--------------------------
S.E.R. No. 48
February 1, 1957
SWITCHING ENGINEERING REPORT
COVERING THE BELL SYSTEM TELEPHONE NUMBERING PLAN -
Case 38931
W.O. Turner
Systems Engineering Dept.
Bell Telephone Laboratories, Incorporated
[the first page is a cover letter from Mr. M.L. Almquist, Director of
Systems Engineering, Bell Telephone Laboratories, 463 West Street, New
York 14, N.Y., CHelsea-3-1000, to Mr. C.M. Mapes, Assistant Chief
Engineer, American Telephone & Telegraph Co., 195 Broadway, New York
7, N.Y., dated March 26, 1957.]
Dear Mr. Mapes:
The attached S.E.R. No. 48 covers a study of the Bell System telephone
numbering plan to determine its adequacy and to explore methods of
economically adapting the plan to meet long-range future requirements.
The study indicates that:
1. The existing numbering plan will reach capacity in about 20 years.
2. Introduction of a pushbutton telephone instrument with suitable features
would permit an economical transition to a new numbering plan with
entirely adequate capacity at such time as all users of direct distance
dialing service are equipped with the new telephone set.
3. Additional capacity will be needed before transition to the final plan
can be made economically. To meet this need, the use of additional
numbering plan area codes of the form 1XX is suggested.
4. The additional code capacity which can be gained by adopting an
all-numeral numbering plan is not essential, but would be of advantage,
particularly to the largest cities.
In preparing this report, we have had the benefit of valuable comments and
suggestions of your people and of many other members of the AT&T Co.
general departments.
(signed) M.L. Almquist
[the report itself now follows]
NOTE:
This report has been prepared to record the results of a Systems
Engineering investigation that has been conducted primarily to provide
background information for use by and guidance of Bell Laboratories
technical organizations. It is not intended that the information be
used as a guide for development as such information, when and if
required, will be contained in a Prospectus. Copies of this report are
being distributed within the Laboratories and, on a limited basis, to
the American and Western Companies.
ACKNOWLEDGEMENT:
The writer wishes to express his appreciation of the helpful comments
and suggestions contributed by many members of the Laboratories and
AT&T Co. staffs during the preparation of this report. In particular,
the advice and assistance given by Mr. L.K. Palmer of the AT&T Co.,
O&E Department, and the data on possible growth in population and
telephones furnished by W. Hodgkinson, Jr. of the AT&T Co. Chief
Statistician's Division, have been invaluable.
TABLE OF CONTENTS:
I. Purpose of Study
II. Requirements
III. Capacity of Present Plan
IV. Proposed Action
V. The All-Numeral Numbering Plan
VI. Summary
Attached:
Appendix - Capacity Requirements
Figure 1 - United States - Population and Telephones Projected to Year 2000
Figure 2 - Bell System Numbering Plan - Number of Office Codes Required
Projected to the Year 2000
I. PURPOSE OF STUDY:
The study covered by this report estimates the possible future
requirements of the Bell System telephone numbering plan, reviews the
adequacy of the present plan in the light of these requirements, and
suggests a course of action which will economically adapt the present
plan to care for long-range future requirements.
II. REQUIREMENTS:
The future requirements of the Bell System numbering plan will be
determined by the geographical area covered by the plan, and by the
number of telephones and number of office codes required for all
purposes in that area.
This study assumes that the Bell System plan will include the United
States, Canada, Mexico, Alaska, and the Hawaiian Islands. In addition,
it is assumed to be desirable, if not essential, that capacity be
provided within the plan for at least operator dialing to Great
Britain and perhaps the rest of Western Europe.
The number of office codes required in this area at a given time will
depend upon the total number of telephones, the percentage of these
telephones identified by telephone numbers, and the average number of
telephone numbers per telephone office. In order to plan a course of
action which will economically care for future requirements, an
estimate must be made of the rate of growth in office codes. Then the
probable exhaust date of the present plan may be estimated and used as
the basis for a program which will provide additional capacity when
needed without unreasonable inconvenience to telephone users or heavy
expenditures by the Telephone Companies in modifications of plant.
The rate of growth selected should be a safe upper limit rather than a
minimum, since the penalties to customers and to the telephone
industry are small if the growth is overestimated, but might be great
if requirements increase much faster than planned for. With this in
mind, a compound annual rate of growth in office codes of 4.0% has
been assumed. This is based on an estimated annual rate of growth of
1.8% in population and 5.1% in telephones, and assumes inward dialing
to all PBX extensions except those of hotel PBX's.
At this rate of growth there will be by the year 2000 a total of
130,000 office codes in the United States, Canada, and Mexico, for a
development of 550-million telephones. (See the report, "Population
and Telephones in the Year 2000", memorandum dated July 16, 1956, by
W. Hodgkinson, Jr., of the AT&T Co., Chief Statisticians Department.)
This compares with approximately 22,000 office codes serving
56-million telephones in the United States and Canada at the beginning
of 1955. The rates of growth in population and telephones are shown on
Figure 1, and the corresponding rate of growth in office codes in
Figure 2. The derivation of the estimate of growth in office codes is
given in the Appendix.
III. CAPACITY OF PRESENT PLAN:
The present plan is a 10-digit numbering plan, consisting of a 3-digit
area code, a 3-digit office code and a 4-digit station number.
Area codes are all-numeral. They are distinguished by having either
the numeral "0" or the numeral "1" as the second digit. There are 152
of these codes available after eliminating those starting with the
numerals "0" and "1", and those conflicting with existing service
codes.
Office codes comprise the first two letters of the central office name
followed by a numerical digit. There are 640 possible codes of which
40 do not correspond to suitable office names and are used only for
radiotelephone service. Of the remaining 600, the 60 ending in the
numeral "0" are considered less desirable because of possible
confusion with the letter "O". The maximum number of office codes per
area is thus between 540 and 600.
In practice, the average number of office codes per area is well below
the maximum for several reasons:
1. Numbering plan area boundaries follow state boundaries. The state is the
basic area; less populous states each comprise one area and the larger
states are subdivided into as many areas as necessary to accommodate
their telephone development. This, of course, reduces the average number
of codes per area well below the capacity.
2. Where a telephone community of interest crosses an area code boundary,
the codes of the offices involved may be reserved in both numbering
plan areas.
3. An adequate reserve of unused codes should be available in each area for
future growth.
At present, the average number of offices per area is about 200. In
the Companies' 20-year view, this will grow to about 300, and should
increase further to a level of about 350 in the ultimate.
The ultimate capacity of the present numbering plan is estimated as:
152 x 350 = 53,000 offices. At the assumed rate of growth in
requirements, this capacity will be reached by 1977 if no area codes
are reserved for Great Britain or Western Europe (see Figure 2). If
some 20 codes are so reserved, capacity will be reduced to 46,000
offices in North America, and this capacity will be reached in 1973.
This estimate is in close agreement with the Companies' own view. In a
forecast for the year 1975 approximately, they estimate that 42,000
office codes in 128 numbering plan areas will be required. This does
not include 5000 codes which would be required for in-dialing to all
PBX extensions, except at hotel PBX's, nor any reservation of codes
for Mexico, Alaska, Hawaii or Great Britain. Adding 17 area codes for
in-dialing to PBX's and one each for Alaska and Hawaii to the
Companies' estimate results in a need for 147 area codes exclusive of
Mexico and Great Britain.
To sum up: Assuming as high a rate of growth in code requirements as
can reasonably be expected, and expansion of the area covered to
include Mexico, Alaska and the Hawaiian Islands with some modest
provision for operator dialing of calls to Western Europe, the
capacity of the present numbering plan may be expected to exhaust at
about the year 1975.
IV. PROPOSED ACTION:
If the rate of growth in office code requirements in the year 1975 and
beyond turns out to be the order of magnitude assumed in this study, a
major increase in the number of office codes available will be
necessary to prevent frequent changes in customer dialing procedures
and modifications of telephone plant as more capacity is needed. It is
proposed that this increase be achieved by providing a "Distance"
button on all telephones. Customers will be instructed to operate
this button before dialing 10-digit calls. The local switching
systems, on receiving the signal generated by this button, will
recognize the first three digits following as an area code and the
second group of three as an office code. In this way, identical codes
can be used as area codes and also as office codes.
With this plan, it naturally follows that switchboard operators as
well as customers will have to take some action on each call to permit
the switching system to distinguish between area and office codes. It
is therefore assumed that each switchboard position at which distance
calls are completed will be equipped with two KP keys, one for 7-digit
and the other for 10-digit calls.
The provision of this "Distance" button has been agreed upon as a
requirement for the pushbutton telephone now under study by the
Laboratories and American Company. When pushbutton telephones with
this feature are in use by all customers having direct distance
dialing privileges, the number of area codes available will be
increased from the present 152 to a total of 872.
This total is made up as follows:
Codes starting with the digits 2 to 9, inclusive ............... 800
Codes starting with the digit 1, excluding those
reserved for Plant and Traffic Administrative purposes (*) ..... 72
---
TOTAL ..... 872
(*) The access code "11", and the various local service codes, will
not conflict with area codes, since they will not be dialed following
the operation of the "Distance" button. The availability of these 72
"1XX" codes is also dependent upon the provision in standard
electronic switching systems (using pushbutton telephones) of some
means of indicating repertory calls, sequence calls, and inter-PBX
extension calls other than by the initial digit "1" which is proposed
for the Morris trial.
[The Morris trial was an experimental Electronic Central Office (ECO)
which eventually became ESS. There were experiments and plans being
developed even as early as the 1950's for "Custom Calling" features
and their dialing and numbering codes to be used in the Electronic
office.]
Assuming an average of 350 office codes per area, the plan would have
capacity for about 300,000 offices, which is adequate for some decades
beyond the year 2000.
Introduction of the "Distance" button as a feature of the pushbutton
telephone rather than as an addition to present dial telephones avoids
very heavy modification costs, but it also imposes a delay of some 25
to 35 years before the new telephones are in universal use and the
potential additional code capacity can be realized. Since it has been
estimated that area code relief may be required in about 20 years,
some inexpensive means of obtaining a moderate increase in available
area codes with the present dial telephone must be found. for this
purpose, it is proposed that all local and toll common control systems
be arranged to accept three-digit area codes starting with the numeral
"1".
No modification of non-senderized step-by-step systems is required,
since users of these systems will be required to precede all distance
calls with the code "11" or "112", and the area code digits will be
registered in a toll system instead of the local system.
Of the 100 possible 3-digit codes starting with the numeral "1", there
will be available 72 for use as area codes after eliminating those
conflicting with the "11" access code, with local service codes, and
with codes used by Plant and Traffic in administering toll systems
[i.e., the ten 10X system codes Plant Test, the ten 11X local service
codes as well as the 11XX system codes for "leave word" operators, and
the eight 1N1 system codes for operators (such as 121 Inward, 131
Information, 141 Rate & Route, etc.)] These 72 added to the present
152 will make available a total of 224 area codes, which at an average
of 350 offices per area will give capacity for 70,000 to 80,000
offices, depending upon the number of area codes used for
transatlantic calls. This capacity should last until 1984 or 1987,
affording an interval of almost 30 years for economical transition
from the present to the new telephone.
If service codes and test codes of the type "1NN" [where the second
and third digits are *identical*, such as] (122, 133, 144, etc.) are
adopted for common control systems, the number of "1XX" codes
available as area codes will be reduced to 64. However, 8 codes of the
"N11" type now used as service codes in common control systems would
be released for use as area codes, bringing the total net gain back to
72.
V. THE ALL-NUMERAL NUMBERING PLAN:
It is evident from the foregoing that it is not necessary to take
advantage of the greater code capacity of all-numeral numbering in
order to expand the Bell System numbering plan to meet requirements of
the year 2000 and beyond. However, Laboratories' studies suggest that
the all-numeral plan may have advantages other than increased code
capacity. It therefore seems worthwhile to outline in this report a
possible program based on the adoption of an all-numeral plan. (See
the report, "An Evaluation of All-Numeral Dialing", by A.H. Inglis,
J.E. Karlin, and W.O. Turner, dated March 1, 1956).
The full advantage of the all-numeral plan in speed and accuracy is
realized when the letters can be eliminated from the dial plate of the
dial telephone, or from the buttons of the pushbutton telephone. If a
new pushbutton set is to be introduced, and if an all-numeral plan is
to be adopted, the buttons of the new set should therefore preferably
be engraved with numbers only. Letters cannot be dispensed with until
central office names are eliminated in all communities which can be
dialed from any offices equipped for direct distance dialing. Such a
conversion, involving among other things, resetting of type in
telephone directories serving these communities, extensive publicity,
and customer education, requires times for adequate planning and
careful execution. In order to keep at a minimum the number of
pushbutton telephones which must display both letters and numerals if
all-numeral dialing is adopted, an early decision in this matter is
obviously desirable.
Were all-numeral dialing adopted, the increase in office codes per area
would be as follows:
1. Present dial telephones: the 60 codes ending in "zero", now considered
undesirable, would be fully useful. The 40 codes for which no suitable
office names are available would also become available. The average
number of offices per area might increase from 350 to 400.
2. Pushbutton telephones with "Distance" keys: all 872 codes available as
area codes could also be used as office codes. The average number of
offices per area would of course be well below this maximum because of
the factors discussed in Section 3, but an average of 575 per area
should be realizable.
A comparison of possible programs with and without all-numeral numbering
follows:
PRESENT PLAN ALL-NUMERAL PLAN
Office Exhaust Office Exhaust
Capacity Date Capacity Date
1. Present
152 Area Codes 46,000-53,000 1973-76 53,000-60,000 1976-80
2. Add 72
"1XX" Area Codes 70,000-80,000 1984-87 80,000-90,000 1987-90
3. Adopt Pushbutton
Telephone with 300,000 beyond 2000 500,000 beyond 2000
"Distance" Button
From a numbering plan standpoint, the added capacity gained through
adoption of all-numeral numbering would be of advantage in two ways.
The excess over-all capacity of the system beyond a safe reservation
for North American needs could be assigned to other continents, thus
permitting a universal 10-digit numbering plan covering as much of the
world as wished to participate in the plan.
The expansion of the maximum number of office codes per area from 600
to almost 900 will become important to rapidly expanding metropolitan
areas. At the average rate of increase in office code requirements
assumed in this report, the following metropolitan exchange or zone
areas will each outgrow the capacity of one letter-numeral numbering
plan area before the year 2000:
Boston, Chicago, Cleveland, Detroit, Los Angeles, New York, Philadelphia,
San Francisco-Oakland.
One additional point in favor of an all-numeral numbering plan should
be mentioned. It appears to be practically essential if customer
dialing is ever to be extended to reach non-English speaking
countries, since differences in the arrangement of letters on the
telephone instruments, in pronunciation and spelling, and in the
alphabets themselves, appear otherwise to be insurmountable.
VI. SUMMARY:
Action suggested as a result of this study is as follows:
1. Proceed with the development of a pushbutton telephone equipped with a
"Distance" button to be operated before pulsing 10-digit numbers. When
these telephones are in use by all customers who have the direct
distance dialing privilege, the supply of area codes can be expanded to
a number adequate for a period well beyond the year 2000.
2. As an interim measure to provide adequate area code capacity until the
new telephones are in general use, introduce a new series of area codes
of the form "1XX". Use of these codes in addition to existing area codes
will provide an interval of some 30 years for economical transition to
the new telephone.
3. Consider the advisability of adopting an all-numeral numbering plan, in
order to gain the full capacity inherent in the adoption of the new
telephone with the "Distance" button. Continued use of central office
names would permit realization of only part of the potential capacity of
the plan.
APPENDIX - CAPACITY REQUIREMENTS
1. FACTORS DETERMINING REQUIREMENTS:
The requirements a numbering plan must meet are determined by the
period of years it is designed to last; by the geographical area to be
covered; and by the number of telephones and number of office codes
required for all purposes in the area at the end of the period. The
following sections outline the basis for estimating these factors for
the purposes of this study.
2. YEAR SELECTED FOR CAPACITY ESTIMATE:
The year 2000 was selected for the basis of the capacity estimate.
Forecasting for such a long period ahead is unusual in the Bell
System, but there are good reasons for doing so in this instance:
Number changes are annoying and frequently expensive to customers.
Business stationery and advertising must be changed. Orders for
products or services may be delayed or lost. Lists of frequently
called numbers must be revised. If the number or type of characters in
the telephone number are changed, customers must change their dialing
habits. Until customers become accustomed to the new plan, telephone
service in general deteriorates.
Customers will undergo these inconveniences with good grace if they
are convinced that the change is necessary and in their own best
interests. Frequent changes, however, must appear capricious or due
to poor planning on the part of the telephone company.
If the present system has to be changed, it is only fair to present
customers to adopt a plan which can reasonably be expected to last for
a generation. The objective is not to make a second change until the
inconvenience of the first has been forgotten.
The telephone industry itself needs a numbering system which will be
stable for a long period as a background for current and future
planning in order to avoid frequent and costly changes.
Numbering plans have impact upon operating procedures, accounting
methods, the design of switching equipment and of the telephone
instrument itself. Considering only the telephone instrument, if a
new numbering system requires a new instrument, the planning,
development and early trials leading to production may well take 5
years; and the replacement of all outmoded instruments on an
economical basis 20 years more. Thus, planning for such a numbering
should start at least 25 years before the capacity is exhausted, and
the new system itself must be adequate for a period of at least 50
years in the future.
3. AREA COVERED BY THE NUMBERING SYSTEM:
The area covered by this study consists of the United States, Canada,
Mexico, Alaska, and the Hawaiian Islands.
Direct distance dialing from San Diego into Mexico would be desirable
now, if the numbering system permitted. As Mexico and the United
States increase their community of interest, dialing across the
Mexican border will become more and more attractive.
Codes are now reserved for Alaska [907] and Hawaii [808].
In addition, the plan should provide for the inclusion of area codes
for Great Britain and perhaps other parts of Europe. This requirement
is impossible to estimate with any precision at this time, but an
adequate margin for intercontinental dialing is a very real
requirement.
4. NUMBER OF TELEPHONES IN THE YEAR 2000:
It is well to point out that what is needed for this purpose is an
_outside_ figure. An under-estimate of growth would advance the date
when a new major change would have to be made, and the economic
penalty to the telephone industry might be enormous.
The estimate used for this study is a total of 550,000,000 telephones
in North America in the year 2000. This compares with 57,000,000
telephones in 1955. Of these, 500,000,000 are assumed to be in the
United States and about 50,000,000 in Canada and Mexico.
The source of the estimate given above is a memorandum prepared by
Mr. W. Hodgkinson, Jr., of the Chief Statistician's Division of the
American Telephone and Telegraph Co., dated July 16, 1956. It should
be emphasized that these data were furnished as safe outside limits
for use in designing a numbering plan. The use of the data as
estimates of telephone development is the writer's own responsibility.
In case any reader feels that the assumed number of telephones in the
United States at the year 2000 is too optimistic, he is referred to
Mr. B.T. Miller's article "The Course of America is Upward", in the
Spring 1956 issue of the {Bell Telephone Magazine}. The following
excerpts from Mr. Miller's article are pertinent:
"We in the telephone business have ... geared our activities to
customers' _needs_ ... We have done very little to cater to this huge
market in the sense that other businesses have ... We have not tried
to find out ... what are the communication possibilities of the
average family ... When we have done so, we have made an honest effort
to meet their _wants_, we shall certainly be amazed at the potential
... This could mean that by 1965 or 1970, when we have 45 million
homes in Bell System territory, we could have a hundred million (*)
telephones in these homes instead of the thirty million or so that we
now have."
(*) Assuming the present ratio of business to residence telephones,
this is equivalent to a total of 140 million telephones. The estimate
used as a basis for this study forecasts a total of 115 million
telephones in the United States in 1970, including non-Bell as well as
Bell territory.
5. NUMBER OF OFFICE CODES IN YEAR 2000:
5.1 United States:
Both the existing and the proposed numbering plans limit the number of
telephone numbers per central office to a maximum of 10,000. In urban
areas, the average number of working telephone numbers per office will
be less than this maximum, since recently changed and disconnected
numbers numbers cannot be immediately reassigned and since in each
building, there will be partially equipped growing units which will
bring down the average per office. A review of the 1955 edition of
"Metropolitan Exchange Data", published by the O&E Dept. of the AT&T
Co. shows that under actual conditions, the average telephone office
in urban areas has 7000 working numbers. For the purposes of this
study, it is assumed that in the year 2000, the average office in
urban areas will serve 7000 telephone numbers.
In rural (*) areas, it will not be possible to approach the average
number of telephones per office found in urban areas, since a high
percentage of the exchanges will serve areas with a hundred or a few
scores of households. In these rural areas, the average number of
telephones per office will be affected more strongly by the average
size of the community served than by the maximum capacity of any one
office.
(*) The word "rural" here is used as defined by the Census Bureau, and
includes places of under 2500 population. These offices will therefore
serve many telephones not classified as "rural" by the telephone
industry.
The average number of telephones per office in rural areas for the year
2000 is estimated as follows:
a. In 1954, the average number of telephones per office in the United
States, considering only "rural" places of less than 2500 population,
was 175.
b. The estimate for the year 2000 assumes that the average number of
residential telephones per household will increase from the 1954 figure
by a factor of 4.5.
c. The average number of telephones per office in places under 2500
population in the year 2000 will be 175 x 4.5, or 800.
In order to use the figures of 7000 telephone numbers per office in
urban areas, and 800 telephones per office in rural areas, some
estimate must be made for the year 2000 of the number of telephones in
the two types of areas, respectively. For this study, it is assumed
that there will be 40 million rural population in the year 2000. This
amounts to 12% of the total population compared with 36% rural
population in 1950, and assumes a decline of 14 million in total rural
population. It is further arbitrarily assumed that there will be no
business telephones in rural areas. The average number of residence
telephones per hundred population for the year 2000 has been estimated
at 106. This gives a total of 42,500,000 telephones in rural areas. At
800 telephones per central office, this would require 53,000 central
offices to serve the rural areas.
Of the remaining 457,500,000 telephones in urban areas, 143 million
are assumed to be business and the rest residence. For the purpose of
this report, it is assumed that all business telephones [including
"extensions" in a PBX] will require numbers, but that in the residence
classes, only main telephones will be numbered. It is further assumed
that the kinds of residence service offered, perhaps Visiphone
[i.e. Picturephone] and other new services, will result in the
equivalent of 1.5 residence main telephones per household, or a total
of 144 million residence main telephones in urban areas.
Inward dialing to PBX extensions is assumed. Of the 143 million
business telephones, 76.5 million will be non-PBX, requiring a number
per telephone. In addition, 12.5 million PBX trunks will occupy
numbered terminals in the central offices. This total of 89 million
business numbers, added to the 144 million residence numbers, will
require 33,300 office codes in urban areas exclusive of those required
for in-dialing to PBX extensions.
Numbers for PBX extensions are assumed to be supplied in blocks of
100. According to a study by Mr. C.M. Conway, of the AT&T Co., O&E
Department, 2500 office codes would be required for the 1955 telephone
development, were in-dialing to extensions in effect in all cases,
except hotel PBX's. On this basis, 23,800 office codes will be
required for this purpose in the year 2000.
The following table summarizes office code requirements for the year
2000, compared with 1955 requirements:
1955 2000
---- ----
Rural 10,505 53,000
Urban 10,400 33,300
In-Dialing to PBX Extensions 0 23,800
------- -------
Total 20,905 110,100
Obviously, not all the factors which will determine the actual number
of office codes in the year 2000 have been included in this
estimate. The use of line concentrators with electronic switching
systems to serve small communities might drastically reduce the number
of office codes required in rural areas. However, the vast
preponderance of these rural communities will be served by non-Bell
companies, and it is not possible now to forecast the extent to which
these companies will adopt the line concentrator principle. On the
other hand, new services not even on the horizon now may be in common
use by the year 2000, and result in a much greater demand for office
codes than forecast here. For purposes of this study, then, the
estimate of 110,000 office codes will be used.
5.2 Canada and Mexico:
According to the estimates used for this study, Canada and Mexico will
have less than 10% of the telephones in North America in 2000. Since
data for Canada and Mexico are not entirely reliable, a "broad-brush"
estimate for these two countries seems as likely to be correct as the
more detailed computations made for the United States.
Canada had 3.9 million telephones and about 2860 office codes in 1955,
or 1400 telephones per office code; about half the average for the
United States. In view of the vast undeveloped areas yet to be
settled, it is not likely that development in terms of telephones per
central office will catch up with that of the United States in the
next 44 years. A more reasonable assumption is that Canada's
development in this respect in the year 2000 will equal that of the
United States today. For want of a better assumption, this may be
applied to Mexico as well.
On the above basis, the 51.6 million telephones in Canada and Mexico
would require 19,000 office codes in the year 2000. Perhaps 1000 more
codes would be required for in-dialing to PBX extensions, for a total
of 20,000.
5.3 Summary:
It is estimated that there will be 100,000 office codes required for
the United States, and 20,000 for Canada and Mexico in the year 2000,
for a total of 130,000.
[There are two "figures" -- graphical charts -- as indicated in the
table of contents, as well as mentioned in the main text. They are not
included in this text-based transcription for TELECOM-Digest,
however.]
[end-of-document]
...............................................................
Date: Thu, 26 Dec 1996 16:25:22 -0800
From: "Mark J. Cuccia" <mcuccia@mailhost.tcs.tulane.edu>
Subject: History - 1958 Numbering Document
The following is a transcript of a Technical Memorandum from Bell
Telephone Laboratories. It is dated April 28, 1958, and authored by
Mr. W.O. Turner. It's reference code is MM-58-312-1, and carries the
subject: History of the Bell System Telephone Numbering Plan - Case
38931.
A copy was recently *loaned* to me by one of my contacts now retired
from Bellcore who had started with Bell Labs in the late 1950's. Any
additional comments of mine are noted enclosed within square brackets
[].
MARK J. CUCCIA PHONE/WRITE/WIRE/CABLE: HOME: (USA) Tel: CHestnut 1-2497
WORK: mcuccia@mailhost.tcs.tulane.edu |4710 Wright Road| (+1-504-241-2497)
Tel:UNiversity 5-5954(+1-504-865-5954)|New Orleans 28 |fwds on no-answr to
Fax:UNiversity 5-5917(+1-504-865-5917)|Louisiana(70128)|cellular/voicemail
---------------------------
The history of the Bell System telephone numbering plan is not well
documented, and not many individuals are left in active service who
have had long continuous contact with this problem. There is renewed
interest currently in the numbering plan situation because the
capacity of the present plan is rapidly being approached and because
discussion of the pushbutton telephone naturally leads to
consideration of the environment in which it will be used. For these
reasons it has seemed appropriate to the writer to set down this
account. Events antedating 1927 are based on documents in the files;
subsequent history is based on the writer's personal recollections and
those of Mr. L.K. Palmer of the AT&T Co., who since about 1930 has had
the responsibility for the administration of the plan.
By the summer of 1916, work on the Western Electric power driven full
mechanical telephone switching system [Panel] had progressed to the
point where preliminary cost comparisons could be made with the
Strowger [SxS] system which was in use in a number of cities and towns
operated by [*non* connecting competitive] independent telephone
companies. These comparisons indicated that the Strowger system would
be cheaper for the small single office towns. The Western Electric
[Panel] system would be more economical in the multi-office cities
where its large access switch and flexible trunking arrangements were
of most advantage.
This conclusion was disheartening to the Bell System people, because
there was uncertainty in their minds whether telephone customers in
large cities would be willing to dial their own telephone calls, and
if not, there would be very little demand for the Western Electric
system. In a small city served by one office, no telephone number
needed to consist of more than 4-digits, but in the multi-office
cities, it would be necessary to prefix office codes to the 4-digit
station numbers to distinguish between offices, and as many as 6 or 7
digits might need to be dialed on each call.
Two questions worried the planners: Would the customers be willing to
dial as many as 7 digits in placing a telephone call, and if so, could
they do this accurately enough so that the grade of service they would
get would compare favorably with manual service? Obviously, a
favorable answer to these two questions depended largely upon the
ability of the planners to evolve a numbering plan which the customers
could use easily and accurately.
Existing dial communities operated by independent companies were small
enough so that telephone numbers of 4 or 5 numerical digits were
sufficient to designate all the telephones. A small number of
telephones operated by the [independent] Home Telephone Co. in Los
Angeles [even] had [as long as] 6-numeral listings. Therefore, the
Bell System people could not depend upon independent company
experience for guidance in solving their problem. The most obvious
course would be to follow independent company practice and number
telephones in the larger cities with 5, 6, or 7-numeral telephone
numbers depending on the size of the city. One school of thought,
however, held that only the line number should be designated by
numerals and that the added office code digits should be made up of
arbitrary letters.
[Actually, in the early decades of the century, some cities with local
dial step-by-step systems of competitive non-connecting independent
telephone companies were using a combination of both letters and
numerals on five or six digit local numbers. The first digit was
represented by a letter, and it identified the particular central
office in that city, of the independent telephone company's dial
system. Most of these used 'unique' lettering on the dial, and not
standardized until much later, beginning in the 1920's, when the
independent and Bell companies began to co-ordinate and consolidate
their local dial systems. Some of this continued into the mid-40's,
when Bell of Pennsylvania took-over the non-connecting competitive
Keystone System in Philadelphia, which had its own unique lettered
dialface. Calgary (Alberta) in Canada also continued to have its own
unique lettered dialface until the 1950's, when it changed over to the
North American standard dialface, as it needed to be properly
incorporated into the DDD network of the USA and Canada.]
Proponents of the all-numeral scheme argued that it was just an
extension of the existing all-numeral scheme used by [some] of the
independent companies, and that the addition of letters on the dials
would result in more errors, since the space would have to be shared
by letters and numerals, thus making all [of the] characters less
legible. Proponents of the letter-numeral scheme felt that a
combination of letters and numerals would be easier to remember and
dial, and that this advantage would outweigh the decrease in
legibility. There was general agreement that there would not be room
for more than one-letter-per-fingerhole on the dial, so that all
office codes would be made up by suitable selections from the ten
available letters. A suggestion that the number of fingerholes be
increased to 15 with letters only in the additional 5 holes appears
not to have been seriously received.
[Most of the independent systems using letters had such
one-letter-per- digit on their dialfaces at that time. Sometimes, it
represented the first letter of the 'name' of a central office
building or neighborhood; others were simply 'arbitrary' letters which
had no 'attachment' to a 'name'. Some systems had 'sequential' or
'increasing' letters - i.e. 1=A, 2=B, etc.; others had 'arbitrary'
arrangements - i.e. 1=A, 2=M, 3=S, 4=W, 5=E, etc. There were some
independent dial systems which had *more* than ten finger- holes on
their dials, where the 'additional' fingerholes (and associated
dialpulses) above the 'ten' were for reaching the operator,
long-distance, or other special functions.]
It was obvious that the full mechanical system would have to be
introduced in multi-office cities on an office-by-office basis, so
that for the period of transition, provision would have to be made for
dial customers to dial manual telephone numbers as well as dial
numbers. Because the proposed form of numbering for dial offices was
different from that used for manual offices, the situation appeared to
present a difficult stumbling block to the smooth introduction of the
new dial system. One possible plan was to change all telephone numbers
in the city to the full mechanical form before the first dial office
was put in service. This would mean abandoning the traditional central
office names, and substituting letter or numeral codes for them. This
wholesale change of numbering affecting customers not to be cutover to
dial as well as those who were, might be extremely distasteful to the
public and would create a bad environment for the introduction of the
new system. Moreover, it was felt that telephone operators would have
more difficulty in understanding and passing arbitrary codes than
central office names so that the service would become less
satisfactory.
A second possibility was to retain the present form of numbering,
using central office names for existing and new offices, but in
addition, to assign to all stations both manual and dial, a second
number made up of all numerals, or arbitrary letters and
numerals. With this plan, it would be necessary either to show two
numbers for every customer in the directory, or to provide two
separate directories, one for use by dial customers, and one for use
by manual customers. The inconvenience to the public, and expense to
the telephone company of either plan are obvious.
While debate was continuing on the merits and demerits of the various
numbering schemes, it was decided to attempt to get some experimental
evidence as to the accuracy with which customers might dial numbers
containing more than 5 characters. Accordingly, in the Fall of 1916,
there was set up at 195 Broadway equipment which would be recorded by
a printing telegraph circuit so that the accuracy of dialing might be
checked. Numbers of 4 to 8 digits inclusive were dialed by test
subjects. In the early stages of the trial, the digits dialed were all
numerals. In a report on the preliminary phases of the trial, dated
November 13, 1916, it was stated that dialing of numbers consisting of
letters and digits was about to start. The files do not contain,
however, any comparisons of accuracy as between all numerals and
letter-numerals, nor do they state specifically that the
letter-numeral combinations were in fact tried. The results of the
trial were summarized in a memorandum dated July 25, 1917. This is of
some historical interest since it may be a record of the first
experimental test of customer dialing performance made by the Bell
System; therefore, it is attached in full as Appendix I. Conclusions
were that the overall error rate ranged from 1.3% with 4-digit
numbers, to 4.1% with 7-digit numbers, and then fell to 2.5% for
8-digit numbers.
The low rate of errors with 8-digit numbers was explained as follows:
"Up to and including the 7-digit numbers, the majority of the people
dialing looked up the number and then dialed it without looking back
in the directory or on the list, while on the 8-digit numbers, nearly
everyone took the number in two parts". It was noted that a large
fraction of the errors were caused by sending in a preliminary digit
caused by a quick movement of the switch-hook before dialing, and when
these were deducted from the total, the per-cent dialing errors
reported as follows:
No. of Digits % Dialing Errors
4 0.82
5 0.95
6 1.80
7 3.14
8 1.95
By the Summer of 1917, the early concept that all ten-positions on the
dial were available for use as office code digits had been
modified. Agreement seems to have been reached that the zero digit
should be reserved for calling the operator, and that the "1" digit
should not be used to start office codes, because of the danger of
reaching the wrong office inadvertently when preliminary pulses were
caused by careless operation of the switch-hook. The desk-stand
["candle-stick" phone] in use at that time was particularly vulnerable
to this kind of error. While there is no explicit statement to this
effect in the files, it might be reasonable to assume that the results
of the dialing tests given above were the basis for the decision to
protect against preliminary pulses.
While the dialing tests indicated that there was hope that customer
dialing accuracy might be sufficient so that dial service would
compare favorably with manual service, no progress had been made by
the middle of 1917 on the problem of devising a smoother way of
introducing dial numbering into a multi-office city. On September 7,
1917, a major break-through occurred. Under this date,
Mr. W.G. Blauvelt wrote a memorandum outlining a plan of retaining
central office names and instructing customers to dial the first 1, 2,
or 3 letters of the central office name, in addition to the digits of
the station number. The number of digits dialed would, of course,
depend on the number of offices in the city. This plan would avoid the
problem of double listings or the alternative problem of complete
abandonment of central office names. Mr. Blauvelt admitted that it
would be necessary to change a large proportion of the existing
central office names in each city, since with only
one-letter-per-fingerhole (he assumed a [single] letter in each of the
ten holes), the codes corresponding to many of the existing central
office names could not be dialed.
During the year following the date of Mr. Blauvelt's suggesting, the
AT&T people held discussions with interested officials of most of the
Bell System Operating Telephone Companies, and the ultimate decision
reached almost unanimously was that the Blauvelt scheme should be
adopted in spite of the penalty involved in changing a large portion
of office names.
It was felt that the plan using central office names was inferior to a
plan using arbitrary code letters, or to an all numeral plan; but the
relative ease of transition from manual to dial outweighed all other
disadvantages. It was pointed out that if dialing errors due to
mis-spelling of names remained at a high level, the names could be
dropped once all manual offices [in that city] were eliminated
[i.e. cutover to dial].
On August 30, 1918, a memorandum was written by Mr. W.D. Sargent which
contained the first proposal to associate more than one letter with
each fingerhole in the dial. Mr. Blauvelt's original numbering plan
proposal was modified by this suggestion, and was eventually patented
(Pat. 1,439,723, issued December 26, 1922). Mr. Sargent's specific
proposal was to associate two letters with each of the ten fingerholes
and, in addition, to use the numeral zero as the letter "O" ("oh"),
thus getting a total of 21 letters. This proposal directed thinking
and experiment in the direction of various arrangements of letters and
numerals on the dial to obtain maximum legibility, and out of these
experiments, there evolved the conviction that it would be feasible to
associate 3 letters with each of the numerals 2 to 9, inclusive, thus
obtaining a total of 24 letters which could be used in office
codes. By leaving out only the letters Q and Z, it was found that
central office name changes could be reduced to a minimum, the changes
being made only in those codes where two names started with the same
letters or with letters equivalent to the same numeral code.
A good deal of discussion took place as to the best arrangement of
letters and numerals, and the best coloring of background and
characters to promote legibility; but by August 14, 1919, a memorandum
was written submitting samples of dial markings employing black
letters and red figures on a white background with the dial
arrangement almost identical with that ultimately adopted for
multi-office cities.
Thus, the pattern of the Bell System numbering plan was set, and
because of the ingenuity of the early planners, our large cities have
gone through the transition from all manual to all dial, with a
minimum of inconvenience to the customers. The general pattern was
about as follows, although there were exceptions:
Number of Number of Type of
Offices Digits in Number
in a City a Number
1 4 4321
more than 1, 5 2-4321
less than (and mixed (and
(ultimate) 8 4 and 5) 4321)
more than 5,
less than 6 ADams-4321
(ultimate) 64
more than
ultimate 64 7 ADAms-4321
One further major change was made in the 7-digit numbering plan. In
the 1920's, New York City found itself badly cramped for office code
combinations for its rapidly expanding number of offices, and the New
York Company suggested that 3-letter/4-numeral listings be changed to
2-letter/ 5-numeral listings. ADA-4321 became AD-2-4321. This would
permit one name to serve eight offices; in fact, if the numerals "0"
and "1" were used as the third office code digits, each name could be
used ten times, and the maximum code capacity for a 7-digit city would
increase from 512 to 640. Moreover, some codes hitherto unusable
could now be used; for example, the code 777 is unusable with a
3-letter system, but forms the perfectly good central office name
PResident-7 with the 2-letter/5-numeral system. [Actually, 777 was
used for SPRing, one of the earliest *manual-era* central offices
used/named in New York City. It became SPring-7, when New York City
went to 2L-5N circa 1930.] Other advantages, such as the possibility
of retaining community names for suburban communities served by more
than one office, were evident. This plan was adopted by New York City
and Northern New Jersey in 1930, and eventually all 7-digit cities
followed suit. [However, even earlier, in the early 1920's, Los
Angeles and the Southern California area had 7-digit numbers of the
2L-5N format, without ever having gone through the 3L-4N phase. In
addition to 2L-5N seven dialpull numbers, the Los Angeles metro area
also had mixed six dialpull 2L-4N numbers.]
As telephone growth outstripped early predictions, many local
numbering systems burst at their seams. Four-numeral cities grew to 4
and 5, then to straight 5 numerals. When 5-numeral cities outgrew this
plan, 2-numeral office codes were added and the plan became 5 and
6-numeral. Six-digit cities with 2L-4N numbers evolved into 6 and
7-digit cities with 2-letters and 4 or 5 numeral [station] numbers. By
1940, there were some 12 different varieties of local numbering in the
Bell System, including some using only one letter and 4 or 5 numerals.
With the end of World War II, the Bell System plunged into a program
of extending operator toll dialing to nationwide proportions. Early
attempts at extended operator toll dialing were limited not only by
inadequate equipment design, but by the necessity for dialing an
arbitrary code to route the call through each intermediate switching
point in its path. Thus, an operator might have to dial as many a 12
digits before dialing the called number if there were 4 intermediate
switching points on the route. Satisfactory dialing accuracy under
these conditions seemed to be too much to hope for.
The No. 4 type toll crossbar system could be given the ability to
translate, change, and spill-forward codes dialed into it, and this
made possible a universal nationwide numbering plan with each
subscriber station given its own unique code and number in the
national plan. At the AT&T Co. the writer and Mr. L.K. Palmer were
charged with the responsibility of evolving this universal nationwide
numbering plan, in co-operation with the Bell Laboratories' engineers.
Requirements set up for the plan were:
Minimum number of digits to cover the United States and Canada and
provide for 40 years' growth.
Uniform number of digits to be dialed on long-distance calls.
All existing local numbering plans to fit into the national plan
without significant change in local numbering, which would increase
the difficulty of dialing local calls.
In addition, it seemed highly desirable that the plan be suitable for
direct distance dialing by customers at such time as facilities for
this became available. This additional requirement was accepted by
management, but only on the basis that the ease and simplicity of
operator dialing was not to be reduced thereby, and that no local
number changes were to be forced to gain this objective alone.
The nationwide plan which was evolved is the now familiar ten-digit plan
consisting of a three-numeral area code preceding a seven-digit number.
Local telephone numbers of less than 7-digits were to be built up to
7-digit numbers by the toll operators by following these rules:
4-numeral numbers:
dial arbitrary 3-numeral code assigned to the town, and 4 numerals
5-numeral numbers:
dial [first] 2-letters of the town name, and 5-numerals
6-numeral numbers:
dial initial letter of the town name, and 6-numerals
2-L/4-N numbers:
dial 3-letters of the office name, and 4-numerals
Thus, for the bulk of the traffic, to all but the smaller towns, the
full 7-digit telephone number could be built up by the operator from
the town name and number furnished by the calling party.
Considerable thought was given to the problem of dividing the United
States and Canada into numbering plan areas. Using the basic 2-L/5-N
numbering plan, there would be capacity in each area for a maximum of
640 offices with no conflicts between office codes.
There were something under 20,000 telephone offices in the area to be
covered by the nationwide plan. One hundred areas could be designated
by a 2-digit area code series; and if area boundaries could be
arbitrarily set so as to allocate an average number of offices to each
area regardless of density of telephone development, these 100 codes
would suffice until the number of offices had increased
three-fold. This would permit adoption of a 9-digit universal
numbering plan. It was immediately apparent, that this ideal was
impossible of attainment, for several reasons.
In toll operating practice, it is customary to maintain on the
switchboard keyshelf adequate information to permit the operators to
determine the route for 90% or more of the calls they handle without
reference to the special route operator. If the boundaries of
numbering plan areas were to be purely arbitrary, without relation to
any natural or political boundaries, the job of determining the area
code for a particular called point would be immensely complicated, and
would almost certainly increase the delay and cost involved in
handling toll calls by making it necessary to refer to the route
operator on more than 10% of the calls. Moreover, there are definitely
outlined local calling areas around the larger cities, and numbering
plan area boundaries should not be drawn to cut across these areas.
As an aid to memory in determining the area code of a called place,
the device obviously most useful was to establish a relationship
between the area code and the state or province in which the office is
located. One area would cover each of the less populous states, while
the larger states would have to be divided into two or more areas
each. This was the plan adopted; early estimates indicated that the
number of codes needed over a 40-year period would be about 100.
The form of numbering plan area code adopted was dictated, in the last
analysis, by past history. Because of the decision, back in 1918, not
to assign letters to dial positions one and zero, since these digits
would not be used to start office names, it followed that the second
letter of office codes never corresponded to the numerals "1" and
"0". Therefore, if numbering plan area codes included a "1" or "0" as
their second digit, the toll switching equipment could readily be
designed to examine the second digit received, and if it were a "0" or
"1", to recognize that the first three digits received were to be
translated as an area code, not an office code. This meant a 3-digit
area code instead of a 2-digit area code; but after conflicts with
codes reserved for other purposes were set aside [the eight "N11"
3-digit local service codes], there remained 152 such codes available
for numbering plan area use. The 100 code capacity of a 2-digit code
system had seemed perilously limited, so the 3-digit code pattern
seemed amply justified. Thus the present pattern evolved: From any
place in the United States and Canada where the necessary facilities
are available, the number CHelsea-3-1000 in New York City can be
reached by dialing 212-CH3-1000.
Hardly had the pattern been set for nationwide operator toll dialing,
when the decision was made to press forward with the development of
facilities to permit nationwide customer direct distance
dialing. Because this possibility had been kept in mid throughout the
development of the nationwide numbering plan, no major changes in the
plan itself needed to be made. Customer use of the plan, however, did
emphasize certain difficulties which the operators had already
encountered, and sparked the next stage in the evolution of the Bell
System numbering plan.
The next stage of evolution consisted of two innovations: the adoption
of universal 2L-5N numbering for all local offices reached by direct
distance dialing, and the introduction of a standardized list of
central office names, chosen on the basis of ease of spelling and
pronunciation.
Although the rules for building-up the existing 4, 5, 6, and 7-digit
telephone numbers to uniform 7-digit numbers were relatively simple
for trained operators to use, customers could not be expected to
remember them. No more should be asked of the general public than to
dial the called area code followed by the called number as
listed. This meant uniform listings of 7-digits across the continent,
in small towns as well as large cities. And as local dialing areas
were expanded to conform to the widening areas of social and business
interest in growing communities, the 7-digit local numbering plan
simplified the dialing of calls to nearby communities as well as
distant ones. By changing at one move to the 7-digit plan, the gradual
shifts from 4-digits, to 5, to 6, and to 7, with the attendant
annoyance to customers and expense to telephone companies, were
stopped once and for all.
Almost from the inception of operator toll dialing, difficulty was
experienced with the spelling of unfamiliar office names, which may
have had rich local significance, but were virtually unknown away from
their immediate locality. An early example of the need for a
standardized list of names came at the time the Indianapolis toll
crossbar switching machine was placed in service. An analysis of the
calls which were misrouted through the toll office showed that some
70% to 80% were due to mis-spelling two Indianapolis office names:
IMperial and RIley. The name IMperial was frequently [mis]spelled [and
mis-dialed as] EMperial; and as for the RIley office, named after the
famous Hoosier poet, it was apparent that outside his home state, most
people were more familiar with the good Irish name Reilly than with
the poet's name. There was no recourse but to give these two offices
new names which would be mis-spelled less frequently.
Another difficulty that had to be straightened out was the existence of
three central offices in different cities of the Bell System spelled
HIland, HYland, and HIghland, respectively. Even the traditional office
name CYnwyd, near Philadelphia, was abandoned.
As experience with distance dialing accumulated, it became apparent
that another difficulty had to be straightened out. That difficulty
arose from the use of town names as central office names. Lafayette,
California is a relatively small town, just outside Oakland. The name
is in the approved list of central office names, so why not use the
town name as an office name? But consider the plight of an operator
asked to complete a call to "Lafayette, California, 3-1211." Has she
complete information? Is the town name missing, or the office name? To
avoid the chance of mis-routing the call, she must cross-examine the
customer, which annoys the customer, and delays the call. So an
arbitrary name is added; the customer asks for: "Lafayette,
California, ATlantic-3-1211" and the call goes through without
delay. Obviously, the name ATlantic has no significance in a town
overlooking San Francisco Bay, but uniformity has been achieved, and
uniformity helps eliminate errors.
Much local pride in historically significant names has had to be put
aside, and telephone office names, for the sake of mechanical
progress, are becoming colorless and standardized.
Even with much progress in eliminating hard-to-spell names, sources of
confusion remained. One of the most frequent errors was substitution
of the numeral "0" for the letter "O" as the second letter of names as
"MOntrose," In 1955, a group of Bell Laboratories' engineers and
scientists met at the request of Mr. A.B. Clark to consider whether
anything might be done to improve dialing accuracy. As a result of
this meeting, a program of laboratory tests was organized to compare
the speed and accuracy of dialing of two systems - the existing system
using central office names, and an all-numeral system. The all-numeral
system was selected for comparison rather than one using arbitrary
letters and numerals, because it is, of course, essentially the
simplest possible numbering system.
Results of the Laboratories' tests were published on April 2, 1956, in
the form of a report "An Evaluation of All-Numeral Dialing", by
Messrs. A.H. Inglis, J.E. Karlin, and W.O. Turner. Conclusions were
that with the all-numeral system, errors would be fewer and dialing
faster; and that from a memory standpoint, there was little choice
between systems except that the average person would take a few days
longer to memorize a 7-numeral number than one consisting of an office
name and 5-numerals. With pushbutton telephones, the speed and
accuracy advantage of the all-numeral system becomes relatively
greater. The tests did not include dialing of ten-digit numbers, as in
direct distance dialing.
Since the establishment of the nationwide numbering plan, new
numbering plan areas have been created at such a rate that in 1957,
there are 113 in existence. This increase in area requirements is
partly the result of inefficient use of office codes in the less
developed parts of the country, and partly due to accelerated growth,
particularly in the fast developing territory of the U.S. south and
west, and Canada. A 1956 survey of the Companies' own forecasts
indicated a requirement of some 130 area codes by the time telephone
development doubles again. This estimate did not take into account
three factors which by this time were definitely in the picture: The
probable extension of the numbering plan into Western Europe and
possibly beyond; increased numbers of office codes required for inward
dialing to PBX extensions and other new services [paging, mobile,
datasets/ modems]; and the possibility of greatly stimulating the
demand for telephone numbers by aggressive merchandizing of telephone
service. Obviously, the 152-area capacity of the numbering plan,
which had looked so safe 10 years before, required re-examination.
This re-examination was undertaken by the writer, and resulted in
S.E.R. No.48, "The Bell System Telephone Numbering Plan," released on
March 26, 1957. Conclusions of the study were as follows:
Capacity of the present numbering plans may be reached in about 20
years.
Limited additional capacity can be provided on a temporary basis by
using 3-digit area codes beginning with the numeral "1".
For permanent relief, the pushbutton telephone, when introduced,
should be equipped with a "Distance" key, to be operated before
setting up all 10-digit calls. With this means of identifying 10-digit
calls, all combinations of three numericals can be used for area codes
- a total of 1000.
Further relief could be obtained by adopting all-numeral telephone
numbers. The maximum number of office codes per area would be
substantially increased, which would particularly benefit the larger
cities.
These proposals have the obvious weakness that long-term expansion of
the numbering plan capacity depends upon the universal substitution of
pushbutton telephones for dial telephones within a 20-year
period. This might turn out to be uneconomical for both the Bell and
non-Bell telephone companies. Discussions of this point among AT&T
Co. and Laboratories' engineers gave rise to a suggestion which is
under review by the AT&T Co. at the time this memorandum is written;
that is, that when further code capacity is needed, either the digit
"1" or the digit "0" be set aside for use as a prefix to be dialed
before all 10-digit calls. The arguments for and against each of these
digits have been summarized by AT&T Co. engineers and discussions
leading to agreement on a choice of digits are currently under
way. This prefix would serve the same purpose as preliminary operation
of a "Distance" key at pushbutton telephones, and would be applicable
to both types of sets. Additional code capacity would no longer be
dependent upon the elimination of dial telephones, but the total
number of available office codes would be reduced. For example, use of
the digit "1" as a prefix to indicate a 10-digit number to follow
would preclude its use as the first digit of an office code.
Meanwhile, in the Directory Departments of the telephone companies,
there has spread an innovation which has a bearing on the evolution of
the numbering plan. As cities grow and telephone directories get
bulkier, each fraction of an inch of directory space becomes more
precious. The directory people looked with covetous eyes on the space
taken up by the central office name, repeated for each listing in the
directory. Cautiously at first, they dropped out all but the first two
letters of the central office name in directories covering but a few
offices, showing in a box on each page the list of office names and
their two-letter codes. The saving in space could be taken in less
pages per directory, or it could be translated into larger and more
legible type. No adverse reaction came from the public - who use the
2L-5N system in advertising and other public displays anyhow - so the
system is being extended to the larger cities, and the list of names
and corresponding codes is being moved to the information section at
the front of the directory.
This development may well lead to further changes in central office
names, or their abandonment entirely. Consider two perfectly good
central office names, ADams and ATlantic, both on the approved
standard list. Now, drop the names, and continue to use the code
letters AD and AT. On passing these by word of mouth between
customers, and between customers and operators, how many times will
"AD" come out as "AT"? Each such translation prepares the ground for
one or more dialing errors. It seems inevitable that if names are
dropped, letters too must be dropped, or else the list of permissible
letters reduced to eight or so that are not easily confused
phonetically with each other or with numerals. The Australian Post
Office has already started the transition from a combination of
arbitrary letters and numerals in their large cities, to a straight
7-numeral plan. In Europe, elimination of letters from local numbers
is being considered as a means of minimizing the language difficulties
accompanying the introduction of direct international dialing.
The arguments for all-numeral dialing are compelling enough so that
the AT&T Co., early in 1958, embarked upon a full-scale trial in
Witchita Falls, Texas. This moderate-sized city has had a 5-numeral
numbering plan and was scheduled to be changed to the conventional
2L-5N plan. Instead, it has been changed to 7-numerals. Indications so
far are that the customers are satisfied with the 7-numeral plan and
that their performance compares favorably with that of customers in
other similar places where the transition has been to the 2L-5N
plan. [It might have been possible that the first digit of the two new
initial digits didn't conflict with the first digit of the five-digit
local number. I assume that Witchita Falls was a SxS city at that
time, and installed 'digit-absorbing' equipment in its SxS first
selectors to 'absorb' the first two new dialed digits. Customers
might have continued to be able to dial only the five-digit portion of
the telephone number for many more years, as occurred in many
mid-sized SxS situations.]
Further experience with the 7-numeral plan is, of course, desirable,
and the acid-test of acceptability will be to convert to this plan,
some city now accustomed to dialing 2L-5N. Plans for such a trial are
under discussion.
[However, in the mid-1960's the "Anti-Digit-Dialing Leagues" were
formed, the first one in the San Francisco Bay Area. It went as far as
even involving a lawsuit against Pacific Telephone & Telegraph, with
the ADDL plaintiffs using attorney Marvin Belli.]
And so, central office names have become a vestige of the past,
without significance to the people who use them, and a source of
errors and confusion. If we try to project the lessons of 40 years of
history into the future when international dialing may well be
common-place, it seems to the writer, at least, that the evolution of
the Bell System numbering plan will not be complete until we arrive at
the ultimate simplicity of an all-numeral numbering plan.
(signed) W.O. Turner
NY-312-WOT-BC
Att. Appendix
APPENDIX:
Tests in Dialing Telephone Numbers with Varying Numbers of Digits.
July 25, 1917
(Taken from O&E Department File 1606, Vol. I)
An automatic telephone with the usual dial at its base was connected
with a printing telegraph machine located in a room on the floor below
the one in which the telephone was located. Two sets of tests were
made. On the first set of tests, lists of numbers with four, five,
six, seven, and eight digits were pasted in a New York City telephone
directory, and lists of the names and addresses corresponding to the
names and addresses in the directory associated with the lists, were
also prepared.
A number of different people were selected approximating as nearly as
possible an average group of telephone users. These people were given
lists of names and addresses containing 25 names with four-digit
numbers, 25 with five-digit numbers, etc., copies of which are
attached. They were also given the attached set of instructions on how
to operate the automatic telephone.
The numbers as sent were recorded on the printing telegraph machine,
and a record was kept of the accuracy with which the numbers were
sent, together with the time required to send the lists with the
varying number of digits. A quick movement of the switch-hook caused
digit one to be printed by the machine, and it was not possible to
decide in many cases, whether these digits would have resulted in
wrong numbers under ordinary operating conditions of an automatic
telephone system. These extra digits are shown separately below, but
are included in the total errors and the per-cent total errors. The
results of the first set of tests are shown in the following table:
Time per
No. of Number to
people Calls Digits Dialing Extra Total Percnt Look up in
using Dialed in Errors Digit Errors Total Directory
telephone Number Errors and Dial
18 450 4 5 4 9 2.0 % 44.3
17 425 5 4 6 10 2.4 % 43,5
19 475 6 8 2 10 2.1 % 42.3
17 425 7 17 5 22 5.2 % 51.5
16 400 8 7 5 12 3.0 % 53.0
---- -- -- -- AVG:
TOTALS: 2175 41 22 63 2.9 %
The second set of tests were made similar in every way to those
summarized above, with the exception that the calling parties were
given lists with the telephone numbers associated with the name,
rather than being instructed to look up the numbers in the telephone
directory. A copy of the lists used in this second set of tests is
attached and the results of the tests is summarized below:
Time per
No. of Number to
people Calls Digits Dialing Extra Total Percnt Look up in
using Dialed in Errors Digit Errors Total Directory
telephone Number Errors and Dial
21 525 4 3 1 4 0.8 % 15.3
21 525 5 5 4 9 1.7 % 20.7
21 525 6 10 1 11 2.1 % 21.7
20 500 7 12 4 16 3.2 % 24.3
21 525 8 11 0 11 2.1 % 26.4
---- -- -- -- AVG:
TOTALS: 2600 41 10 51 1.9 %
The two sets of tests are combined below:
No. of
people Calls Digits Dialing Extra Total Percnt Percent
using Dialed in Errors Digit Errors Total Dialing
telephone Number Errors Errors
39 975 4 8 5 13 1.3 % 0.82 %
38 950 5 9 10 19 2.0 % 0.95 %
40 1000 6 18 3 21 2.1 % 1.80 %
37 925 7 29 9 38 4.1 % 3.14 %
37 925 8 18 5 23 2.5 % 1.95 %
The extra digit errors were undoubtedly independent of the number of
digits in the numbers dialed, so that the final column "percent
dialing errors" is the one giving the best data as to the effect on
the accuracy of the number of digits per telephone number. Up to and
including the seven-digit numbers, the majority of the people dialing,
looked up the number and then dialed it without looking back in the
directory or on the list, while on the eight-digit numbers nearly
everyone took the number in two parts - the first five-digits were
dialed, and then the calling party looked in the directory or on the
list for the last three numbers and dialed them. This, I think,
accounts for the lesser number of errors on the eight than on the
seven-digit numbers.
(no signature shown)
Attached:
Lists of Names and Addresses
Set of Instructions
Copy of Lists used in Second Test
(None of these attachments are included in the file)
[end-of-document]
------------------------------------------------
[TELECOM Digest Editor's Note: Parts three and four of this
telephone numbering history series will follow. PAT]
================================================================================
From: TELECOM Digest Editor <ptownson@massis.lcs.mit.edu>
Newsgroups: comp.dcom.telecom
Subject: Telephone Numbering History - Part II
Message-ID: <97.01.04.133uyge46@massis.lcs.mit.edu>
Date: Sat, 04 Jan 1997 13:30:00 EST
Reply-To: TELECOM Digest Editor <ptownson@massis.lcs.mit.edu>
This is part two in the two part series circulated earlier today
on Bell System numbering history submitted by Mark Cuccia. The
first article covered 1957-58 (and earlier notes); this one
covers 1960-70.
PAT
Date: Thu, 26 Dec 1996 16:25:24 -0800
From: "Mark J. Cuccia" <mcuccia@mailhost.tcs.tulane.edu>
Subject: History - 1960 Numbering Document
The following is a transcript of several mimeographed pages of notes,
from a Bell System conference on the US/Canada Nationwide Numbering
Plan. It is 'unauthored' as well as undated, but I assume it is from
1959 or 1960. In the following, there are several references to
material presented on a "Vugraph", i.e., an overhead transparency
projector. I don't think that any of those "Vugraph" charts are
'themselves' included here, but are probably somewhat described in the
notes.
An original copy of the following was recently *loaned* to me by a man
who is now retired from Bellcore, and who had been with Bell Labs
since the late 1950's. Any additional notes of mine are enclosed
within square brackets [].
MARK J. CUCCIA PHONE/WRITE/WIRE/CABLE: HOME: (USA) Tel: CHestnut 1-2497
WORK: mcuccia@mailhost.tcs.tulane.edu |4710 Wright Road| (+1-504-241-2497)
Tel:UNiversity 5-5954(+1-504-865-5954)|New Orleans 28 |fwds on no-answr to
Fax:UNiversity 5-5917(+1-504-865-5917)|Louisiana(70128)|cellular/voicemail
-----------------------------------
NUMBERING PLANS
The first numbering plan crisis occurred in 1917 when dial service was
started for large cities. A numbering plan was needed then that would
handle a mixture of manual and dial offices. W.G. Blauvelt provided
the solution with the 3-letter/4-number plan. Later this became the
present 2L-5N plan. The historical need for a lettered dial ceased
when the large cities became substantially all dial. However, the
present DDD numbering plan has extended the usefulness of the lettered
dial.
The present plan of numbering for DDD was devised in the later 1930's
and early 1940's. In 1947, 86 area codes were assigned in the U.S. and
Canada. By 1957, 113 area codes were assigned. This is a growth
factor of 27 codes in 10 years. As of December 1959, 117 codes were
assigned from a possible maximum of 152. If growth continues at the
same rate, all codes will be used up about 1972, and in the year 2000,
229 codes will be in use. This does not include an allowance for
dialing to other parts of the world.
The demand for NPA codes can be estimated by a more logical process,
that is, by basing it on population forecasts. We have several
estimates based on forecasts by the AT&T Co. statisticians, by Fortune
Magazine, and by George Washington University. They are shown on this
first Vugraph.
An interesting side issue developed out of this analysis. Bell System
Central Office station fill in 1925 was slightly under 2000 stations
per central office, including [PBX] extensions. By 1957, 32 years
later, the fill was slightly above 5000 stations. This 2.5 fold
improvement in fill indicates that a large part of the telephone
growth was absorbed by additions, rather than by the opening of new
offices.
The telephone industry in 1957, Bell and Independent, in the U.S. and
Canada, had a fill of slightly under 3000 stations, including [PBX]
extensions. At the past rate and type of growth, this will result in a
fill of 10,000 stations by the year 2000. Since a large part of this
fill (residence extensions) does not have a requirement for line
numbers, this figure is quite plausible.
The implication drawn from this is that it is likely that most of the
telephone growth of the next 40 years will also be absorbed by
additions to central offices at existing wire centers.
The present ABX [NNX] numbering plan for local offices provides a
maximum of 640 codes per [NPA] area. If each code must have a useable
name, the maximum is about 600. If, in addition, codes cannot end in
[the numerical zero] "0" because of possible customer confusion with
the letter "O", the maximum is 540. On the basis of present code
assignments to NPA's, in the DDD Reference Guide the fill averages 134
central office codes per [NPA] area. When DDD is fully implemented,
the Guide will list about 60% more names. By the year 2000, the fill
will average about 390 codes per NPA. Thus there does not appear to
be an immediate necessity for planning for more codes per NPA. All
this is premised only on the needs we see now and includes
consideration of arrangements for PBX in-dialing, national Enterprise
service [what would become automated 800 Toll-Free Service], data
transmission [new dial-up services using modems over the DDD network;
also automation of existing manual TWX Service, using N10 form NPA
codes], and similar items.
There is a definite need for more area codes, however, as already
shown on the first graph. The present 152 capacity will exhaust
sometime between 1967 and 1975 depending on the forecast used. Plans
for expanding this capability are as numerous as individuals
consulted. A few years ago, it was proposed to add the 1XX series to
the present series to gain added capacity of at least 64 codes. This
would move the congest dates out to a period from 1980 to
1996. Another possible added series is the AB0 [NN0] series, of which
19 have already been assigned to central office codes, but which can
be recaptured. This moves us out to from 1989 to beyond the year
2000. If the AB1 [NN1] series were recaptured for area codes, the
earliest exhaust date would be 1997.
There is one additional possibility, namely the 0XX series. This would
add 64 more codes, but would require either a delay on all "0" Operator
calls, or a changed operator code.
This kind of approach we have just been discussing was dropped because
of the feeling that we should set up a plan that would carry us a long
way into the future - well past the year 2000.
New numbering plan arrangements are expected to provide for more than
increased capacity. Examples are:
1. Barrier codes to prevent locally intended calls from reaching toll
points in error. [i.e. the use of 1+]
2. Access codes to provide access to the centralized CAMA equipment in
SxS areas. [i.e. the use of 112+ initially, later to become simply 1+]
3. Reduction in six-digit translation. This [six-digit translation] has
been costly and is desirable [than] to split areas to reduce the amount
of six-digit translation.
4. Elimination of non-uniformity in service codes [i.e. conversion of 11X
service codes in SxS areas to the "common-control type" of N11 service
codes] and provision of a code for DDD information. [i.e. the use of
(NPA)-555-1212]
5. A code for dialing person and collect calls. [i.e. 0+ dialing]
6. Arrangements that will be compatible with mechanized Enterprise Service,
a special type of collect call [i.e. future 800 Toll-Free]
7. Possible codes for dialing to foreign countries.
[809 for the Caribbean; 903 for the northwest Mexican border; 90-5 access
to Mexico City; proposed inclusion of Europe, Latin America, Australia
into the Bell System DDD network and numbering plan. The latter proposed
inclusion of countries was dropped, as Europe, and later the CCITT was
developing a worldwide 'country code' numbering plan of its own. 011+
would become the access code for dialing out of the NANP]
This brings us up to recent history, the plans that have been
proposed, and the present favorite plan.
Work was started in 1956 to provide more uniformity in dialing, and
reduced effort in calling nearby NPA's. As a result of this work, the
"11" plan was presented to the President's Conference of October
1956. This plan proposed the use of "11" instead of "112" in SxS
areas, and the allied use of "1X" for calling nearby NPA's. There was
no consideration of person dialing. An important consideration was to
get uniformity without senderization of SxS. The plan was covered in
an AT&T Co. letter to the field on January 28, 1957. It didn't take -
but here it is on this Vugraph.
During 1957, study work was started on an expanded numbering
plan. This work was stimulated by Mr. Turner's SER-48 [Switching
Engineering Report] issued February 1, 1957, which included that more
capacity would be needed in about 15 or 20 years.
In May 1957, a conference was held by AT&T Co. people to discuss the
need for co-ordinated development of certain items. Among the items
discussed were access, area, and service codes. Increased area code
capacity was one of the items. It was pointed out that the
Laboratories were handicapped by lack of a plan. The O&E conferees
agreed that quick action was necessary and that a plan would be
forthcoming in a few months.
A formal committee of O&E and Laboratories people (O. Myers from
Labs.) was set up on March 4, 1958 to recommend a plan. The outcome of
the committee's work was the presentation of 3 plans to which a fourth
was later added at the request of the New York Telephone Company.
These were to be appraised and priced and one was to be selected. The
plans are shown on these Vugraphs.
On September 8, 1958, Mr. C.A. Armstrong wrote Mr. F.J. Singer to
confirm the formation of a joint O&E-Labs. working group to estimate
cost factors of the three basic dialing plans. This group farmed out
the job of determining what changes were required to implement the
several plans. A group of development and systems engineers studies
the nature of the changes required and wrote a memorandum on the
subject.
Pricing on the detailed basis suggested by this memorandum would have
taken too much time and effort. It was, therefore, agreed with the O&E
that short-cut methods would be employed. A group of four O&E and Labs
people, therefore, priced out the plans using these short-cuts.
Here is a summary of the results on this Vugraph. The important thing
is that they are all roughly a stand-off if person-to-person dialing
is included.
A series of three meetings was held in February and March of this year
for presenting the report of the pricing committee. At the last, held
on March 23, 1959, Messrs. Busch, Singer and Ritchie were present.
As a result of these meetings, Plans 2 and 4 were dropped, and Plan 1
was agreed upon for common control [Panel, Crossbar, and at-that-time
still experimental/developing Electronic Switching] areas. For
step-by-step areas, the long term plan was left open but it appeared
that the "112" access code should be dropped at the time person
dialing was started. An option was suggested that would permit
retaining the "112" access code until pushbutton dialing with the "d"
button was introduced.
[There was a proposal for a touchtone button labeled 'd' which would
have probably been the present '*' or '#' button. It would have been
entered as an access code for all ten-digit DDD calls. This would have
allowed probably all one-thousand possible XXX format codes to be used
as area codes, however this plan was dropped, since it would have
taken way to long for everyone in the US and Canada to be cut-over to
touchtone, before any 'additional' format area codes could have been
assigned.]
It was also suggested that a conference be held with the New York
Telephone Co. to acquaint them with the study results and the
tentative decisions. This was held and the New York Co. objected to
the proposed plans. Their objection was based on N.Y.Co. plans to
publish area codes in directories.
The Manhattan directory to be issued this June will list the area code
for each foreign listing instead of the statement "ask operator for
[location name], etc." By January 1, 1963, there will be 100% area
code listing for the New York City area. PBX attendants are being
trained to accept area code placement and the commercial people have
agreed to billing on an area code basis, i.e., there will be no place
names. Customers will be urged to print their complete telephone
numbers on letterhead and in advertising.
[THAT LAST SENTENCE CAN *NEVER* BE UNDERSTATED, IMO.]
There are several advantages to this, as follows:
1. Rate and route effort will be reduced. About $2,000,000 per year will be
saved in New York City.
2. Operating effort will be saved on foreign listings.
3. It takes care of the boundary problem, i.e. telling the customer what to
dial when the called customer is on or near an NPA boundary.
In order for the plan to work properly, the New York Co. feels that
the person prefix must be placed *ahead* of the area code, not after
the first digit. [There was another proposal that "10+" would be
dialed for collect and person type calls. This was changed to "0+"
before actually being implemented. Maybe this is what was referred to
with the 0+ prefix being dialed 'first'?]
On March 5, 1959, or just before, Mr. Singer called Mr. Mapes who
promised to get out a letter with a tentative answer very shortly. The
answer the letter will contain is shown on this Vugraph. We do not
have the letter as yet, however.
If we do not get an answer by the middle of the year, the person
project will be delayed.
[end-of-document]
----------------------------------
Date: Thu, 26 Dec 1996 16:25:27 -0800
From: "Mark J. Cuccia" <mcuccia@mailhost.tcs.tulane.edu>
Subject: History - 1970 Numbering Document
The following document is a transcript of a memo from Mr. Charles Clos
to to Mr. F.J. Turner of Bell Labs. It is dated May 15, 1970, and it
regards some of the history behind the dialing plan used in the Bell
System, and how individual local switching systems shaped the dialing
plan.
A copy of this document was recently *loaned* to me by one of my
contacts who is now retired from Bellcore/Bell-Labs. Any notes of mine
are enclosed in square brackets [].
MARK J. CUCCIA PHONE/WRITE/WIRE/CABLE: HOME: (USA) Tel: CHestnut 1-2497
WORK: mcuccia@mailhost.tcs.tulane.edu |4710 Wright Road| (+1-504-241-2497)
Tel:UNiversity 5-5954(+1-504-865-5954)|New Orleans 28 |fwds on no-answr to
Fax:UNiversity 5-5917(+1-504-865-5917)|Louisiana(70128)|cellular/voicemail
--------------------------------
The Bell System's future numbering plan which is currently being
implemented was approved in 1959. It is the best plan that the study
committee was able to arrive at and obtain universal approval. When I
became chairman of that committee, I stopped the sniping on all other
plans. I have arrived at three Numbering Plan Principles:
1. Everyone is an expert on numbering.
2. Everyone's numbering plan has a twist to it that makes it better than
everyone else's plan.
3. Future Requirements are always underestimated.
The committee met to consider one plan at a time, assuming it to be
the most desirable plan. At such sessions, the objective was to make
to plan viable. To everyone's surprise, we found that the four major
plans under study required equivalent central office modifications and
each cost about the same. The decision then hinged on choosing that
plan which was least objectionable.
Before delving into some details of the numbering plan, a bit of
historical and philosophical background seems desirable to show how
prior decisions and switching hardware developments shaped the
numbering plan.
MANUAL SYSTEMS:
It appears that manual systems left no visible effect on the numbering
plan, unless we consider that the "B" operator's reach, with the old
plugs and jacks, limited the size of a manual office to 10,500
numbers. This limitation probably carried over and resulted in the
10,000 number limitation in common control offices.
STEP-BY-STEP SYSTEMS:
Step-by-step systems almost produced a lasting effect on the numbering
plan, in that these systems cater to open-ended numbering. The
invention of the digit absorbing selector made it economically
tolerable to convert step-by-step systems using fewer than 7 digits
and/or open-ended numbering to a closed system of 7 digits.
OPEN VS. CLOSED NUMBERING:
The great debate between open-ended and closed numbering was settled
in favor of the latter (except in Germany where it was settled in
favor of the former), chiefly because the switching hardware in common
control [Panel, Crossbar, ESS, etc] systems needs an indication when
the last digit has been dialed. The cheapest way to provide this
indication is to fill up the register and immediately advance the
call. If, however, one digit less might be permitted, the register
must wait for a prescribed time awaiting another possible digit. This
is costly for two reasons, namely, because of the timing circuit and
due to the need to provide extra registers to allow for the waiting
times. There are other solutions, such as the use of pre-translators,
the recognition of certain digits in certain places, etc. But these
likewise are costly.
BLAUVELT'S INVENTION:
In 1917, Mr. W.G. Blauvelt got a patent on the use of letters on the
dial (other than for party-line letters). He made it possible to
introduce dial offices in an orderly fashion in the larger
multi-office manual cities. Imagine the customer problems if, in
1922, all manual offices in New York City had suddenly become known by
numerical prefixes. Blauvelt showed how to get the equivalent of a
numerical prefix by the simple expedient of dialing "letters
suggestive of the office name." The first dial office in New York City
was named PENnsylvania and existing manual offices became known as
BEEkman, ORChard, etc.
2L-5N NUMBERING:
The size of New York City in the early 1920's was such that a 3-letter
prefix sufficed. In fact, this was thought to last for a long time to
come. However, in the early 1930's, the New York Telephone Company
started to exhaust its supply of suitable central office
names. According to Mr. Chester E. Brooks, Mr. H.C. Carpenter (Staff
V.P.) called a conference at which all sorts of solutions were
debated. The choice narrowed down to 2L-5N or [completely numerical]
7D, with the former winning by one vote.
THE LETTER "Z":
In the early 1940's, the New York Telephone Company studied toll
dialing for the Greater New York Metropolitan area. One plan assumed
the use of toll directing codes, such as JZ, LZ, and WZ for dialing
calls to New Jersey, Long Island, and Westchester, respectively. Since
J and L are in the same pull [of the dial, namely the '5'], this plan
was not viable. However, for several years, dials were manufactured
with a 'Z' in the tenth [zero] pull.
[Another reason for the letter 'Z' on the digit '0' is that some
people might have tried to dial operator-connected manually translated
toll-free reverse charge "Zenith" calls. By putting the 'Z' on the
dial with the '0'/zero for Operator, customers trying to *dial* ZEnith
would be dialing '03', which would connect them with the operator on
the first digit dialed, '0'. Same applies for customers trying to dial
'ZEro' for the Operator.]
AB1X [NN1X] CODES:
Another plan studied by the New York Telephone Company assumed a great
big dialing area, out to Brewster in the North, and Riverhead in the
East. To make this plan viable, Mr. K.E. Bassett proposed the use of
ABX [NNX] and AB1X [NN1X] type central office codes. At that time,
there were a great many manual offices still in existence in the
suburbs. Bassett's idea was to name the offices "CArmel Eleven",
"WHitehall Twelve", etc. and at the same time, keep [existing New York
City] designations, such as WHitehall Four. The use of "teens"
designations was for operators when passing calls manually. The
presence of a "1" in the third-pull [of the dial] would alert the dial
office to expect 8 digits. All dial offices in New York City had a
capacity for 8 digits. [Party-lines in manual offices in the city
would have seven-digits of the 2L-5N format, plus a single letter to
identify the particular party on that line. All eight digits would be
pulled out on the dial by a calling party from a dial central office.]
His plan was proposed years before the first AB1 [NN1] type codes were
assigned anywhere in the Bell System, which occurred in Pittsburgh
around 1950. [Actually, there were some seven-digit 2L-5N cities using
NN1 central office codes in the 1930's. Some places even had some NN0
central office codes at that time, although this was highly
discouraged.]
Bassett's proposal would have doubled the number of available local
codes in an NPA. When he presented his plan to '195' [AT&T's
Headquarters office building was at 195 Broadway in New York City],
Mr. C.G. Miller of Bell Laboratories objected because the dial offices
in Chicago did not have a capacity of 8 digits. The proposal was
dropped. I cite this incident in support of Mr. R.W. Ferguson's
viewpoint that sometimes a small insignificant decision, made in a
casual way, has a far-reaching effect. If Bassett's idea had been
adopted, we would never have had to make the big study in
1958-9. However, in 1944, there were only a few people who foresaw DDD
and the early congestion of the numbering plan.
DDD CODES:
The first mention of A1X [N1X] and A0X [N0X] type NPA codes seems to
have been made by Mr. F.F. Shipley in a memorandum dated July 28,
1944. In a second memorandum dated January 16, 1945, he grossly
underestimates the future NPA code requirements. He says, " ... it has
been estimated that 53 numbering areas are sufficient." Yet, in 1947,
86 codes were initially assigned to the NPA's in the USA and Canada.
ANC:
All Number Calling was studied by the Laboratories around 1956. Once a
large multi-office city is on an all dial basis, there is no real need
to continue with the use of Blauvelt's invention, namely, the lettered
dial. ANC makes more local codes available because it removes the
restriction of impossible names for certain of the codes. The use of
arbitrary letters would, of course, accomplish the same thing. The
opposition to ANC came chiefly from customers who had a fond
attachment to a particular name, e.g., BUtterfield-8, RIverside-9,
etc.
SER No. 48:
In 1957, Mr. W.O. Turner, in Switching Engineering Report No. 48
estimates that 130,000 central office codes would be required in the
year 2000 for the United States, Canada and Mexico. He estimated that
23,800 codes would be needed for in-dialing to PBX's. The underlying
concern in his report was that the NPA codes may be expected to
exhaust at about the year 1975. No concern regarding an early exhaust
of NNX codes was indicated, yet today it seems that NNX codes in two
NPA's will exhaust before the supply of NPA codes exhausts.
LOCAL OFFICE CODES:
When I undertook the big study in 1958, I obtained from earlier
correspondence, so-called ultimate central office code requirements
for each NPA. The following were the ten NPA's having the highest
future code requirements:
Number of Local
Office Codes as
State/ Shown in the DDD Number of Projected Ultimate
NPA Province Reference Guide Local Central Office Codes
305 FL 249 634
504 LA 155 622
519 ON 059 596
705 ON 029 586
312 IL 319 575
404 GA 126 570
617 MA 357 555
813 FL 073 547
217 IL 048 530
314 MO 209 525
Note the gross underestimate for the 212 and 213 NPA's which are missing
from the above list.
CAMA SENDERS:
In the mid 1950's, the CAMA senders were being developed for crossbar
tandem. Mr. M.E. Maloney got quick acceptance from someone in '195'
Traffic that the CAMA senders can be limited to 10-digits. Actually,
Maloney asked if the Laboratories could ignore the stations delay
digit, the 8th-digit in areas where manual numbers exceeding 10,000
still existed or where party line letters were used. He should have
asked: will we ever need 11-digits? By 1958, that "innocuous"
decision helped to keep us from studying an 8-digit local code
plan. [However, there were situations in the 1960's and 70's where
eleven-digits total were MF keypulsed between switches in the network,
or MF keypulsed by operators, including: routing of calls to Mexico
(using 180+ the eight-digit Mexican national number), and routing of
certain TWX calls (conversions from 4-row terminal to 3-row terminal
connections, where a leading digit '0' is internally keyed to the
ten-digit "3-row" TWX number NPA-NNX-XXXX.)]
"112" PREFIX:
CAMA in step-by-step areas was made feasible with the "112"
prefix. Thus, on a ten-digit call, the customer dialed 13 digits,
three of which were needed to route to the CAMA tandem. There was
considerable public criticism of this arrangement. The development of
a double headed trunk made it possible to dial CAMA calls in
step-by-step areas by means of the single digit prefix "1+". [i.e., if
the second digit following the initial '1' is another '1', the call
would have routed to the 11X miscellaneous or service codes, while if
the second digit were an 'N' i.e. a '2' through '9', it would indicate
the first digit of an NPA code. However, many SxS areas began to
reconfigure their local switch routings to adopt the traditionally
'common-control' style N11 service codes, thus eliminating the use of
all 11X form service codes, and able to adopt the 1+ toll prefix
without having to use this 'double-headed' trunking method.]
PLANS STUDIED:
Each of the four plans studied in 1958-9 assumed 7-digit local
numbering. No serious consideration was given to any plan involving
more digits. In retrospect, this indicates that there was a lack of
adequate information that the local office codes in New York City and
Los Angeles would exhaust before the supply of NPA codes ran out.
One of the plans studied provided for 4-digit NPA codes, but did not
provide for local codes beyond the present limit of 640. The other
three plans assumed a common supply of 792 codes, used interchangeably
as NPA or as local office codes. They differed amongst themselves with
respect to the prefix to be used when a code is used as an NPA
code. As long as the number of NPA codes plus the number of local
office codes in the fullest NPA do not exceed 792 codes, there is no
need for a prefix or indicator to clear up the ambiguity created when
an interchangeable code is dialed. Timing after the 7th-digit is one
way to resolve the ambiguity. The operation of an end-of-dialing
button ['#'] is another.
1+ DIALING:
The simplest possible prefix is the digit "1". Since it dovetailed
into the replacement of the "112" Prefix in step-by-step areas by the
Prefix "1", "1+" dialing became the plan accepted in 1959. Basically,
"1+" dialing should be used only on ten-digit calls, and not on
7-digit calls. However, the use of "1+" dialing in step-by-step CAMA
areas is for the purpose of gaining access to the toll billing
equipment, and, in common-control areas, to reduce toll-billing errors
- not to discriminate between ten-digit and 7-digit calls. Hence, the
"1+" dialing plan has an anomaly that can only be resolved, when
interchangeable codes come into being, by timing after the seventh
digit to await an eighth digit. This is perhaps the weakest spot in
the "1+" dialing plan, namely, the use of "1+" on 7-digit toll calls,
with timing delaying the completion of such calls. We have looked at
this problem as one that will eventually go away when local charging
areas are expanded to include a whole NPA.
[To this day, the debate continues of what the '1+' *really* means,
and how/when/where/if it should/could/must/might be used; i.e. whether
"1+" is permitted or required.]
0+ DIALING:
The "1+" dialing plan assumed "10+" dialing for person-to-person,
etc. type calls. This was changed to "0+" dialing by a management
decision. To discriminate between "Dial-0" calls to Operators and "0+"
calls, timing is employed after [dialing] the "0". This delays
completion of calls to the assistance operator.
CONTEMPORARY PERIOD:
This concludes the narrative, bringing it to the period with which you
are familiar, namely, the decisions and events concerning emergency
codes [which was originally planned to be "00" to reach the Operator,
so as to avoid delaying for a time-out if a single "0" had been
dialed; however, 911 to reach an "emergency center" was later
implemented], international access codes [011/01+], the directory
assistance code [use of "555-1212" in each and every NPA, for customer
dialing to a distant city's directory information operator/center],
and the unblocking of the 'E' digit for dialing into Mexico City.
[Mexico City had numbers also of the form N1X-xxxx and N0X-xxxx in the
early 1970's, when 90-5+ was proposed for customer DDD calling to
there, well before Los Angeles (213) and New York City (212) had such
local codes in the later 1970's. All Common-control toll switches, as
well as such local #5XB, #1XB, etc. switches, had to allow a '1' or
'0' in the 'E' or fifth position dialed by a customer for dialing to
90-5 Mexico City. i.e. The fifth position of: 905-N0X or
905-N1X. Unblocking of customer access to a 'E' digit of '0' or '1'
also meant that certain internal and operator dialed 0XX/1XX codes had
to be modified, as there could have been a code conflict --
i.e. Kp+312+115X+St had indicated a "115X" leave-word operator in the
Chicago (312) area code, as there would have been no local central
office codes of the N0X/N1X format. However, when a local area started
to have such N0X/N1X central office codes, 312-115X could be a local
seven-digit number. So, all 'leave-word' operator codes had an extra
"dummy digit of '1'" tacked onto the end -- 115X-1. So, Kp+312-115X+St
would indicate a local seven-digit number for the operator to dial,
while Kp+312+115X-1+St would indicate a "115X-1" leave-word operator
in Chicago to be reached.]
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