Hi,
well, first of all, it's not the people at Wireshark that came up with this, but the people that specified RFC3550. So maybe it would be better to ask them.
Now, just to give my 2 cents, what i think is that jitter is a measure that should *increase* both for packets beeing too late (with respect to their expected arrival time) as well as for packets being too early. Since both are problematic, both should contribute positively (i.e. increasing the "badness") to the overal jitter value.
Otherwise put, you shouldnt be able to buy you good jitter by buying back bad late packets with bad early packets.
That's certainly the reasoning behind taking the absolute value of the difference.
regards,
Lars
________________________________
From: wireshark-users-bounces@xxxxxxxxxxxxx [mailto:wireshark-users-bounces@xxxxxxxxxxxxx] On Behalf Of NITIN GOYAL
Sent: mardi 20 mars 2012 13:35
To: Community support list for Wireshark
Subject: [Wireshark-users] Jitter calculation in WIreshark
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
