocol.

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: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it ). 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.

Call/session control

Several roles of Session Initiation Protocol (SIP) servers or proxies, collectively called Call Session Control Function (CSCF), are used to process SIP signalling packets in the IMS.

• A Proxy-CSCF (P-CSCF) is a SIP proxy that is the first point of contact for the IMS terminal. It can be located either in the visited network (in full IMS networks) or in the home network (when the visited network isn't IMS compliant yet). Some networks may use a Session Border Controller for this function. The terminal discovers its P-CSCF with either DHCP, or it is assigned in the PDP Context (in General Packet Radio Service (GPRS).
  • it is assigned to an IMS terminal during registration, and does not change for the duration of the registration
  • it sits on the path of all signalling messages, and can inspect every message
  • it authenticates the user and establishes an IPsec security association with the IMS terminal. This prevents spoofing attacks and replay attacks and protects the privacy of the user. Other nodes trust the P-CSCF, and do not have to authenticate the user again.
  • it can also compress and decompress SIP messages using SigComp, which reduces the round-trip over slow radio links
  • it may include a Policy Decision Function (PDF), which authorizes media plane resources e.g. quality of service (QoS) over the media plane. It's used for policy control, bandwidth management, etc. The PDF can also be a separate function.
  • it also generates charging records

• A Serving-CSCF (S-CSCF) is the central node of the signalling plane. It is a SIP server, but performs session control too. It is always located in the home network. It uses Diameter Cx and Dx interfaces to the HSS to download and upload user profiles — it has no local storage of the user. All necessary information is loaded from the HSS.
  • it handles SIP registrations, which allows it to bind the user location (e.g. the IP address of the terminal) and the SIP address
  • it sits on the path of all signaling messages, and can inspect every message
  • it decides to which application server(s) the SIP message will be forwarded, in order to provide their services
  • it provides routing services, typically using Electronic Numbering (ENUM) lookups
  • it enforces the policy of the network operator
  • there can be multiple S-CSCFs in the network for load distribution and high availability reasons. It's the HSS that assigns the S-CSCF to a user, when it's queried by the I-CSCF.

• An I-CSCF (Interrogating-CSCF) is another SIP function located at the edge of an administrative domain. Its IP address is published in the Domain Name System (DNS) of the domain (using NAPTR and SRV type of DNS records), so that remote servers can find it, and use it as a forwarding point (e.g. registering) for SIP packets to this domain. The I-CSCF queries the HSS using the Diameter Cx interface to retrieve the user location (Dx interface is used from I-CSCF to SLF to locate the needed HSS only), and then routes the SIP request to its assigned S-CSCF. Up to Release 6 it can also be used to hide the internal network from the outside world (encrypting part of the SIP message), in which case it's called a THIG (Topology Hiding Inter-network Gateway). From Release 7 onwards this "entry point" function is removed from the I-CSCF and is now part of the IBCF (Interconnection Border Control Function). The IBCF is used as gateway to external networks, and provides NAT and Firewall functions (pinholing).
  
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