Wireshark-dev: Re: [Wireshark-dev] RTP Jitter calculation (Telephony->RTP Stream Analysis)
Again, with the line break in the right place (i hope)
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)
-----Original Message-----
From: wireshark-dev-bounces@xxxxxxxxxxxxx [mailto:wireshark-dev-bounces@xxxxxxxxxxxxx] On Behalf Of RUOFF, LARS (LARS)** CTR **
Sent: mercredi 6 avril 2011 18:42
To: Developer support list for Wireshark
Subject: Re: [Wireshark-dev] RTP Jitter calculation (Telephony->RTP Stream Analysis)
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@xxxxxxxxxxxxx [mailto:wireshark-dev-bounces@xxxxxxxxxxxxx] 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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