IMS means IP Multimedia Subsystem is an essential part of the all-IP UMTS architecture and Relevant for WiMAX for the same usage and also for WiMAX to UMTS Interworking. This is part of the WiMAX Forum architecture.

IP Multimedia Subsystem (IMS) is a Service Delivery Architecture and standardized architecture to provide Internet Protocol (IP)-based mobile and fixed multimedia services. IMS architecture has evolved over the past few years. Today, IMS could allow operators who own different types of networks with varying architectures to offer the same services to all of their customers.

Interoperability Protocols

What is CSCF?

CSCF is Call Session Control Function which provides session control for subscribers accessing services within the IM (IP Multimedia) CN. In essence the CSCF is a SIP Server.

It has responsibility for interacting with network databases such as the HSS for mobility and AAA (Access, Authorization and Accounting) Servers for security. There are several CSCF servers which cover in below image.

CSCF servers

IMS Concept:  In Rel.5, services controlled in home network (by S-CSCF) but executed anywhere (home, visited or external network) and delivered anywhere, IMS Specs up to Rel 8 are about!