The word telephone, from the Greek roots tēle, “far,” and phonē, “sound,” was applied as early as the late 17th century to the string telephone familiar to children, and it was later used to refer to the megaphone and the speaking tube, but in modern usage it refers solely to electrical devices derived from the inventions of Alexander Graham Bell and others. Within 20 years of the 1876 Bell patent, the telephone instrument, as modified by Thomas Watson, Emil Berliner, Thomas Edison, and others, acquired a functional design that has not changed fundamentally in more than a century. Since the invention of the transistor in 1947, metal wiring and other heavy hardware have been replaced by lightweight and compact microcircuitry. Advances in electronics have improved the performance of the basic design, and they also have allowed the introduction of a number of “smart” features such as automatic redialing, call-number identification, wireless transmission, and visual data display. Such advances supplement, but do not replace, the basic telephone design. That design is described in this section, as is the remarkable history of the telephone’s development, from the earliest experimental devices to the modern digital instrument.
Alexander Graham Bell demonstrating the ability of the telephone to transmit sound by electricity from Salem to Boston, Mass., 1887.Library of Congress, Washington, D.C.
Working components of the telephone
As it has since its early years, the telephone instrument is made up of the following functional components: a power source, a switch hook, a dialer, a ringer, a transmitter, a receiver, and an anti-sidetone circuit. These components are described in turn below.
Power source
In the first experimental telephones the electric current that powered the telephone circuit was generated at the transmitter, by means of an electromagnet activated by the speaker’s voice. Such a system could not generate enough voltage to produce audible speech in distant receivers, so every transmitter since Bell’s patented design has operated on a direct current supplied by an independent power source. The first sources were batteries located in the telephone instruments themselves, but since the 1890s current has been generated at the local switching office. The current is supplied through a two-wire circuit called the local loop. The standard voltage is 48 volts.
actor portraying Alexander Graham BellAn actor portraying Alexander Graham Bell in a short film, 1930.The Bettmann Archive
Cordless telephones represent a return to individual power sources in that their low-wattage radio transmitters are powered by a small (e.g., 3.6-volt) battery located in the portable handset. When the telephone is not in use, the battery is recharged through contacts with the base unit. The base unit is powered by a transformer connection to a standard electric outlet.
Switch hook
The switch hook connects the telephone instrument to the direct current supplied through the local loop. In early telephones the receiver was hung on a hook that operated the switch by opening and closing a metal contact. This system is still common, though the hook has been replaced by a cradle to hold the combined handset, enclosing both receiver and transmitter. In some modern electronic instruments, the mechanical operation of metal contacts has been replaced by a system of transistor relays.
Nineteenth-century telephones typically contained a transmitter that had to be in an upright position for proper operation, with the receiver located in an attachment that rested on a hook when not in use. The tall profile of AT&T's desk set, such as the 1897 model shown here, led many people to call them “candlestick” phones.Courtesy of AT&T Bell Laboratories/AT&T Archives
When the telephone is “on hook,” contact with the local loop is broken. When it is “off hook” (i.e., when the handset is lifted from the cradle), contact is restored, and current flows through the loop. The switching office signals restoration of contact by transmitting a low-frequency “dial tone”—actually two simultaneous tones of 350 and 440 hertz.
The dialer is used to enter the number of the party that the user wishes to call. Signals generated by the dialer activate switches in the local office, which establish a transmission path to the called party. Dialers are of the rotary and push-button types.
The traditional rotary dialer, invented in the 1890s, is rotated against the tension of a spring and then released, whereupon it returns to its position at a rate controlled by a mechanical governor. The return rotation causes a switch to open and close, producing interruptions, or pulses, in the flow of direct current to the switching office. Each pulse lasts approximately one-tenth of a second; the number of pulses signals the number being dialed.
In push-button dialing, introduced in the 1960s, the pressing of each button generates a “dual-tone” signal that is specific to the number being entered. Each dual tone is composed of a low frequency (697, 770, 852, or 941 hertz) and a high frequency (1,209, 1,336, or 1,477 hertz), which are sensed and decoded at the switching office. Unlike the low-frequency rotary pulses, dual tones can travel through the telephone system, so that push-button telephones can be used to activate automated functions at the other end of the line.
In both rotary and push-button systems, a capacitor and resistor prevent dialing signals from passing into the ringer circuit.
Ringer
The ringer alerts the user to an incoming call by emitting an audible tone or ring. Ringers are of two types, mechanical or electronic. Both types are activated by a 20-hertz, 75-volt alternating current generated by the switching office. The ringer is commonly activated in two-second pulses, with each pulse separated by a pause of four seconds.
The traditional mechanical ringer was introduced with the early Bell telephones. It consists of two closely spaced bells, a metal clapper, and a magnet. Passage of alternating current through a coil of wire produces alternations in the magnetic attraction exerted on the clapper, so that it vibrates rapidly and loudly against the bells. Volume can be muted by a switch that places a mechanical damper against the bells.
In modern electronic ringers, introduced in the 1980s, the ringer current is passed through an oscillator, which adjusts the current to the precise frequency required to activate a piezoelectric transducer—a device made of a crystalline material that vibrates in response to an electric current. The transducer may be coupled to a small loudspeaker, which can be adjusted for volume.
The ringer circuit remains connected to the local loop even when the telephone is on hook. A larger voltage is necessary to activate the ringer because the ringer circuit is made with a high electrical impedance in order to avoid draining power from the transmitter-receiver circuit when the telephone is in use. A capacitor prevents direct current from passing through the ringer once the handset has been lifted off the switch hook.
Transmitter
The transmitter is essentially a tiny microphone located in the mouthpiece of the telephone’s handset. It converts the vibrations of the speaker’s voice into variations in the direct current flowing through the set from the power source.
In traditional carbon transmitters, developed in the 1880s, a thin layer of carbon granules separates a fixed electrode from a diaphragm-activated electrode. Electric current flows through the carbon against a certain resistance. The diaphragm, vibrating in response to the speaker’s voice, forces the movable electrode to exert a fluctuating pressure on the carbon layer. Fluctuations in the carbon layer create fluctuations in its electrical resistance, which in turn produce fluctuations in the electric current.
In modern electret transmitters, developed in the 1970s, the carbon layer is replaced by a thin plastic sheet that has been given a conductive metallic coating on one side. The plastic separates that coating from another metal electrode and maintains an electric field between them. Vibrations caused by speech produce fluctuations in the electric field, which in turn produce small variations in voltage. The voltages are amplified for transmission over the telephone line.
Receiver
The receiver is located in the earpiece of the telephone’s handset. Operating on electromagnetic principles that were known in Bell’s day, it converts fluctuating electric current into sound waves that reproduce human speech. Fundamentally, it consists of two parts: a permanent magnet, having pole pieces wound with coils of insulated fine wire, and a diaphragm driven by magnetic material that is supported near the pole pieces. Speech currents passing through the coils vary the attraction of the permanent magnet for the diaphragm, causing it to vibrate and produce sound waves.
Through the years the design of the electromagnetic system has been continuously improved. In the most common type of receiver, introduced in the Bell system in 1951, the diaphragm, consisting of a central cone attached to a ring-shaped armature, is driven as a piston to obtain efficient response over a wide frequency range. Telephone receivers are designed to have an accurate response to tones with frequencies of 350 to 3,500 hertz—a dynamic range that is narrower than the capabilities of the human ear but sufficient to reproduce normal speech.
Anti-sidetone circuit
The anti-sidetone circuit is an assemblage of transformers, resistors, and capacitors that perform a number of functions. The primary function is to reduce sidetone, which is the distracting sound of the speaker’s own voice coming through the receiver from the transmitter. The anti-sidetone circuit accomplishes this reduction by interposing a transformer between the transmitter circuit and the receiver circuit and by splitting the transmitter signals along two paths. When the divided signals, having opposite polarities, meet at the transformer, they almost entirely cancel each other in crossing to the receiver circuit. The speech signal coming from the other end of the line, on the other hand, arrives at the transformer along a single, undivided path and crosses the transformer unimpeded.
The anti-sidetone circuit also matches the low electrical impedance of the telephone instrument’s circuits to the higher electrical impedance of the telephone line. Impedance matching allows a more efficient flow of current through the system.
Development of the telephone instrument
Early sound transmitters
Beginning in the early 19th century, several inventors made a number of attempts to transmit sound by electric means. The first inventor to suggest that sound could be transmitted electrically was a Frenchman, Charles Bourseul, who indicated that a diaphragm making and breaking contact with an electrode might be used for this purpose. In the 1850s Italian American inventor Antonio Meucci had electrical devices in his home called telettrofoni that he used to communicate between rooms, though he did not patent his inventions. By 1861 Johann Philipp Reis of Germany had designed several instruments for the transmission of sound. The transmitter Reis employed consisted of a membrane with a metallic strip that would intermittently contact a metallic point connected to an electrical circuit. As sound waves impinged on the membrane, making the membrane vibrate, the circuit would be connected and interrupted at the same rate as the frequency of the sound. The fluctuating electric current thus generated would be transmitted by wire to a receiver, which consisted of an iron needle that was surrounded by the coil of an electromagnet and connected to a sounding box. The fluctuating electric current would generate varying magnetic fields in the coil, and these in turn would force the iron needle to produce vibrations in the sounding box. Reis’s system could thus transmit a simple tone, but it could not reproduce the complex waveforms that make up speech.
Gray and Bell: the transmission of speech
The first devices
In the 1870s two American inventors, Elisha Gray and Alexander Graham Bell, each independently, designed devices that could transmit speech electrically. Gray’s first device made use of a harmonic telegraph, the transmitter and receiver of which consisted of a set of metallic reeds tuned to different frequencies. An electromagnetic coil was located near each of the reeds. When a reed in the transmitter was vibrated by sound waves of its resonant frequency—for example, 400 hertz—it induced an electric current of corresponding frequency in its matching coil. This coil was connected to all the coils in the receiver, but only the reed tuned to the transmitting reed’s frequency would vibrate in response to the electric current. Thus, simple tones could be transmitted. In the spring of 1874 Gray realized that a receiver consisting of a single steel diaphragm in front of an electromagnet could reproduce any of the transmitted tones. Gray, however, was initially unable to conceive of a transmitter that would transmit complex speech vibrations and instead chose to demonstrate the transmission of tones via his telegraphic device in the summer of 1874.
Bell, meanwhile, also had considered the transmission of speech using the harmonic telegraph concept, and in the summer of 1874 he conceived of a membrane receiver similar to Gray’s. However, since Bell too had no transmitter, the membrane device was never constructed. Following some earlier experiments, Bell postulated that, if two membrane receivers were connected electrically, a sound wave that caused one membrane to vibrate would induce a voltage in the electromagnetic coil that would in turn cause the other membrane to vibrate. Working with a young machinist, Thomas Augustus Watson, Bell had two such instruments constructed in June 1875. The device was tested on June 3, 1875, and, although no intelligible words were transmitted, “speechlike” sounds were heard at the receiving end.
An application for a U.S. patent on Bell’s work was filed on February 14, 1876. Several hours later that same day, Gray filed a caveat on the concept of a telephone transmitter and receiver. A caveat was a confidential, formal declaration by an inventor to the U.S. Patent Office of an intent to file a patent on an idea yet to be perfected; it was intended to prevent the idea from being used by other inventors. At this point neither Gray nor Bell had yet constructed a working telephone that could convey speech. On the basis of its earlier filing time, Bell’s patent application was allowed over Gray’s caveat. On March 7, 1876, Bell was awarded U.S. patent 174,465. This patent is often referred to as the most valuable ever issued by the U.S. Patent Office, as it described not only the telephone instrument but also the concept of a telephone system.
Bell, Alexander Graham: telephoneAlexander Graham Bell filing the patent for his telephone at the United States Patent Office on February 14, 1876, two hours before declaration of a rival device by Elisha Gray. Bell's telephone is on the table to the right.Photos.com/Thinkstock
The search for a successful transmitter
Gray had earlier come up with an idea for a transmitter in which a moving membrane was attached to an electrically conductive rod immersed in an acidic solution. Another conductive rod was immersed in the solution, and, as sound waves impinged on the membrane, the two rods would move with respect to each other. Variations in the distance between the two rods would produce variations in electric resistance and, hence, variations in the electric current. In contrast to the magnetic coil type of transmitter, the variable-resistance transmitter could actually amplify the transmitted sound, permitting use of longer cables between the transmitter and the receiver.
Again, Bell also worked on a similar “liquid” transmitter design; it was this design that permitted the first transmission of speech, on March 10, 1876, by Bell to Watson, which Bell transcribed in his lab notes as “Mr. Watson—come here—I want to see you.” The first public demonstrations of the telephone followed shortly afterward, featuring a design similar to the earlier magnetic coil membrane units described above. One of the earliest demonstrations occurred in June 1876 at the Centennial Exposition in Philadelphia. Further tests and refinement of equipment followed shortly afterward. On October 9, 1876, Bell conducted a two-way test of his telephone over a 5-km (2-mile) distance between Boston and Cambridgeport, Massachusetts. In May 1877 the first commercial application of the telephone took place with the installation of telephones in offices of customers of the E.T. Holmes burglar alarm company.
The poor performance of early telephone transmitters prompted a number of inventors to pursue further work in this area. Among them was Thomas Alva Edison, whose 1886 design for a voice transmitter consisted of a cavity filled with granules of carbonized anthracite coal. The carbon granules were confined between two electrodes through which a constant electric current was passed. One of the electrodes was attached to a thin iron diaphragm, and, as sound waves forced the diaphragm to vibrate, the carbon granules were alternately compressed and released. As the distance across the granules fluctuated, resistance to the electric current also fluctuated, and the resulting variations in current were transmitted to the receiver. Edison’s carbon transmitter was sufficiently simple, effective, cheap, and durable that it became the basis for standard telephone transmitter design through the 1970s. Development of the modern instrumentThe telephone instrument continued to evolve over time, as can be illustrated by the succession of American instruments described below. The concept of mounting both the transmitter and the receiver in the same handle appeared in 1878 in instruments designed for use by telephone operators in a New York City exchange. The earliest telephone instrument to see common use was introduced by Charles Williams, Jr., in 1882. Designed for wall mounting, this instrument consisted of a ringer, a hand-cranked magneto (for generating a ringing voltage in a distant instrument), a hand receiver, a switch hook, and a transmitter. Various versions of this telephone instrument remained in use throughout the United States as late as the 1950s. As is noted in the section Switching, the telephone dial originated with automatic telephone switching systems in 1896.
The first commercial telephone 1877AT&T archives
Desk instruments were first constructed in 1897. Patterned after the wall-mounted telephone, they usually consisted of a separate receiver and transmitter. In 1927, however, the American Telephone & Telegraph Company (AT&T) introduced the E1A handset, which employed a combined transmitter-receiver arrangement. The ringer and much of the telephone electronics remained in a separate box, on which the transmitter-receiver handle was cradled when not in use. The first telephone to incorporate all the components of the station apparatus into one instrument was the so-called combined set of 1937. Some 25 million of these instruments were produced until they were superseded by a new design in 1949. The 1949 telephone was totally new, incorporating significant improvements in audio quality, mechanical design, and physical construction. Push-button versions of this set became available in 1963.
AT&T desk telephone with E1A handset, 1928.Courtesy of AT&T Bell Laboratories/AT&T Archives
AT&T combined desk telephone, 1937.Courtesy of AT&T Bell Laboratories/AT&T Archives
AT&T “500”desk telephone, 1949.Courtesy of AT&T Bell Laboratories/AT&T Archives
Modern telephone instruments are largely electronic. Wire coils that performed multiple functions in older sets have been replaced by integrated circuits that are powered by the line voltage. Mechanical bell ringers have given way to electronic ringers. The carbon transmitter dating from Edison’s time has been replaced by electret microphones, in which sound waves cause a thin, metal-coated plastic diaphragm to vibrate, producing variations in an electric field across a tiny air gap between the diaphragm and an electrode. The telephone dial has given way to the keypad, which can usually be switched to generate either pulses similar to those of the dial mechanism or dual-tone signals as in AT&T’s Touch-Tone system. Finally, a number of other features have become available on the telephone instrument, including last-number recall and speed-dialing of multiple telephone numbers.
AT&T Touch-Tone telephone, 1968.Courtesy of AT&T Bell Laboratories/AT&T Archive
Cordless telephones are devices that take the place of a telephone instrument within a home or office and permit very limited mobility—up to 100 metres (330 feet). Because they communicate with a base unit that is plugged directly into an existing telephone jack, they essentially serve as a wireless extension to existing home or office wiring. The first cordless phones employed analogmodulation methods and operated over a pair of frequencies, 1.7 megahertz and 49 megahertz. Beginning in the 1980s, cordless phones operated over a pair of frequencies in the 46- and 49-megahertz bands, and in the late 1990s phones operating in the 902–928-megahertz band began to appear. These phones employed either analog modulation, digital modulation, or spread-spectrum modulation. Some digital cordless telephones now operate in the gigahertz region—for example, 5.8 gigahertz. Generally speaking, each successive generation of cordless phones has offered improved quality and range to the consumer.
In a number of countries throughout the world, a wireless service called the personal communication system (PCS) is available. In the broadest sense, PCS includes all forms of wireless communication that are interconnected with the public switched telephone network, including mobile telephone and aeronautical public correspondence systems, but the basic concept includes the following attributes: ubiquitous service to roving users, low subscriber terminal costs and service fees, and compact, lightweight, and unobtrusive personal portable units.
The first PCS to be implemented was the second-generation cordless telephony (CT-2) system, which entered service in the United Kingdom in 1991. The CT-2 system was designed at the outset to serve as a telepoint system. In telepoint systems, a user of a portable unit might originate telephone calls (but not receive them) by dialing a base station located within several hundred metres. The base unit was connected to the PSTN and operated as a public (pay) telephone, charging calls to the subscriber. Because of its limited coverage, the CT-2 system went out of service, giving way to the popular GSM digital cellular system (see mobile telephone).
Meanwhile, the European Conference on Posts and Telecommunications (CEPT) had begun work on another personal communication system, known as DECT (Digital Enhanced Cordless Telecommunications, formerly Digital European Cordless Telephone). The DECT system was designed initially to provide cordless telephone service for office environments, but its scope soon broadened to include campus-wide communications and telepoint services. By 1999 DECT had reached 50 percent of the European cordless market.
In Japan a PCS based loosely on the DECT concepts, the Personal Handy-Phone System (PHS), was introduced to the public in 1994. The PHS became popular throughout urban areas as an alternative to cellular systems. Supporting data traffic at 32 and 64 kilobits per second, it could perform as a high-speed wireless modem for access to the Internet.
In the United States in 1994–95 the Federal Communications Commission (FCC) sold a number of licenses in the 1.85–1.99-gigahertz region for use in PCS applications.
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The Telephone Network
In order to understand the many concepts represented in the public switched telephone network (PSTN), it is helpful to review the processes that take place in the making of a single call on a traditional wired telephone. To make a call, a telephone subscriber begins by taking the telephone “off-hook”—in the process, signaling the local central office that service is requested. The central office, which has been monitoring the telephone line continuously (a process known as attending), responds with a dial tone. Upon receiving the dial tone, the customer enters the called party’s telephone number. The central office stores the entered number, translates the number into an equipment location and a path to that location, and tests whether the called party’s line is already in use (or “busy”). The called party’s number may lie in the same central office (in which case the call is designated intraoffice), or it may lie in another central office (requiring an interoffice call). If the call is intraoffice, the central office switch will handle the entire call process. If the call is interoffice, it will be directed either to a nearby central office or to a distant central office via a long-distance network. In the case of interoffice calls, a separate signaling network is employed to coordinate the call progression through a multitude of switches and telephone trunks. Assuming, however, that the call is an intraoffice call, if the called party’s line is busy and does not have call waiting (in which the current call can be suspended), the telephone switch will return a busy signal until the calling party returns to the “on-hook” condition. If the called party’s line is not busy or does have call waiting, it will be alerted, or “rung.” At the same time that the line is rung, an audible signal will be returned to the calling party to indicate that ringing is taking place. If the called party answers by going off-hook, ringing will be discontinued and a voice path will be established through the switching system to both the calling and called parties. The voice path is maintained until either party goes back on-hook. At that moment the voice path is disconnected, and call charging is recorded.
From the example described above, it is evident that telephone systems consist of four major components:
Switching, between telephone sets and between trunks, as required.
Signaling, between the telephone sets and the central offices as well as between central offices when needed.
Transmission, between the central switching office and subscribers’ telephone sets and also between central offices.
Each of these major components of a telephone system is discussed in turn in this section.
Switching systems
Manual switching
From the earliest days of the telephone, it was observed that it was more practical to connect different telephone instruments by running wires from each instrument to a central switching point, or telephone exchange, than it was to run wires between all the instruments. In 1878 the first telephone exchange was installed in New Haven, Connecticut, permitting up to 21 customers to reach one another by means of a manually operated central switchboard. The manual switchboard was quickly extended from 21 lines to hundreds of lines. Each line was terminated on the switchboard in a socket (called a jack), and a number of short, flexible circuits (called cords) with a plug on both ends of each cord were also provided. Two lines could thus be interconnected by inserting the two ends of a cord in the appropriate jacks.
A manual central switchboard in an American city, c. 1900.Encyclopædia Britannica, Inc.
Electromechanical switching
The idea of automatic switching appeared as early as 1879, and the first fully automatic switch to achieve commercial success was invented in 1889 by Almon B. Strowger, the owner of an undertaking business in Kansas City, Missouri. The Strowger switch consisted of essentially two parts: an array of 100 terminals, called the bank, that were arranged 10 rows high and 10 columns wide in a cylindrical arc; and a movable switch, called the brush, which was moved up and down the cylinder by one ratchet mechanism and rotated around the arc by another, so that it could be brought to the position of any of the 100 terminals. The ratcheting action on the brush gave Strowger’s invention the common name step-by-step switch. The stepping movement was controlled directly by pulses from the telephone instrument. In the original systems, the caller generated the pulses by rapidly pushing a button switch on the instrument. Later, in 1896, Strowger’s associates devised a rotary dial for generating the necessary pulses. (The rotary dialing system is described below in Rotary dialing.)
A crossbar-type electromechanical telephone switching system from the 1960s.Courtesy of AT&T Bell Laboratories/AT&T Archives
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