Wireshark-users: [Wireshark-users] Jitter calculation in WIreshark
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From: NITIN GOYAL <nitinkumgoyal@xxxxxxxxx>
Date: Tue, 20 Mar 2012 18:05:06 +0530
Hi

I have a query regarding the Jitter calculation method in Wireshark.

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

Now, here the absolute value of inter-arrival jitter has been taken into consideration. My query is why the absolute value has been taken when the jitter could be negative also and i think if we take the negative jitter into consideration also, we will get the much actual value rather than the value we are taking at present

Also, when we plot the jitter distribution graph using the above method, it wont be centered around zero as we have made all the values positive and that graph wont look realistic.

Can someone clarify my query?

Regards
Nitin