Re: MSC-News: Partial and Total ordering of MSC

Subject: Re: MSC-News: Partial and Total ordering of MSC
From: Oystein Haugen (
Date: Thu Nov 05 1998 - 10:02:51 GMT


Aldo Buratti skrev:

I have a clarification request about the partial ordering of MSC events.

Let's talk of the MSC "event_ordering" taken from Z.120 (ver 10/96),
chapter 4, Fig.4.1a
(You can look at the attached GIF copy named
[NOTE: the frame of the MSC has been replaced with a fictitious axis
named "ENV"]

As you can read on the Z.120 document, just above the figure, the
following partial relations are listed:
1) out(m2) < in(m2)
2) out(m3) < in(m3)
3) out(m4) < in(m4)
4) in(m1) < out(m2) < out(m3) < in(m4)
5) in(m2) < out(m4)

Now, we can start the discussion:
i)  relations 1,2,3 are the trivial relations of each message, BUT, WHY
the relation "out(m1) < in(m1)" has not been listed ?
ii) relation 4 ...sorry... I don't understand !!
  In my opinion it would be:
        out(m1) < out(m2) < out(m3) [total order of axis ENV],
  PLUS  in(m1) < in(m4)             [total order of axis A]

You must remember that the environment has no order. Actually there are no events on the environment at all in the formal interpretation. We cannot know what happens outside our MSC frame. If you think about it you will find this reasonable if though you sometimes know in which order environment events take place, but this must be described in another MSC referring to the one you use.

I'm not sure of the intended meaning of the "<" relation, but I argue
that the original partial order relations are inconsistent with the
original picture.
Maybe, the correct picture would be the second attached picture named

What's right and what's wrong ?

Now I get very puzzled. The figure is _not_ what you say it is. Below I have shown how it should look like and what was actually sent to ITU. I would be very interested to learn that in your version it is different. (image no longer available 27May216)

A second question:
In chapter 2.4, it is stated that:
1) "If an incoming event and an outgoing event are on the same point of
an instance axis, then it is interpreted as if the incoming event is
drawn above the outgoing event"
2) "It is not allowed to draw two or more outgoing events on the same
3) "It is not allowed to draw two or more incoming events on the same

I agree with (1) (it's a useful shorthand) , but what's the reason of
the rules (2) and (3) ??
In my opinion these are unnecessary restrictions (and in my experience I
saw a lot of such patterns).

Don't you think that these widely used patterns (even in an informal
way), can be legitimated by adopting an "implicit" coregion ?

This could have been another interpretation, but we decided rather to disallow the construction because we were not sure that your interpretation would be the only intuitive one.

In other words, two incoming/outcoming events on the same point, should
be treated as events occurring at "the same time", that is in a random
order, that is like in a coregion.

"In the same time" has absolutely no meaning in MSC. Again the reason for disallowing the construction was the risk of misinterpretation from human beings.

 I hope to hear your opinions, soon

You have it !

Oystein Haugen
Rapporteur MSC

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