Subject: jitter-ipdv

From: Demichelis Carlo <[email-masked]>
Date: Tue, 24 Nov 1998 10:02:54 +0100

I would like to add some considerations about the problems related 
to jitter.
There are some different possibilities of defining jitter, and I 
don't think that among them there is a definition better that another.
Each definition describes a property or an effect of the jitter.
I have considered the definitions that I quickly presented in my
presentation in Los Angeles, that were:

+ 1-point PDV (defined as 1-point CDV for ATM cells as you can find
  in ITU-T Rec. I.356): inter-arrival times measured against an 
  expected arrival time. This definition supposes packets sent at a 
  constant rate and it seems to better describe the flow that the 
  network behavior.
+ 2-point PDV (defined as 2-point CDV for ATM cells as you can find
  in ITU-T Rec. I.356): difference in delay with respect a reference
  delay. This is a weak idea of variation, since it is a delay minus
  an offset, you need to define a reference delay for each path and
  also the conditions in which this delay is measured (or defined),
  and therefore is connected to the physical situation of the networks
  that can change quite frequently. It is strongly dependent from 
  synchronization and does not describe how fast or irregular is the 
  variation. (I had problems with this definition also when we
  worked on it in ITU - do you remember, Ken? unluckily it was 
  Friday morning -).
+ Variation with respect an opportune "local" average. This is an
  interesting definition, even if a bit more complex to calculate,
  and is the difference between each delay and the straight line 
  that best fits the delays registered in a given period. It is
  similar to the 2-point PDV, but the reference delay is represented
  time by time by a point of the straight line. Also in this case we
  don't have a description of how fast or irregular is the variation,
  but the reference delay is properly defined. The slope of the 
  straight line provides information about momentary constant 
  variations (over short periods the average of the variation can be 
  not zero, and connected with variation in traffic)
+ IPDV as it is defined in the Draft. Of course, since I'm working
  for a network operator, I have in mind a sub-set of problems 
  related to a set of applications. To produce something more general
  comments are essential.

Probably it is not sufficient to define only one metric for describing
all what is of interest about the network behavior. IPDV is a sort of 
"differential" analysis of the network, and by "integration" you get
exactly the definition of 2-point PDV (integration constant is the 
first delay of the sample) and this supports the feeling that 
2-point PDV does not contain a strong idea of variatiion.

What is certanly missing is a document of Terminology and one of 
Methodology about delay and its variation. A Draft was proposed and
the idea was accepted by BMWG. I would like to underline that we
need it urgently.

Finally, I am tying to transfer the IPDV metric from benchmarking 
to real traffic by using the RTFM tools, but in this case the 
problem is data reduction. It is, maybe, possible to use a sampling
of the measurement. Also in this case, the real traffic environment
could suggest something about definitions.

Bye
Carlo