Re: Arpanet outage


haverty@ccv.bbn.com
16 Dec 86 08:09:50 EST (Tue)


Dan,

It's misleading to think that you are ordering a "trunk" from a
communications supplier. What you are buying is a plug at one
site through which you can pass bits, which appear by some magic
at the plug you have bought at the other site. Assuming that
there is a physical wire between the two with any particular
characteristics other than what is specified in the service
offering (e.g., BER, speed, conditioning) is a dangerous
practice.

The nice network maps we all draw are topological, not physical.
We've often deduced physical characteristics from observed
behavior, and seen this kind of thing in many networks. I
remember one in particular that had a microwave "sweeper" on a
tower, which swept a beam in a circle to hit N other microwave
stations around the horizon; the observed effect of this was a
propagation delay of about 100 msec., which is far too short for
any normal satellite trunk, and far too long for any normal
terrestrial circuit. I also remember a backhoe in a farmer's
field in Illinois which dug up N of our carefully redundantized
trunks with a single flip of the scoop.

I think in most cases even if you figure out something about the
physical implementation, there is no guarantee that it will be
the same next week. Vendors do offer some options that you can
specify, usually at extra cost, like a guaranteed terrestrial
routing to control delay; I think you can also specify separate
physical routes for different circuits in some cases.

Jack



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