Re: CRASH!

John Ingham (johningh@tafe.sa.edu.au)
Tue, 25 Feb 1997 10:06:11 +0930


Brad Klein said...

>I don't know how the mhz ratings compare between macs and dos machines but
>if I am understanding the above statement correctly, my 7100/80av is the
>reason my fps rate is so poor, even though I am on a very crowded T1.
>100mhz or less = poor image quality. Am I following this correctly?

You say that you are"on a very crowded T1" - presumably via a LAN - but
what network connection are YOU using on your 7100 Brad, Ethernet or
Apple's Localtalk? And what are you comparing your results to - another
CU-SeeMe user on the same LAN, or perhaps are your expectations based on
dedicated Videoconferencing codecs or maybe on broadcast TV?

If your LAN connection is via your Localtalk port then that is likely to be
the weak link in the chain. Don't quote me, but I understand that the
maximum through-put via Localtalk is quite limited - about 56 kbs??? If
you are trying to run CU-SeeMe faster than your network connection you are
bound to get lost packets and inferior results. With CU-SeeMe if you
over-run your available bandwidth you will ALWAYS get degradation and
poorer results than if you cap to within the available bandwidth.

You say that your T1 is very crowded, and you quote 100 mhz or less. Quite
a lot less actually because a T1 connection's maximum theoretical
throughput is only about 1.5 million bits per second - nothing like 100 mhz
(at the risk of comparing apples with oranges!) Even if you're running an
Ethernet LAN connection, at (say) 100 kbs just one CU-SeeMe connection is
taking up a sizable chunk of the available bandwith on an already "very
crowded T1", and Ethernet is notorious for rapid degradation due to packet
collisions as usage increases.

It is some time ago that I did the test, but running my 7100/80av on an
Ethernet with CU-SeeMe capped at 100 kbs gave reasonable quality results
with sound in a point-to-point connection with someone on my own LAN. But
it depends a lot on your expectations. I wasn't expecting to see
full-screen, full 30 fps motion. What are you expecting to see?

John Ingham
Supervisor, Technical Operations, CALS (Centre for Applied Learning Systems)
Adelaide Institute of TAFE (Training and Further Education)
GPO Box 1872 Adelaide South Australia 5001
Ph: +61 8 8207 8550 Fax: +61 8 8207 8552
Email: johningh@tafe.sa.edu.au
Web Site: http://www.tafe.sa.edu.au/video-conf/
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