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 landline 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 microphone A2 produces a modulated electric current which varies its frequency and amplitude 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 via a larger network , where it passes through the coil of the receiver A3.
Along with the microphone and speaker, additional circuitry is incorporated to prevent the incoming speaker signal and the outgoing microphone signal from interfering with each other. This is accomplished through a hybrid coil A3. The incoming audio signal passes through a resistor A8 and the primary winding of the coil A3 which passes it to the speaker A1. Since the current path A8 — A3 has a far lower impedance than the microphone A2 , virtually all of the incoming signal passes through it and bypasses the microphone.
At the same time the DC voltage across the line causes a DC current which is split between the resistor-coil A8-A3 branch and the microphone-coil A2-A3 branch. The DC current through the resistor-coil branch has no effect on the incoming audio signal. This causes a small portion of the microphone output to be fed back to the speaker, while the rest of the AC current goes out through the phone line.
A communication device for sailing vessels The Telephone was the invention of a captain John Taylor in This instrument used four air horns to communicate with vessels in foggy weather.
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Credit for the invention of the electric telephone is frequently disputed. New controversies over the issue still arise from time to time. That first patent by Bell was the master patent of the telephone, from which other patents for electric telephone devices and features flowed. Wooden wall telephone with a hand-cranked magneto generator.
A candlestick phone. Modern sound-powered emergency telephone.
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However telephones would not be demonstrated there until , with a set of telephones from Bell. This uses multiple vibrating steel reeds in make-break circuits. Gray decides to abandon his caveat. Watson, come here, I want to see you. The patent , was granted 3 May , after a year delay because of litigation. Edison was granted patent , for a carbon granules transmitter in Early commercial instruments Early telephones were technically diverse.
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The sound-powered 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. 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.
The earliest dynamic telephones also had only one port opening for sound, with the user alternately listening and speaking or rather, shouting into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient but also more expensive. At first, the benefits of a telephone exchange were not exploited. Instead telephones were leased in pairs to a subscriber , who had to arrange for a telegraph contractor to construct a line between them, for example between a home and a 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. 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.
Large wall telephones in the early 20th century usually incorporated the bell, and separate bell boxes for desk phones dwindled away in the middle of the century. Rural and other telephones that were not on a common battery exchange had a magneto 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.
Some local farming communities that were not connected to the main networks set up barbed wire telephone lines that exploited the existing system of field fences to transmit the signal. In the s a new smaller style of telephone was introduced, packaged in three parts. Cradle designs were also used at this time, having a handle with the receiver and transmitter attached, now called a handset , separate from the cradle base that housed the magneto crank and other parts.
Ericsson DBH ca. Telephone used by American soldiers WWII, Minalin, Pampanga , Philippines 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.
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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. 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 shows the direct connection of the transmitter to the line, while the receiver was induction coupled. In local battery configurations, when the local loop was too long to provide sufficient current from the exchange, the transmitter was powered by a local battery and inductively coupled, while the receiver was included in the local loop.
The dial switch in the base interrupted the line current by repeatedly but very briefly disconnecting the line 1 to 10 times for each digit, and the hook switch in the center of the circuit diagram disconnected the line and the transmitter battery while the handset was on the cradle.
In the s, telephone sets were developed that combined the bell and induction coil with the desk set, obviating a separate ringer box. The rotary dial becoming common-place in the s in many areas enabled customer-dialed service, but some magneto systems remained even into the s. After World-War II, the telephone networks saw rapid expansion and more efficient telephone sets, such as the model telephone in the United States, were developed that permitted larger local networks centered around central offices.
The invention of the transistor in dramatically changed the technology used in telephone systems and in the long-distance transmission networks. With the development of electronic switching systems in the s, telephony gradually evolved towards digital telephony which improved the capacity, quality, and cost of the network. The development of digital data communications method, such as the protocols used for the Internet , it became possible to digitize voice and transmit it as real-time data across computer networks , giving rise to the field of Internet Protocol IP telephony, also known as voice over Internet Protocol VoIP , a term that reflects the methodology memorably.
VoIP has proven to be a disruptive technology that is rapidly replacing traditional telephone network infrastructure. An IP desktop telephone attached to a computer network, with touch-tone dialing From a customer perspective, IP telephony uses a high-bandwidth Internet connection and specialized customer premises equipment to transmit telephone calls via the Internet, or any modern private data network.
The customer equipment may be an analog telephone adapter ATA which interfaces a conventional analog telephone to the IP networking equipment, or it may be an IP Phone that has the networking and interface technology built into the desk-top set and provides the traditional, familiar parts of a telephone, the handset, the dial or keypad, and a ringer in a package that usually resembles a standard telephone set. In addition, many computer software vendors and telephony operators provide softphone application software that emulates a telephone by use of an attached microphone and audio headset, or loud speaker.
Despite the new features and conveniences of IP telephones, some may have notable disadvantages compared to traditional telephones. Another problem in Internet-based services is the lack of a fixed physical location, impacting the provisioning of emergency services such as police, fire or ambulance, should someone call for them. By the end of , there were a total of nearly 6 billion mobile and fixed-line telephone subscribers worldwide. This included 1. Bluetooth is a wireless technology standard for exchanging data over short distances using short-wavelength UHF radio waves in the ISM band from 2.
Invented by telecom vendor Ericsson in , [5] it was originally conceived as a wireless alternative to RS data cables. It can connect several devices, overcoming problems of synchronization. Bluetooth is managed by the Bluetooth Special Interest Group SIG , which has more than 25, member companies in the areas of telecommunication, computing, networking, and consumer electronics.
The Bluetooth SIG oversees development of the specification, manages the qualification program, and protects the trademarks. The idea of this name was proposed in by Jim Kardach who developed a system that would allow mobile phones to communicate with computers. At the time of this proposal he was reading Frans G. Bluetooth uses a radio technology called frequency-hopping spread spectrum. Bluetooth divides transmitted data into packets, and transmits each packet on one of 79 designated Bluetooth channels.
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Originally, Gaussian frequency-shift keying GFSK modulation was the only modulation scheme available. Since the introduction of Bluetooth 2. Bluetooth is a packet-based protocol with a master-slave structure.