From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Newsgroups: comp.dcom.lans.fddi
Subject: Re: Daisy chaining SAS FDDI workstations
Date: 9 Apr 92 02:26:38 GMT
Organization: Silicon Graphics, Inc.  Mountain View, CA

In article <1992Apr8.210100.23022@amd.com>, shah@brahms.amd.com (Amit Shah) writes:
> In article <j9dqmr8@sgi.sgi.com> vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:
> >
> >Actually, running without PCM is a lot simpler than running with PCM.  You
> >just put your MAC into the ring whenever you're ready, and bypass yourself
> >the rest of the time, either with an optical bypass or electrically (e.g.
> >PHY repeat-mode)....
> 
> Not as simple as it sounds. 

Well, for me it was as simple as it sounds, to me.  I mean it is very easy
to turn on a MAC (including a FORMAC) and do frames.  I've done it, as no
doubt have many others while bringing up FDDI implementations.  However,
many people are unwilling to consider the utility of that mode.

CMT needs lots of timers and at least two state machines (if you count the
bit signally separately).  It's incompariably more work, even if you use
the DEC/Motorola ELM.  The CMT machine(s) don't count RMT, PCI, and all of
the other state machines that you don't need if your hardware never fails.
Of course, your hardware will always eventually fail.

> >For those to whom that paragraph doesn't make sense:  the SMT standard,
> >including PCM, made FDDI vastly more complicated.
> 
> Why do I get the impression that you hate the SMT document????

I don't hate the document.  The recent SMT document is not bad.

What I hate is the so called Standards Process, where things are drug out
for eons, where things are done in a caricature worse than any good hearted
person would imagine from the phrase "design by committtee", where talented
consultents literally spend 3 to 5 days every other month talking down to
and otherwise publically belittling the intelligence and ability of 250
other people to read and remember previous tirades, where the hopes of new
companies to sell network management products and the timidity of delagates
representing "users" delayed things for 2 years, where almost everyone
(including !!me!!) behaves publically in ways that cannot be described with
printable language, and all in something close to Robert's Rules of Order.

I'm not too hot on silly extra features like duplicate address detection
for a network using the same address asignment rules as ethernet.  (There
are many other, more likely, undetectable-by-SMT ways your hardware can
trash the ring than by deciding to use the wrong MAC address.)

> >The justification for PCM and SMT in general is "fault domain isolation",
> >locating whatever is broken when things don't work right.
> 
>If implemented ***correctly*** then the justification is not just "fault domain
> isolation"; it is also fault detection and (possibly) correction.

SMT is supposed to do fault domain isolation, correcty implemented or not.
There is very little fault detection, unless you consider generating alarms
based on bit and frame error counts; maybe that counts as "fault
diagnosis."  SMT has little to do with fault correction.  I don't consider
handling duplicate station addresses by letting the bad stations shut down
"fault correction".  It's at most "fault compensation."  Using the Link
Error Monitor to turn off a noisey link doesn't correct the faulty
hardware, although it may keep the ring up (if it doesn't sausage the
ring.)


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