I have a Polycom VS 4000 IP-based video conferencing system and a
Zydacron Z360 ATM-based desktop video conferencing system. When I try
to establish a video conferencing session between the two, only audio
can be heard at both sides. Video, however, can't be seen from the
Zydacron side. It can only be seen at the Polycom's side. What is the
reason for this, what is a solution and what other IP-based video
conferencing system can be used with interoperability with the
Zydacron.

I am not familiar with both, but I glanced at the spec of both.
I saw that Polycom is superior than Zydacron. Anything that Zydacron
can send, Polycom will be able to receive, but Polycom has extra
protocols that Zydacron cannot receive. I think this might be the
problem, as you stated.

Try to force Polycom to use lower fps encoding (to 30fps or even
15fps), and disable its H.263 protocol for video. Try to use QCIF
because Zydacron has it, and disable FCIF because I'm not sure
Zydacron's CIF is the same as FCIF.

Because sound can be heard from both side, and one side was able to
see video, then the problem is not with the connection.

Hope this helped.

Yes it's a know problem. The work-around is to use the vic application
for video display on the Zydacron station. When installing the VRVS
software package, vic should have been one of the applications that
was installed. Although the H.323 video (Zydacron OnWAN, Polycom
ViaVideo etc. as the case may be) cannot be *transmitted* by means of
vic, nevertheless the VRVS conference video can be *received* and
displayed with vic, when the H.323 is set for "MultiVideo" mode.

The procedure is, after having started the H.323 software, and
successfully joined the conference via the "JOIN" button, use the
"VIDEO MODE" button
in which you should select the "Multi-Video" mode, then [CLOSE]. This
should cause the initial window of vic to open, with thumbnails of the
other participants. As usual in vic, you click on the desired
thumbnail(s) to get a reasonable sized window of the other
participant(s).