H.323 and SIP have many similarities but also some differences. Both allow two-party and multiparty calls using both computers and telephones as end points. Both support parameter negotiation, encryption, and the RTP/RTCP protocols. A summary of the similarities and differences given. Although the feature sets are similar, the two protocols differ widely in philosophy. H.323 is a typical, heavyweight, telephone-industry standard, specifying
the complete protocol stack and defining precisely what is allowed and what is forbidden. This approach leads to very well defined protocols in each layer, easing the task of interoperability. The price paid is a large, complex, and rigid standard that is difficult to adapt to future applications.
In contrast, SIP is a typical Internet protocol that works by exchanging short lines of ASCII text. It is a lightweight module that interworks well with other Internet protocols but less well with existing telephone system signaling protocols. Because the IETF model of voice over IP is highly modular, it is flexible and can be adapted to new applications easily. The downside is potential interoperability problems, although these are addressed by frequent meetings where different
implementers get together to test their systems.
Voice over IP is an up-and-coming topic. Consequently, there are several books on the subject already. A few examples are . The May/June 2002 issue of Internet Computing has several articles on this topic.
