Wireshark-users: [Wireshark-users] Abnormal RTP Jitter Value

From: "T.Shimizu" <shimizu.takao@xxxxxxxxxxxxxx>
Date: Fri, 1 Sep 2006 14:35:24 +0900

Hi, All.

In Ethereal V0.99, /Statistics/RTP/Show All Streams,
The case that the said value is abnormaly large is found.

For Example, VoIP 20ms G.711;

   RTP Time Stamp = 2^32-1(FFFFFFFF) at Seq1
   and
   RTP Time Stamp = 159 at Seq2
   RTP Time Stamp = 319 at Seq3
      foliows +160 by Seq.

   In this case, Jitter Value is shown as  about 10^6 msec in
/Statistics/RTP/Show All Streams.

It seems that Ethereal calculate the Jitter Value as follows;

   Delta between  Seq1 and Seq2 :  159-(2^32-1)  = |-(2^32-160)|

However, TimeStamp means + Signed Time Unit, therefore, the calculation may be better

as follows;

   Delta between  Seq1 and Seq2 :

                   159-(2^32-1)  = -(2^32-160)

                                      = +2^32 -(2^32-160)

= 160 =

  In other words,

      In case of

          Timestamp(Seq=N+1) - Timestamp(N) <0

      It is necessary to Normalize Delta by adding 2^32.


Bset Redards