The Discussion Of QoS In VoIP And The Realization Of VoIP System Based LAN

In recent years the multimedia communication services based IP networks as a new force suddenly rises, in particular VoIP1 (Voice-over-IP). The tendency of its development is becoming more and more stronger. Because of its cheaper price and the universality and flexibility of IP networks, VoIP,the transmission of packetized voice over IP networks, is gaining much attention as a possible alternative to conventional Public Switched Telephone Networks(PSTN). Firstly, this dissertation discusses the basic theory of VoIP, the several models of its construction, and in detail sets forth its main protocols such as H.323, SIP, RTP/RTCP, at the same time points out these protocols' characteristic and the differences of each other. During the following dissertation, we present the key technology about VoIP. The appearance of VoIP brings the impact on the conventional PSTN. Through this compare, we will get clearer about the strongpoint and shortcoming of VoIP in the market.The discussion of QoS(Quality of Service) in VoIP is our working center. Impairments present on IP networks, namely jitter and delay can lead to the loss of packets at the receiving end. This packet loss degrades the speech quality. It's very popular for IT industry circles to evaluate the QoS of Network Layer, but it's seldom to talk about the QoS of Application Layer. Now, this dissertation originally bring forward the Evaluation Model of QoS Based "cask theory" and "risk theory" In the Application Layer.At last, we design a simple PC-PC VoIP system based LAN (Local Area Network), and analyses the future development of VoIP.