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RE: [AVT] Clock skew and RTP

I also have a question related to this topic, for those who have looked into
the clock accuracy of various vendors. Since local clocks with higher
inaccuracy can contribute to the clock sckew problem, what is an acceptable
(commonly used) clock accuracy (for exmple 50 ppm), used by manufacturers of
Real-time streaming multimedia applications over IP (such as Voice over IP).
Is there an IETF recommendation in this regard?

Thanks,

Mehrdad Abrishami
Principle Engineer
LSI Logic
240-314-1642


-----Original Message-----
From: avt-admin@ietf.org [mailto:avt-admin@ietf.org]On Behalf Of
Dominique Fober
Sent: Friday, October 18, 2002 4:40 AM
To: avt@ietf.org
Subject: [AVT] Clock skew and RTP


I would like to know if there is any mecanism and/or payload defined
to take account of the clock skew problem.  To be more explicit, here is
a brief description of what I mean by 'clock skew problem':

Clocks on different stations are not running at the same frequency
(we have commonly measured clock deviations up to 1 per 10000 ms).
For real-time transmissions, it results in a problem similar to a
consumer / producer problem.
Let's take an example with a sender host A (the producer) and a receiver
host B (the consumer):
if the consumer consumes the events faster than produced (the B clock is
running faster than the A clock), it will unavoidably lack data in a
given delay,
if the consumer consumes the events slower than produced, (the B clock is
running slower than the A clock) it will unavoidably accummulate more and
more data, resulting sooner or later in storage capacity problems.

Real-time streaming multimedia applications are generally buffering
up to several seconds of data in order to make up for the transport
latency variations. Therefore side effects of the clock skew aren't
noticeable unless the transmission lasts several hours.

What's the common way to solve this problem (if there is one) ?
are there any mecanism and/or payload defined in RTP ?

----------------------------------------------
Dominique Fober               <fober@grame.fr>
----------------------------------------------
GRAME - Centre National de Creation Musicale -
9 rue du Garet  69001 Lyon France
tel:+33 (0)4 720 737 06 fax:+33 (0)4 720 737 01
http://www.grame.fr


_______________________________________________
Audio/Video Transport Working Group
avt@ietf.org
https://www1.ietf.org/mailman/listinfo/avt

_______________________________________________
Audio/Video Transport Working Group
avt@ietf.org
https://www1.ietf.org/mailman/listinfo/avt