ia terminal area Figure 6.4 The IETF multimedia model. Nevertheless, it does not depend in any of the above for its functionality and operation. SIP transparently supports name mapping and redirection services, thereby allowing the implementation of ISDN and IN telephony subscriber services, and enabling personal mobility. Technically SIP has the following characteristics called and calling peers can specify their preference of where they would like calls to be connected; use of user@domain as call addresses and http look alike messages; only deals with tracking down users and delivering a call to an endpoint, i.e. it is orthogonal to other signalling protocols; uses servers for redirection (redirect server), user location tracking (registrar), fork request (proxy server); it does not have address initiation and termination like H.323, but it is widely ac- cepted; The UMTS Network and Radio Access Technology simple and easy to implement by IP developers. SIP supports five phases of establishing and terminating multimedia calls: user location determination of the end system for connection; user capabilities determination of the media and media parameters for usage; user availability determination of the willingness of the called party to engage communications; call setup ringing establishment of call parameters at both called and calling party; call handling including transfer and termination of calls. SIP can also initiate multi-party calls using a multi-point control unit (MCU) or fully meshed interconnection instead of multi-cast. SIP vs. H.323 H.323 Standards ITU-TSG - 16 IETF MMusic Properties Based on H.320 conferencing and Based on Web principals (Internet ISDN Q.931 legacy friendly) Difficult to extend and update Easily to extend and update No potential beyond telephony Readily extensible beyond telephony Complex, monolithic design Modular simplistic design Standards H.450.x series provides minimal No real end-device feature standard status feature set (pure peer approach) Adding mixed peer/stimulus ap- Ma