Evolved Packet System (EPS): The LTE and SAE Evolution of 3G UMTS

2.4 The IMS Subsystem

The IMS (IP Multimedia Subsystem) is a generic platform offering IP-based multimedia services. IMS provides functions and common procedures for session control, bearer control, policy and charging. From a network architecture perspective, IMS has to be considered as an access-type agnostic overlay to the Packet Core.

Of course, IMS can be deployed on top of UTRAN access as well as over the 2G GPRS/EDGE network. Because of its clear orientation towards efficient packet transmission support, Evolved UTRAN will add a lot of value to IMS and allow more quality-demanding services to be supported in a more efficient way.

The rest of the section is not to provide a full description of the IMS subsystem. The objective here is to describe the main network entities, as an introduction to the detailed service description which can be found in Chapter 6.

2.4.1 The Session Control Function

The CSCF (Call Session Control Function) plays a key role in IMS architecture, for establishing, terminating or modifying IMS sessions. The CSCF is actually a specific type of SIP (Session Initiation Protocol) server. SIP is the base protocol developed by the IETF for the management of IP-based multimedia sessions and services; it is described in the following document:

• RFC3261, ‘SIP: Session Initiation Protocol’.

A more detailed description of the SIP protocol in the scope of IMS session control is provided in Chapter 4.

The CSCF can play three different roles in the IMS network: the ...