Based Sip Voip System And The User Agent

VoIP is a packet-based and digital-based technique built on IP network. People can communicate with each other through Internet. Because of its low price and relatively good voice quality, VoIP is appreciated by more and more users. Since 1995 when it was brought forward, it has been developing for several years and getting lots of improvements such as audio quality , QoS(quality of service) , terminals, access techniques and so on. Nowadays, VoIP can not only provide audio service, but text, video and many other new functions that combined with Internet.The paper firstly introduces the background and history of VoIP , together with its principals , critical techniques, general architecture and related standards.Signaling protocol for initiating, managing and terminating multimedia sessions between users in Packet-Based Network is the most important part in VoIP system. At present, SIP raised by IETF and H.323 by ITU are two mainstream-used VoIP signaling protocols. After evaluating the performance of the two signaling architectures we conclude that SIP is more suitable in TCP/IP network.Providing personal mobility ability is the most critical functions of SIP. In orderto provide the function, two critical techniques——registration and location——and their implementations are introduced in detail. Based on this, implementations of terminal mobility ability and session mobility ability are provided.The development and implementation of SIP-based communication system needs a terminal device that interacts with users. The terminal device in SIP system is called User Agent. The function and quality of User Agent affect the popularization of SIP system and the promotion of other software production. This paper adopts modularization design method, and realizes an intelligent user agent software terminal in C++. The User Agent has the functions of establishing and terminating calls, supporting basic speech coding, and recording SIP messages and so on.