The 3GPP IP Multimedia Subsystem (IMS) technology provides an architectural framework for delivering IP based multimedia services. IMS enables telecom service providers to offer a new generation of rich multimedia services across both circuit switched and packet switched networks. IMS offers access to IP based services independent of the access network e.g. wireless access (GPRS, 3GPP’s UMTS, LTE, 3GPP2’s CDMA2000) and fixed networks (TISPAN’s NGN)

IMS defines a architecture of logical elements using SIP for call signaling between network elements and Provides a layered approach with defined service, control, and transport planes. Some of IMS high level requirements are noted below:

The application plane provides an infrastructure for the provision and management of services, subscriber configuration and identity management and defines standard interfaces to common functionality.

The IMS control plane handles the call related signaling and controls transport plane. Major element of control plane is the Call Session Control Function (CSCF) , which comprises Proxy-CSCF (P-CSCF),  Interrogating-CSCF (I-CSCF) and Serving-CSCF (S-CSCF). The CSCF (Call/Session Control Function) is essentially a SIP server.

The IMS transport plane provides a core IP network with access from subscriber device over wireless or wireline networks.