The original North American area codes were established by the American Telephone and Telegraph Company (AT&T) in 1947, after the demonstration of regional Operator Toll Dialing during the World War II period. The program had the goal of speeding the connecting times for long-distance calling by eliminating intermediary telephone operators. Expanding this technology for national use required a comprehensive and universal, continent-wide telephone numbering plan.
The new numbering plan established a uniform destination addressing and call routing system for all telephone networks in North America which had become an essential public service.[1] It had the eventual benefit of direct distance dialing (DDD) by telephone subscribers, a feature by which a caller may, without operator assistance, call any other user outside the local calling area (typically requiring extra digits to be dialed).
The initial Nationwide Numbering Plan of 1947 established eighty-six numbering plan areas (NPAs) that conformed mainly to existing U.S. state and Canadian provincial boundaries, though fifteen states and provinces were subdivided further. Forty NPAs were mapped to entire states or provinces. Each NPA was identified by a three-digit area code used as a prefix to each local telephone number. The United States received seventy-seven area codes, and Canada nine. The initial system of numbering plan areas and area codes was expanded rapidly during the ensuing decades, and established the North American Numbering Plan (NANP).
History
Early in the 20th century, the American and Canadian telephone industry had established criteria and circuits for sending telephone calls across the vast number of local telephone networks on the continent to permit users to call others in many remote places in both countries. By 1930, this resulted in the establishment of the General Toll Switching Plan, a systematic method, with technical specifications, for routing calls between two major classes of routing centers, Regional Centers and Primary Outlets,[2] as well as thousands of minor interchange points and tributaries. Calls were forwarded manually between stations by long-distance operators who used the method of ringdown to command remote operators to accept calls on behalf of customers. This required long call set-up times with several intermediate operators involved. When making a call, the originating party would typically have to hang up and be called back by an operator once the call was established.
The introduction of the first Western Electric No. 4 Crossbar Switching System in Philadelphia to commercial service, in August 1943,[3] automated the process of forwarding telephone calls between regional switching stations. For the Bell System this was a beginning for the ability of their long-distance operators to dial calls directly to distant telephones.[4] While automatic switching decreased the connection times from as much as fifteen minutes to approximately two minutes for calls between far-away locations, each intermediate operator still had to determine special routing codes unique to their location for each call. To make a nationwide dialing network an efficient, practical reality, a uniform nationwide numbering plan was needed so that each telephone on the continent had a unique address that could be used independently from where a call originated. Such a system is called destination routing.
With this goal, AT&T developed a new framework during the early 1940s, termed Operator Toll Dialing, which was begun by the installation of a newly developed toll switching system in Philadelphia in 1943. In 1945, the American Telephone and Telegraph Company declared this effort a major post-war project for the Bell System,[5] and proceeded with periodical communications to the general telecommunication industry via the Dial Interexchange Committee of the United States Independent Telephone Association (USITA),[6] which disseminated the project's progress to its members via industry journals and conference contributions. The planning transitioned to implementation, when Ralph Mabbs presented the results in a talk at the Fiftieth Anniversary Meeting of the Independent Telephone Association, on October 14, 1947.[7]
A fundamental requirement for the success of automated toll dialing was a new telephone numbering plan, which became known as the Nationwide Numbering Plan. This numbering plan accounted initially for seventy-seven area codes in the United States and nine in Canada. With the construction of technical infrastructure for automated toll dialing, the allocations needed to be changed in many states, adding numerous additional area codes during the next decade. By 1975, the numbering plan was known as the North American Numbering Plan, as efforts were in progress to expand the system beyond the United States and Canada.
Numbering plan requirements
Building a nationwide network in which any telephone could be dialed directly required a systematic numbering system that was easy to understand and communicate. Local telephone numbers varied greatly across the country, from two or three digits in small communities, to seven in the large cities.
By the time the Bell Laboratory engineers began efforts to involve the broader industry bodies in 1945, definite concepts had been developed for Operator Toll Dialing. A crucial requirement was the conversion of all participating telephone networks to a universal numbering plan. In 1947, Ralph Mabbs recalled the specifications for this numbering plan as follows:[7]
A distinctive telephone number for each telephone in the United States and Canada.
The minimum number of digits which will provide for growth and new services.
Minimum changes in customers' numbers.
Minimum changes in local dialing practice.
Least cost for equipment changes.
Minimum reference by operators to bulletins and route guides to gain speed of service advantages.
Provisions for operators to directly reach other operators at distant toll centers.
Telephone dial number card of c.1948 with the local telephone number 4-5876 in Atlantic City, NJ, using the central office prefix 4, later converted to AT4Face of a 1939 rotary telephone dial with the telephone number LA-2697, which includes the first two letters of Lakewood, New Jersey, as the central office prefix, later converted to LA6.
Based on the precedent and experience with the large-city dial systems in the nation, the designers decided to direct all telephone companies in the nation to standardize the local telephone networks to seven-digit local telephone numbers before they could participate with Operator Toll Dialing. This required few or no changes in the nation's largest cities, but in the smaller communities the shorter telephone numbers had to be padded with additional digits in a transparent, easily understandable manner, so that extra digits were not always needed when dialing other local subscribers. By 1955, AT&T disseminated a formal publication of network documentation, specifications, and recommendations to the telephone industry, entitled Notes on Nationwide Dialing.[7][8][9]
Central office prefixes
Most automatic dial switching systems were designed since the early 1920s to provide service for as many as ten-thousand subscriber lines. Each of these switching systems constituted a local telephone exchange, formally known as a central office. Therefore, each telephone connected to a central office had a unique four-digit line or station number.
In larger communities that required multiple central offices to account for the service need of their population, extra digits were added to the telephone number, preceding the line number. Such extra digits were dialed when calling a telephone connected to another switching system in the same city or in a nearby community, and served as routing codes to those central offices. Central office prefixes had already been used in the cities' dial systems since the 1920s, and were typically dialed by subscribers as the initial letters of the exchange name, but only the largest of cities used three digits or letters. This practice and familiarity was preserved in the initial formulation of the new numbering plan, but was standardized to a format of using two letters and one digit in the prefix, resulting in the format 2L–5N (two letters and five numerals) for the subscriber telephone number.[9]
For most cities, this conversion required the addition of extra digits or letters to the existing central office prefix. For example, the Atlantic City, New Jersey, telephone number 4-5876 was converted to AT4-5876 during the 1950s. Complete replacement of existing prefixes was necessary in the case of conflicts with another office in the state. Duplication of central office names, or an identical mapping of two different names to digits, was not uncommon. In practice, the conversion of the nation to this numbering plan took decades to accomplish and was not complete before the alphanumeric number format was abandoned during the 1960s in favor of all-number calling (ANC).
In addition to the central offices that provided the subscriber lines for each telephone (wire centers), the toll routing system included special switching facilities that routed long-distance calls between end offices. Each of these toll offices also received an assignment of a unique three-digit toll office code. To reach another operator in another central office or toll office, an operator dialed only the office code of the destination.
Numbering plan areas
By 1945, initial concepts for a nationwide numbering plan anticipated a division of the continent into between fifty and seventy-five numbering plan areas.[10] For this size of the network, a unique two-digit code for each numbering plan area (NPA) would have been sufficient. However, AT&T wanted to preserve existing dialing practices by only dialing the local number for local calls; it was therefore necessary to distinguish the NPA codes from central office codes automatically by the switching system. Central office codes already could not have the digits 0 and 1 in either of the first two positions, because no letters were mapped to those to represent exchange names. This was the opportunity for distinction, but only when 0 or 1 were used in the second position, because switching systems already suppressed single loop interruption (corresponding to 1) automatically, and 0 was used to reach an operator or long-distance desk.[9]
Therefore, numbering plan area codes, often termed just area codes, were defined to have three digits, with the middle digit being 0 or 1. Area codes with the middle digit 0 were assigned to numbering plan areas that comprised an entire state or province, while jurisdictions with multiple numbering plan areas received area codes having 1 as the second digit. This pattern lasted only until 1956, when New Jersey's "201" was divided.
The geographic layout of numbering plan areas across the North American continent was chosen primarily according to national, state, and territorial boundaries in Canada and the United States.[7] While originally considered, no numbering plan area in the United States included more than a single state, but in Canada NPA 902 comprised all three provinces of The Maritimes in the far east. The largest states, and some states with suitable call routing infrastructure were divided into smaller entities, resulting in fifteen states and provinces that were subdivided further, creating 46 NPAs. Forty NPAs were assigned to entire states or provinces.
1947 division of the North American continent into 86 numbering plan areas (boundaries approximate)[7]
The original configuration of the North American Numbering Plan assigned eighty-six area codes in October 1947, one each to every numbering plan area.
California (northern, but not including Sacramento)
Assignment plan
States with more than two NPAs
New York
212, 315, 518, 716, 914
Illinois
217, 312, 618, 815
Ohio
216, 419, 513, 614
Pennsylvania
215, 412, 717, 814
Texas
214, 512, 713, 915
California
213, 415, 916
Iowa
319, 515, 712
Michigan
313, 517, 616
States and provinces with two NPAs
Indiana, Kansas, Massachusetts,
Minnesota, Missouri, Wisconsin, Ontario, Quebec
The number of central offices that could be effectively installed with a numbering plan with two letters and one digit for the central office code was expected to be approximately five hundred, because acceptable names for central offices had to be selected carefully to avoid miscommunication.[9] States or provinces that required this many offices had to be divided into multiple smaller areas. Next to size, another important aspect was the existing infrastructure for call routing, which had developed during preceding decades independently of state or municipal boundaries. Since traffic between numbering plan areas would require special Class-4 toll switching systems, planning the divisions avoided cutting busy toll traffic routes, so that most toll traffic remained within an area, and outgoing traffic from one area would not be tributary to toll offices in an adjacent area.[11][9] However, it was recognized already in 1930 that too little granularity in the allocation pattern introduced expensive traffic back-haul requirements, conceivably resulting in congestion of the routes to the centers.[2]
Consideration of several assignment patterns resulted in the configuration that was publicized as a map in October 1947.[7] The three-digit codes were assigned to numbering plan areas in seemingly random manner, avoiding consecutive, nearly-consecutive, or just very similar codes in neighboring numbering plan areas to avoid customer confusion,[12] even when located in the same jurisdiction. This criterion was not always achieved, however. A more geography-based enumeration method had been examined earlier, but was discarded. Thus, it would not have been possible to locate the approximate geographic location of a numbering plan area by its code alone. The plan divided New York into five areas, the most of any state. Illinois, Ohio, Pennsylvania, and Texas were assigned four area codes each, and California, Iowa, and Michigan received three. Eight states and provinces were divided into two NPAs.
Rotary dialing telephones enabled customers to dial stations on their own exchange. The Bell Company allowed customers to dial direct to foreign exchanges beginning in 1951 in the US. However, switchboard operators were used for most long-distance calls until this became the norm in the 1960s and 1970s.
The pattern of this assignment of area codes is shown in the following tabulation. This method of arrangement, which is known to have been in use at Bell Laboratories,[13] shows clearly that area codes were assigned not entirely in random order, but by filling the table in diagonal manner from the top left corner, containing the low-numbered area codes, toward the center and lower right corner. Such a pattern suggests that the designers intended to maintain the same degree of randomness in digits for the remaining, yet unassigned codes. The first area code (201) was given to the entire state of New Jersey, the state with the greatest population density in the nation. Despite its density, the state was not subdivided until about a decade later. In fact, in the group of single-NPA states, having the middle digit 0, all of the low-number codes were assigned to the mid-Atlantic states around New Jersey, i.e. the District of Columbia, Connecticut, Maryland, Delaware, and Rhode Island, all states in the top of the list of states with the greatest population densities.[14]
Because telephones of this era, e.g. the Western Electric 302 desk rotary phone or the M3 354 wall telephone, were designed to send pulses or clicks to the central office'sswitching station, smaller digits were quicker to dial. This makes the fastest-dialing area code 212 (5 total clicks), followed by 312 and 213 (6 clicks). The area code 605 is much slower than these, at 21 clicks. Many large US cities generally had fast-dialing area codes with the middle digit 1. Touch-tone dialing, introduced later, eliminated the wait.
Single-NPA States and Provinces
N
N00
N01
N02
N03
N04
N05
N06
N07
N08
N09
2
†
NJ
DC
CT
MB
AL
WA
ME
ID
3
†
MD
DE
CO
WV
FL
SK
WY
4
†
RI
NE
AB
GA
OK
MT
5
†
AR
KY
OR
LA
NM
6
†
MS
AZ
NH
BC
SD
7
†
ND
NV
VA
NC
8
†
UT
VT
SC
9
†
TN
NB NS PE
Multi-NPA States and Provinces
N10
N11
N12
N13
N14
N15
N16
N17
N18
N19
†
†
NY
CA
TX
PA
OH
IL
MN
†
†
IL
MI
MO
NY
KS
IN
IA
†
†
PA
MA
WI
CA
ON
QC
OH
†
†
TX
OH
QC
IA
MI
NY
†
†
MN
ON
OH
MI
MA
IL
†
†
IA
TX
WI
NY
PA
†
†
IN
PA
IL
MO
†
†
KS
NY
TX
CA
Notes: † Special dialing codes (not permitted as area codes)–Area codes could not begin with N=0 or 1, or end in 0
In this table, the assignments of the nine area codes to the Canadian provinces are highlighted by a blue background. The red fields are the NPAs that hosted the Regional Centers for toll-switching established in the General Toll Switching Plan of 1929:[2] New York City (212), Los Angeles (213), Dallas (214), Chicago (312), St. Louis (314), and San Francisco (415) in the multi-NPA states, and in Denver (303) and Atlanta (404) in states with just a single area code each. These NPAs are clustered in the upper left corner, especially in the multiple-NPA table, but area codes 303 and 404 also fit neatly into the corresponding white positions of the N1X table (not Regional Centers), so that the Regional Center formed an almost closed block when ignoring the middle digit.
These assignments of these Regional Centers was complemented by assignments to the toll offices (in orange fields) in Detroit (313), serving the busy toll route to Toronto,[15] and in Philadelphia (215), which had been chosen for the first cut-over into commercial service of a No. 4 Crossbar toll switch in 1943 for regional toll service.[16]
The codes of the forms N00, N10, and N11, where N is 2 through 9, were not available for assignment as area codes at the time, but were reserved as special codes,[9] leaving a total of 136 possible combinations. The series N00 was used later for non-geographic numbers, starting with intrastate toll-free 800 numbers for Inward Wide Area Telephone Service (WATS) in 1965.[17]N10 numbers became teletypewriter exchanges,[18] and N11 were used for special services, such as information and emergency services.
Implementation and expansion
For several years, area codes could be used only for Operator Toll Dialing by long-distance operators on routes between toll offices equipped with trunk code translation equipment. This absent, operators still had to rely on route operators and office-specific trunk codes, or employ the previous method of ringdown forwarding between intermediate operators.[7] For entering the destination codes and telephone numbers into newly designed machine-switching equipment, long-distance operators did not use the slow rotary dials, but a ten-button key set, operating at least twice as fast, which transmitted multi-frequency (MF) tone pulses over regular voice channels to the remote switching stations.[19][20] Such channels were incapable of transmitting the direct-current pulses of a rotary dial beyond a single link. All existing toll switching offices, many still using direct-control (step-by-step) methods, had to be supplemented with components to permit MF signaling and automatic route selection.
In December 1948, AT&T advanced the new long-distance technology with the cutover of new crossbar switching systems for toll-dialing in New York and Chicago,[21] which added new technology to the No. 4 Crossbar Switching System, first installed in Philadelphia in 1943. This enabled operators to make calls directly to distant telephones without additional operators en route to some three hundred cities, and resulted in the handling of about ten percent of all Bell System long-distance calling by Operator Toll Dialing.[19] On average, it took about two minutes for any long-distance call to be completed to its destination. As foreseen and stated in 1949, the target goal for call completion, after full implementation of the system across the nation, was one minute.
As new switching systems came online and design and manufacture of other routing infrastructure proceeded, the implementation of the new numbering plan advanced and several numbering plan areas were redrawn or added during subsequent years. In 1948, northern Indiana received an extra area code (219) for its Chicago suburbs by dividing area code 317.[22] In 1950, southwest Missouri, with a new toll-center in Joplin, received area code 417, a change that provided more central offices in Kansas City. In 1951, the number of area codes grew to ninety: the State of New York gained area code 516 on Long Island, and Southern California received area code 714, to reduce the numbering plan area of Los Angeles.[23]
Area code handbook issued by many telephone companies in 1962 to promote the newly introduced direct distance dialing.
By 1951, a new Class-5 switching system, the No.5 Crossbar, had proven successful in Media, Pennsylvania, and automatic message accounting (AMA) became available for billing telephone calls. The switching system was capable of handling the dialing of as many as eleven digits by subscribers.[24] In Englewood, New Jersey, such a system was linked to a toll-class switch with the first commercial transistor circuitry that enabled the system to automatically translate area codes into toll trunk codes.[25] On November 10, 1951, the system commenced a customer trial of direct distance dialing (DDD) from this single location in the country.
From the customer dialing experience in the Englewood DDD trials, Bell Laboratories engineers gained the confidence to predict that customers would use the new numbering plan with a reasonable degree of convenience and accuracy.[26] After the success of these trials,[23][27] expansion of the numbering plan accelerated with new crossbar systems and four new area codes in 1953, and seven in 1954. By the end of the decade thirty-one new area codes had been created in addition to the initial allotment of 1947.[28] to satisfy the post-war surge in demand for telephone service.
While the first customer-dialed call using an area code was made in 1951, it took nearly fifteen years after the 1947 announcement of the new numbering plan that direct distance dialing was common in most cities. By then, some of the initial criteria for assignment, such as the 0/1 rule for single/multiple NPA assignments in a given state had to be abandoned by new requirements from population shifts and growth of communication services. In 1960, AT&T engineers, estimating that the capacity of the numbering plan would be exhausted by 1975,[28] prepared for the next major advance in the evolution of the network by eliminating central office names, and introducing all-number calling (ANC). ANC, once supplemented by interchangeable central office codes during the 1970s, increased the number of central office prefixes possible in each numbering plan area from 540 to an eventual maximum of 792.
A telephone switchboard is a device used to connect circuits of telephones to establish telephone calls between users or other switchboards. The switchboard is an essential component of a manual telephone exchange, and is operated by switchboard operators who use electrical cords or switches to establish the connections.
Direct distance dialing (DDD) is a telecommunications service feature in North America by which a caller may, without operator assistance, call any other user outside the local calling area. Direct dialing by subscribers typically requires extra digits to be dialed as prefixes to the directory telephone number of the destination. International Direct Distance Dialing (IDDD) extends the system beyond the geographic boundaries of the North American Numbering Plan (NANP).
The North American Numbering Plan is a telephone numbering plan for twenty-five regions in twenty countries, primarily in North America and the Caribbean. This group is historically known as World Zone 1 and has the telephone country code 1. Some North American countries, most notably Mexico, do not participate with the NANP.
411 is a telephone number for local directory assistance in Canada and the United States. Until the early 1980s, 411 – and the related 113 number – were free to call in most jurisdictions.
A toll-free telephone number or freephone number is a telephone number that is billed for all arriving calls. For the calling party, a call to a toll-free number from a landline is free of charge. A toll-free number is identified by a dialing prefix similar to an area code. The specific service access varies by country.
Subscriber trunk dialling (STD), also known as subscriber toll dialing, is a telephone numbering plan feature and telecommunications technology in the United Kingdom and various Commonwealth countries for the dialling of trunk calls by telephone subscribers without the assistance of switchboard operators.
A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reachable by a system of destination code routing. Telephone numbering plans are defined in each of the administrative regions of the public switched telephone network (PSTN) and in private telephone networks.
Seven-digit dialing is a telephone dialing procedure customary in some territories of the North American Numbering Plan (NANP) for dialing telephone numbers in the same numbering plan area (NPA). NANP telephone numbers consist of ten digits, of which the leading three are the area code. In seven-digit dialing it is not necessary to dial the area code. The procedure is also sometimes known as local format or network format.
In telecommunications, a long-distance call (U.S.) or trunk call is a telephone call made to a location outside a defined local calling area. Long-distance calls are typically charged a higher billing rate than local calls. The term is not necessarily synonymous with placing calls to another telephone area code.
Area codes 202 and 771 are telephone area codes in the North American Numbering Plan (NANP) for Washington, D.C.
A telephone exchange name or central office name was a distinguishing and memorable name assigned to a central office. It identified the switching system to which a telephone was connected, and facilitated the connection of telephone calls between switching systems in different localities.
Area codes 408 and 669 are telephone area codes in the North American Numbering Plan (NANP) in the U.S. state of California. The numbering plan area comprises most of Santa Clara County and Northern Santa Cruz County, and includes Gilroy, Morgan Hill, Saratoga, Los Gatos, Monte Sereno, Milpitas, Sunnyvale, Santa Clara, Cupertino, Campbell, and San Jose.
The expansion of the North American Numbering Plan (NANP) is the anticipated requirement for providing more telephone numbers to accommodate future needs beyond the pool of ten-digit telephone numbers. Ten-digit telephone numbers have been in use in the United States and Canada in long-distance telephone service since the 1950s. An October 2020 analysis estimated that the numbering plan would not be exhausted until after 2050.
The Number One Crossbar Switching System (1XB), was the primary technology for urban telephone exchanges served by the Bell System in the mid-20th century. Its switch fabric used the electromechanical crossbar switch to implement the topology of the panel switching system of the 1920s. The first No. 1 Crossbar was installed in the PResident-2 central office at Troy Avenue in Brooklyn, New York which became operational in February 1938.
Telephone numbers in Canada follow the fixed-length format of the North American Numbering Plan (NANP) of a three-digit area code, a three-digit central office code, and a four-digit station or line code. This is represented as NPA NXX XXXX.
PSTN network topology is the switching network topology of a telephone network connected to the public switched telephone network (PSTN).
A telephone exchange, also known as a telephone switch or central office, is a crucial component in the public switched telephone network (PSTN) or large enterprise telecommunications systems. It facilitates the interconnection of telephone subscriber lines or digital system virtual circuits, enabling telephone calls between subscribers.
In the North American Numbering Plan (NANP), interchangeable NPA and central office codes constituted a change in numbering plan design and policy, to mitigate exhaustion of the numbering resources of the ten-digit telephone numbers used in the closed numbering plan of the NANP.
In conventional landline telephony, a non-dialable toll point or toll station was a lone station or line serving a rural subscriber many miles from the nearest central office. As it had no home telephone exchange and therefore no local calling area, no customer could dial its number; all connections to it had to be obtained manually by the long distance operator.
Operator Toll Dialing was a telephone call routing and toll-switching system for the Bell System and the independent telephone companies in the United States and Canada that was developed in the 1940s. It automated the switching and billing of long-distance calls. The concept and technology evolved from the General Toll Switching Plan of 1929, and gained technical merits by the cutover of a new type of crossbar switching system in Philadelphia to commercial service in August 1943. This was the first system of its kind for automated forwarding of calls between toll switching centers, but it served customers only for regional toll traffic. It established initial experience with automatic toll switching for the design of a nationwide effort that was sometimes referred to as Nationwide Operator Toll Dialing.
References
↑ Hill, A.W. (1926-09-11). "Dawn of Unified Telephone Service". Telephony. 99 (11): 29. The old, and in the main, disastrous cutthroat methods formerly existing between the Bell and Independent companies are relegated to the past. The public has come to see the situation in its true light. and more and more growing is the demand for a universal telephone system, regardless of who the owner of the particular unit serving their section may be.
↑ "Bell System distributes Notes on Nationwide Dialing". Telephony. 148 (6): 55. 1955-02-05.
1 2 3 4 5 6 'Notes on Nationwide Dialing(PDF) (Report). Department of Operation and Engineering, AT&T. 1955. Archived(PDF) from the original on 2014-10-14. Retrieved 2022-04-28– via Exploding the Phone.
↑ Erickson, Andrew (March 2021). "New In Town". American University Magazine. Retrieved January 7, 2022. (NANPA] relies on a tried-and-true methodology that considers factors like customer confusion.
↑ Korn, F.A.; Ferguson, James G. (1950). "Number 5 Crossbar Dial Telephone Switching System". AIEE Transactions. 69 (1): 244–254. doi:10.1109/T-AIEE.1950.5060147. S2CID51637587. The Number 5 Crossbar System ... can be used for as few as four digits in a subscriber number, or as many as 11 digits may be used to code the called subscriber completely. (abstract)
↑ P. Mallery, Transistors and Their Circuits in the 4A Toll Crossbar Switching System, AIEE Transactions, September 1953, p.388
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