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From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Newsgroups: comp.dcom.lans
Subject: Re: Real-Time Over LAN
Summary: don't hold your breath for synchronous transmit FDDI
Message-ID: <66098@sgi.sgi.com>
Date: 4 Aug 90 01:52:46 GMT
References: <4426@baird.cs.strath.ac.uk>
Sender: vjs@rhyolite.wpd.sgi.com
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 39

In article <4426@baird.cs.strath.ac.uk>, tony@cs.strath.ac.uk (Tony O'Grady) writes:
>...
> For FDDI, there are separate Synchronous and Asynchronous transmit queues.
> However, the Asynchronous queue is FIFO so the expected transmission order
> from within a station will not occur, i.e., low priority messages will be
> transmitted BEFORE high priority messages queued later.


Would you like to guess what is still not defined in the SMT standard which
was recently letter balloted?  Yes, there is still no "synchronous bandwidth
allocation protocol."  I do not see how manually allocated sync bandwidth,
is more useful than the mostly automatically allocated async bandwidth.

It has been written in stone that no vendor will be criticised for
implementing no more than "SMT 5.1" from last year, which lacked many
things in following drafts.  This means that those who choose not to rush
out implementations of whatever protocol is chosen are not supposed to be
critisized by those who are waiting for the final standard.
(Yeah.  I also have this bridge...)

If you set the token rotation time very low/fast, limit the number of
stations on the ring to much less than the maximum 500, and the stations
that you care about do good things with work in their async. queue, will
you get reasonable latency?  If your frames absolutely, positively have to
be delivered in the next 500 microseconds, wouldn't such a small,
controlled ring be necessary and sufficient?


Were the "guaranteed latency" claims of a few years ago just marketting
hipe from those people trying to prove 802.5, MAP, etc. faster than
ethernet, driven by a need to sell what they and no one else had?  Were the
MAP guarantees as silly as those offered for FDDI?  (Yes, in Real Life
non-speciallized FDDI rings, the worst case latency between opportunties to
transmit will be something like 6 orders of magnitude longer than the time
required to transmit one frame.  As has been said, that's "geologic.")


Vernon Schryver
vjs@sgi.com