Videoconferencing at UVa
A mismatch in video compression protocol between endpoints can cause lack of video signal.
H.323 is comprised of several different sub-standards. One of these is the video compression method. The two referrenced in endpoints are H.263 and H.264. As might be guessed, H.263 is an older protocol than H.264. In fact, it originated with ISDN devices and was migrated to H.323 devices. By the time H.264 was developed, however, H.323 (Internet-bsased videoconferencing) was already established, and endpoint hardware was already available that used H.263. So the solution was to allow use of either protocol.
As a result, there are instances when endpoints from different manufacturers (and perhaps different hardware eras) do not agree on which video compression standard is going to be used. Some clients use a scheme that will support both standards, and the endpoints will "agree" on which one to use. But if the endpoints do not agree, one or both sites may get a blank (or green or black) video display, yet their audio works in both directions. This is unlike a firewall blockage, which will disrupt both video and audio.
When this occurs, it is not usually irreparable. The conference administrators need to agree which protocol to use, and then ensure that their equipment uses that protocol. Since H.263 was involved, it is safe to agree to use that protocol. If an endpoint only supports H.263, it won't even have an option to use H.264, so there will be no explicit setting that can be found.
The interface for setting the option will depend on the particular make and model of the endpoint. "Video settings" is a prime suspect.