guration information. The second requirement is met using a "preloaded" Route header field in requests. Normally a Route header is inserted in a request when a proxy requests it using a Record-Route header field. However, it is possible for a configured user agent to include a Route header field. If the Route header contains the URI of the home proxy, the request will be routed to the home proxy after the local proxies have been traversed, meeting the requirement for outgoing requests. For incoming requests, the double registration technique will result in both the home and local proxies being traversed by incoming requests. This will result in a call flow similar to Figure 9.2 but with the home proxy server forwarding the INVITE instead of redirecting. These SIP mobility capabilities are well suited to use over a wireless net- work such as 802.11 in a home, office, or public space. As roaming agreements allow such wireless "hotspots" to be linked up in metropolitan areas, this will provide a wireless service. However, commercial wireless providers plan a spe- cific purpose wireless telephony network using SIP. For some of their business and service requirements, some SIP extensions have been developed, which will be discussed in this chapter. Wireless and 3GPP Wireless SIP clients may also make use of voice codecs such as the iLBC [6], which is highly tolerant to packet loss, which may be experienced in a heavily loaded 802.11 network. 9.3 3GPP Architecture and SIP The 3GPP architecture uses SIP in the IP Multimedia Core Network Subsystem (IMS). A simplified architecture is shown in Figure 9.5. The elements of the IMS architecture are listed in Table 9.1. The specification of SIP in 3GPP is described in 3GPP TS 24.229 and in [7]. User equipment Proxy-CSCF Interrogating-CSCF Serving-CSCF (SIP UA) visited network home network home network (local SIP proxy) (home SIP proxy) (home services SIP proxy) Figure 9.5 3GPP IMS architecture. Table 9.1 IMS Elements Abbreviation Name P-CSCF Proxy - Call Session Control Functi