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Thực hiện tiếng nói qua IP (P3)

EVOLUTION OF VoIP SIGNALING PROTOCOLS1 This chapter reviews the existing and emerging VoIP signaling and call control protocols. In PSTN networks, ISUP (ISDN user part) and TCAP (transaction capabilities application part) messages of the SS7 protocol [1] are commonly used for call control and interworking of services. The first generation (released in 1996) of VoIP signaling and media control protocols, such as ITU-T’s H.225/H.245—defined under ITU-T’s H.323 umbrella protocol [2]—was intended to o¤er LAN-based real-time VoIP services | Implementing Voice over IP. Bhumip Khasnabish Copyright 2003 John Wiley Sons Inc. ISBN 0-471-21666-6 3 EVOLUTION OF VoIP SIGNALING PROTOCOLS1 This chapter reviews the existing and emerging VoIP signaling and call control protocols. In PSTN networks ISUP ISDN user part and TCAP transaction capabilities application part messages of the SS7 protocol 1 are commonly used for call control and interworking of services. The first generation released in 1996 of VoIP signaling and media control protocols such as ITU-T s H.225 H.245 defined under ITU-T s H.323 umbrella protocol 2 was intended to offer LAN-based real-time VoIP services. These protocols already had the proper ingredients such as support of ISUP messaging for call control to support interworking with PSTN networks as well. Consequently there was a flurry of networking activities to deliver VoIP services in LAN or within enterprises and to offer long-haul inter-LATA and international transport of VoIP. The latter is also known as cheap and wireless quality long-distance voice service over wireline network using IP. However the telecom service providers found the following two problems with version 1 of the H.323 protocol a. Many of the desired and advanced PSTN-domain call features and services could not be easily implemented using H.323v1 because of its lack of openness i.e. all of the procedures are internally defined and b. Scalable implementation was neither feasible nor cost-effective because it needed call state full proxies. 1 The ideas and viewpoints presented here belong solely to Bhumip Khasnabish Massachusetts USA. 32 EVOLUTION OF VoIP SIGNALING PROTOCOLS 33 These problems motivated ITU-T to release the second version of H.323 in 1998. H.323-v2 supports lightweight call setup runs over UDP instead of using multiple TCP sessions per call and declares many of the mandatory features and protocols of H.323v1 to be optional 3 . But in 1999 IETF released the first version of its Internet paradigm Web .