Editing Pulse dialing

{{Short description|Telephone address signaling protocol}}

[[Image:Model500Telephone1951.jpg|thumb|right|The [[model 500 telephone|model 500]] rotary dial telephone by [[Western Electric]] was a pulse-dialing instrument.]]

'''Pulse dialing''' is a [[Signaling (telecommunications)|signaling]] technology in [[telecommunications]] in which a direct current [[local loop]] circuit is interrupted according to a defined [[code|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 [[Pulse (signal processing)|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 train]]s 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 switch]]ing 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]].<ref>{{Cite Appletons'|wstitle=Roosevelt, Nicholas I.|year=1900|author=Isa Carrington Cabell |short=x}}</ref>

The first commercial automatic telephone exchange, designed by [[Almon Brown Strowger]], opened in [[La Porte, Indiana|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.

==Rotary dial==
The first rotary dials worked by creating current interruptions on two separate direct current circuits (three-wire system) to actuate stepping motors in a vertical and horizontal (rotational) switch arrangement. Operating the dial error-free required smooth rotary motion of the finger wheel by the user, but was found as too unreliable. Later dials after c. 1907 produced just a single pulse train on a 2-wire circuit as the user rotated the dial to the finger stop starting at a different position for each digit transmitted. This mechanism was 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==
[[File:Bakelittelefon 1947a.jpg|thumb|Swedish phone with one pulse for 0]]
The mechanical nature of the stepping switches or relays employed at the switching system 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.<ref>AT&T Specification No. 4566, February 1926, p.113</ref> The British (GPO, later [[Post Office Telecommunications]]) standard for  [[Strowger switch]] exchanges has been ten impulses per second (allowable range 7 to 12) and a 66% break ratio (allowable range 63% to 72%).<ref>J. Atkinson, ''Telephony'' Volume 1, p. 142 (1948, Pitman, London)</ref><ref>Current UK standard  [http://www.sinet.bt.com/sinet/SINs/pdf/351v4p6.pdf BT SIN 351] {{Webarchive|url=https://web.archive.org/web/20160804180952/http://www.sinet.bt.com/sinet/SINs/pdf/351v4p6.pdf |date=2016-08-04 }}</ref>

In most switching systems 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 numeral system|unary]], excepting the digit 0. Exceptions to this are Sweden, with one pulse for 0, two pulses for 1, and so on, and New Zealand, with ten pulses for 0, nine pulses 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 ten digits in a sequence of up to ten pulses are known as ''decadic dialing'' systems.

Some switching systems used digit registers that doubled the allowable pulse rate up to twenty 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 access lines to the [[panel switch]] in the 1920s, 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 with off-normal switches on the dial to exclude the receiver from the circuit, or greatly attenuated them by electrical means with a [[varistor]] connected across the receiver.

[[Image:GPO 232 Telephone without bell set 26.JPG|thumb|right|British (GPO) Type 232 telephone of 1932]]

==Successors==
It was recognized as early as the 1910s that push buttons were faster than rotary dials, when the first [[panel switch]]es used keys for dialing at the operator stations. In the 1940s, Bell Laboratories conducted field trials of pushbutton telephones for customer dialing to determine accuracy and efficiency. However, the technology of using mechanical reed relays was too unreliable until transistors transformed the industry. In 1963, the Bell System introduced to the public [[dual-tone multi-frequency]] (DTMF) technology under the name Touch-Tone, which was a trademark in the U.S. until 1984.<ref>The Trademark Electronic Search System on the [[U.S. Patent and Trademark Office]] [http://www.uspto.gov/ web site] shows the trademark with serial number 72109459, registered 1962-09-04 and cancelled 1984-03-13.</ref> The Touch-Tone system used [[push-button telephone]]s. In the decades after 1963, rotary dials were gradually phased out on new telephone models in favor of keypads and the primary dialing method to the central office became touchtone dialing. Most central office systems still support rotary telephones today. Some keypad telephones have a switch or configuration method for the selection of tone or pulse dialing.

[[Mobile telephone]]s and 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 Identifier]]s (URI) for addressing, instead of digits alone.

==Switch-hook dialing==
As pulse dialing is achieved by interruption of the [[local loop]], it was possible to dial digits by rapidly tapping, i.e. depressing, the switch hook the corresponding number of times at approximately ten taps per second.<ref>{{Cite book|url=https://bnrg.cs.berkeley.edu/~randy/Courses/CS39K.S13/anarchistcookbook2000.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://bnrg.cs.berkeley.edu/~randy/Courses/CS39K.S13/anarchistcookbook2000.pdf |archive-date=2022-10-09 |url-status=live|title=Anarchists Cookbook v2000 - BNRG|chapter=147.Phone Dial Locks -- How to Beat'em by The Jolly Roger|date=2000}}</ref> However, many telephone makers implemented a slow switch hook release to prevent rapid switching.

In the United Kingdom, it was once possible to make calls from [[payphone|coin-box phones]] by tapping the switch hook without depositing coins. Unlawfully obtaining a free telephone call was deemed a criminal offence, [[abstracting electricity]]<ref>{{Cite web |title=Abstracting electricity |work=Oxford Reference |access-date=6 October 2020 |url= https://www.oxfordreference.com/view/10.1093/oi/authority.20110803095345117}}</ref> from the [[General Post Office]], which operated the telephone system, and several cases were prosecuted.

In popular culture, tapping was shown in the film ''[[Red Dragon (2002 film)|Red Dragon]]'' when prisoner [[Hannibal Lecter]] dialed out on a telephone without dialing mechanism. The technique was also used by the character Phantom Phreak in the film ''[[Hackers (film)|Hackers]]''.

==References==
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{{Telecommunications}}

[[Category:Obsolete technologies]]
[[Category:Telephony signals]]