Ethernet ring

Alex McKenzie (mckenzie@LABS-N.BBN.COM)
Thu, 31 Mar 88 9:16:00 EST

I believe the product in question was designed with the following goals:

1) Use Ethernet interface, so customer could use any standard Ethernet card in
    the attached equipment.

2) Use FO transmission medium for less bulk and greater resistance to
   environmental electrical issues (noise, grounding, wiretapping)

The ring topology was chosen as the easiest way to get the signal around to all
the stations with FO. The above is based on my fuzzy recollection of a
conversation with the vendor a few years ago.

As I recall, the taps are active, which makes them cost more. Collisions
happen within the taps, where the signals are regenerated, rather than on the
"wire". This means that the Ethernet cable length restrictions don't apply,
since those restrictions are required only to insure that if one station sees a
collision, all stations will. I think it also means Ethernet packet size
restrictions are not required, but since the intent was to use standard
Ethernet boards, this restriction is retained.

If you want to use Ethernet and FO, another alternative is the FO "star
couplers" sold by Siecor and Codenoll, among others. In this approach the
light signals from a number of fiber runs are combined optically in a fused
glass central location. We are using one of these at BBN as the "backbone" to
which each of our many coax Ethernets is connected (via a bridge). The biggest
disadvantage to this approach is that any changes are harder than one would
like, due to the careful consideration that has to be given to the light level
on each arm of the star (the signals have to be well enough balanced so that a
collision between the strongest and the weakest is noticed, and the signal
input on each arm has to be strong enough so that 1/n-th of it is strong enough
to be detected by the receiver which is fartherst away).

A third approach is to install coax Ethernet in every machine cluster and
bridge each one to one of the "Ethernet in a box" devices; the bridges can be
connected via FO cable. This uses FO for the long runs, while keeping the cost
of attachment of individual devices low. This approach is currently my
personal favorite.

Hope this helps,
Alex McKenzie

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