ot sufficient to support many emerging multimedia services. Moreover, due to the use of licensed spectrum, the data services provided by 3G systems are very expensive. Contrarily, some other wireless access technologies such as wire- less local area networks (WLAN) can provide high-speed Internet access in C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security small areas like cafes or airports at a much lower cost because they operate on unlicensed spectrum. These access technologies can be a good complement to 3G mobile communication systems. Due to advances in signal processing tech- nologies and microelectronics, a contemporary mobile device can be equipped with multiple radio interfaces that enable it to be connected to multiple net- works. As a result, it is anticipated that the next generation (NG) wireless communication systems will integrate different wireless access technologies together and provide user "Always Best Connected" (ABC) services [1] via the heterogeneous wireless access networks. The 3rd Generation Partnership Project (3GPP) has released several specifications for 3G wireless network development. Apart from the circuit-switched (CS) and packet-switched (PS) domains, 3GPP has defined a third signalling domain named IP Multimedia Subsystem (IMS) in release 5 to provide some administration services such as quality of service (QoS)- provisioning and charging [2]. IMS can provide access-independent services via standard access interfaces. As shown in Figure 11.1, it is foreseeable that, in next generation (NG) wireless systems, heterogeneous wireless access networks are interconnected by the IP core network for data transport and by the IMS for signalling exchanges. The signalling protocol operating in IMS is a text- format protocol named Session Initiation Protocol (SIP) [3] that was initially developed by the Internet Engineering Task Force (IETF) to invite users to join multimedia sessions or conferences over the Internet. Currently, the func- tionality of S