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).

All this is taken care of by so-called Multipoint Control Units or MCUs. ESnet has two such MCUs from Codian.

• mcu2.es.net provides the 85X service
• mcu3.es.net provides the 88X service

The Codian MCU is fairly sophisticated, and allows for streaming (in which conferences can be followed from the Web, rather than using specific hardware. Note, however, that this does not allow to transmit information). Use their respective URLs above to make use of this feature. A Quick Reference guide to usage of the Codian MCU is available, as well as instructions from ESnet.

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

Furthermore, it also specifies the T.120 protocol for data transfers, and protocols for MCUs. The gatekeeper is an additional component that deserves mention. As stated in the ECS user guide, the gatekeeper address (198.128.3.86 in the ESnet case; there are more that one could register to, and they may be able to forward information from one to another) is to be used in the system settings of user equipment. User equipment is identified by its E.164 number (also called its IP alias). The gatekeeper uses this to know to/from which IP address to route audio/video signals. A more extensive tutorial on the H.323 protocol is provided by the International Engineering Consortium. Further information regarding H.323 may also be found on the H.323 information site (this includes links to hardware equipment, etc.), or on the Wikipedia.