The switches used to route telephone calls, which were once electromechanical, are now largely replaced by sophisticated digital electronic switching systems. The electronic switches are much more flexible because they can be programmed to provide new services. The latest generation of switches have made a number of new features possible. Users, for example, can read the telephone number of the calling party on a display device if they choose to subscribe to a "caller ID" service. In "call waiting," audio signals let a person already on a telephone know that someone else is trying to reach that person. Subscribers can also program the telephone switches to forward their calls automatically to another number ("call forwarding"). Other features include voice mailboxes and the ability to make three-way conference calls.
The problems associated with long-distance and intercity telephone service have been met with increasing success. The telephone lines used include the ordinary open wire lines, lead-sheathed cables consisting of many lines, and coaxial and fiber-optic cables. Coaxial and fiber-optic cables are typically placed underground, but other cables may be either overhead or underground. Transmission of telephone messages over long distances is often accomplished by means of radio and microwave transmissions. In some cases microwaves are sent to an orbiting communications satellite (see satellite, artificial) from which they are relayed back to a distant point on the earth. Cellular telephone systems allow small, low-power portable radio transceivers access to the telephone network; some cellular models provide access to the Internet. The incorporation of microelectronics and digital technology has led to the inclusion of unrelated applications in telephones, such as alarm clocks, calculators, and voice memos for recording short verbal reminders. A camera phone is a cellular phone that has photo taking and sending (to another camera phone or computer) capability. Similarly, a videophone transmits and receives real-time video images.
With the advent of the Internet, computer programs have been developed that allow voice communications across long distances, bypassing conventional carriers. The programs, which often require a computer equipped with a telephone or cable modem, microphone, and speakers, compress the voice message into digital signals. In other cases, a special adapter is used to allow a standard telephone to access the Internet directly though a cable modem or other broadband connection, or an Internet telephone (IP phone) may be used instead. The digital signals may be transmitted over the Internet to another computer, which must have another copy of the same program, or to a telephone. If a connection is established with another computer, the second program decompresses the digital signals and plays the sound almost instantaneously. The advantage of using the Internet is that under current tariffs no long-distance charges accrue on a computer to computer call, regardless of the length of the conversation. The disadvantages are the inferior sound quality on dialup connections and, in some cases, the need to have computers that are running the same program and the need to establish a connection between those computers.
In 1984 a federal court ordered American Telephone and Telegraph Company (AT&T) to divest its Bell Telephone operating companies (the "Baby Bells") after the court ruled that AT&T held a monopoly over U.S. telephone service. Since then, the regional operating companies and new competitors for long-distance service have grown through acquisitions and mergers. By 2007, AT&T (formerly SBC Communications, a Baby Bell, which acquired AT&T and adopted the name, and then merged in 2006 with Bell South, another Baby Bell) was the largest U.S. long-distance provider, followed by Verizon Communications (a Baby Bell that merged with MCI), and Sprint. Meanwhile, the seven Baby Bells that had been formed in 1984 were reduced to three, AT&T, Verizon, and Qwest Communications International. The distinctions between types of telephone providers, which had been created by the AT&T breakup, had disappeared, with telephone companies offering local and long-distance service in various locations, and owning wireless carriers and offering high-speed Internet service as well. At the same time these companies were also facing increasing challenges from cable television companies that offered Internet-based (VoIP) phone service over a broadband connection and independent VoIP companies, such as Vonage and Skype.
See T. B. Costain, Chord of Steel: The Story of the Invention of the Telephone (1960); A. M. Noll, Introduction to Telephones and Telephone Systems (2d ed. 1991).
A traditional landline telephone system, also known as "plain old telephone service" (POTS), commonly handles both signaling and audio information on the same twisted pair of insulated wires: the telephone line. Although originally designed for voice communication, the system has been adapted for data communication such as Telex, Fax and Internet communication. The signaling equipment consists of a bell, beeper, light or other device to alert the user to incoming calls, and number buttons or a rotary dial to enter a telephone number for outgoing calls. A twisted pair line is preferred as it is more effective at rejecting electromagnetic interference (EMI) and crosstalk than an untwisted pair.
A calling party wishing to speak to another party will pick up the telephone's handset, thus operating a button switch or "switchhook", which puts the telephone into an active state or "off hook" by connecting the transmitter (microphone), receiver (speaker) and related audio components to the line. This circuitry has a low resistance (less than 300 Ohms) which causes DC current (48 volts, nominal) from the telephone exchange to flow through the line. The exchange detects this DC current, attaches a digit receiver circuit to the line, and sends a dial tone to indicate readiness. On a modern telephone, the calling party then presses the number buttons in a sequence corresponding to the telephone number of the called party. The buttons are connected to a tone generator that produces DTMF tones which are sent to the exchange. A rotary dial telephone employs pulse dialing, sending electrical pulses corresponding to the telephone number to the exchange. (Most exchanges are still equipped to handle pulse dialing.) Provided the called party's line is not already active or "busy", the exchange sends an intermittent ringing signal (generally over 100 volts AC) to alert the called party to an incoming call. If the called party's line is active, the exchange sends a busy signal to the calling party. However, if the called party's line is active but has call waiting installed, the exchange sends an intermittent audible tone to the called party to indicate an incoming call.
When a landline phone is inactive or "on hook", its alerting device is connected across the line through a capacitor, which prevents DC current from flowing through the line. The circuitry at the telephone exchange detects the absence of DC current flow and thus that the phone is on hook with only the alerting device electrically connected to the line. When a party initiates a call to this line, the ringing signal transmitted by the telephone exchange activates the alerting device on the line. When the called party picks up the handset, the switchhook disconnects the alerting device and connects the audio circuitry to the line. The resulting low resistance now causes DC current to flow through this line, confirming that the called phone is now active. Both phones being active and connected through the exchange, the parties may now converse as long as both phones remain off hook. When a party "hangs up", placing the handset back on the cradle or hook, DC current ceases to flow in that line, signaling the exchange to disconnect the call.
Calls to parties beyond the local exchange are carried over "trunk" lines which establish connections between exchanges. In modern telephone networks, fiber-optic cable and digital technology are often employed in such connections. Satellite technology may be used for communication over very long distances.
In most telephones, the transmitter and receiver (microphone and speaker) are located in the handset, although in a speakerphone these components may be located in the base or in a separate enclosure. Powered by the line, the transmitter produces an electric current whose voltage varies in response to the sound waves arriving at its diaphragm. The resulting current is transmitted along the telephone line to the local exchange then on to the other phone (via the local exchange or a larger network), where it passes through the coil of the receiver. The varying voltage in the coil produces a corresponding movement of the receiver's diaphragm, reproducing the sound waves present at the transmitter.
A Lineman's handset is a telephone designed for testing the telephone network, and may be attached directly to aerial lines and other infrastructure components.
The early history of the telephone is a confusing morass of claim and counterclaim, which was not clarified by the huge mass of lawsuits which hoped to resolve the patent claims of individuals. The Bell and Edison patents, however, were forensically victorious and commercially decisive.
Early telephones were technically diverse. Some used a liquid transmitter, some had a metal diaphragm that induced current in an electromagnet wound around a permanent magnet, and some were "dynamic" - their diaphragm vibrated a coil of wire in the field of a permanent magnet or the coil vibrated the diaphragm. This dynamic kind survived in small numbers through the 20th century in military and maritime applications where its ability to create its own electrical power was crucial. Most, however, used the Edison/Berliner carbon transmitter, which was much louder than the other kinds, even though it required an induction coil, actually acting as an impedance matching transformer to make it compatible to the impedance of the line. The Edison patents kept the Bell monopoly viable into the 20th century, by which time the network was more important than the instrument.
Early telephones were locally powered, using either a dynamic transmitter or by the powering of a transmitter with a local battery. One of the jobs of outside plant personnel was to visit each telephone periodically to inspect the battery. During the 20th century, "common battery" operation came to dominate, powered by "talk battery" from the telephone exchange over the same wires that carried the voice signals.
Early telephones used a single wire for the subscriber's line, with ground return used to complete the circuit (as used in telegraphs). The earliest dynamic telephones also had only one opening for sound, and the user alternately listened and spoke (rather, shouted) into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient but were more expensive.
At first, the benefits of an exchange were not exploited. Telephones instead were leased in pairs to the subscriber, who had to arrange telegraph contractors to construct a line between them, for example between his home and his shop. Users who wanted the ability to speak to several different locations would need to obtain and set up three or four pairs of telephones. Western Union, already using telegraph exchanges, quickly extended the principle to its telephones in New York City and San Francisco, and Bell was not slow in appreciating the potential.
Signalling began in an appropriately primitive manner. The user alerted the other end, or the exchange operator, by whistling into the transmitter. Exchange operation soon resulted in telephones being equipped with a bell, first operated over a second wire, and later over the same wire, but with a condenser (capacitor) in series with the bell coil to allow the AC ringer signal through while still blocking DC (keeping the phone "off hook"). Telephones connected to the earliest Strowger automatic exchanges had seven wires, one for the knife switch, one for each telegraph key, one for the bell, one for the push button and two for speaking.
Rural and other telephones that were not on a common battery exchange had a magneto or hand-cranked generator to produce a high voltage alternating signal to ring the bells of other telephones on the line and to alert the operator.
In the 1890s a new smaller style of telephone was introduced, packaged in three parts. The transmitter stood on a stand, known as a "candlestick" for its shape. When not in use, the receiver hung on a hook with a switch in it, known as a "switchhook." Previous telephones required the user to operate a separate switch to connect either the voice or the bell. With the new kind, the user was less likely to leave the phone "off the hook". In phones connected to magneto exchanges, the bell, induction coil, battery and magneto were in a separate "bell box." In phones connected to common battery exchanges, the bell box was installed under a desk, or other out of the way place, since it did not need a battery or magneto.
Cradle designs were also used at this time, having a handle with the receiver and transmitter attached, separate from the cradle base that housed the magneto crank and other parts. They were larger than the "candlestick" and more popular.
Disadvantages of single wire operation such as crosstalk and hum from nearby AC power wires had already led to the use of twisted pairs and, for long distance telephones, four-wire circuits. Users at the beginning of the 20th century did not place long distance calls from their own telephones but made an appointment to use a special sound proofed long distance telephone booth furnished with the latest technology.
What turned out to be the most popular and longest lasting physical style of telephone was introduced in the early 20th century, including Bell's Model 102. A carbon granule transmitter and electromagnetic receiver were united in a single molded plastic handle, which when not in use sat in a cradle in the base unit. The circuit diagram of the Model 102 shows the direct connection of the receiver to the line, while the transmitter was induction coupled, with energy supplied by a local battery. The coupling transformer, battery, and ringer were in a separate enclosure. The dial switch in the base interrupted the line current by repeatedly but very briefly disconnecting the line 1-10 times for each digit, and the hook switch (in the center of the circuit diagram) permanently disconnected the line and the transmitter battery while the handset was on the cradle.
After the 1930s, the base also enclosed the bell and induction coil, obviating the old separate bell box. Power was supplied to each subscriber line by central office batteries instead of a local battery, which required periodic service. For the next half century, the network behind the telephone became progressively larger and much more efficient, but after the dial was added the instrument itself changed little until touch tone replaced the dial in the 1960s.
IP telephony uses a broadband Internet connection and IP Phones to transmit conversations as data packets. In addition to replacing POTS(plain old telephone service), IP telephony is also competing with mobile phone networks by offering free or lower cost connections via WiFi hotspots. VoIP is also used on private wireless networks which may or may not have a connection to the outside telephone network.
IP telephony technology transforms many non-telephone electronics devices into unified communications devices which simulate telephone usage, such as adding telephone-like features to portable game devices, digital picture frames, or handheld GPS receivers, typically by incorporating a voice engine. When used on a personal computer, an IP telephone is referred to as a soft phone.