Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!bu-cs!kwe
From: kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England))
Newsgroups: comp.dcom.lans
Subject: Re: Request For Opinions: FDDI follow-up
Message-ID: <29650@bu-cs.BU.EDU>
Date: 15 Apr 89 17:27:40 GMT
References: <4824@charon.unm.edu> <29505@bu-cs.BU.EDU> <1507@Portia.Stanford.EDU> <825@oregon.uoregon.edu> <29548@bu-cs.BU.EDU> <867@oregon.uoregon.edu>
Reply-To: kwe@buit13.bu.edu (Kent England)
Followup-To: comp.dcom.lans
Organization: Boston U. Information Technology
Lines: 34

In article <867@oregon.uoregon.edu> 
dsmith@oregon.uoregon.edu (Dale Smith) writes:
>
>There are two basic techniques used in FDDI for a ring to survive a
>station failure.  The first, and most desirable, technique is that of
>bypass.  If a station detects that it is no longer functioning properly
>or if it loses power, it should go into bypass.  
>
>As vjs@rhyolite.SGI.COM (Vernon Schryver) of Silicon Graphics points out
>in article <30661@sgi.SGI.COM>, you must have bypass or you will never
>be able to survive the loss of two or more stations.  This will become
>extremely important when you start buying your SGI (or SUN or DEC)
>workstations with an FDDI board rather than an ethernet board.  

	FDDI contains specs for ring concentrators, dual and single
ring modes, and the SMT (soon) to make all these options and modes work
together.  Sorry, I still don't think optical bypass is useful.
	I think the typical backbone will be dual ring with wrap mode
as the dominant fault tolerance approach.  You might convince me that
optical bypass would be useful here, but in my two years' experience
with Pronet-80 I don't miss optical bypass.  I haven't heard anyone
else with Pronet-80 asking for bypass.
	I think the typical workstation ring will be single ring,
because people can't afford and don't need dual ring on the
workstation nets.  Just like the IBM-PC token rings, the workstation
nets will be star wired and the concentrator will take stations in and
out of the ring, electrically I imagine.
	If the workstations have to have optical bypass to handle
faults that the concentrators don't handle (or if you don't use
concentrators), then how does FDDI deal with an unplugged optical
connector?  I don't think FDDI deals with optical bypass in the FDDI
connector.  You know, the IBM token ring shorts out when the station is
unplugged and a lot of people have found that to be troublesome.  But
that's simple compared to optical bypass in the FDDI connector.