From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Subject: Re: >>>>Future of IP routers
Organization: Silicon Graphics, Inc.  Mountain View, CA
Date: Thu, 3 Sep 1992 04:34:11 GMT

In article <1992Sep2.182548.18739@amd.com>, shah@angelo.amd.com (Amit Shah) writes:
> ...
> >>Some of us do not think much of an 8 ms.
> 
> This issue has been blown out of proportion....

> If one has a network which is expected to operate at near 100 Mbps sustained,
> then the tuning of TTRT and ring latency is obviously more important than it
> would be for a backbone expecting to carry 20-30% load with minimal bursts.

How big is must a "burst" be before it is "sustained"?  Is a 10 MByte
file a burst?  Even if it takes 20% longer to send?  If the FDDI ring
are like the ethernets I know, idle except for the 30% of the time when
100% staturated?  I'm arguing that modern "workstation" as opposed to
"PC" network loads are very bursty.

> ..
> I have played with a few SBus and EISA cards myself and I have yet to see
> a major impact in a *REAL* environment of an 8 ms. TTRT.  This is simply
> because nobody has loaded the network in a sustained manner...

There may be more EISA buses in the world than VME, S, and GIO buses.
However, my employer hopes that I'm not the only one who uses the stuff
I work on.  

I do not count on other vendors being unable to make fast FDDI
interfaces as quickly as my employer.  Some vendors in were not that
slow in the test labs last year.

Don't judge the whole world by PC's.  PC's still usually don't staturate
one ethernet, but any reasonable workstation can clobber two or three.
Fast FDDI systems will be soon be cheap and common among workstations.


> ...
> In conclusion, although I agree with vernon that 8 ms TTRT can be inefficient
> in some cases, in other cases it may not be.  Because of the dependence of the
>TTRT on the applications, it is difficult to put a broad tar on it and say that
> 40 ms (say) shalt be the value.

I don't claim 8 ms is "inefficient".  I leave defining "efficient"
to others and accept the statement that 8 ms is "efficient."
I claim 8 ms. is "slow."

The evil with 8 ms is not that it is right or wrong on any particular
or even most networks.  It's evil is as I've argued years ago, that by
making 8 ms the default, vendors have made it practically impossible
for customers to choose 9 ms, 16 ms, 40 ms or 165 ms.  That is because
you must wrestle with each such box on your ring to override its PROM.

If they would just tell people in big red letters "8 ms is The One True
Right Value", and tell them to set T_Req=8 on exactly one (1) station
or concentrator, all would be well.  As it is, should such a default-8
concentrator be reset to its default parameters, it will override your
choice of 16 or 40 or whatever, which you have made on your official
FDDI Control Station.


> >So the question is, how many
> >people actually make use of the ability to carry synchronous data?
> >
>Good question.  I know that some people are beginning to use it for voice/video
> sort of applications but most of the nets today use async. only.

I have argued for years that when you need the freedom from jitter and
other problems that sync FDDI gives you then you don't need it.  While
your ring is lightly loaded, async is as jitter free as sync.  While
your ring is heavily loaded, you can't afford sync because your async
traffic will suffer, and so will install another ring.

The significant reasons ethernets are not now commonly used for voice
and video do not include problems with jitter.

I predict that FDDI will be commonly used for synchronous computer
applications at the same time ISDN, ATM and ethernet are.  I also
predict that async FDDI frames will be used, not sync frames.  I do not
predict whether that those uses will be common in my lifetime or no.
Too many better informed people than I have been wrong too many times
about when voice and video would take off in the last 25 years.


Vernon Schryver,  vjs@sgi.com
Newsgroups: comp.dcom.lans.fddi