Subject: [ippm] Jitter Correlation

From: "Alan Clark" <[email-masked]>
Date: Fri, 5 Oct 2001 11:09:23 -0400

There are various circumstances that can lead to correlated high pdv
including self-similarity and short term changes in traffic loading.  By
correlated high pdv we mean that during some period of time either the
packet inter-arrival time varies more significantly OR that the delay
increases momentarily.  We have seen this effect in both simulations of VoIP
services and Internet traces, and it has been reported by others (e.g.
San-qi Li).

The degree of correlation of pdv can have a significant effect on the
performance of some real-time applications.  Many such applications employ a
(fixed or adaptive) jitter buffer to eliminate pdv, replacing it with delay
and packet loss.

In the case of a fixed jitter buffer then a period of high pdv would result
in a burst of packet discards, which would result generally in some
noticeable artifacts from the voice/video decoder.  The degree to which the
packet discards affects subjective quality will depend on their
distribution - as (in general) isolated lost packets can be concealed more
effectively than bursts of lost packets.

In the case of an adaptive jitter buffer then a period of high pdv would
trigger the adaptive jitter buffer to increase delay.  Some initial packets
may be discarded but hopefully the jitter buffer would limit this to a few.
If pdv is NOT correlated, e.g. occasional packets have large delay, then an
adaptive jitter buffer may not help as the delayed packets will be discarded
triggering the jitter buffer to increase its delay but the following packets
may have low delay.

We are currently modeling these effects using NS-2 with several different
network topologies, MMPP traffic models, an adaptive jitter buffer and
VQmon - which provides a plausible VoIP service modeling environment. The
aim of this work is to obtain a better understanding of the nature of pdv
and it's statistical properties, and to derive time series models that can
be used to simulate pdv for different network conditions.  We would be
interested to know if anyone else is working in this area.

With regard to IPPM - the current ipdv draft does not appear to address
jitter correlation although it does mention skew and drift which could also
cause issues with jitter buffer operation.  Is anyone aware of any/
interested in defining pdv metrics that may be useful in measuring this
effect?

Regards

Alan Clark
Telchemy

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