Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!uunet!fibercom!gwg
From: gwg@fibercom.COM (Greg Goodchild)
Newsgroups: comp.dcom.lans
Subject: Re: how far can you run ethernet over fiber
Message-ID: <11338@fibercom.COM>
Date: 31 Oct 90 14:05:54 GMT
References: <7178@cica.cica.indiana.edu> <15866@netcom.UUCP>
Reply-To: gwg@fibercom.COM (Greg Goodchild)
Organization: FiberCom Inc., Roanoke, Virginia
Lines: 41

ssw@cica.cica.indiana.edu (Steve Wallace) asks:

>How far can you run Ethernet over fiber before the collision
>detecting mechanism breaks down?  I came up the following which
>seems too far:

.... math deleted.....

>     11,520 meters is the round trip time.  This divided by 2
>(the sender A must hear sender B transmitting) gives a ball park
>figure (5,760 meters).  I'd give myself a fudge factor of 30
>percent and end up with about 4,000 meters.  Is this close?

      Closer than I thought you would get.  You were on the right track in your
approach but there is a little more to it which is implementation dependent. 
In order to avoid what would sound like an advertisement, let me say that there
exists a nearly five year old product that implements Ethernet over an active
ring.  The maximum allowable ring circumference is 8km and therefore, if pulled
into a straight line, 4km maximum station to station distance.

      While not quite answering your question, Carlo Milono brings up a good
point when:

cmilono@netcom.UUCP (Carlo Milono) writes:

>The practical limits (today) appear to be about half of what your fudged
>number is: 2km.

      Knowing that in this sense "practical" means "inexpensive", yes the usual
limit between active nodes is 2km.  In the above mentioned implementation,
neighboring stations on the ring must be within 2km, but because it is a ring
architecture (and not a star) you can implement a physically more encompassing
network.  So, as long as you have at least four stations on your net, two of 
them can be 4km apart.




-- 
Greg Goodchild                 INTERNET: gwg@fibercom.com
FiberCom, Inc.                 UUCP: ...!uunet!fibercom!gwg