TELECOM Digest OnLine - Sorted: Re: 5- v. 7-digit Dialing

Re: 5- v. 7-digit Dialing
25 Apr 2006 11:37:22 -0700

Neal McLain wrote:

> You keep asking this same question, and I keep giving you the same
> answer: mandatory 7-, 10-, or 11-digit dialing applies to almost every
> ESS office in every state. (I say "almost" because there's probably an
> exception out there somewhere; I've just never heard of one.)

Thanks for the detailed explanation. The areas I wondered about were
the sparse areas with no population growth, areas where no exchanges
would get added. In other words, plenty of "room" so there was no
conflict. I'm not sure such areas exist anymore, although I
understand some western states are zero or even negative population

> - Because every dialing plan has to avoid conflicts
> between local 4- or 5-digit numbers and NNX codes in
> nearby communities reached by 7-digit dialing.

Back in the 1970s I saw small town phone books. Dialing instructions
for between towns could be rather complex, with a variety of odd
access codes required depending on the town. I don't think this was

Further, in some places the dialable area was quite narrow, anywhere
required the operator. Anyway, all of this would simplify the switch
level design.

In the 1970s, when the Bell System introduced dialed direct toll
savings, they prominently stated that they applied to areas that
didn't have DDD.

> Have you followed all this? Or are your eyes glazed over by now? If
> you haven't followed it because it's too complicated, that's my point:
> it is complicated! It's amazing that traffic engineers back in the 50s
> and 60s were able to figure it all out.
> Even more amazing is the fact that they were able to implement it with
> electromechanical devices: Strowger switches and relays.

All very true. Dialing outward could be controlled by special access
codes, as mentioned or simply not even provided for.

But inward dialing was still needed to these little points and
everyone needed a 7 digit unique number within the area code. Some
towns had all toll service passed through a larger next down, that
probably was an SxS too, but doing double duty as a tandem to relay
calls through.

Many people were converted from 5d to 7d at that time, others got new

The Bell System Labs history Pt II switching goes into some of this,
although they don't get into the selector level detail you provided.

We take DDD for granted but as you showed, it was extremely complex.
The Bell System had a wide variety of individual exchanges and
trunking arrangements out there, and the independents had even more
variety. In addition, the layouts were fluid as many suburban
communities were rapidly growing. Indeed, I'm not sure how much DDD
was implemented before 1955 as the Bell System was until then
struggling to meet basic service demands. Wholly new exchanges,
additions to existing ones, new cable plant, new commercial offices,
etc. They had people working out of trailers due to a shortage of
office space.

> I trust this explains why 7-digit (if not 10- or 11-) dialing is now
> mandatory everywhere.

Thanks again for the information.

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