Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!sgi!vjs@rhyolite.SGI.COM
From: vjs@rhyolite.SGI.COM (Vernon Schryver)
Newsgroups: comp.dcom.lans
Subject: Re: Request For Opinions: FDDI follow-up
Summary: optical bypasses needed
Message-ID: <30661@sgi.SGI.COM>
Date: 13 Apr 89 22:45:10 GMT
References: <4824@charon.unm.edu> <29505@bu-cs.BU.EDU> <1507@Portia.Stanford.EDU> <29548@bu-cs.BU.EDU>
Sender: daemon@sgi.SGI.COM
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 21

In article <29548@bu-cs.BU.EDU>, kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England)) writes:
> 	With the dual counter-rotating ring you have the secondary
> ring in standby in case of node failure.  If a node on the ring fails,
> then the adjacent nodes "wrap" and use the primary and secondary paths
> to create a new ring that bypasses the failed node.

Without an optical bypass, how do you survive > 1 fault?  If you have a
nontrivial number of stations on the ring (i.e. hundreds), you are certain
to have > 1 broken computer.  (Take a reasonable MTBF, and remember the
birthday paradox.)

Yes, an optical bypass is expensive.  So are FDDI transcievers, chipsets,
and so on.  If you are playing with FDDI, you don't care about $, at least
not for a while.  Since the vast majority of individual pairs or "work
groups" of computers running operating systems released in Q1 1989 are
unable to eat an entire ethernet, you are using FDDI for reasons other than
simple, end-to-end performance, but that's another paradox.

Vernon Schryver
Silicon Graphics
vjs@sgi.com