Subject: RE: [Ieprep] I-D ACTION:draft-ietf-ieprep-framework-06.txt

From: Scott Bradner <[email-masked]>
Date: Thu, 9 Oct 2003 10:27:56 -0400 (EDT)

> So what
> you really want to say is that, say, "95% of the packets must have a delay
> of less than 300 milliseconds", recognizing that 5% of the packets may get
> dropped if the buffer mechanism at the receiver imposes a consistent 300
> millisecond delay.

that is just the right idea

Scott

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From: "Gunn, Janet" <[email-masked]>
Date: Thu, 9 Oct 2003 10:00:28 -0400

To clarify what I mean.

If you have a long, but consistent delay (say of the order of .5 seconds
(500 milliseconds) in an intercontinental satellite call) it is very
irritating from a conversation point of view- figuring out when it is "your
turn to talk", and avoiding talking over each other.  But the speech quality
is perfectly intelligible.

But if you have a highly variable delay, ranging from, say, 100 milliseconds
to 500 milliseconds, with an average delay of  300 milliseconds, you have a
high probability of "gaps" in the middle of words, even if there is no
packet loss.  You can, of course, buffer all the packets to the equivalent
of 500 milliseconds delay, in which case the speech is intelligible, but
then you gain no advantage from the fact that the average delay is 300
milliseconds instead of 500 milliseconds.

You could thus say that what is important is the "maximum delay" (rather
than the combination of "average delay" and "jitter").  But in the real
world, there is no such thing as an upper bound on delay (except as it is
enforced by dropping "late" packets), but there is a distribution.  So what
you really want to say is that, say, "95% of the packets must have a delay
of less than 300 milliseconds", recognizing that 5% of the packets may get
dropped if the buffer mechanism at the receiver imposes a consistent 300
millisecond delay.

This is what I mean by "jitter is important".  I would be equally satisfied
by saying "distribution of delay" is important,  But neither "average delay"
nor "maximum delay", by themselves, are sufficient criteria. 

-----Original Message-----
From: Scott Bradner [mailto:[email-masked]]
Sent: Wednesday, October 08, 2003 6:54 PM
To: [email-masked]; [email-masked]; Gunn, Janet
Subject: RE: [Ieprep] I-D ACTION:draft-ietf-ieprep-framework-06.txt

> Substance
> 1.2 third para
> I have always understood that jitter was more important than delay for
VoIP,
> but you do not mention it.

I'm not sure that jitter per se is a big issue - the distribution of 
delay is - most of the packets need to get to the other end within 
a reasonable limit - that can be because the basic delay is near the
limit and there is no jitter or the basic latency is less and the jitter
brings the delay to thye limit.

Scott