From: miw@cc.uq.oz.au (Mark I. Williams)
Subject: Re: FASTPAC and AARNET
Organization: Prentice Centre, University of Queensland
Date: Mon, 28 Sep 1992 10:52:25 GMT

Chris.Rusbridge@levels.unisa.edu.au writes:

>Here in the UK there are some interesting experiments being conducted 
>by UCL on packet video over JANET, as a prelude to video over 
>SuperJANET. I watched a demo at the Networkshop, and it was pretty 
>crude stuff. Nevertheless, with slightly improved technology and 
>better bandwidth it would be quite usable for improving interaction in 
>distributed research teams.

What I am relying on when I talk about the effects of delay are human
factor studies (which seem to be generally accepted by the multimedia
folks) saying that the maximum acceptable end-end delay for interactive 
video is about 150 ms. Now videoconferencing achieves this by using
layered coding over channels with no queuing delays (i.e synchronous
channels). 

That is, first of all the quality of the image is adjusted to keep a 
constant data rate.  second, you know that when you send a bit out over 
the interface, it is going to arrive in a timeframe dictated by the speed 
of propagation of light on a bearer. This means you save yourself the 
delay of a receiver buffer.

Constant quality encodings like MPEG are an entirely new ballgame, since
the peak bandwidth required just to keep delay in bounds without
queueing would be about 3-4 Mbps, depending on whom you believe. Then
you have the very high burstiness which plays havoc with the queues on
switching equipment. Since you will very likely be sending this over
some kind of asynchronous channel like cell relay, you then have to
provide buffering at the receiver to smooth out the delay jitter, adding
buffer delay to the delay of the channel. In order to support these
traffic streams, the communications channel has to provide low delay and
low delay jitter. Unfortunately, the Ethernet interface on FASTPAC-10
seems to have a combination of delay jitter and assembly/disassembly
delay that is too large to support interactive video if you apply the 150ms 
criterion. 

Of course, Ethernet interfaces suck big time when it comes to delay
jitter anyway, because they have exponential backoff built into them.
Perhaps a token-ring interface would be better. You could also relax the
delay criterion and provide more buffering, but you might find that
users will reject the result.

This is really not meant as a criticism of FASTPAC-10. After all, AOTC
certainly aren't touting it as a service for carrying video traffic.
However, what I *am* saying is that FASTPAC-10 won't allow AARNet to
carry much in the way of sexy new services. It is, after all, really
only equivalent to a couple of megalinks, as it is a half-duplex medium.
This means that any move by AARNet to FASTPAC can only be justified on
the basis of how it stacks up price-wise. That is *all* I was saying
when it comes to FASTPAC.

I happen to think that the price advantage is there, if a bit marginal.
Geoff thinks it is *too* marginal to be taken seriously at the moment.
So we disagree. That's life.

>More recently, I was involved in a video conferencing 
>trial using 112 Kbps (2*ISDN-US B channels). At first it was unnerving 
>to see the presenter's face freeze as she sent a high resolution 
>document over the line, but it was surprising how quickly we got used 
>to it. More unnerving was the delay introduced by the compression 
>techniques into the audio channel. This was at least twice the 
>satellite delay. Nevertheless I was convinced that such a system was 
>workable.

It sure is. I attended a meeting last Friday where the WA delegates
attended via videoconference. (They couldn't get a ticket east due to
the AFL final). I am convinced that the WA delegates had a much better
deal of it, since I spent 2 days mucking about with planes and crappy
accommodation, while they could have muffins and coffee during the
meeting without disturbing anyone. In addition, they got do do a bit of
workshopping when the cranks got hold of the microphone at the Canberra
end. They also got more than their fair share of the floor becaues of 
their 'disadvantaged' status. :-)

Incidentally, this technology is almost old hat. *Very* nice
videoconferencing hardware is available off-the-shelf today, and you
just plug it into your ISDN microlink or macrolink, and get resolution
and motion according to how many channels you dial. No muss, no fuss, no
sweat. You can have control voice-switched and get the camera to find
the speakers nearly automatically. It just costs around $100 grand per 
station.

>The point here is that I suspect for that sort of application (video 
>conferencing, remote video tutorials, etc)  that the delay introduced 
>by Fastpac will not be significant. It is very likely to be much less 
>than the incremental satellite delay. Yes, there are high quality, 
>rela time video applications for which it would not be suitable. Just 
>don't believe that _all_ video is ruled out.

I don't disagree with you that *some* video can be sent over FASTPAC. In
fact, the folks at Woolongong and AOTC have been running trials sending colour
video at reduced scan rates over FASTPAC. Given a clear Ethernet, one
session only, and the wind behind them they got a result that was almost
'there', but they are the first to admit that it isn't yet a real-world
service. They also got results that were almost as good using a couple
of Wellfleets joined by a Megalink service. I might add that they were
at pains to point out that their results did not constitute a criticism
of FASTPAC. The results are published in the proceedings of the 2nd
Australian Multi-Media Communications, Applications, and Technology
Workshop (July, 1992).

"Desktop Teleconferencing for Broadband Networks"
by H.W. Peter Beadle, John Judge, Greg McFarlane, and Leith Hayes.

If you are talking about real multimedia applications using 2-way
full-motion video it is a different story. What is acceptable to
conditioned operators who want the system to work for them will not be
acceptable to the general public. For that you need a real broadband
service.

I'll get out of your way now,

cheers,
--
Mark Williams         The University of Queensland       miw@cc.uq.edu.au
+61 7 36 54012   (w)  Prentice Centre
+61 7 36 54477  (fax) Qld 4072  Australia
Nullum magnum ingenium sine mixtura dementiae fuit.  -- Seneca