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[ippm] Comments on draft-morton-ippm-delay-var-as-03.txt
General:
(a). It may be useful to
incorporate a reference to ITU-T G.1050. This is an IP impairment model
that is somewhat oriented toward modeling congestion over limited bandwidth
links, providing a useful tool for experimenting with and testing PDV
measurement techniques. G.1050 is based on a time series model that was
originally developed as a potential PDV measurement
algorithm.
(b) ITU-T COM12-D98 "Analysis,
measurement and modelling of jitter" (2003) provided some useful background on
the sources and measurement of jitter. This also pointed out some
significant characteristics of the PPDV performance metrics that affect reported
values with particular patterns of delay (e.g. delay ramps, alternate pairs of
early late packets......)
3.4 Service Level
Comparison
IMHO - percentile based
approaches are particularly suitable for SLA agreements. It is both easy
to specify and easy to implement an SLA measurement of the type "no more than x%
of packets shall arrive later than y milliseconds". This is similar
to (within the scope of?) Y.1540 however reports the percentage of packets
outside some given threshold rather than the threshold that corresponds to some
given percentile.
3.5 Separation of scheduling/
smoothing PDV from PDV introduced by the network.
Some delay variation may be
introduced by packet scheduling for smoothing purposes or the queuing of packets
within sending systems. Smoothing could potentially be introduced by
shaping devices in the network. If packet spacing/ size is
uniform then this smoothing or shaping would not necessarily result in PDV
however for any system that uses irregular or bursty packet
transmission smoothing/ shaping is likely to result in PDV that could
in some cases significantly exceed the PDV due to network congestion.
From the perspective of network performance monitoring it is very
useful to separate the two components of smoothing/ shaping PDV (which
would result in strong correlation between delay variation and the bandwidth of
the test signal) and PDV due to other sources (which "should" be weakly
correlated or uncorrelated with the current bandwidth of the test
signal).
Regards
Alan
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