Re: lost packets

John Ingham (johningh@tafe.sa.edu.au)
Thu, 6 Feb 1997 13:43:45 +0930


Tony Stewart said...

>I am using Enhanced CuSeeMe(demo) with a Pentium 133, 28,800 modem,

>32meg memory. I have got used to no sound and virtually no movement,
but

>I notice that if I click on the icon under a distant video picture, I

>seem to get a large number of so-called 'lost packets'.

>Why am I losing them? Can i ensure that I don't lose any in the
future??

Lost packets are a fact of life with CU-SeeMe I'm sorry to say due to
the difference in the type of protocol used - UDP - as compared with
TCP as used used for e-mail and WWW. In simple terms, UDP is like
regular snail mail - all care taken but no promises made re deliverey -
while TCP is like Registered snail mail where signatures are required
to confirm delivery. This means that UDP is faster than TCP but not as
reliable.

The end result is that if a CU-SeeMe user is sending packets at a rate
faster than <bold>any part of the transmission path from him to
you</bold> some packets will inevitably get lost. For instance the
other user may have his packet cap set too high for his (or your) modem
capacity, or his (or your) Internet Service Provider's link to the
Internet may have insufficient bandwidth, or depending on the path
between you and the other user, congestion on the Internet itself may
be limiting the effective data through-put etc.

Basically, whenever you use an application based on a protocol such as
UDP you have very little control over the number of lost packets.
Operation via a reflector is almost always inferior to direct point to
point connection, particularly if you have more than one window open at
a time.

Finally I note that you say "I have got used to no sound and virtually
no movement". Maybe your expectaions are too high. CU-SeeMe is not
broadcast Television and never will be. If you have a 28,800 bps modem
and the scene you're watching is a person sitting quite still against a
plain back gound, you may see 6 fps maximum. I have seen higher rates
than this only when looking at a stationary non-animate object. That's
it - that's CU-SeeMe! But it IS useful for person to person
communication.

I use CU-SeeMe for talking with friends and we always freeze the
picture when talking - that way at least you can see the other person's
reactions to what you are saying (even if the round trip delay can be
anything from 5 to 30 seconds).

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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