Re: [AVTCORE] RFC3550: RTP Jitter value calculation

[Nataraja Hosahalli <[e-mail]>]

Hi Steve,

Thanks for the clarification. Now I got a better understanding of the significance of the jitter.

In order to understand the topic bit further I have added both the calculations into excel sheet, I have calculated the rr->jitter for 500 iterations with a constant value of "d" for both fixed and floating point formula. 
Please have a quick look on the attached excel sheet. You can edit the cell A6 for a different "d" value.

In the excel results, jitter value with both formulae is differing by 1 RTP TS unit. But I assume it is very much negligible.

Thank you.

Regards,
Nataraj

-----Original Message-----
From: Stephen Casner [mailto:[e-mail]] 
Sent: Wednesday, 26 March, 2014 10:56 PM
To: Nataraja Hosahalli
Cc: Kevin Gross; [e-mail]
Subject: Re: [AVTCORE] RFC3550: RTP Jitter value calculation

On Wed, 26 Mar 2014, Nataraja Hosahalli wrote:

> Hi Kevin,
>
> Yes you are right. I ran both the formulae and also your improved
> formula for 500 times with constant d value, the value rr->jitter is
> converging near to average delay d after couple of iterations (~100
> for d=2 for example).
> However your new formula below is resulting to more accurate average.
> rr->jitter = (s->jitter + 8) >> 4;
> Older formula is almost always lagging by nearly 1 ms (0.5 ms
> floating value) with the average delay.
>
> May be your new formula can be updated in RFC.

That difference in accuracy is unimportant because the value is not
intended to be interpreted quantitatively.  This is an important
paragraph from RFC 3550:

   The interarrival jitter field provides a second short-term measure of
   network congestion.  Packet loss tracks persistent congestion while
   the jitter measure tracks transient congestion.  The jitter measure
   may indicate congestion before it leads to packet loss.  The
   interarrival jitter field is only a snapshot of the jitter at the
   time of a report and is not intended to be taken quantitatively.
   Rather, it is intended for comparison across a number of reports from
   one receiver over time or from multiple receivers, e.g., within a
   single network, at the same time.  To allow comparison across
   receivers, it is important the the jitter be calculated according to
   the same formula by all receivers.

The integer calculation was kept as simple as possible for use in
environments with constrained processing power.  In a modern DSP with
fast floating point calculations, this is not an issue, but 30-40
years ago we had to use fixed-point arithmetic in our array processors
and early DSPs.  The jitter calculation using a scaled value is an
example of that.

                                                        -- Steve

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