VoIP system by virtue of its low tariffs and better voice quality

has become an important telecommunica- tions business

and there is a trend to replace the traditional long-distance business. Many organizations have studied and instituted the IP network protocol standards to the call

but there are two IP telephony signaling and control protocols of the most influential standards. One is ITU recommendation H.323

and the other is the IETF SIP. These two protocols represent very different approaches to the same problem: H.323 embraces the more traditional circuit-switched approach to signaling based on the ISDN Q.931 protocol and earlier H-series recommendations

and SIP favors the more lightweight Internet approach based on HTTP. In this paper

we compare SIP and H.323 on system construction

dependability

complexity

extensibility

scalability and supported services.