H.323 is an ITU VOIP protocol. It was created at about the same time as SIP, but was more widely adopted and deployed earlier. Today, most of the world's VoIP traffic is carried over H.323 networks, with billions of minutes of traffic being carried every month.

H.323's strengths lie in its ability to serve in a variey of roles, including multimedia communication (voice, video, and data conferencing), as well as applications where interworking with the PSTN is vital. H.323 was designed from the outset with multimedia communications over IP networks in mind, making it the perfect solution for real-time multimedia communication over packet-based networks.

• ITU H.323 Page
• Packetizer's H.323 Information Site
• OpenH323 channel driver: asterisk-oh323
• Asterisk H323 channels
• ATcom:H.323 to ISDN Gateway
• Open H.323: Open Source implementation
• OpenH323 Gatekeeper: The GNU Open Source H.323 Gatekeeper
• ISDN2H323: H323 to ISDN Gateway (discontinued)
• IsdnGw: H.323 to ISDN Gateway
• ooh323c: An Open Source C implementation of H.323 stack
• Uniqall Gridborg HMP Proprietary Host Media processing server with H.323 and SIP frontends, and simple ASCII control protocol. It works in both Linux & Windows environments. Its client-server architecture enables you to use any programming or scripting language. It can handle 240 ports on dual processor servers.
• Yate it's free software (open source) that use OpenH323. The H.323 channel in Yate it's considered to be the best free implementation based on OpenH323. Yate also works as a SIP-H323 signalling proxy, for companies who have internal SIP networks and H.323 carriers.

H323 Variables

External H.323 links

• quintum http://www.binnacleita.com/ H.323 gateways
• Detailed H.323 Call Flow Diagram
• H.323 Call Flow Explained Example