Videoconferencing background: Ad-Hoc H.323
The ESnet Ad-Hoc H.323 service
The essential information regarding the ESnet Ad-Hoc H.323 service can be found on the ECS web pages. The information found on this web page is in no way intended to replace that from ECS, and in case of problems, the ECS troubleshooting guide should be consulted first. That said, the following information may be of interest.
The ECS service features three IP-based videoconferencing services, which in the following are denoted as 82X, 85X, and 88X. The notation is appropriate, as they denote numbers to connect to, and in principle, X can stand for any extension: it is up to meeting participants to pick one. The "service" that is provided can be understood to be the interconnections made between various sites participating in a videoconference. These interconnections result in the situation we now consider to be "normal":
- sound from all sites can be heard by everyone;
- the video is typically voice-switched (meaning that the video that is transmitted is normally that of the site that produces the most sound).
- mcu2.es.net provides the 85X service
- mcu3.es.net provides the 88X service
Note that ESnet also provides the possibility for (a limited number of) participants to dial into an Ad-Hoc H.323 meeting using their normal telephone. Instructions can be found here. However, it is strictly forbidden to use this for audio-only conferences! In that case, the MeetingPlace service should be used instead.
The Ad-Hoc service also allows for inter-operability with other systems using H.323 conferencing. This is done using the so-called Global Dialing Scheme or GDS, which allows H.323 Gatekeepers (GKs) to "peer". As a result, users registered with one GK in principle can communicate with users registered with another GK. A brief introduction can be found here. When registered with a different GK, the Ad-Hoc service can be used by prefixing the usual IP numbers with 0011349.
The H.323 protocol
As the title suggests, H.323 is one of the protocols defined by the International Telecommunications Union ITU, conceived to establish a standard that both hardware vendors and software programmers could use as a basis for their work on Packet-based multimedia communications systems. In fact, it is not a single, self-contained protocol, but it rather relies on other protocols to provide desired functionality. The following components are required for H.323 compliance:
- H.245 for exchanging terminal capabilities and creation of media channels
- H.225 for call signaling and call setup
- RAS for registration and other admission control with a gatekeeper
- RTP/RTCP for sequencing audio and video packets
- G.711 for coding and decoding (CODEC) of audio signals
- H.261 / H.263 / H.264 for coding and decoding (CODEC) of video signals