By Phil Hochmuth
Network World, 12/24/01
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The origins of H.323 and Session Initiation Protocol are about as disparate as the histories of the World Wide Web and Ma Bell's century-old network.

The H.323 protocol came on the scene in the mid-'90s as a transmission and session setup protocol for videoconferencing over ISDN networks. It comes out of the International Telecommunication Union, a 54-year-old standards body for technologies and protocols for the international phone network.

SIP, on the other hand, was borne out of the Internet Engineering Task Force (IETF), the governing body that oversees development and standardization of all Internet technologies. SIP inventors built the protocol similar to HTTP, the default protocol now used to view billions of Web pages worldwide.

H.323 and SIP perform the same basic tasks in telephony, such as call setup and tear down (the signaling for call initiation, dial tone and termination), and provide signaling for features such as hold, caller ID and call transferring.

H.323 networks rely on a central server or device for tasks such as call processing and applications, with "dumb devices" (phones) typically deployed as endpoints in the network. SIP operates under a more decentralized network scheme, with some intelligence pushed out to "smart" endpoints, such as SIP phones, PCs and wireless devices that have the intelligence to run applications.

H.323 and SIP are also structured differently. As a text-based protocol, SIP runs on top of IP. With H.323, voice data is encapsulated into IP packets. It is more complex than SIP, making it harder for network managers to troubleshoot and more complex for application developers to work with, experts say.

H.323 was designed for ISDN videoconferencing, not voice over IP, says Tom Valovic, an analyst with IDC. The IETF engineers who crafted SIP always had IP voice in mind.

"SIP is more in the next-generation space. H.323 is more of a bridge between older and newer approaches," he says.