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VoIP and Videoconferencing Protocols

H.323 is the dominant protocol for IP videoconferencing on the market. Many products from many vendors interoperate to form complex systems for education and industry. H.323 interoperates particulary well with H.320 and VoIP applications, so that a single infrastructure can support voice and video both on the IP network and PSTN.
The IETF's Session Initiation Protocol for IP is finding applications in videoconferencing, especially in the software desktop endpoint with integrated instant messaging space. Internetworking products between H.323 and SIP are starting to appear on the market.
H.320 supports videoconferencing over ISDN. This protocol has a long and successful history. Sometimes considered a "legacy" protocol, private industry still relies heavily on this protocol, and it provides an important bridge to the PSTN.
H.239 allows multiple video streams to be transmitted in a single session. This is commonly used for applications where it is desired to display a picture of a presenter, as well as a shot of a classroom, or computer display.
H.248, also referred to as Megaco, is the industry standard media gateway control protocol, allowing a media gateway controller to control all aspects of media flow on a gateway device.
H.321 provides support for videoconferencing over ATM. A number of successful systems have been built upon this technology, though the general scarce deployment of ATM limits the overall reach of these systems.
MGCP, or Media Gateway Control Protocol, is similar in function to H.248, but is simpler and more narrowly focused on enabling a softswitch to control PSTN gateway devices.
H.324 provides support for low bandwidth videoconferencing over PSTN. This protocol enjoys success particularly in Asian mobile markets, as it has been chosen as the protocol of choice for 3G mobile video communications.
The Extensible Messaging and Presence Protocol (XMPP) is the IETF standard for instant messaging. It is based on XML and widely supported in the industry, largely thanks to its early introduction as an enterprise solution and work to make the platform completely open.
H.350 provides directory services for a variety of conferencing protocols including H.323, SIP, H.320 and non-standard protocols. It makes it easy to find and dial users, support identity management to allow for high scalability in deployments, and store call handling preferences. You can try out a sample directory search at the ViDeNet Directory of Directories portal. A brochure on H.350 is available for vendors and universities.
T.120 is a data collaboration conferencing standard often used in conjunction with videoconferencing protocols to provide application sharing, electronic whiteboarding, file transfer, text chat, and other data services.