Jitter calculation in WIreshark

[NITIN GOYAL <nitinkumgoyal () gmail com>]

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

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