Looking at the code, i'm not sure they are even calculating the standard
deviation correctly. They are using this formula:
dev = sqrt( (E (sqr(x)))/N - sqr( (E(x))/N))
standard deviation is defined as
sqrt( 1 / (N) * E sqr((x - E(x)/N) ))
where E represents sigma, or the sum all items from i=1 to n.
The mean deviation is the mean of the absolute deviations, which is
nothing like standard deviation.
MD = 1/N * E abs(x - E(x))
Josh, you are absolutely correct it should be sdev, not mdev. However,
sdev is not being calculated correctly. When they expanded sqr(x -
(E(x))/N) it should be
sqr(x) - 2x E(x)/N + sqr((E(x))/N)
They have
sqr(x) + sqr((E(x))/N)
So they essentially removed E(-2x E(x)/N) from the calculation, giving a
number that is not the standard deviation. Am I off base anywhere here?
This should be an easy one to fix. I've already added a jitter
calculation by retrieving the average of the differences of each
response time. That should be close to the standard deviation isn't it?
Derek
Josh Coates 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 found the answer. It is the standard deviation. mdev stands for mean
deviation.
mdev is the std dev? standard deviation is not the same thing as the mean
deviation.
http://mathworld.wolfram.com/StandardDeviation.html
http://mathworld.wolfram.com/MeanDeviation.html
i didn't look at the source, but what you've excerpted below looks like they
are assigning 'mdev' the stnd deviation, which doesn't look right.
quick, now is your chance to make a contribution to the open source
movement!!
submit a bug and patch: s/mdev/sdev/g on ping_common.c
w00t!
Josh Coates
www.jcoates.org
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of
Derek Burdick
Sent: Friday, June 17, 2005 12:28 AM
To: Provo Linux Users Group Mailing List
Subject: Re: Ping's definition of mdev
I found the answer. It is the standard deviation. mdev stands for mean
deviation.
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
squared.
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.
Thanks,
Derek
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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