Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!blake!oregon!dsmith
From: dsmith@oregon.uoregon.edu (Dale Smith)
Newsgroups: comp.dcom.lans
Subject: Re: Request For Opinions: FDDI follow-up
Message-ID: <867@oregon.uoregon.edu>
Date: 14 Apr 89 21:36:42 GMT
References: <4824@charon.unm.edu> <29505@bu-cs.BU.EDU> <1507@Portia.Stanford.EDU> <825@oregon.uoregon.edu> <29548@bu-cs.BU.EDU>
Followup-To: comp.dcom.lans
Organization: University of Oregon
Lines: 43

In article <29548@bu-cs.BU.EDU>, kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England)) writes:
> In article <825@oregon.uoregon.edu> dsmith@oregon.uoregon.edu  (Dale Smith) writes:
>>FDDI has an 11dB loss budget.  If you want to survive a failed
>>station that has gone into bypass, then you need to figure you'll lose
>>4dB through the bypassed station, giving you 7dB.  But, you have to spit
>>the 7dB between runs on each side of the failed station, giving you a
>>3.5dB budget between any two stations.  Figuring 1dB per inactive
> 
> 	Now this is interesting.  This is the second person I've heard
> talk about optical bypass and FDDI.  I don't understand why you want
> or need optical bypass for FDDI.
> 
> 	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.
> 
> 	Why do you want optical bypass fault tolerance as well?

A basic issue in ring design is that of fault tolerance and reliability. 
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.  This means that the
ring does not reconfigure, but rather that the node is no longer
receiving and retransmitting the optical signals.  If the station fails
and this failure is not detected internally, then the ring must be
reconfigured for it to survive.  In this case, the adjancent nodes
"wrap" to eliminate the failed station.  This "wrapping" is also the
technique used to survive a failed link.

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.  (How
would you like it if your ethernet died when two workstations were
powered off :-).)

Dale Smith			Internet: dsmith@oregon.uoregon.edu
University of Oregon		BITNET: dsmith@oregon.bitnet
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