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:  Optical Fiber Physical Topologies
Message-ID: <825@oregon.uoregon.edu>
Date: 13 Apr 89 14:59:58 GMT
References: <4824@charon.unm.edu> <29505@bu-cs.BU.EDU> <1507@Portia.Stanford.EDU>
Organization: University of Oregon
Lines: 46

In article <1507@Portia.Stanford.EDU>, morgan@Jessica.stanford.edu (RL "Bob" Morgan) writes:
> As my last act at San Francisco State University in 1987, I designed a
> fiber plant to support FO Ethernet (using Codenoll xcvrs) now and FDDI
> very eventually.  I ended up with something that is a lot like what
> Kent just described for his new net at BU:  a set of interconnected
> stars.  In this case it was three stars, each with a "passive" center,
> each serving four to six buildings with 12 multi-mode fibers per
> building.  The stars were connected together with 12 more fibers
> (probably should have been more).  This allowed a logical design that
> is currently very Computer-Center-centric (it's at one of the stars),
> using Ethernet bridges to link the stars, while allowing ring or
> whatever eventually.

At the University of Oregon, we have taken a slightly different approach
and installed a "star of rings" rather than the "ring of stars"
described by Kent of BU and Bob of Stanford (network of SFSU).  We have
three interconnected rings that cover most of our 1 square mile campus. 
From our Computing Center, which is a node in one of the rings, we run
point-to-point to the other rings (thus the star of rings).  As Kent
points out, you must be careful about loss in buildings with no active
devices.  You should engineer for losing 1dB per building that you
patch through.  Actual practice should be somewhere closer to .5dB, but
you should engineer for 1dB.  With FDDI, this can become a big problem
fast.  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
building plus a little loss in the cable, you can see you don't want to
build huge rings.  Note that I have been very conservative in the
figures above and you can get by with a lot more.

Special point-to-point applications also pose problems in a ring
topology.  You can potentially end up using lots of fiber for simple
links.  We have carefully planned where our star presences are
so that we can minimize the amount of fiber required for special
applications (we hope). 

A drawback of running FDDI over a physical star is that a single media
failure can partition your network, whereas no single failure can
partition your FDDI network if you have a true ring.

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