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From: floyd@ims.alaska.edu (Floyd Davidson)
Newsgroups: comp.dcom.telecom
Subject: Re: Interoffice Signalling
Message-ID: <15850@accuvax.nwu.edu>
Date: 7 Jan 91 10:12:36 GMT
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X-Telecom-Digest: Volume 11, Issue 13, Message 5 of 12


In article <15840@accuvax.nwu.edu> bill@toto.info.com (Bill Cerny)
writes:

>In the local switching environment, if two end offices are connected
>by interoffice trunks, how many digits are passed between offices when
>completing an interoffice call?  (assumptions: electronic offices,
>inband signalling, same NPA, same telco)

>Side trivia: were #5 crossbar offices capable of supporting DID?

I can't answer the xbar question.

Because you specified the same NPA they would most likely pass only
the last four digits.  But also it is unlikely that there would be two
switches and only one NPA.

With digital switching it can be arranged in almost any manner that
will uniquely identify the correct routing.  Usually the minimum
number of digits are sent (but not less than four).  There are times
when for some oddball reason more than the minimum required are sent.
Usually that is either future planning, or bad planning.

The same is true of toll trunks to end offices.  Inter-toll trunking
almost always requires all digits to be passed.

With mechanical switches there were other considerations because the
switch may not have been configured to handle some given set of
numbers.  Strange things could happen.  Before digital switches made
it impossible there was such a quirk here in the Fairbanks area.  The
Fairbanks telco (45x), the North Pole (488), and the Eielson AFB telco
all had EAS (Extended Area Service) trunks between each other.  But
there was a grand total of only six from Eielson to North Pole (one
way trunks, six went the other direction too).  From Eielson AFB if
you dialed 488-nnnn you most likely got an all trunks are busy signal.
But for those who knew about it (almost everyone), you dialed 458-nnnn
instead.  It went 26 miles up the road to Fairbanks, grabbed a trunk
to 488 land, and went ten miles back down the same road to North Pole.

It worked because the Eielson switch stripped the first digit,
selected N.P.  or FBK for a second digit of 8 or 5, and sent the last
five digits down the line.  The Fairbanks switch looked at the first
of those five digits and selected either itself (a 6), the other
Fairbanks switch (a 2), or an 8 would send it to North Pole.  For some
reason they did absorb a 7, so it could never be routed back to
Eielson (372 and 377).

Digital switching came in '82 and it became almost impossible to call
between North Pole and Eielson until more trunks were in place.  And
now the Fairbanks telco is using 458 for one of their remotes.


Floyd L. Davidson        floyd@ims.alaska.edu
Salcha, AK 99714         paycheck connection to Alascom, Inc.
When I speak for them, one of us will be *out* of business in a hurry.