Re: Arpanet outage

Mike Muuss (mike@BRL.ARPA)
Fri, 19 Dec 86 0:12:38 EST

Since nobody from DCA has spoken up yet, I'll add a few comments.
As the MILNET is being rebuilt using the "new" IMP packaging
(with link encryption capability), some (most?) of the data
circuits are being moved to DCTN (?Defense Computer Telecomunications
Network?), which is an ISDN-oriented base of somewhat switchable
circuit capabilities. I believe DCTN has a phased implementation
plan, probably with automatic switching happening much later.

My general impression is that DCA and Army Signal Corps (now the
"Information Systems Command") both tend to do a good to excellent
job implementing systems designed around traditional concepts
such as point-to-point circuits, so DCTN is likely to be
a big win. In addition, I suspect that routing of DCTN circuits
is likely to be carefully controlled to prevent excessive
bundling onto single transmission links, precisely for survivability.
(Blind faith here).

What we have seen of DCTN so far is a T1 line terminating in our
Post's Central Office at a D4 channel-bank, with a bunch (7?)
of 56k DDS links from there to the location of the MILNET IMP.
This gives much better signal quality than previous arrangements
where the DDS lines traveled over 5 miles of wire to the town CO.
It does not provide any additional reliability, as everything still
travels over the big black cable from our CO to the town CO.
This cable is especially attractive to heavy earthmoving equipment,
and is neutralized several times each year. Presumably when the
T1 gets to the town CO, it terminates in something resembling
a circuit switch or patch pannel or something (behind another D4
channel bank, of course), so that some alternate routing capability
exists at that point. Of course, it might be that the T1 gets
zipped through a bunch of repeaters to some regional circuit
switch, extending our line of vulnerability a good long way.

Personally, I find the concept of layering a packet switching network
on top of a switchable circuit network rather amusing, but
quite realistic and practical.

More grist for the Rumor Mill, may it grind long and fine...

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