Ping's definition of mdev
derek at burdick.cc
Fri Jun 17 00:27:44 MDT 2005
I found the answer. It is the standard deviation. mdev stands for mean
Derek Burdick wrote:
> I searched the man page and google but can not find a definition for
> mdev, which is a returned statistic when ping exits. Does anybody
> know what it is measuring? I have looked through the code and
> determined mathematically what the algorithm does, but I don't see any
> relevance to networking. Does anybody have any insight? I have
> attached my comments on what the code is doing in case somebody knows
> of a generic use for this algorithm.
> Here is the sample output
> 3460 packets transmitted, 3451 received, 0% packet loss, time 119983ms
> rtt min/avg/max/mdev = 303.108/305.426/347.859/2.979 ms, pipe 11
> Here is what I believe ping_common.c is doing to determine mdev:
> for each packet received and assuming tsum and tsum2 start at 0:
> tsum += triptime
> tsum2 += triptime * triptime
> Once ping finishes
> tsum /= nreceived + nrepeats // Get the average of the triptimes. We
> have 0 repeats so it has no effect here.
> tsum2 /= nreceived + nrepeats // Get the average of the squared
> triptimes. Once again we have 0 repeats
> tmdev = llsqrt(tsum2 - tsum * tsum); // Take the square root of the
> difference of the average squared triptimes and the average triptime
> tmdev is then printed as printf("%ld.%03ld ms", tmdev /1000,
> tmdev%1000); //This is giving us the results in milliseconds
> If anybody has any insight or comments i'd love to hear them.
> p.s. This is not a class assignment. Our sales team wants to sell IP
> phone service to a client in India using the Internet for the
> bandwidth. I was looking for any numbers I could find to help me give
> them a good idea of what to expect.
> I think I will add a Jitter calculation in to ping as that is usually
> a great indication along with packet loss on what to expect with VOIP.
> Thanks again.
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