TCP/IP Fiber Optic Ring Backbone

Stephen M. Carr (
Tue, 29 Mar 88 14:24:22 PST


1. We are in the process of admiring the pros and cons of
implementing a fiber optic ring backbone versus a straight
vanilla 50 Ohm baseband coaxial cable backbone to support IEEE
802.3 CSMA/CD Ethernet and TCP/IP. This backbone would support
other subnets, as well as support a gateway for all subnets into

2. We had somebody propose installation of a fiber optic ring as
the backbone. They were adamant that this is an IEEE 802.3
CSMA/CD Ethernet solution that supports TCP/IP on top of it. I
was taken aback, for I never heard of IEEE 802.3 implemented in a
"ring" topology. Seems senseless to implement IEEE 802.3 in a
"ring" topology to begin with, since IEEE 802.5 Token Ring is
specifically matched to a ring topology.

3. Nevertheless, I am assured that just such a TCP/IP IEEE 802.3
CSMA/CD Ethernet solution in a fiber optic ring environment is
available from Fibercom Corporation.

4. Aside from the fact that at first blush it appears silly to
me to even try to implement IEEE 802.3 in a ring topology, I have
other misgivings.
    a. The cost per device connection would be significantly
    b. A fiber optic ring backbone, even if feasible in an IEEE
802.3 environment, is not as generic as a 50 Ohm baseband coaxial
environment. In a 50 Ohm baseband coaxial environment, I can
procure interface devices from several vendors at least. I fear
that a Fibercom Corporation implementation of IEEE 802.3 in a
fiber ring environment would lock us into procurement of
gateway/router and interface devices from only one firm,
"Fibercom". I wouldn't like to see us being maneuvered into
procurement of a "vendor proprietary" solution with little
flexibility regarding choice of gateway/routers and vendor
interface devices.
    c. Implementation of IEEE 802.3 in a ring topology seems to
me would require something akin to the opposite of IEEE 802.4
Token Bus. In other words, implement me a bus protocol in a ring
topology. Not that IEEE 802.4 doesn't make sense, but it appears
that essentially the MAP folks have implemented a ring protocol
in a bus environment. I am sure they have their reasons, the MAP
community isn't stupid. But what about implementing IEEE 802.3
in a ring topology? Is this for real? I confess, I am ignorant.
    d. I would think that in a ring topology, be it fiber or
coaxial cable, by definition requires an order of magnitude
greater configuration management overhead, in the sense that you
can't just pick a 2 meter mark on an Ethernet cable and screw in
a tap. Adding or deleting nodes in a ring topology seems to me
requires very involved physical and software configuration
management processes. Whereas management of a baseband Ethernet
appears relatively simple, requiring no extraordinary services or
talent. Hmmm, maybe somebody is setting themselves up for a LAN
configuration management follow on contract.
    e. Fault isolation. I had been warned a long time ago about
the problems encountered in fault isolating and repairing a ring
topology. A baseband coaxial cable plant seems so much more
straightforward and maintainable.
    f. We are talking about building a fiber optic backbone for
one large building approximately 2 football fields long, and one
football field wide. It is a one story structure. Since we do
not exceed the limits of 500 meter segment and 2.5 kilometer span
restrictions of 50 Ohm baseband coaxial IEEE 802.3, I don't
understand what advantage a fiber optic ring is going to buy us
in terms of distance. Besides, most if not all of the subnets
within this building are going to be other 50 Ohm baseband IEEE
802.3 gatewayed/routed/bridged into this backbone.
    g. To the best of my knowledge, the FDDI (Fiber Optic
Distributed Device Interface) standard has yet to be promulgated,
and is still a draft standard. If we wanted to maintain
interoperbility and conform to standards, I would think we should
be picking FDDI as a target for a fiber optic backbone. Right
now, I am not even sure of the correct physical specifications
for an FDDI cable. Is this cast in concrete? Personally, I
would avoid entertaining fiber optic backbones until such time
that the FDDI standard is promulgated.

5. We have precious little networking experience, but this fiber
optic ring solution seems to have gained a lot of popularity. I
fear that people, in their zeal to become state of the art, are
throwing caution, reliability, interoperability, and the KISS
principle to the wind.

6. Can anybody comment and shed light on this situation? Does
anybody have first hand experience with such a TCP/IP Fibercom
Corporation IEEE 802.3 Ethernet fiber optic ring? My apologies
for any statements above which may appear patently stupid from a
technical perspective. Your candid response citing pros/cons and
errors in my logic would be greatly appreciated.

                       Very Respectfully,
                           Steve Carr
                          LCDR, SC, USN

Navy Management Systems Support Office (Code 42A)
Naval Air Station
Norfolk, Virginia 23511-6694

Commercial: (804) 445-2171, 445-1595
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