snipped-for-privacy@bbs.cpcn.com responded:
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 --
- 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 --
- 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 --
- 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 itself).
- 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 first selectors:
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 designated "A".
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 them?
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
856 numbers.I trust this explains why 7-digit (if not 10- or 11-) dialing is now mandatory everywhere.
Neal McLain