Re: RTP Jitter calculation (Telephony->RTP Stream Analysis)

["RUOFF, LARS (LARS)** CTR **" <lars.ruoff () alcatel-lucent com>]

Hi,
Oh my, it's been a long time since I have written that code and i didn't check the diffs that have been contributed 
since, but i'll give it a try...

(Code uses real time units instead of RTP timestamp units but that doesn't matter for the argument)

Saved values from previous packet:
statinfo->time == R(i-1)
statinfo->lastnominaltime == S(i-1)
statinfo->jitter == J(i-1)

Value from current packet:
current_time == Ri
nominaltime == Si

Thus...
expected_time == R(i-1) + (Si - S(i-1))
current_diff == |Ri - expected_time| == |Ri - (R(i-1) + (Si - S(i-1)))| == |D(i-1,i)|
current_jitter == (15 * J(i-1) + |D(i-1,i)|) / 16 == J(i-1) - J(i-1)/16 + |D(i-1,i)|/16 == J(i)

Bingo!

Regards,
Lars
 

-----Original Message-----
From: wireshark-dev-bounces () wireshark org [mailto:wireshark-dev-bounces () wireshark org] On Behalf Of Chaswi 
Przellczyk
Sent: mercredi 6 avril 2011 17:12
To: Wireshark Dev
Subject: [Wireshark-dev] RTP Jitter calculation (Telephony->RTP Stream Analysis)


Dear all,

I'm trying to match
---------------------------
http://wiki.wireshark.org/RTP_statistics
---------------------------
How jitter is calculated

Wireshark calculates jitter according to RFC3550 (RTP):

If Si is the RTP timestamp from packet i, and Ri is the time of arrival in RTP timestamp units for packet i, then for 
two packets i and j, D may be expressed as

    * D(i,j) = (Rj - Ri) - (Sj - Si) = (Rj - Sj) - (Ri - Si) 

The interarrival jitter SHOULD be calculated continuously as each data packet i is received from source SSRC_n, using 
this difference D for that packet and the previous packet i-1 in order of arrival (not necessarily in sequence), 
according to the formula

    * J(i) = J(i-1) + (|D(i-1,i)| - J(i-1))/16 

---------------------------
to the following code from tap-rtp-common.c (rtp_packet_analyse):
---------------------------
                if( !statinfo->first_packet )
                { /* Calculate the current jitter(in ms) */
                        expected_time    = statinfo->time + (nominaltime - statinfo->lastnominaltime);
                        current_diff     = fabs(current_time - expected_time);
                        current_jitter   = (15 * statinfo->jitter + current_diff) / 16;

                        statinfo->delta  = current_time - (statinfo->time);
                        statinfo->jitter = current_jitter;
                        statinfo->diff   = current_diff;
                }
                statinfo->lastnominaltime = nominaltime;

---------------------------

And I can't find (m)any similarities between the two.

Can anyone lend a hand?

Thanks!
CP.

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