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Introduction
The plan is to submit a paper based on this document. The initial draft of the paper is in Google docs. This is a project suggested by Bebo White to build and validate a PingER Measurement Agent (MA) based on an inexpensive piece of hardware called a Raspberry Pi (see more about Raspberry Pi) using a linux distribution as the Operating System (see more about Raspbian). If successful one could consider using these in production: reducing the costs, power drain (they draw about 2W of 5V DC power compared to typically over 100W for a deskside computer or 20W for a laptop) and space (credit card size). This is the same type of power required for a smartphone so appropriate off the shelf products including a battery and solar cells are becoming readily available. Thus the Raspberry could be very valuable for sites in developing countries where cost, power utilization and to a lesser extent space may be crucial.
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In this report the jitter is represented by the Inter Packet Delay (IPD), the absolute values of the IPF (Abs(IPD)) and the Stdev(IPD).
Methodology
Measurements
We Since we believed, both a priori and from observations, that the major impact on the measurements was the network and not the servers' hardware or OS, we chose to make detailed analysis of PingER measurements between two representative MAs at SLAC and two sites well separated from SLAC and hence with very different RTTs. The MAs at SLAC were:measurements to and from two MAs at SLAC.
The Dell Poweredge 2650 bare metal pinger.slac.stanford.edu server running Red Hat Linux 2.6.32-504.8.1.el6.i686 with specs.
Code Block 103cottrell@pinger:~$uname -a Linux pinger 2.6.32-504.8.1.el6.i686 #1 SMP Fri Dec 19 12:14:17 EST 2014 i686 i686 i386 GNU/Linux
The Raspberry Pi pinger-raspberry.slac.stanford.edu an armv61 running Gnu Linux (see above) with specs.
Code Block pi@pinger-raspberry ~ $ uname -a Linux pinger-raspberry.slac.stanford.edu 3.18.11+ #781 PREEMPT Tue Apr 21 18:02:18 BST 2015 armv6l GNU/Linux
Both were in the same building at SLAC, i.e. roughly at latitude 37.4190 N, longitude 122.2085 W, but on different floors. The machines are about 30 metres apart or about 0.0003 msec based on the speed of light in a direct fibre.
The measurements used were made:
- between pinger.slac.stanford.edu and pinger-raspberry.slac.stanford.edu, and
- from both pinger.slac.stanford.edu and pinger-raspberry.slac.stanford.edu to targets at varying distances and hence varying minimum RTTs from SLAC.
- from two representative MAs at sitka.triumf.ca and ping.cern.ch to pinger.slac.stanford.edu and pinger-raspberry.edu
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To find the probability of the distributions overlapping use a nomogram of mean differences versus error ratios given in Overlapping Normal Distributions. John M. Linacre for normal distributions.
We also use the Z Tests
Kolmogorov-Smirnov Test
The Kolmogorov-Smirnov test (KS-test) tries to determine if two datasets differ significantly. The KS-test has the advantage of making no assumption about the distribution of data. In other words it is non-parametric and distribution free. The method is explained here and makes use of an Excel tool called "Real Statiscs". The tests were made using the raw data and distributions, both methods had similar results except for the 100Bytes Packet that had a great difference in the results. The results using raw data says both samples does not come from the same distribution with a significant difference, however if we use distributions the result says that only the 1000Bytes packet does not come from the same distribution. Bellow you will find the graphs for the distributions that were created and the cumulative frequency in both cases plotted one above other (in order to see the difference between the distributions).
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