A crowded telephony field

Session Initiation Protocol (SIP) may be the protocol of the moment when it comes to Internet telephony, but it doesn't have the field all to itself.

The main incumbent is H.323, a standard developed by the International Telecommunication Union and first published in 1996. It is a "heavy" standard that includes just about everything necessary for delivering multimedia conferencing applications, whereas SIP is a "light" protocol that needs extensions to give it the kind of functionality that H.323 provides.

"H.323 is very robust and thick and includes such things as a codex to handle video," said Joan Vandermate, a vice president for Siemens Enterprise Networks. "You don't get that with SIP, but is it something you really need for IP?"

Also, she said, H.323 was initially designed for traditional telephony environments, whereas SIP was built from the ground up for the Internet and is as simple for developers to use as HTML.

However, H.323 has a base of knowledgeable users and developers. For example, Siemens has been shipping H.323 products for four years now, and Polycom Inc., an industry leader in the conferencing market, has more than 100,000 H.323 videoconferencing systems in place around the world.

Another important protocol in the IP telephony market is the Media Gateway Control Protocol, designed as a bridge between the circuit-based public switched telephone network and the packet-switched IP networks of the Internet. Proprietary protocols, such as Cisco Systems Inc.'s Skinny Client Control Protocol, have also played a role in the early adoption of voice-over-IP phones and servers.

Whether SIP will ever completely supplant these protocols in Internet telephony is unclear, but "it clearly has all of the momentum right now," said Doug Montgomery, manager of the Internetworking Technologies Group at the National Institute of Standards and Technology.