IMS Description

The 3GPP system in its first release (R99) was designed to be backward compatible with the existing GSM circuit switched infrastructure. As the costs to procure and maintain the very proprietary hardware of circuit switched systems are very high, and much of the bandwidth used to transmit user data is lost, 3GPP system slowly evolves towards an all-IP core network, i.e. the existing circuit switched infrastructure will be outdated and replaced by IP-based hardware, open, scalable, and cheaper to procure and maintain, due to a more competitive market. Moreover, most of new services and applications developed in the market place are IP based.

Thus, through REL-4 and REL-5 of the 3GPP specifications, more and more importance is given to IP based architecture and the IP multimedia subsystem (IMS) is introduced, firstly to handle classical circuit switched services (like voice) over IP, secondly to handle all multimedia services provided to a subscriber.

The IP Multimedia Subsystem is an extension of the PS core Network, intended to become independent of the PS-CN from REL-6 on. It uses the Session Initiation Protocol (SIP) to setup, maintain and terminate voice and multimedia sessions.

Examples of multimedia sessions

1.        The user is in a voice communication, and receives an incoming IP video communication. The user decides not to accept the communication, but diverts the incoming video to a messaging system.  Further, the user is given an indication that there is a video message in his mail box

2.        The user is in a voice communication, and receives an incoming video communication. The user decides to accept the communication but wishes to switch between the two communications.

3.        The user is idle in a network and not involved in a communication.  The user modifies his user profile to divert all voice communications other than those from high priority, pre-identified callers (e.g. his boss).  In this scenario all emails and text messages continue to be received regardless of the sender.

More examples excerpt from 3GPP TS 22.228 "Service requirements for the IP Multimedia Core Network Subsystem (Stage 1)"

User Authentication

User identification is based on a unique private identity, only used by the UE (User Equipment) and the core network (comparable in function to the IMSI) and public identities, used by third parties to address a user and probably written on business cards and the like.

Authentication uses the principles of UMTS AKA, the 3GPP authentication scheme. The authentication is separated from UMTS authentication and the secret keys and functions are independent from those used in the UMTS subscription, but can be the same. The idea is that a UE starts with establishing a PDP context with the IMS (see GPRS), thus is identified and authenticated in the PS domain, before it identifies and authenticates in the IMS domain. When, in future, IMS is access independent, one can think of a UE connecting to IMS via WLAN or a fixed LAN, thus requiring specific access credentials for the access network, while gaining access to IMS services by satisfying IMS specific authentication requirements.

ISIM architecture

The ISIM is specified in 3GPP TS 31.103 "Characteristics of the ISIM Application".

It's architecture is based on the USIM.