D-link i2Eye between two Cisco Pix 501's -- premature timeout

Hello,

My church nationally uses a Cisco Pix 501 as a firewall/router in each
of its buildings that have a broadband connection. We would like to
establish a video phone call between two of our buildings for the
purpose of offering a supplemental gospel instruction class to a few
members in a remote area that would like to participate despite their
limited numbers. Locally our budget is limited, so we are experimenting
with a pair of D-Link i2Eye DVC-1100 wireless broadband video phones for
this project.

Each building has an ADSL line with a single dynamically assigned IP
address. I have assigned the D-Link i2Eye video phones static private IP
addresses and made static assignments in the Pix to map ports on the
external interface to coresponding ports on the internal IP. The D-Link
video phones are H323 compatible, using TCP port 1720 to setup the call
and TCP/UDP ports 15328 through 15333 to do the audio/video magic. It
also connects to an LDAP directory server to register itself so that you
can use the speed dial feature to call the other one without having to
determine the current IP address, which is helpful considering the
less-technically savy instructor that will be operating this device on
the other end. I had to open up all of these ports on the Pix's
access-list.

In order to get this device to operate I found that it was also
necessary to turn on a couple of "fixup" commands for LDAP and H323. I
also found it necessary to increase the timeout setting for H323 from
the default of 5 minutes to instead be a few hours, as we will need the
call to last nearly 2 hours.

After doing all of this the video phones work and the quality is pretty
well. However, after a random number of minutes they hang up on each
other and say the connection was lost. You can immediately redial and
connect back up, but this is very disruptive. So far I haven't found a
pattern. Sometimes it'll last as little as 90 seconds. Other times it
will last over half an hour. I am getting 0% packet loss, and don't
experience any other connectivity problems besides this.

I wish the D-Link device would give a better description of why the
connection was lost. Unfortunately I'm not really sure what I'm trying
to fix, and am afraid I might be fixing something that's not broken. Are
there any other settings on the Pix that would be causing the H323
connection to timeout after a seemingly random number of minutes,
usually between :05 and :35?

D-link suggested that I might try adjusting the MTU settings being that
both connections are through PPPoE over DSL. I found them to be at the
default of 1500 on the Pix, so I have tried adjusting this to 1492 on
one end. Doing the same on the other end will be my next step, but I
will have to go back out there and havne't had a chance yet. I'm not
really too familer as to sure how H323 handles itself with MTU--will two
different MTU settings at each end result in different packet sizes for
the same call, or will the protocal limit itself to the smaller of the
two automatically?

I'll keep bumping down the MTU settings at each end and testing this
thing, but I'm wondering if there are any other things I could have
overlooked.

Thanks,

Jeff Phillips