> Many small towns required only four- or five-digit dialing locally
> long after the "conversion" took place. By "long," I mean 10 or 15
> years or more.
> A friend of mine reported is update NY state town worked like that.
> But once they went to ESS it no longer worked.
> I wonder if this still applies to sparsely populated places in say
> Wyoming and the Dakotas where there is little population growth.
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.)
You've stated that an ESS switch can accommodate 5-digit dialing, and I
agree. However, as I've noted previously on this list, doing so makes
it difficult to devise dialing plans to interconnect two or more
switches without running into conflicts between local 5-digit numbers
and nearby 7-digit numbers. Furthermore, it makes it extremely
difficult, if not impossible, to add more NNXs as the community grows.
Here are links to my previous posts on this subject:
CARBONDALE, ILLINOIS, 1971 -- http://tinyurl.com/nd4m4
Carbondale is (or was) a simple situation -- it avoided conflicts by
segregating functions on separate levels:
- Levels 3, 7, and 9: local 5-digit numbers
- Levels 4 and 5: repeatedly-absorbed ("AR") digits.
- Levels 6 and 8: NNXs in nearby communities.
- Level 2: unused.
There were no absorbed-once ("A") digits.
ANN ARBOR, MICHIGAN -- http://tinyurl.com/jtg3f
Prior to about 1953, Ann Arbor presented a similar situation -- it
avoided conflicts by segregating functions on separate levels:
- Levels 2, 3, and 5: local 5-digit numbers
- Levels 6, 7, 8, and 9: local 4-digit numbers
- Level 4: NNXs in nearby communities.
There were no absorbed ("A" or "AR") digits.
Around 1953, "AR" and "A" digits were added, and the situation became:
- Levels 2, 3, and 5: local 5-digit numbers.
- Level 6: repeatedly-absorbed ("AR") digit.
- Level 8: absorbed-once ("A") digit -- see note below.
- Level 4: NNXs to nearby communities.
- Levels 7 and 9: unused.
Note how the "A" digit 8 was used to resolve conflicts:
- 668 (or just 8) followed by 6, 7, 8, or 9 was a local 5-digit number.
- 668 (or just 8) followed by 1, 2, 3, 4, 5, or 0 was absorbed and ignored.
CENTERVILLE, IOWA, 1975 -- http://tinyurl.com/8axyn
Centerville was an even more complicated situation:
- Level 6: absorbed-once ("A") digit -- see note below.
- Levels 5 and 8: repeatedly-absorbed ("AR") initial digits.
- Level 4: NNXs to nearby communities (plus one located in Centerville
- Levels 2, 3, 7 and 9: unused.
Note how the "A" digit 6 was used to resolve conflicts:
- 856 (or just 6) followed by 2,3,6,8, or 9 was a local 5-digit number.
- 856 (or just 6) followed by 5 was 658-XXXX in Cincinnati.
- 856 (or just 6) followed by 1,4,7, or 0 was absorbed and ignored.
So, you might ask, why didn't the telco just segregate all Centerville
numbers on separate levels, like GTE did in Carbondale?
- 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.
- Because every dialing plan has to consider how the
local dialing plans in nearby communities avoid
conflicts between *their* local (4- or 5-digit) numbers
and the NNX codes used by *their* nearby communities.
- Because every NNX in an area code has to be unique.
A telco can't pick an NNX just because it's convenient
for the local dialing plan if it's already in use
somewhere else in the area code.
And ultimately, because all dialing plans within an area code form a
continuous web of inter-community 7-digit dialing, each one of which has
to avoid local conflicts.
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.
The key to it all was the development of "A" and "AR" levels on the
A = The selector absorbs the specified digit once only; on
the next digit, it "trunks on all levels." This digit
must be dialed once (and only once) in order to reach
certain specified second digits. However, it is absorbed
(ignored) for any other second digit.
AR = The selector absorbs the specified digit repeatedly
unless a digit has been absorbed previously on a level
All this is discussed in detail in "Notes on Distance Dialing,"
Section 4, "Typical Trunking Diagrams for Step-by-Step Offices,"
published by AT&T Engineering and Network Services Department, Systems
Planning Section, 1975. A PDF of Appendix A1 (the trunking diagram of
a hypothetical SxS switch) is posted at
Now fast forward a couple of decades, and replace all those old
electromechanical switches with ESS switches. Can you program an ESS
to emulate "A" and "AR" first selector levels in order to resolve the
old conflicts? Sure.
But what about new conflicts created by new NXXs (no longer called
NNXs) that have appeared during those two decades? How do you resolve
How, for example, would you add 895 to Centerville? If you follow the
existing numbering plan, you would have to add it *after* the "A" and
"AR" digits (similar to the way 437 was added). But doing that would
create a conflict with existing numbers in the form 856-895X. The
only way to avoid this conflict is to implement 7-digit dialing for
I trust this explains why 7-digit (if not 10- or 11-) dialing is now