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Pulse dialing (or dialling in British English) is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. This lends the method the often used name loop disconnect dialing. In the most common variant of pulse dialing, decadic dialing, each of the ten arabic numerals are encoded in a sequence of up to ten pulses. The most common version decodes the digits 1 through 9, as one to nine pulses, respectively, and the digit 0 as ten pulses. Historically, the most common device to produce such pulse trains is the rotary dial of the telephone, lending the technology another name, rotary dialing.
The pulse repetition rate was historically determined based on the response time needed for electromechanical switching systems to operate reliably. Most telephone systems used the nominal rate of ten pulses per second, but operator dialing within and between central offices often used pulse rates up to twenty per second.
Early automatic exchanges
Automatic telephone exchange systems were developed in the late 19th and early 20th century. For identification, telephone subscribers were assigned a telephone number unique to each circuit. Various methods evolved to signal the desired destination telephone number for a telephone call directly dialed by the subscriber. An automatic switch-hook was designed by Hilborne Roosevelt.
The first commercial automatic telephone exchange, designed by Almon Brown Strowger, opened in La Porte, Indiana on 3 November 1892, and used two telegraph-type keys on the telephone, which had to be operated the correct number of times to control the vertical and horizontal relay magnets in the exchange. But the use of separate keys with separate conductors to the exchange was not practical. The most common signaling system became a system of using direct-current pulse trains generated in the telephone sets of subscribers by interrupting the single-pair wire loop of the telephone circuit.
Strowger also filed the first patent for a rotary dial in 1891. The first dials worked by direct, forward action. The pulses were sent as the user rotated the dial to the finger stop starting at a different position for each digit transmitted. Operating the dial error-free required smooth rotary motion of the finger wheel by the user, but was found as too unreliable. This mechanism was soon refined to include a recoil spring and a centrifugal governor to control the recoil speed. The user selected a digit to be dialed by inserting a finger into the corresponding hole and rotated the dial to the finger stop. When released from this position, the dial pulsing contacts were opened and closed repeatedly, thus interrupting the loop current in a pattern on the return to the home position. The exchange switch decoded the pattern for each digit thus transmitted by stepping relays or by accumulation in digit registers.
Pulse rate and coding
When electromechanical switching system were still in use, the current pulses generated by the rotary dial on the local loop operated electrical relays in the switches at the central office. The mechanical nature of these relays and the loop capacitance, affecting pulse shape, generally limited the speed of operation, the pulsing rate, to ten pulses per second.
The specifications of the Bell System in the US required service personnel to adjust dials in customer stations to a precision of 9.5 to 10.5 pulses per second (pps), but the tolerance of the switching equipment was generally between 8 and 11 pps. The British (BPO, later Post Office Telecommunications) standard for Strowger exchanges was 10 impulses per second (allowable range 7 to 12) and a 66% break ratio (allowable range 63% to 72%)
In most countries one pulse is used for the digit 1, two pulses for 2, and so on, with ten pulses for the digit 0; this makes the code unary, excepting the digit 0. Exceptions to this are: Sweden (example dial), with one click for 0, two clicks for 1, and so on; and New Zealand with ten clicks for 0, nine clicks for 1, etc. Oslo, the capital city of Norway, used the New Zealand system, but the rest of the country did not. Systems that used this encoding of the 10 digits in a sequence of up to 10 pulses, are sometimes known as decadic dialing systems.
Some later switching systems used digit registers which doubled the allowable pulse rate to 20 pulses per second, and the inter-digital pause could be reduced as the switch selection did not have to be completed during the pause. These included some Crossbar systems, the later version (7A2) of the Rotary system, and the earlier 1970s stored program control exchanges.
In some telephones, the pulses may be heard in the receiver as clicking sounds. However, in general, such effects were undesirable and telephone designers suppressed them by mechanical means with off-normal switches on the dial, or greatly attenuated them by electrical means with a varistor connected across the receiver.
As pulse dialing is achieved by interruption of the local loop, it was in principle possible to dial a telephone number by rapidly tapping, i.e. depressing, the switch hook the corresponding number of times for each digit at approximately ten taps per second. However, many telephone makers implemented a slow switch hook release to prevent rapid switching.
In the United Kingdom, it used to be possible to make calls from coin-box phones (payphones) by tapping the switch hook without depositing coins. A person caught tapping could be charged with 'abstracting electricity' from the General Post Office and several cases were prosecuted under this offence.
In popular culture, tapping was used in the film Red Dragon as a way for prisoner Hannibal Lecter to dial out on a phone with no dialing mechanism. This method was also used by the character 'Phantom Phreak' to call 'Acid Burn' when taken to prison in the film Hackers.
It was recognized as early as the 1940s that faster dialing and more accurate dialing could be achieved with push-button systems, but the technology was too unreliable in customer trials until transistor technology transformed the industry. In 1963, the Bell System officially introduced dual-tone multi-frequency (DTMF) technology under the name Touch-Tone to the general public. Touch-Tone was a trademark in the U.S. until 1984. The Touch-Tone system used push-button telephones. In the decades following, pulse dialing was gradually phased out as the primary dialing method to the central office, but many systems still support rotary telephones today. Some models of keypad telephones have a switch for the selection of tone or pulse dialing.
Mobile telephones as well as most voice-over-IP systems use out-of-band signaling and do not send any digits until the entire number has been keyed by the user. Many VoIP systems are based on the Session Initiation Protocol (SIP) which uses a form of Uniform Resource Identifiers (URI) for addressing, instead of digits alone.
- Isa Carrington Cabell (1900). "Roosevelt, Nicholas I.". Appletons' Cyclopædia of American Biography.
- AT&T Specification No. 4566, February 1926, p.113
- J. Atkinson, Telephony Volume 1, p.142 (1948, Pitman, London)
- The Trademark Electronic Search System on the U.S. Patent and Trademark Office web site shows the trademark with serial number 72109459, registered 1962-09-04 and canceled 1984-03-13.