Subject: IPDV clock terminology

From: "Matthew J Zekauskas" <[email-masked]>
Date: Tue, 10 Aug 1999 11:06:43 -0400

I was re-reading the IPDV draft yesterday and found that one term
bothered me as I was reading the draft, and that was the reuse of
the Framework term "skew".

Carlo (I believe) coined the terms "skew" and "drift", where skew was
the fixed offset and drift was the variable offset of two clocks.

How about just calling the fixed offset "offset"?

--Matt

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From: "Philip F. Chimento" <[email-masked]>
Date: Tue, 10 Aug 1999 17:01:07 +0200

Hi Matt: 
Ok. That is fine with me. As I said in Oslo I need to go over those 
sections carefully and clarify them (and make sure that the 
terminology is in conformance with the framework). Any other 
suggestions like this are welcome. 
Regards, Phil 
-- 
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       ___o___  Phil Chimento                       tel.: +31 53 4894331
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From: Andy Scherrer <[email-masked]>
Date: Tue, 10 Aug 1999 11:00:51 -0500 (CDT)

> suggestions like this are welcome. 

I noted that there is now some progress in writing down a definition of
`jitter'. 

But we should keep in mind that there are many different quantities of
variation that we will want to measure even in delay, since variation in
latency along a real life path is introduced by many different sources.
Some of these are independent, others are nested heirarchically, etc.

So it will be useful to talk about an overall variation in a data set, as
well as the various components that make up that total. This begs for a
notation.

Beyond the sources of variation, there are different representations of
it, e.g., some sort of range (percentiles), absolute deviations, summed
deviations, squared deviations, etc. 

I suggest that the notation should allow for specifying both the computed
quantity and the source of variation. 

So, for example, if you measure the Mean Squared Error of a sample along
one segment of 19-hop path, you may want to call it something like,
		MSE(hop 2)

But the summed absolute deviations would be:
		SAD(hop 2)

[Note that MSE is standard statistical notation for that quantity, while
SAD is not.]

Andy Scherrer
Statistician
Matrix Information and Directory Services, Inc.
1106 Clayton Lane, Ste. 501W, Austin, TX 78723
http://www.mids.org/
+1-512-451-7602

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From: "Philip F. Chimento" <[email-masked]>
Date: Wed, 11 Aug 1999 09:12:56 +0200

> Some of these are independent, others are nested heirarchically, etc.
Right; I tried to lay out in Oslo (and its on the slides) what I thought
the sources of the variation are. And yes, you are right, there are a lot 
of different quantities that can be used for "delay variation". 

> deviations, squared deviations, etc. 
I think that these can easily be defined as derivative measures from the 
basic IPDV metric. I think that probably the main criterion for defining them 
will be whether someone thinks that they are useful. In any case, I think 
that a couple of paragraphs are warranted that explain the different 
metrics that could be derived and what their meaning is. 

> one segment of 19-hop path, you may want to call it something like,
I don't quite understand the notation here. What does hop 2 signify ? 
 
Regards, Phil

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From: Andy Scherrer <[email-masked]>
Date: Wed, 11 Aug 1999 09:45:46 -0500 (CDT)

> metrics that could be derived and what their meaning is. 

OK -- mainly what I was getting at was it might be useful to think on how
to allow for the specification of many sources of variation (those deemed
important) in a consistent way, but not really to try and list & define
every possible important source.

> I don't quite understand the notation here. What does hop 2 signify ? 

I omitted a sentence of explanation... 
(hop2) merely specifies the entity to which the quantity applies, or the
source of variation. I was thinking of "hop 2" as the second segment on a
path -- just a vague and arbitrary example. It probably would have been
better for me to use something like,
		MSE(router 1.2.3.4)  
indicating that part of the total variation which was due to the influence
of that router. 

Hopefully that's a little clearer.

Andy