Subject: [AVT] Question concearning the buffering in VIC

From: "Nikos Laoutaris" <[email-masked]>
Date: Wed, 17 Oct 2001 14:19:35 +0300

Hello 

I have a question concearning the playout adaptation algorithm used by the
VIC MBone tool. I've read that VIC presents frames as soon as they arrive
and that it does not implement a dejitter buffer (at least in the basic
version,i.e., without intermedia synchronization with some audio tool).

Does this mean that if an arriving frame finds upon its arrival other frames
in the playout buffer will still be immediatelly presented and the other
frames will be discarded? Even without initial buffering, a dejitter buffer
may build up as a consequence of buffer underflows (followed by batches of
arriving frames that form the dejitter buffer). My understanding is that vic
is "delay preserving", it presents frames as soon as they arrive and at that
point discards frames that are buffered ahead (late frames). Can you please
confirm that?

The alternative would be to operate in a "data preserving manner"; start
without dejitter buffer and let underflows build up the dejitter buffer in
response to time varying jitter. In any time instant the buffer will be able
to absorb delays that are not larger than the largest delay up to that
instant. Larger delays will cause underflows and subsequently will build up
the dejitter buffer.  

I've looked at the basic reference for vic (McCanne, Jacobson, "vic: A
Flexible Framework for Packet Video") but did not find information on the
implemented playout algorithm. Also i have looked at the source code but it
is difficult to locate the file/function where the playout point is
scheduled. 

Thank you

Nikos Laoutaris
Communication Networks Laboratory,
Department of Informatics & Telecommunications,
University of Athens, Greece
e-mail : [email-masked]
www : http://www.cnl.di.uoa.gr/~laoutaris/laoutaris.html
tel. : +301 7275341 fax : +301 7275601