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105-406: In a pure time-slot interchange switch, there is only one physical input, and one physical output. Each physical connection is an opportunity for a switching fabric to fail. The limited number of connections of this switch is therefore valuable in a large switching fabric, because it makes this type of switching very reliable. The disadvantage of this type of switch is that it introduces a delay into

210-545: A CCITT standard. Similar schemes were used in the Americas and in some European countries including Spain. Digit strings between switches were often abbreviated to further improve utilization. For example, one switch might send only the last four or five digits of a telephone number . In one case, seven digit numbers were preceded by a digit 1 or 2 to differentiate between two area codes or office codes, (a two-digit-per-call savings). This improved revenue per trunk and reduced

315-446: A panel switch and a manual switchboard. Probably the most common form of communicating dialed digits between electromechanical switches was sending dial pulses , equivalent to a rotary dial 's pulsing, but sent over trunk circuits between switches. In Bell System trunks, it was common to use 20 pulse-per-second between crossbar switches and crossbar tandems. This was twice the rate of Western Electric/Bell System telephone dials. Using

420-459: A central office (C.O.) is a common carrier switching center Class 5 telephone switch in which trunks and local loops are terminated and switched. In the UK, a telephone exchange means an exchange building, and is also the name for a telephone switch. With manual service , the customer lifts the receiver off-hook and asks the operator to connect the call to a requested number. Provided that

525-495: 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

630-484: A circuit connecting a dialed call through an electromechanical switch had DC continuity within the local exchange area via metallic conductors. The design and maintenance procedures of all systems involved methods to avoid that subscribers experienced undue changes in the quality of the service or that they noticed failures. A variety of tools referred to as make-busy s were plugged into electromechanical switch elements upon failure and during repairs. A make-busy identified

735-412: A digit receiver (part of an element called an Originating Register ) would be connected to a call just long enough to collect the subscriber's dialed digits. Crossbar architecture was more flexible than step offices. Later crossbar systems had punch-card-based trouble reporting systems. By the 1970s, automatic number identification had been retrofitted to nearly all step-by-step and crossbar switches in

840-517: A failed switch element. A trouble reporting card system was connected to switch common control elements. These trouble reporting systems punctured cardboard cards with a code that logged the nature of a failure. Electromechanical switching systems required sources of electricity in form of direct current (DC), as well as alternating ring current (AC), which were generated on-site with mechanical generators. In addition, telephone switches required adjustment of many mechanical parts. Unlike modern switches,

945-446: A formal publication of network documentation, specifications, and recommendations to the telephone industry, entitled Notes on Nationwide Dialing . 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

1050-404: A higher-level system design of the switching fabric. Hence the technology yielding T determines n for a given t. T also limits t for a given n. Real switching fabrics have real requirements for n and t, and therefore since T must be an actual number set by a possible technology, real switches cannot be arbitrarily large n or small t. In higher-speed switches, the limit from T can be halved by using

1155-404: A horizontal panel containing two rows of patch cords, each pair connected to a cord circuit . When a calling party lifted the receiver, the local loop current lit a signal lamp near the jack. The operator responded by inserting the rear cord ( answering cord ) into the subscriber's jack and switched their headset into the circuit to ask, "Number, please?" For a local call, the operator inserted

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1260-411: A hundred pair cable between switches, for example. Conductors in one common circuit configuration were named tip, ring, ear (E) and mouth (M). Tip and ring were the voice-carrying pair, and named after the tip and ring on the three conductor cords on the manual operator's console. In two-way trunks with E and M signaling , a handshake took place to prevent both switches from colliding by dialing calls on

1365-485: A lamp on the operator's switchboard, signaling the operator to perform service. In the largest cities, it took many years to convert every office to automatic equipment, such as a panel switch . During this transition period, once numbers were standardized to the 2L-4N or 2L-5N format (two-letter exchange name and either four or five digits), it was possible to dial a number located in a manual exchange and be connected without requesting operator assistance. The policy of

1470-399: A manual office, having listings such as Hillside 834 or East 23, was recognizable by the format in which the second letter was not capitalized. Rural areas, as well as the smallest towns, had manual service and signaling was accomplished with magneto telephones, which had a crank for the signaling generator. To alert the operator, or another subscriber on the same line, the subscriber turned

1575-426: A more expensive, less reliable two-port RAM. In these designs, the read and write usually occur at the same time. The switch must still arbitrate when there is an attempt to read and write a RAM slot at the same time. This is normally done by avoiding the case in the control software, by rearranging the connections in the switching fabric. (E.g. see Nonblocking minimal spanning switch ) In packet-switching networks,

1680-407: 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

1785-810: 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. 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

1890-407: A private telephone exchange is termed a private branch exchange (PBX), which connects to the public switched telephone network. A PBX serves an organization's telephones and any private leased line circuits, typically situated in large office spaces or organizational campuses. Smaller setups might use a PBX or key telephone system managed by a receptionist, catering to the telecommunication needs of

1995-436: A single line. When calling a party, the operator used code ringing, a distinctive ringing signal sequence, such as two long rings followed by one short ring. Everyone on the line could hear the signals, and could pick up and monitor other people's conversations. Automatic exchanges , which provided dial service , were invented by Almon Strowger in 1888. First used commercially in 1892, they did not gain widespread use until

2100-526: A telephone with a dial tone . Telecommunication carriers also define rate centers for business and billing purposes, which in large cities, might encompass clusters of central offices to specify geographic locations for distance measurement calculations. In the 1940s, the Bell System in the United States and Canada introduced a nationwide numbering system that identified central offices with

2205-453: A time-slot interchange switch is often combined with two space-division switches to implement small network switches. In telephone switches, time-slot interchange switches usually form the outer layer of the switching fabric at a central office's switch. They take data from time-multiplexed T-1 or E-1 lines that serve neighborhoods. The T-1 or E-1 lines serve the subscriber line interface cards (SLICs) in local neighborhoods. The SLICs serve as

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2310-466: A trunk as idle. Trunk circuitry hearing a 2,600 Hz tone for a certain duration would go idle. (The duration requirement reduced falsing .) Some systems used tone frequencies over 3,000 Hz, particularly on SSB frequency-division multiplex microwave radio relays . On T-carrier binary digital transmission systems, bits within the T-1 data stream were used to transmit supervision. By careful design,

2415-476: A unique three-digit code, along with a three-digit numbering plan area code (NPA code or area code), making central office codes distinctive within each numbering plan area. These codes served as prefixes in subscriber telephone numbers. The mid-20th century saw similar organizational efforts in telephone networks globally, propelled by the advent of international and transoceanic telephone trunks and direct customer dialing. For corporate or enterprise applications,

2520-522: 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. By 1945, initial concepts for a nationwide numbering plan anticipated a division of the continent into between fifty and seventy-five numbering plan areas. 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

2625-579: A wide range of advanced services. Local versions were called ARE11 while tandem versions were known as ARE13. They were used in Scandinavia, Australia, Ireland and many other countries in the late 1970s and into the 1980s when they were replaced with digital technology. Original North American area codes The original North American area codes were established by the American Telephone and Telegraph Company (AT&T) in 1947, after

2730-402: A year later. In 1887 Puskás introduced the multiplex switchboard . . Later exchanges consisted of one to several hundred plug boards staffed by switchboard operators . Each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve (3-conductor) jacks, each of which was the local termination of a subscriber 's telephone line . In front of the jack panel lay

2835-413: Is shown in the following tabulation. This method of arrangement, which is known to have been in use at Bell Laboratories, 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

2940-452: Is usually a location for the desired output time-slot. The current value of the counter also selects the RAM data to forward in the current output time slot. Then the counter is incremented to the next value. The switch repeats the algorithm , eventually sending data from any input time-slot to any output time-slot. To minimize connections, and therefore improve reliability, the data to reprogram

3045-468: The Bell System stated that customers in large cities should not need to be concerned with the type of office, whether they were calling a manual or an automatic office. When a subscriber dialed the number of a manual station, an operator at the destination office answered the call after seeing the number on an indicator , and connected the call by plugging a cord into the outgoing circuit and ringing

3150-737: The Bell Telephone Company in Boston in 1877. The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to Berlin under the direction of Heinrich von Stephan . George W. Coy designed and built the first commercial US telephone exchange which opened in New Haven, Connecticut in January, 1878, and the first telephone booth was built in nearby Bridgeport . The switchboard

3255-665: 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

Time-slot interchange - Misplaced Pages Continue

3360-470: The Western Electric 1ESS switch , Northern Telecom SP1 , Ericsson AXE, Automatic Electric EAX-1 & EAX-2, Philips PRX /A, ITT Metaconta, British GPO/BT TXE series and several other designs were similar. Ericsson also developed a fully computerized version of their ARF crossbar exchange called ARE. These used a crossbar switching matrix with a fully computerized control system and provided

3465-546: The crossbar switch . Circuits interconnecting switches are called trunks . Before Signalling System 7 , Bell System electromechanical switches in the United States originally communicated with one another over trunks using a variety of DC voltages and signaling tones. Today, those simple digital signals have been replaced by more modern coded digital signals (typically using binary code). Some signaling communicated dialed digits. An early form called Panel Call Indicator Pulsing used quaternary pulses to set up calls between

3570-565: 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 . 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's switching station , smaller digits were quicker to dial. This makes

3675-415: The telephone when the user removes the handset from the switchhook or cradle. The exchange provides dial tone at that time to indicate to the user that the exchange is ready to receive dialed digits. The pulses or DTMF tones generated by the telephone are processed and a connection is established to the destination telephone within the same exchange or to another distant exchange. The exchange maintains

3780-409: The Bell System required continuous maintenance, such as cleaning. Indicator lights on equipment bays alerted staff to conditions such as blown fuses (usually white lamps) or a permanent signal (stuck off-hook condition, usually green indicators). Step offices were more susceptible to single-point failures than newer technologies. Crossbar offices used more shared, common control circuits. For example,

3885-414: The Bell System. Electronic switching systems gradually evolved in stages from electromechanical hybrids with stored program control to the fully digital systems. Early systems used reed relay -switched metallic paths under digital control. Equipment testing, phone numbers reassignments, circuit lockouts and similar tasks were accomplished by data entry on a terminal. Examples of these systems included

3990-652: The NPAs that hosted the Regional Centers for toll-switching established in the General Toll Switching Plan of 1929: 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

4095-573: 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. On average, it took about two minutes for any long-distance call to be completed to its destination. As foreseen and stated in 1949,

4200-474: The Strowger's typical 10 pps—typically about 20 pps. At a later date many also accepted DTMF "touch tones" or other tone signaling systems. A transitional technology (from pulse to DTMF) had converters to convert DTMF to pulse, to feed to older Strowger, panel, or crossbar switches. This technology was used as late as mid-2002. Many terms used in telecommunication technology differ in meaning and usage among

4305-404: The United States , which included Alaska , and Hawaii, did not receive area codes at first, nor did the territories of Canada or Newfoundland and Labrador, which was a British dominion at the time. 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

Time-slot interchange - Misplaced Pages Continue

4410-471: The appropriated bits did not change voice quality appreciably. Robbed bits were translated to changes in contact states (opens and closures) by electronics in the channel bank hardware. This allowed direct current E and M signaling, or dial pulses, to be sent between electromechanical switches over a pure digital carrier which did not have DC continuity. Bell System installations typically had alarm bells, gongs, or chimes to announce alarms calling attention to

4515-407: The automation of telephone circuit switching. While there were many extensions and adaptations of this initial patent, the one best known consists of 10 levels or banks, each having 10 contacts arranged in a semicircle. When used with a rotary telephone dial , each pair of digits caused the shaft of the central contact "hand" of the stepping switch to first step (ratchet) up one level for each pulse in

4620-445: 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: Based on the precedent and experience with the large-city dial systems in the nation, the designers decided to direct all telephone companies in

4725-474: The call only if intermediate trunk lines were available between all the centers at the same time. In 1943 when military calls had priority, a cross-country US call might take as long as 2 hours to request and schedule in cities that used manual switchboards for toll calls. On March 10, 1891, Almon Brown Strowger , an undertaker in Kansas City, Missouri , patented the stepping switch , a device which led to

4830-474: The centers. Consideration of several assignment patterns resulted in the configuration that was publicized as a map in October 1947. 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, even when located in the same jurisdiction. This criterion

4935-437: The circuit, allowing them to handle another call, while the caller heard an audible ringback signal, so that that operator would not have to periodically report that they were continuing to ring the line. In the ringdown method, the originating operator called another intermediate operator who would call the called subscriber, or passed it on to another intermediate operator. This chain of intermediate operators could complete

5040-439: 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

5145-419: The connection until one of the parties hangs up. This monitoring of connection status is called supervision. Additional features, such as billing equipment, may also be incorporated into the exchange. The Bell System dial service implemented a feature called automatic number identification (ANI) which facilitated services like automated billing, toll-free 800-numbers , and 9-1-1 service. In manual service,

5250-472: 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 . After the success of these trials, expansion of the numbering plan accelerated with new crossbar systems and four new area codes in 1953, and seven in 1954. By

5355-581: The crank to generate ringing current. The switchboard responded by interrupting the circuit, which dropped a metal tab above the subscriber's line jack and sounded a buzzer. Dry cell batteries, normally two large N°. 6 cells in the subscriber's telephone, provided the direct current for the transmitter. Such magneto systems were in use in the US as late as 1983, as in the small town, Bryant Pond, Woodstock, Maine . Many small town magneto systems featured party lines , anywhere from two to ten or more subscribers sharing

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5460-665: 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. It had

5565-468: The destination station. For example, if a dial customer calling from TAylor 4725 dialed a number served by a manual exchange, e.g., ADams 1383-W, the call was completed, from the subscriber's perspective, exactly as a call to LEnnox 5813, in an automated exchange. The party line letters W, R, J, and M were only used in manual exchanges with jack-per-line party lines. In contrast to the listing format MAin 1234 for an automated office with two capital letters,

5670-501: 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, which added new technology to

5775-406: 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. 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

5880-406: The end of the decade thirty-one new area codes had been created in addition to the initial allotment of 1947. 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

5985-473: The enterprise. In the era of the electrical telegraph, its principal users were post offices, railway stations, the more important governmental centers (ministries), stock exchanges, very few nationally distributed newspapers, the largest internationally important corporations, and wealthy individuals. Despite the fact that telephone devices existed before the invention of the telephone exchange, their success and economical operation would have been impossible on

6090-549: 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

6195-532: 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. The original configuration of the North American Numbering Plan assigned eighty-six area codes in October 1947, one each to every numbering plan area. The territories of

6300-493: The faster pulsing rate made trunk utilization more efficient because the switch spent half as long listening to digits. DTMF was not used for trunk signaling. Multi-frequency (MF) was the last of the pre-computerized methods. It used a different set of tones sent in pairs like DTMF. Dialing was preceded by a special keypulse (KP) signal and followed by a start (ST). Variations of the Bell System MF tone scheme became

6405-484: 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. In this table, the assignments of the nine area codes to the Canadian provinces are highlighted by a blue background. The red fields are

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6510-921: The first cut-over into commercial service of a No. 4 Crossbar toll switch in 1943 for regional toll service. 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, 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. N10 numbers became teletypewriter exchanges , and N11 were used for special services, such as information and emergency services. 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

6615-435: The first decade of the 20th century. They eliminated the need for human switchboard operators who completed the connections required for a telephone call . Automation replaced human operators with electromechanical systems, and telephones were equipped with a dial by which a caller transmitted the destination telephone number to the automatic switching system. A telephone exchange automatically senses an off-hook condition of

6720-424: The first digit and then to swing horizontally in a contact row with one small rotation for each pulse in the next digit. Later stepping switches were arranged in banks, the first stage of which was a linefinder . If one of up to a hundred subscriber lines (two hundred lines in later linefinders) had the receiver lifted "off hook", a linefinder connected the subscriber's line to a free first selector, which returned

6825-399: The front cord of the pair ( ringing cord ) into the called party's local jack and started the ringing cycle. For a long-distance call, the operator plugged into a trunk circuit to connect to another operator in another bank of boards or at a remote central office. In 1918, the average time to complete the connection for a long-distance call was 15 minutes. Early manual switchboards required

6930-404: The impetus for the creation of a new industrial sector. As with the invention of the telephone itself, the honour of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange was Hungarian Tivadar Puskás in 1877 while he was working for Thomas Edison . The first experimental telephone exchange was based on the ideas of Puskás, and it was built by

7035-541: 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, 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

7140-674: 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, and proceeded with periodical communications to the general telecommunication industry via the Dial Interexchange Committee of the United States Independent Telephone Association (USITA), which disseminated

7245-408: The latter term originating from the Bell System . A central office typically refers to a facility that houses the inside plant equipment for one or several telephone exchanges, each catering to a specific geographical region. This region is sometimes known as the exchange area. In North America, the term wire center may be used to denote a central office location, indicating a facility that provides

7350-459: 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

7455-514: 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, automated

7560-634: 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, and in Philadelphia (215), which had been chosen for

7665-477: 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

7770-431: The new configuration instantly to all the connected ICs. In a time-slot interchange (TSI) switch, two memory accesses are required for each connection (one to read and one to store). Let T be the time to access the memory. Therefore, for a connection, 2T time will be taken to access the memory. If there are n connections and t is the operation time for n lines, then t=2nT which gives n=t/2T t and n normally come from

7875-399: The number is in the same central office, and located on the operator's switchboard, the operator connects the call by plugging the ringing cord into the jack corresponding to the called customer's line. If the called party's line is on a different switchboard in the same office, or in a different central office, the operator plugs into the trunk for the destination switchboard or office and asks

7980-476: The number of digit receivers needed in a switch. Every task in electromechanical switches was done in big metallic pieces of hardware. Every fractional second cut off of call set up time meant fewer racks of equipment to handle call traffic. Examples of signals communicating supervision or call progress include E and M signaling , SF signaling, and robbed-bit signaling. In physical (not carrier) E and M trunk circuits, trunks were four wire. Fifty trunks would require

8085-432: The operator answering (known as the "B" operator) to connect the call. Most urban exchanges provided common-battery service, meaning that the central office provided power to the subscriber telephone circuits for operation of the transmitter, as well as for automatic signaling with rotary dials . In common-battery systems, the pair of wires from a subscriber's telephone to the exchange carry 48V (nominal) DC potential from

8190-543: The operator knows where a call is originating by the light on the switchboard jack field. Before ANI, long-distance calls were placed into an operator queue and the operator asked the calling party's number and recorded it on a paper toll ticket. Early exchanges were electromechanical systems using motors, shaft drives, rotating switches and relays . Some types of automatic exchanges were the Strowger switch or step-by-step switch, All Relay, panel switch , Rotary system and

8295-408: The operator to operate listening keys and ringing keys, but by the late 1910s and 1920s, advances in switchboard technology led to features which allowed the call to be automatically answered immediately as the operator inserted the answering cord, and ringing would automatically begin as soon as the operator inserted the ringing cord into the called party's jack. The operator would be disconnected from

8400-594: The outer space-division switches of a modern wired telephone system. Telephone switch 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. The terminology used in telecommunications has evolved over time, with telephone exchange and central office often used interchangeably,

8505-427: The part being worked on as in-use, causing the switching logic to route around it. A similar tool was called a TD tool. Delinquent subscribers had their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's office equipment on Crossbar systems or line group in step-by-step switches. The subscriber could receive calls but could not dial out. Strowger-based, step-by-step offices in

8610-462: The previous method of ringdown forwarding between intermediate operators. 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. Such channels were incapable of transmitting

8715-491: 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. 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

8820-583: 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. A fundamental requirement for the success of automated toll dialing was a new telephone numbering plan, which became known as

8925-679: The same schema and structure of the contemporary telegraph, as prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and communicated with only a single other telephone (such as from an individual's home to the person's business ). A telephone exchange is a telephone system for a small geographic area that provides the switching (interconnection) of subscriber lines for calls made between them. Telephone exchanges replaced small telephone systems that connected its users with direct lines between each and every subscriber station. Exchanges made telephony an available and comfortable technology for everyday use and it gave

9030-409: 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

9135-414: The same trunk at the same time. By changing the state of these leads from ground to −48 volts, the switches stepped through a handshake protocol. Using DC voltage changes, the local switch would send a signal to get ready for a call and the remote switch would reply with an acknowledgment (a wink) to go ahead with dial pulsing. This was done with relay logic and discrete electronics. These voltage changes on

9240-593: 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. 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

9345-545: 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. 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

9450-414: The signals. When a packet (or byte, on telephone switches ) comes to the input, the switch stores the data in RAM in one sequence, and reads it out in a different sequence. Switch designs vary, but typically, a repeating counter is incremented with an internal clock . It typically wraps-around to zero. The RAM location chosen for the incoming data is taken from a small memory indexed by the counter. It

9555-457: The subscriber a dial tone to show that it was ready to receive dialled digits. The subscriber's dial pulsed at about 10 pulses per second, although the speed depended on the standard of the particular telephone administration. Exchanges based on the Strowger switch were eventually challenged by other exchange types and later by crossbar technology. These exchange designs promised faster switching and would accept inter-switch pulses faster than

9660-545: The subscriber telephone number. 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,

9765-457: The switch is usually programmed via a single wire that threads through the entire group of integrated circuits in a printed circuit board. The software typically compares the data shifted-in with the data shifted-out, to verify that the ICs remain correctly connected. The switching data entered into the ICs is double-buffered. That is, a new switch set-up is shifted-in, and then a single pulse applies

9870-481: The system beyond the United States and Canada. 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

9975-502: 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. In 1950, southwest Missouri, with

10080-400: The telephone company end across the conductors. The telephone presents an open circuit when it is on-hook or idle. When a subscriber's phone is off-hook, it presents an electrical resistance across the line which causes current to flow through the telephone and wires to the central office. In a manually operated switchboard, this current flowed through a relay coil, and actuated a buzzer or

10185-434: The trunk circuit would cause pops or clicks that were audible to the subscriber as the electrical handshaking stepped through its protocol. Another handshake, to start timing for billing purposes, caused a second set of clunks when the called party answered. A second common form of signaling for supervision was called single-frequency or SF signaling . The most common form of this used a steady 2,600 Hz tone to identify

10290-451: The various English speaking regions. For the purpose of this article the following definitions are made: A central office originally was a primary exchange in a city with other exchanges service parts of the area. The term became to mean any switching system including its facilities and operators. It is also used generally for the building that houses switching and related inside plant equipment. In United States telecommunication jargon,

10395-602: 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 , as well as thousands of minor interchange points and tributaries. Calls were forwarded manually between stations by long-distance operators who used

10500-593: Was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations. Charles Glidden is also credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878. In Europe other early telephone exchanges were based in London and Manchester , both of which opened under Bell patents in 1879. Belgium had its first International Bell exchange (in Antwerp )

10605-415: Was capable of handling the dialing of as many as eleven digits by subscribers. 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. On November 10, 1951, the system commenced a customer trial of direct distance dialing (DDD) from this single location in

10710-535: Was expected to be approximately five hundred, because acceptable names for central offices had to be selected carefully to avoid miscommunication. 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

10815-532: 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). Early in the 20th century, the American and Canadian telephone industry had established criteria and circuits for sending telephone calls across

10920-556: 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. The pattern of this assignment of area codes

11025-523: 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

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