ling. On the other hand, it doesn't perform any signaling translation at the application layer. It has two components - Transmitting-SGW and Receiving-SGW. The T-SGW provides signaling translation between MSC Server and PSTN while R-SGW interacts with the HLR for R99 or pre-R99 subscription data. 8.2.1.3 Release 5 3GPP Release 5 supports HSDPA (High Speed Downlink Packet Access), IP transport in radio access network and IP Multimedia Subsystem (IMS) and was functionally frozen in June 2002. Figure 8.3 shows the core network architecture of 3GPP Rel-5 where all the new elements are IMS-specific (IP Multimedia Subsystem). The rest of the Rel-5 core network architecture is a replica of Rel-4 model. At the heart of IMS is SIP (Session Initiation Protocol), which provides a centric signaling mechanism for the delivery of multimedia services over IMS. SIP has emerged as the signaling and session control standard for the packet switched networks. It is compatible with both IPv4 and IPv6 which makes it an ideal choice for packet-sized voice and multimedia services. SIP was standardized by IETF (Internet Next Generation Mobile Communications Ecosystem Server Networks P-CSCF Server Server PSTN/Legacy Network CS Domain PSTN/Legacy PS Domain IM-MGW Network lu-ps IP Multimedia Subsystem Figure 8.3 3GPP Release 5 core network architecture. Reproduced from Willie C 2002 John Wiley and Sons, Ltd [1] Engineering Task Force) in early 1999 and it has been chosen by both 3GPP and 3GPP2 as the signaling protocol for their packet telephony services. It is an application layer protocol that can establish, modify, and terminate multimedia sessions over the Internet [2]. IMS is primarily developed for the mobile 3G devices communicating over IP with embedded SIP clients. The service requirements of the 3GPP IMS are specified in 3GPP TS 22.228 [7] and the technical details can be found in 3GPP TS 23.228 [8]. Since IMS is associated with the delivery of applications the details of IMS are thus provided in Chapter 9; in this section we brie