 between the protocols in call setup, and some niche markets that H.323 currently dominates, SIP, with its text encoding, presence and instant message extensions, and Internet architecture, is poised to be the signaling and "rendezvous" protocol of choice for Internet devices in the future. References "Packet-Based Multimedia Communications Systems," ITU Recommendation H.323, 2000. Levin, O., "H.323 Uniform Resource Locator (URL) Scheme Registration," RFC 3508, 2003. Rosenberg, J., and H. Schulzrinne, "A Comparison of SIP and H.323 for Internet Teleph- ony," Network and Operating System Support for Digital Audio and Video (NOSSDAV), Cambridge, England, July 1998. SIP: Understanding the Session Initiation Protocol Schulzrinne, H., and C. Agboh, "Session Initiation Protocol (SIP)-H.323 Interworking Requirements," IETF Internet-Draft, Work in Progress, 2003. Agrawl, C., "Session Initiation Protocol (SIP) - H.323 Interworking," " IETF Internet- Draft, Work in Progress, 2000. Wireless and 3GPP The mobility features of SIP have been discussed in earlier chapters. In this chapter, those aspects will be explored further. In 2000 the Third Generation Partnership Project [1] adopted SIP as their call signaling protocol starting with Intelligent Multimedia core Subsystem (IMS) Release 5. Since then, a number of extensions and usage documents have been authored describing their planned use of SIP; these will be discussed in this chapter. The future direction of wire- less SIP will be discussed in the final section of this chapter. IP Mobility There are a number of different types of mobility that will be discussed in this chapter, which include terminal mobility, personal mobility, and service mobil- ity [2]. Terminal mobility is the ability of an end device to maintain its connec- tion to the Internet as it moves around and possibly changes its point of connection. Personal mobility is the ability to have a constant address (identi- fier) across a number of devices. Finally, service mobility is the ability of a user to keep