Telephone as a method of communication
Credit for the invention of the electric telephone is frequently disputed. As with other influential inventions such as radio , television , the light bulb , and the computer , several inventors pioneered experimental work on voice transmission over a wire and improved on each other's ideas. 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.
Early telephones were technically diverse. Some used a water microphone , 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. The sound-powered dynamic variants 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. During the 20th century, telephones powered from the telephone exchange over the same wires that carried the voice signals became common. 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 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. Exchange operation soon resulted in telephones being equipped with a bell in a ringer box , 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 " on hook ". Telephones connected to the earliest Strowger switch 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. 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 or " ringer box ". 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. 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 soundproofed 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 type desk set. 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 commonplace 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.
Ericsson DBH ca. Modern sound-powered emergency telephone. One type of mobile phone , called a cell phone. 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. A January Newsweek article suggested that Internet telephony may be "the next big thing.
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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. Unless the IP telephone's components are backed up with an uninterruptible power supply or other emergency power source, the phone ceases to function during a power outage as can occur during an emergency or disaster when the phone is most needed.
Traditional phones connected to the older PSTN network do not experience that problem since they are powered by the telephone company's battery supply, which will continue to function even if there is a prolonged power outage. 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.
Unless the registered user updates the IP phone's physical address location after moving to a new residence, emergency services can be, and have been, dispatched to the wrong location. This included 1. From Wikipedia, the free encyclopedia. For other uses, see Telephone disambiguation. For other uses, see Phone disambiguation. For the handheld personal computer, see Smartphone. This article's lead section may be too long for the length of the article. Please help by moving some material from it into the body of the article.
Please read the layout guide and lead section guidelines to ensure the section will still be inclusive of all essential details. Please discuss this issue on the article's talk page. July Further information: Telephone call. Main articles: History of the telephone and Timeline of the telephone.https://okicicuqip.tk/pajac-single-sauna.php
Communication Systems/Telephone System
Main article: Timeline of the telephone. Play media. Video shows the operation of an Ericofon. Telephones portal. In Chisholm, Hugh. Cambridge University Press. Google Books. Kingsbury published in The Telephone and It's Several Inventors: A History. Jefferson, NC: Historical first patents: University of Michigan: Scarecrow Press. Retrieved Archived from the original on Archived from the original on January 18, Science in Our World: Certainty and Controversy". Retrieved 5 July Coaxial cable Fiber-optic communication Optical fiber Free-space optical communication Molecular communication Radio waves Transmission line.
Space-division Frequency-division Time-division Polarization-division Orbital angular-momentum Code-division. Today most switches are digital in nature and operate mainly in the time domain. However, some degree of space switching is always required since signals must ultimately be routed from one line to another. Networks always provide some form of concentration.
That is to say, not all customers can be handled simultaneously. This is similar to highway systems where the roads are designed to handle not all vehicles simultaneously, but only a certain peak load. This contains the system intelligence and customer database. It knows who the customers are, what they want, and how to provide the service they require. In a step x step [step by step] switch, the intelligence is fully distributed and there is no central control, whereas in a crossbar facility, all of the intelligence is resident in a central controller or computer.
In all modern systems the intelligence is somewhat distributed, with various functional blocks contributing to the decision making process. At onetime there was a sharp distinction between computers and telecom switches, but today this division is less clear, and central controllers may be regarded as a specialized computer. The steps in completing a local telephone call are as follows:. Line cards are the single most common component in a telephone office. It is a very complex device that contains a wide range of technologies. This acronym describes the functional requirements of a standard telephone line interface.
The tip and ring leads of the telephone set are wired through some protection devices to the line interface located in the peripheral module. This interface must perform the following functions:. Recently however, they have also become available for the central office environment as well. Most domestic appliances are powered from an electric utility grid. The notable exception to this is the telephone. This is because the telephone should still operate in the event of a power failure. Indeed, the telephone is vital in case of disaster or emergency. The telephone office provides a nominal volt dc feed to power the phone.
This magnitude is considered the maximum safe dc operating potential. A negative potential was chosen to reduce corrosive action on buried cables. Multi-function telephones cannot always be powered from the telephone exchange and often require an alternate power source. If the electric power fails, the complex phone cannot function to full capacity.
The telephone exchange can sense the local power outage through the telephone loop and switches to POTS only service. The POTS loop requires a nominal v at 20 — ma dc to maintain a voice and signaling path. The earpiece in the handset does not require biasing, but the carbon microphone does. Subscriber signaling is performed by temporarily placing a short circuit on the loop thus changing the loop current, which is then sensed at the central office. There are several ways to provide loop current, the simplest being a resistor in series with a battery.
Although this method is quite complex, it has become quite popular with the advent of high voltage bipolar technology.
The Importance of Telephone Communication in Business
One of the more difficult requirements to meet is the dB longitudinal line balance requirement. To achieve this, the impedance to ground on each side of the loop, must match within 0. This is easy to do with laser trimmed thick film resistors, but a bit tricky with current sources. A standard telephone requires a minimum of about 20 ma. In actual practice, the loop is generally limited to W. The maximum loop length is determined by the wire gauge.
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The two major types of over-voltage that can occur are lightning strikes and power line contact. In both cases, the circuit must either recover or fail-safe. Under no circumstances can a surge be allowed to propagate further into the system, or create a fire. Since these devices take a finite time to respond, high-speed diodes are also used at the line circuit inputs.
Ringing is often provided by means of a dedicated ringing generator that is connected onto the loop by means of a relay. It is possible to generate ringing voltages at the line interface if the current generators have a high enough voltage source available to them. Or alternately, a switching converter with step up capability can be place on the interface. On rural party lines, ringing codes of long and short rings are sometimes used.
In the U. In such cases, each telephone ringer is tuned to its own frequency. Other methods use positive and negative battery voltages or apply ringing on either the tip or ring side of the line with respect to ground. The central office must supervise the loop in order to identify customer requests for service. A request for service is initiated by going off-hook.
This simply draws loop current from the CO. Loop current at the far-end is monitored during ringing to enable the CO to disconnect the ringing generator when the phone is answered. The office continues to monitor the loop current at both ends of the connection throughout the call, to determine when the call is terminated by hanging up. Signaling is a way to inform the CO what the customer wants. The two basic signaling methods used in customer loops are dial pulse and touch-tone. It is interesting to note that preferred customer loop signaling method in analog exchanges is digital, while the preferred method in digital exchanges is analog!
Two tones are used to perform the signaling function to eliminate the possibility that speech be interpreted as a signal. At one time DTMF decoders were costly and bulky devices located in a common equipment bay, but today with the advent of LSI technology, this function can be performed on a chip. It also reduces the noise and crosstalk levels at the receiver.
Advantages and Disadvantages of Telephone
There are two principal international standards used to implement coding algorithms today: Although the actual compression algorithms are continuous functions, the codec approximates them by linear segments. Another important difference between the European and North American codecs, can be seen by the position of the decision threshold and its digital value. When a telephone call is placed between Europe and North America, it is essential that all of these differences be accounted for.
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It is possible to regenerate the analog voice by passing it through the same type of codec that originally processed it, and then re-code with the other. An alternate approach is to use lookup tables that translate the binary values of one system to the other. Since the highest frequency passed is about 3. Note that these are well above the Nyquist rate but considerably below the Shannon-Hartley limit. It is ironic that although the telephone system has been updated to digital technology, the telephone set and loop has remained analog.
Since each transmitted sample is 8 bits long, the analog voice signal is encoded into a 64 kbps binary steam. This rate determines the basic channel data rate of most other digital communications systems.
Personal and Immediate
By bypassing the codec, it is possible to send 64 kbps customer data through the telephone system. However, because of old style signaling schemes still in use, digital data rates are often limited to 56 kbps. A diplexer performs a bi-directional 2-wire to 4-wire conversion. It allows two unidirectional electrical paths to be combined into a single bi-directional one, and vice versa. It is advantageous to separate transmit and receive portions of the signal since it is easier to make unidirectional amplifiers, filters, and logic devices.
There are several ways to split transmit and receive paths, the simplest method uses a single core hybrid transformer. For a single core hybrid with a center-tapped secondary, the impedance relationships for proper operation conjugate matching are:. This last requirement can be satisfied by adjusting the impedances Z 1 - Z 4 to make the currents equal. From this we observe that signals injected into any port emerge only at adjacent ports but not at the opposite one. In a properly balanced single core hybrid the typical throughput or insertion loss is about 3.
It has better performance than the single core device, but is bulkier and more expensive. All telecom equipment is tested and characterized against standard impedance terminations. These impedances are based on line surveys and are approximate equivalent circuit representations of the outside cabling plant. For this reason, these networks vary from country to country.
In order to maintain a high degree of service As a result, a separate test buss and access relay is provided on a line interface. Tests may be performed in a bridged mode or with the loop and line card disconnected from each other. These tests are generally automated and are conducted late at night when there is little chance that the customer will request service, thus interrupting the test.
Some of the scheduled tests may include:.
Some other tests that may be performed when commissioning a line or when a complaint is lodged, include:. By placing two hybrids back to back, it is possible to create a bidirectional amplifier or repeater. The total gain in the 4-wire path within the repeater must not exceed the combined transhybrid loss of the transformers.
If this happens, the circuit will oscillate or sing. The physical path between any two customers on a space switch is not shared with anyone else. Crosspoints made from electromechanical relays have been used to perform the interconnection, but newer systems use semiconductors. In a crossbar matrix, the number of inputs and outputs do not have to be equal thus facilitating either concentration or expansion. In any case, a total of NxM crosspoints are required. Although only one contact is shown, many systems require two contacts if they keep the integrity of the tip and ring leads throughout the system.
The major disadvantage of this type of switch is the rapid increase of crosspoints as the system size increases. The number of crosspoints increases as N 2 and yet only a maximum of N crosspoints can be active at any given time. Failure of a single crosspoint prevents communication between the two devices sharing that crosspoint.
One way to avoid the cost penalties associated with full matrices, is to organize the contacts into smaller groups. This impacts the call processing since the switch controller must manage several contacts per connection. Furthermore, the connection between any two subscribers may take any one of a number of paths, thus further complicating the decision making process. The following sketch shows a simple three-stage space switch. It should be remembered that since a full duplex connection is required, a second switch supplying the return path must be provided. The number of crosspoints required is dramatically reduced in this multiple stage switch in comparison to a square matrix.
The central stage allows several ways to make a connection between two subscribers therefor, single crosspoint failures can be bypassed. This results in a more flexible and reliable system, but demands a more complex control structure. A non-blocking network is capable of finding a path between any idle input to any idle output.
This does not mean that the system be able to handle simultaneously all customer requests for service. In such a case, the system may overload but the customer is not able to distinguish the difference between blocking and overload. The following illustration shows how blocking can occur. The solid lines represent connections in service.
The connections in service are not necessarily the optimum routing and may have been forced by the prior connections. Note that in this case, it is not possible for the last customer on switch 1: To overcome this, an additional center stage can be added. To prevent blocking, 2n-1 center arrays are necessary.
As the total number of lines in increase, the total number of crosspoints can be approximated by:. A non-blocking network of this type is known as a Clos switch, after its inventor. Its basic characteristics include:. Time domain switching is simply an application of time domain multiplexing and may be performed on analog or digital signals. Any number of inputs may be sequentially routed to a single output. This technique increases the transmission link utilization and can be modified to support circuit switching. If a multiplexer is placed at the input, a demultiplexer is placed at the output.
This system can be used to multiplex either analog or digital signals.
Each customer is assigned a unique switch, but all customers share the same internal signal path.