Re: Relationship between fps and Kbps (long)

John Ingham (johningh@tafe.sa.edu.au)
Fri, 3 Jan 1997 10:17:38 +1030


Don Castella asked...
>SO John... What settings would you recommend for dialup ISDN at 64Kbps? I
use a Connectix Color QuickCam with WP Enhanced CU in color or B/W on
reflectors. Currently I set the Xmit cap to 80 and the receive to 64...
seems to work
great. <snip>

It is important to realise that while YOU may be receiving a great picture,
you may appear to the other participants to have a badly "shattered"
picture if you are sending at too great a rate for them to receive. When
working via a Reflector you need to set your transmission rate for the
lowest common denominator. If some of your participants are connected via
28,800 modems (and with public reflectors they invariably are), then if you
cap your transmitter at 80 Kbps (the maximum recommended by some reflector
managers) you will be received clearly by only a few others.

In my post I said that the two ends of a CU-SeeMe contact signal to each
other with data on receive error rate and thus "throttle" back transmission
when necessary. However, it has been my experience that operation via a
Reflector interrupts this end-to-end "handshaking" and unless you manually
cap your transmit Kbps rate to quite a low rate (eg 10 Kbps) you can easily
cause problems at the other end and never know it (unless the other party
complains to you). Setting your *Receive* cap however to 60 Kbps or more
is a good idea if you have the bandwidth because all your open video
windows share your available bandwidth to you.

>My only lament is that ECU seems to continue to receive video
>streams long after you close unwanted user windows on a reflector
>connection. Hence all 64K may be in use with closed windows...

In my original post I stated that you could improve received audio by
closing the receive video window. (I should have gone on to say ALL video
windows when connected to a reflector ). Anyway, a reflector WILL continue
to send data to you for a time after you have closed all windows because it
takes time for your data on your open windows to reach the reflector and
even more time for the return journey back from the reflector.

You should be aware that delays via the Internet can be quite long. I have
experienced up to 30 seconds round trip on a point to point call to the USA
from Australia, and although this length of delay is unusual, it does give
you some idea of the problem. And having a wide bandwidth connection will
effect overall throughput (for downloading Web pages etc.) but doesn't
reduce end-to-end delay which is much more important for interactive
applications such as CU-SeeMe.

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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