From: walvdrk@research.ptt.nl (Kees van der Wal +31 70 332 6295)
Subject: Re: Curious attitude ...
Organization: PTT Research, The Netherlands
Date: Thu, 30 Jul 1992 17:45:56 GMT

In article <nu696eo@rhyolite.wpd.sgi.com>, vjs@rhyolite.wpd.sgi.com (Vernon
Schryver) writes: 

>> In article <22062@venera.isi.edu>, finn@isi.edu (Greg Finn) writes:
>>...
>> > 	No.  Multicast is NOT broadcasting.  A routing tree is
>> > constructed so that the traffic flows according to what is ideally a
>> > minimum-cost spanning tree.  Hardly broadcast I think.

>> I agree. The essence is that there's hardly any real time contraints when 
>> doing none-interactive multicast.

> Wasn't the IETF multicast "interactive"?

I don't have any idea what IETF is. A group of people looking at a presentation
on a videoscreen is not my understanding of "interactive". 


> If you don't have some real time constraints, pictures get objectionably
> jerky and voice is hard to understand.

What I meant to say is that if the information (picture, voice) is only going 
into a single direction (from performer to audience) there's no problem to 
smooth the variances in the delay. So no jerky pictures or 
difficult-to-understand voice. For such an application I don't mind a total
delay of (say) several seconds. 

If the audience is supposed to talk back ("interaction") I _do_ mind about the 
delay.


> 50 milliseconds is not at all hard to hit on typical ether-internets,
> with workstations acting as routers.  (I know a little of such an
> internet with a hundred or so ethernets, some FDDI and TR and a few
> thousand workstations, and a little more about the hardware and
> software of those workstations.)  Sometimes round trips get worse than
> 50 msec, but then I've noticed my telephone tends to not work very well
> shortly before the building is rattled by thunder.

Hmm, that's interesting information. Though not at all in line with what I
experience (as a user) from the network here. But a major network-overhaul is 
due for this weekend ...

But, what determines the performance (sustainable througput & delay) in such a
network? My ignorant guess is, it's not the number of workstations but the
amount of data these workstation produce, the attainable througput of (such a
workstation-based) router and the number of routers that have to be passed. 

With all respect for data-people in general, for me it's a "small" network.
Doing similar things in a world-wide network might make some difference.


> Do you guys think that all of the "multi-media" people are without a
> clue or a hope?  (I think many lack one or the other, but few lack
> both.)
> 
> I'm trying to politely suggest that you telephony guys should get out
> more.  See other parts of the world.  Good data networking people
> generally know they don't know about telephony, but it is strange the
> obverse does not seem to apply.  (I won't claim in public to be good,
> but I will claim to not know too much about telephones.)

This looks (to me) as a flame against "you guys". Did "we" say anything wrong?
Then "I" apologize. I never suggested (at least I didn't mean to suggest) there
would be no hope for the world without ATM (or "telephony guys" for that
matter). 


Regards, <kees>

Kees  van der Wal		       e-mail: J.C.vanderWal@research.ptt.nl
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