Ken Pogran (email@example.com)
Tue, 16 Dec 86 9:13:04 EST
Your question to Andy, "How on earth does it come to happen that
7 'trunks' are 'routed' through one fiber optics cable" is more
properly addressed to the common carriers whose circuits the
ARPANET uses, rather than to the packet switching folks.
Here we are in the world of circuits leased from common carriers,
where economies of scale (for the carriers!) imply very high
degrees of multiplexing. As the customer of a common carrier,
you specify the end points that you'd like for the circuit, and
the carrier routes it as he sees fit. This is a personal
opinion, and not a BBNCC official position, but I think it's safe
to say that without spending a lot of extra money, and citing
critical national defense needs, it's going to be hard to get a
carrier to promise -- and achieve! -- diverse physical routings
for a given set of leased circuits. I would also venture the
opinion that there are lots of places in the U. S. where there's
only one physical transmission system coming into the area that
can provide the 56 Kb/s Digital Data Service that the ARPANET (and
MILNET, and ...) uses.
An implication of this is that almost any wide-area network
(doesn't matter whose, or what the technology is) is going to be
somewhat more vulnerable to having nodes isolated than its
logical map would suggest.
In fairness to the common carriers (are there any in the
audience?), the higher the degree of multiplexing, the more
well-protected the carrier's facilities are, and the more
attention is paid to issues of automatic backup (carriers call
this "protection") and longer-term rerouting of circuits when
there's an outage (carriers call this "restoration"). So an
outage of the type that's been discussed ought to be a very
low-prob event. Kind of like wide-spread power failures ...
Hope this discussion helps.
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