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The Telecom Digest for September 18, 2011
Volume 30 : Issue 234 : "text" Format
====== 30 years of TELECOM Digest -- Founded August 21, 1981 ======
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Date: Sat, 17 Sep 2011 07:35:45 -0700 (PDT)
From: Wes Leatherock <wleathus@yahoo.com>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: EXchanges (was: area code named beer)
Message-ID: <1316270145.13927.YahooMailClassic@web111726.mail.gq1.yahoo.com>
--- On Fri, 9/16/11, Thor Lancelot Simon <tls@panix.com> wrote:
> In article <8fe68be8-b2cf-4d9a-9528-4089c8be51fd@l4g2000vbv.googlegroups.com>,
> HAncock4 <withheld@invalid.telecom-digest.org> wrote:
[snip]
> >DDD required that all subscribers have a uniform ten digit number--
> >three digit area code and seven digit local number. In 1950 a great
> >many subscribers in smaller towns and cities had phone numers with
> >three, four, five, or six digits, and all had to be converted to seven
> >digits. This was a major undertaking that took years to complete.
Seven-digit numbers were required for operator toll dialing, too. Operator
toll dialing was already widespread before the first DDD was offered to
subscribers.
> Not such a major undertaking. In roughly 1980 I lived in a rural area
> of New York State that still had 4 digit dialing. Extension to a
> longer number can be done automatically -- it is fully deterministic,
> you are just adding predetermined digits.
>
> My number was 687-XXXX for external purposes, but for in-town callers,
> simply XXXX would do. The "687", obviously, was just added onto the
> front when numbers went to 7 digits -- and there were towns near us
> with switches still configured to allow 3-digit dialing, which had had
> four digits prefixed to the front, too.
Generally the switch was configuired to accept the full seven-digit
number on local calls, too, but was not required on local calls.
Wes Leatherock
wleathus@yahoo.com
wesrock@aol.com
Date: Sat, 17 Sep 2011 10:14:40 -0500
From: Neal McLain <nmclain@annsgarden.com>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: EXchanges (was: area code named beer)
Message-ID: <4E74B960.3070001@annsgarden.com>
Thor Lancelot Simon in Message-ID: <j50lap$38p$1@reader1.panix.com>,
quoting HAncock4 <withheld@invalid.telecom-digest.org>, wrote:
>> DDD required that all subscribers have a uniform ten digit
>> number--three digit area code and seven digit local number.
>> In 1950 a great many subscribers in smaller towns and cities
>> had phone numbers with three, four, five, or six digits, and
>> all had to be converted to seven digits. This was a major
>> undertaking that took years to complete.
> Not such a major undertaking. In roughly 1980 I lived in a
> rural area of New York State that still had 4 digit dialing.
> Extension to a longer number can be done automatically -- it
> is fully deterministic, you are just adding predetermined
> digits.
>
> My number was 687-XXXX for external purposes, but for in-town
> callers, simply XXXX would do. The "687", obviously, was just
> added onto the front when numbers went to 7 digits -- and
> there were towns near us with switches still configured to
> allow 3-digit dialing, which had had four digits prefixed to
> the front, too.
I agree with Hancock4 -- it was indeed a major undertaking.
In the example you cite, the situation was straightforward: simply
adding 687 worked because there were no local numbers beginning with 6,
7, or 8, and because 687 was available within the area code. The local
switch was configured to distinguish the absorbed ("predetermined")
digits from actual numbers by segregating them on separate levels.
Without knowing the specifics of that particular switch, I can't specify
the dialing plan, but it must have looked something like this:
1 = access for non-local numbers + vertical service codes
2XXX = local number, blank level, or nearby community
3XXX = local number, blank level, or nearby community
4XXX = local number, blank level, or nearby community
5XXX = local number, blank level, or nearby community
6 = absorbed by digit absorber
7 = absorbed by digit absorber
8 = absorbed by digit absorber
9XXX = local number, blank level, or nearby community
0 = operator (or access code)
The digit absorber in this case would have been type "AR" meaning
"absorb repeatedly." You could have dialed 6, 7, or 8 repeatedly, in
any order, with no affect. You could have dialed 687-0 and reached the
operator. Or 8888877776666786886786780. Did you ever try it?
As I've noted in previous posts here in T-D, similar situations existed
elsewhere. Example:
CARBONDALE, ILLINOIS, 1971 -- http://tinyurl.com/nd4m4
Like your hometown in New York, Carbondale 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.
But the situation was more complicated in larger communities where it
was not possible to avoid conflicts by segregating functions on separate
levels. In such cases, a different type of digit absorber, type "A",
was used. The distinction:
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".
Two examples:
ANN ARBOR, MICHIGAN -- http://tinyurl.com/jtg3f
- 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:
- 8 followed by 6, 7, 8, or 9 was a local 5-digit number.
- 8 followed by 1, 2, 3, 4, 5, or 0 was absorbed and ignored.
CENTERVILLE, IOWA, 1975 -- http://tinyurl.com/8axyn
- 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:
- 6 followed by 2,3,6,8, or 9 was a local 5-digit number.
- 6 followed by 5 was 658-XXXX in Cincinnati.
- 6 followed by 1,4,7, or 0 was absorbed and ignored.
So, you might ask, why didn't the telcos just segregate all Ann Arbor
and Centerville numbers on separate levels, like GTE did in Carbondale?
- Because every dialing plan has to avoid conflicts
between local 3-, 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 (3-, 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, relays, and time-delay relays.
This technique 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
http://tinyurl.com/29bgqm6
All this reminds me of a story. Back in the '50s when I was an
undergrad at U of M in Ann Arbor, I was living in a residence hall known
as East Quad. In those days, each residence hall had its own manual
PBX. All calls were dialed by the PBX operator to prevent any
unauthorized calls. Residents were not permitted to make inside calls.
I had a friend named John who lived in West Quad, which had incoming
number 2-4401. West Quad residents were not permitted to make inside
calls either.
John figured out that if he asked the operator for 8-2440, the operator
would dial it like any other 8-XXXX number. He'd then flash of the
switchhook on his room phone, effectively dialing 1. Thus, the central
office saw 82-4401, ignored the "A"-digit 8, and sent the call right
back to the West Quad PBX. John could make an inside call.
This technique worked well on busy evenings when student operators were
on duty. But John screwed up: he tried it on a weekday afternoon when
the Chief Operator was on duty. She recognized his voice, and politely
informed him the inside calls were not permitted. I've always wondered
if she figured out how he did it.
Neal McLain
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