Consultant TATA Consultancy Services, Ltd.
Introduction This paper considers the Internet protocol multimedia subsystem (IMS) technical architecture and its importance in adapting to various applications (e.g., push to talk over cellular, session initiation protocol [SIP] instant messaging presence leverages extension [SIMPLE], streaming, multiplayer games). The third-generation partnership project (3GPP) has proposed IMS in Release 5 on Internet protocol version 6 (IPv6), but it is negotiated for IPv4 because the industry is not ready to migrate to IPv6 now. IMS is a person-to-person service that enriches the way people communicate with each other by combining voice, image, and video into a single session. IMS uses SIP and session description protocol (SDP) underneath architecture to communicate between various IMS components. 3GPP IMS Release 6, target phase 2, calls for IMS enhancement for POC, group management, conferencing, messaging, interworking with CS network, and lawful intercept. 3GPP IMS Release 7, target phase 3, calls for further IMS enhancement for emergency calls, system enhancement for fixed broadband access to IMS, multimedia telephony, and combination of CS bearer with IMS. Open Mobile Alliance (OMA) is developing IMS application-enabler specifications and certifying the enabler with the Global Certification Forum (GCF) while providing conformance and interoperability test cases. IMS Overview IMS is defined by 3GPP as a new subsystem, i.e., a new mobile network infrastructure that enables the convergence of data, speech, and mobile network technology over an IP–based infrastructure. IMS was designed to fill the gap between the existing traditional telecommunications technology and Internet technology, which increased bandwidth alone cannot do. This will allow operators to offer new, innovative services that shareholders and end users are expecting. The architecture of IMS specifically helps enable and enhance real-time multimedia mobile services such as rich voice services, video telephony, messaging, conferencing, and push services. 1
IMS enables these user-to-user communication services via a number of key mechanisms such as session negotiation and management, quality of service (QoS) and mobility management. However, IMS enables much more than just real-time user-to-user services. Architecture and Features IMS is designed to provide a number of key functionalities required to enable new IP services via mobile networks. This new realm of IP services must take into account the complexity of multimedia, constraints of the underlying network, management of mobility, and management of the multitude of emerging applications. Although IMS was designed for mobile networks, it can also be used to provide services for fixed networks at the same time, providing unique mixtures of services with transparency for the end user. Figure 1 gives a diagrammatic view of the IMS architecture in the context of the radio network and its importance for application server. It also shows the interworking possibility with public switched telephone networks (PSTNs). Application Servers
Push to Talk over Cellular Visited Network
Multi Player Games
ISC Interface PDF I-BCF HSS
ISC Interface PDF I-BCF HSS
Figure 1: IMS Architecture
Proxy Call Session Control Function (P–CSCF) This is the first point of contact in the IMS for a user element (UE). The P–CSCF may be in the home or visited network. SIP register/invite methods are initially terminated to P–CSCF, and it ensures that registration/session requests are passed to the correct home network or that SIP messages are passed to the correct serving CSCF (S–CSCF). 2
The major role of P–CSCF is to route information to the correct S–CSCF so it can support the...