Telecommunications.

Telecommunications. I INTRODUCTION Telecommunications, devices and systems that transmit electronic or optical signals across long distances. Telecommunications enables people around the world to contact one another, to access information instantly, and to communicate from remote areas. Telecommunications usually involves a sender of information and one or more recipients linked by a technology, such as a telephone system, that transmits information from one place to another. Telecommunications enables people to send and receive personal messages across town, between countries, and to and from outer space. It also provides the key medium for delivering news, data, information, and entertainment. Telecommunications devices convert different types of information, such as sound and video, into electronic or optical signals. Electronic signals typically travel along a medium such as copper wire or are carried over the air as radio waves. Optical signals typically travel along a medium such as strands of glass fibers. When a signal reaches its destination, the device on the receiving end converts the signal back into an understandable message, such as sound over a telephone, moving images on a television, or words and pictures on a computer screen. Telecommunications messages can be sent in a variety of ways and by a wide range of devices. The messages can be sent from one sender to a single receiver (pointto-point) or from one sender to many receivers (point-to-multipoint). Personal communications, such as a telephone conversation between two people or a facsimile (fax) message (see Facsimile Transmission), usually involve point-to-point transmission. Point-to-multipoint telecommunications, often called broadcasts, provide the basis for commercial radio and television programming. II HOW TELECOMMUNICATIONS WORKS Telecommunications begin with messages that are converted into electronic or optical signals. Some signals, such as those that carry voice or music, are created in an analog or wave format, but may be converted into a digital or mathematical format for faster and more efficient transmission. The signals are then sent over a medium to a receiver, where they are decoded back into a form that the person receiving the message can understand. There are a variety of ways to create and decode signals, and many different ways to transmit signals. A Creating and Receiving the Signal Devices such as the telegraph and telephone relay messages by creating modulated electrical impulses, or impulses that change in a systematic way. These impulses are then sent along wires, through the air as radio waves, or via other media to a receiver that decodes the modulation. The telegraph, the earliest method of delivering telecommunications, works by converting the contacts (connections between two conductors that permit a flow of current) between a telegraph key and a metal conductor into electrical impulses. These impulses are sent along a wire to a receiver, which converts the impulses into short and long bursts of sound or into dots and dashes on a simple printing device. Specific sequences of dots and dashes represent letters of the alphabet. In the early days of the telegraph, these sequences were decoded by telegraph operators (see Morse Code, International). In this way, telegraph operators could transmit and receive letters that spelled words. Later versions of the telegraph could decipher letters and numbers automatically. Telegraphs have been largely replaced by other forms of telecommunications, such as electronic mail (e-mail), but they are still used in some parts of the world to send messages. The telephone uses a diaphragm (small membrane) connected to a magnet and a wire coil to convert sound into an analog or electrical waveform representation of the sound. When a person speaks into the telephone's microphone, sound waves created by the voice vibrate the diaphragm, which in turn creates electrical impulses that are sent along a telephone wire. The receiver's wire is connected to a speaker, which converts the modulated electrical impulses back into sound. Broadcast radio and cellular radio telephones are examples of devices that create signals by modulating radio waves. A radio wave is one type of electromagnetic radiation, a form of energy that travels in waves. Microwaves are also electromagnetic waves, but with shorter wavelengths and higher frequencies. In telecommunications, a transmitter creates and emits radio waves. The transmitter electronically modulates or encodes sound or other information onto the radio waves by varying either the amplitude (height) of the radio waves, or by varying the frequency (number) of the waves within an established range (see Frequency Modulation). A receiver (tuner) tuned to a specific frequency or range of frequencies will pick up the modulation added to the radio waves. A speaker connected to the tuner converts the modulation back into sound. Broadcast television works in a similar fashion. A television camera takes the light reflected from a scene and converts it into an electronic signal, which is transmitted over high-frequency radio waves. A television set contains a tuner that receives the signal and uses that signal to modulate the images seen on the picture tube. The picture tube contains an electron gun that shoots electrons onto a photo-sensitive display screen. The electrons illuminate the screen wherever they fall, thus creating moving pictures. Telegraphs, telephones, radio, and television all work by modifying electronic signals, making the signals imitate, or reproduce, the original message. This form of transmission is known as analog transmission. Computers and other types of electronic equipment, however, transmit digital information. Digital technologies convert a message into an electronic or optical form first by measuring different qualities of the message, such as the pitch and volume of a voice, many times. These measurements are then encoded into multiple series of binary numbers, or 1s and 0s. Finally, digital technologies create and send impulses that correspond to the series of 1s and 0s. Digital information can be transmitted faster and more clearly than analog signals, because the impulses only need to correspond to two digits and not to the full range of qualities that compose the original message, such as the pitch and volume of a human voice. While digital transmissions can be sent over wires, cables or radio waves, they must be decoded by a digital receiver. New digital telephones and televisions are being developed to make telecommunications more efficient. Personal computers primarily communicate with each other and with larger networks, such as the Internet, by using the ordinary telephone network. Increasing numbers of computers rely on broadband networks provided by telephone and cable television companies to send text, music, and video over the Internet at high speeds. Since the telephone network functions by converting sound into electronic signals, the computer must first convert its digital data into sound. Computers do this with a device called a modem, which is short for modulator/demodulator. A modem converts the stream of 1s and 0s from a computer into an analog signal that can then be transmitted over the telephone network, as a speaker's voice would. The modem of the receiving computer demodulates the analog sound signal back into a digital form that the computer can understand. B Transmitting the Signal Telecommunications systems deliver messages using a number of different transmission media, including copper wires, fiber-optic cables, communication satellites, and microwave radio. One way to categorize telecommunications media is to consider whether or not the media uses wires. Wire-based (or wireline) telecommunications provide the initial link between most telephones and the telephone network and are a reliable means for transmitting messages. Telecommunications without wires, commonly referred to as wireless communications, use technologies such as cordless telephones, cellular radio telephones, pagers, and satellites. Wireless communications offer increased mobility and flexibility. In the future some experts believe that wireless devices will also offer high-speed Internet access. C Wires and Cables Wires and cables were the original medium for telecommunications and are still the primary means for telephone connections. Wireline transmission evolved from telegraph to telephone service and continues to provide the majority of telecommunications services. Wires connect telephones together within a home or business and also connect these telephones to the nearest telephone switching facility. Other wireline services employ coaxial cable, which is used by cable television to provide hundreds of video channels to subscribers. Much of the content transmitted by the coaxial cable of cable television systems is sent by satellite to a central location known as the headend. Coaxial cables flow from the headend throughout a community and onward to individual residences and, finally, to individual television sets. Because signals weaken as distance from the headend increases, the coaxial cable network includes amplifiers that process and retransmit the television signals. D Fiber-Optic Cables Fiber-optic cables use specially treated glass that can transmit signals in the form of pulsed beams of laser light. Fiber-optic cables carry many times more information than copper wires can, and they can transmit several television channels or thousands of telephone conversations at the same time. Fiber-optic technology has replaced copper wires for most transoceanic routes and in areas where large amounts of data are sent. This technology uses laser transmitters to send pulses of light via hairthin strands of specially prepared glass fibers. New improvements promise cables that can transmit millions of telephone calls over a single fiber. Already fiber optic cables provide the high capacity, 'backbone' links necessary to carry the enormous and growing volume of telecommunications and Internet traffic. E Radio Waves Wireless telecommunications use radio waves, sent through space from one antenna to another, as the medium for communication. Radio waves are used for receiving AM and FM radio and for receiving television. Cordless telephones and wireless radio telephone services, such as cellular radio telephones and pagers, also use radio waves. Telephone companies use microwaves to send signals over long distances. Microwaves use higher frequencies than the radio waves used for AM, FM, or cellular telephone transmissions, and they can transmit larger amounts of data more efficiently. Microwaves have characteristics similar to those of visible light waves and transmit pencil-thin beams that can be received using dish-shaped antennas. Such narrow beams can be focused to a particular destination and provide reliable transmissions over short distances on Earth. Even higher and narrower beams provide the high-capacity links to and from satellites. The high frequencies easily penetrate the ionosphere (a layer of Earth's atmosphere that blocks low-frequency waves) and provide a high-quality signal. F Communications Satellites Communications satellites provide a means of transmitting telecommunications all over the globe, without the need for a network of wires and cables. They orbit Earth at a speed that enables them to stay above the same place on Earth at all times. This type of orbit is called geostationary or geosynchronous orbit because the satellite's orbital speed operates in synchronicity with Earth's rotation. The satellites receive transmissions from Earth and transmit them back to numerous Earth station receivers scattered within the receiving coverage area of the satellite. This relay function makes it possible for satellites to operate as "bent pipes"--that is, wireless transfer stations for point-to-point and point-to-multipoint transmissions. Communications satellites are used by telephone and television companies to transmit signals across great distances. Ship, airplane, and land navigators also receive signals from satellites to determine geographic positions. III TELECOMMUNICATIONS SYSTEMS Individual people, businesses, and governments use many different types of telecommunications systems. Some systems, such as the telephone system, use a network of cables, wires, and switching stations for point-to-point communication. Other systems, such as radio and television, broadcast radio signals over the air that can be received by anyone who has a device to receive them. Some systems make use of several types of media to complete a transmission. For example, a telephone call may travel by means of copper wire, fiber-optic cable, and radio waves as the call is sent from sender to receiver. All telecommunications systems are constantly evolving as telecommunications technology improves. Many recent improvements, for example, offer high-speed broadband connections that are needed to send multimedia information over the Internet. A Telegraph Telegraph services use both wireline and wireless media for transmissions. Soon after the introduction of the telegraph in 1844, telegraph wires spanned the country. Telegraph companies maintained a system of wires and offices located in numerous cities. A message sent by telegraph was called a telegram. Telegrams were printed on paper and delivered to the receiving party by the telegraph company. With the invention of the radio in the early 1900s, telegraph signals could also be sent by radio waves. Wireless telegraphy made it practical for oceangoing ships as well as aircraft to stay in constant contact with land-based stations. B Telephone The telephone network also uses both wireline and wireless methods to deliver voice communications between people, and data communications between computers and people or other computers. The part of the telephone network that currently serves individual residences and many businesses operates in an analog mode, uses copper wires, and relays electronic signals that are continuous, such as the human voice. Digital transmission via fiber-optic cables is now used in some sections of the telephone network that send large amounts of calls over long distances. However, since the rest of the telephone system is still analog, these digital signals must be converted back to analog before they reach users. The telephone network is stable and reliable, because it uses its own wire system that is powered by low-voltage direct current from the telephone company. Telephone networks modulate voice communications over these wires. A complex system of network switches maintains the telephone links between callers. Telephone networks also use microwave relay stations to send calls from place to place on the ground. Satellites are used by telephone networks to transmit telephone calls across countries and oceans. C Teletype, Telex, and Facsimile Transmission Teletype, telex, and facsimile transmission are all methods for transmitting text rather than sounds. These text delivery systems evolved from the telegraph. Teletype and telex systems still exist, but they have been largely replaced by facsimile machines, which are inexpensive and better able to operate over the existing telephone network. The Internet increasingly provides an even more inexpensive and convenient option. The teletype, essentially a printing telegraph, is primarily a point-to- multipoint system for sending text. The teletype converts the same pulses used by telegraphs into letters and numbers, and then prints out readable text. It was often used by news media organizations to provide newspaper stories and stock market data to subscribers. Telex is primarily a point-to-point system that uses a keyboard to transmit typed text over telephone lines to similar terminals situated at individual company locations. See also Office Systems: Communications; Telegraph: Modern Telegraph Services. Facsimile transmission now provides a cheaper and easier way to transmit text and graphics over distances. Fax machines contain an optical scanner that converts text and graphics into digital, or machine-readable, codes. This coded information is sent over ordinary analog telephone lines through the use of a modem included in the fax machine. The receiving fax machine's modem demodulates the signal and sends it to a printer also contained in the fax machine. D Radio Radios transmit and receive communications at various preset frequencies. Radio waves carry the signals heard on AM and FM radio, as well as the signals seen on a television set receiving broadcasts from an antenna. Radio is used mostly as a public medium, sending commercial broadcasts from a transmitter to anyone with a radio receiver within its range, so it is known as a point-to-multipoint medium. However, radio can also be used for private point-to-point transmissions. Two-way radios, cordless telephones, and cellular radio telephones are common examples of transceivers, which are devices that can both transmit and receive point-to-point messages. Personal radio communication is generally limited to short distances (usually a few kilometers), but powerful transmitters can send broadcast radio signals hundreds of kilometers. Shortwave radio, popular with amateur radio enthusiasts, uses a range of radio frequencies that are able to bounce off the ionosphere. This electrically charged layer of the atmosphere reflects certain frequencies of radio waves, such as shortwave frequencies, while allowing higher-frequency waves, such as microwaves, to pass through it. Amateur radio operators use the ionosphere to bounce their radio signals to other radio operators thousands of kilometers away. E Television Television is primarily a public broadcasting medium, using point-to-multipoint technology that is broadcast to any user within range of the transmitter. Televisions transmit news and information, as well as entertainment. Commercial television is broadcast over very high frequency (VHF) and ultrahigh frequency (UHF) radio waves and can be received by any television set within range of the transmitter. Televisions have also been used for point-to-point, two-way telecommunications. Teleconferencing, in which a television picture links two physically separated parties, is a convenient way for businesspeople to meet and communicate without the expense or inconvenience of travel. Video cameras on computers now allow personal computer users to teleconference over the Internet. Videophones, which use tiny video cameras and rely on satellite technology, can also send private or public television images and have been used in news reporting in remote locations. Cable television is a commercial service that links televisions to a source of many different types of video programming using coaxial cable. The cable provider obtains coded, or scrambled, programming from a communications satellite, as well as from terrestrial links, including broadcast television stations. The signal may be scrambled to prevent unpaid access to the programming. The cable provider electronically unscrambles the signal and supplies the decoded signals by cable to subscribers. Television users with personal satellite dishes can access satellite programming directly without a cable installation. Personal satellite dishes are also a subscriber service. Fees are paid to the network operator in return for access to the satellite channels. Most television sets outside of the United States that receive programming use different types of standards for receiving video signals. The European Phase Alternative Line standard generates a higher-resolution picture than the sets used in the United States, but these television sets are more expensive. Manufacturers now offer digital video and audio signal processing, which features even higher picture resolution and sound quality. The shape of the television screen is changing as well, reflecting the aspect ratio (ratio of image height to width) used for movie presentation. F Global Positioning and Navigation Systems The United States Global Positioning System (GPS) and the Russian Global Orbiting Navigation Satellite System (GLONASS) are networks of satellites that provide highly accurate positioning information from anywhere on Earth. Both systems use a group of satellites that orbit around the north and south poles at an altitude of 17,500 km (10,900 mi). These satellites constantly broadcast the time and their location above Earth. A GPS receiver picks up broadcasts from these satellites and determines its position through the process of triangulation. Using the time information from each satellite, the receiver calculates the time the signal takes to reach it. Factoring in this time with the speed at which radio signals travel, the receiver calculates its distance from the satellite. Finally, using the location of three satellites and its distance from each satellite, the receiver determines its position. GPS services, originally designed for military use, are now available to civilians. Handheld GPS receivers allow users to pinpoint their location on Earth to within a few meters. One type of navigational tool used in automobiles integrates a GPS receiver with an intelligent compact disc player capable of displaying road maps and other graphical information. Upon receiving the GPS location data, the CD player can pinpoint the location visually on one of the road maps contained on disc. G Personal Computers Personal computers use telecommunications to provide a transmission link for the delivery of audio, video, text, software, and multimedia services. Many experts believe that the convergence of these services will generate consumer demand for new generations of high-speed, broadband networks. Currently, the delivery of most of these audio, video, and text services occurs over existing telephone connections using the Internet. Some computers connect directly to the digital portion of the telephone network using the Integrated Services Digital Network (ISDN) or Digital Subscriber Lines (DSL), but this requires special equipment at user locations. Telephone and cable television companies must also make upgrades to their lines so that they can handle high-speed data transmission. In many locations companies and individuals with high-speed data requirements now have the option of securing DSL service from telephone companies and cable modem service from cable television companies. Electronic mail, or e-mail, is a key attraction of the Internet and a common form of computer telecommunications. E-mail is a text-based message delivery system that allows information such as typed messages and multimedia to be sent to individual computer users. Local e-mail messages (within a building or a company) typically reach addressees by traveling through wire-based internal networks. E-mail that must travel across town or across a country to reach the final destination usually travels through the telephone network. Instant messaging is another key feature of computer telecommunications and involves sending text, audio, or video data in real time. Other computer telecommunications technologies that businesses frequently use include automated banking terminals and devices for credit card or debit card transactions. These transactions either bill charges directly to a customer's credit card account or automatically deduct money from a customer's bank account. H Voice Over Internet Protocol (VOIP) Voice Over Internet Protocol (VOIP) is a method for making telephone calls over the Internet by sending voice data in separate packets, just as e-mail is sent. Each packet is assigned a code for its destination, and the packets are then reassembled in the correct order at the receiving end. Recent technological improvements have made VOIP almost as seamless and smooth as a regular telephone call. In February 2004 the Federal Communications Commission (FCC) ruled that VOIP, like e-mail and instant messaging, is free of government regulation as long as it involves communication from one computer to another. The FCC did not rule on whether VOIP software that sends voice data from a computer directly to a regular telephone should be regulated. Such services became available in the early part of the 21st century and are expected to become widely available. They require a broadband connection to the Internet but can reduce telephone charges significantly while also offering for free additional services such as call waiting, caller identification, voice mail, and the ability to call from your home telephone number wherever you travel. IV HISTORY Communicating over long distances has been a challenge throughout history. In ancient times, runners were used to carry important messages between rulers or other important people. Other forms of long-distance communication included smoke signals, chains of searchlights and flags to send a message from one tower to another, carrier pigeons, and horses. Modern telecommunications began in the 1800s with the discovery that electricity can be used to transmit a signal. For the first time, a signal could be sent faster than any other mode of transportation. The first practical telecommunications device to make use of this discovery was the telegraph. A The Telegraph Beginning in the mid-1800s, the telegraph delivered the first intercity, transcontinental, and transoceanic messages in the world. The telegraph revolutionized the way people communicated by providing messages faster than any other means provided at the time. American art professor Samuel F. B. Morse pursued an interest in electromagnetism to create a practical electromagnetic telegraph in 1837. Morse partnered with Alfred Vail and was able to commercialize the technology with financial support from the U.S. government. In 1843 Morse built a demonstration telegraph link between Washington, D.C., and Baltimore, Maryland. On May 24, 1844, the network was inaugurated for commercial use with the message, "What hath God wrought!" Telegraph use quickly spread; the first transcontinental link was completed in 1861 between San Francisco, California, and Washington, D.C.Railroad companies and newspapers were the first major telegraphy users. Telegraph lines were constructed parallel to railroad beds. Telegraphy helped the railroads manage traffic and allowed news organizations to distribute stories quickly to local newspapers. Within a few years, several telegraph companies were in operation, each with its own network of telegraph wires. Consolidation occurred in the telegraph industry (as it has in numerous telecommunications industries), and by the 1870s the Western Union Telegraph Company emerged as the dominant operator. B Commercial Growth of the Telephone In 1876 American inventor Alexander Graham Bell ushered in a new era of voice and sound telecommunication when he uttered to his assistant the words, "Mr. Watson, come here; I want you," using a prototype telephone. Bell received the patent for the first telephone, but he had to fight numerous legal challenges to his patent from other inventors with similar devices. Bell was able to make his prototype telephone work, and this enabled him to attract financial backers, and his company grew. The telephone was a vast improvement over the telegraph system, which could only transmit coded words and numbers, not the sound of a human voice. Telegraph messages had to be deciphered by trained operators, written down, and then delivered by hand to the receiving party, all of which took time. The telephone transmitted actual sound messages and made telecommunication immediate. Improved switching technology (used to transfer calls from one local network to another) meant individual telephones could be connected for personal conversations. The first commercial telephone line was installed in Boston, Massachusetts, in 1877. Early telephones required direct connections to other telephones, but this problem was solved with telephone exchange switches, the first of which was installed in New Haven, Connecticut, in 1878. A telephone exchange linked telephones in a given area together, so a connection between the telephone and the exchange was all that was needed. Telephones were much more convenient and personal than telegrams, and their use quickly spread. By 1913 telephone lines from New York City to San Francisco had been established, and by 1930 radio signals could transmit telephone calls between New York and London, England. Eventually, long-distance telephone service in the United States was consolidated into one company, the American Telephone and Telegraph Company (now known as AT&T, Inc.), which was a regulated monopoly. C The Emergence of Broadcasting Telephones and telegraphs are primarily private means of communications, sending signals from one point to another, but with the invention of the radio, public communications, or point-to-multipoint signals, could be sent through a central transmitter to be received by anyone possessing a receiver. Italian inventor and electrical engineer Guglielmo Marconi transmitted a Morse-code telegraph signal by radio in 1895. This began a revolution in wireless telegraphy that would later result in broadcast radios that could transmit actual voice and music. Radio and wireless telegraph communication played an important role during World War I (1914-1918), allowing military personnel to communicate instantly with troops in remote locations. United States president Woodrow Wilson was impressed with the ability of radio, but he was fearful of its potential for espionage use. He banned nonmilitary radio use in the United States as the nation entered World War I in 1917, and this stifled commercial development of the medium. After the war, however, commercial radio stations began to broadcast. By the mid-1920s, millions of radio listeners tuned in to music, news, and entertainment programming. Television got its start as a mass-communication medium shortly after World War II (1939-1945). The expense of television transmission prevented its use as a two-way medium, but radio broadcasters quickly saw the potential for television to provide a new way of bringing news and entertainment programming to people. For more information on the development of radio and television, see Radio and Television Broadcasting. D Government Regulation The number of radio broadcasts grew quickly in the 1920s, but there was no regulation of frequency use or transmitter strength. The result was a crowded radio band of overlapping signals. To remedy this, the U.S. government created the Federal Communications Commission (FCC) in 1934 to regulate the spreading use of the broadcast spectrum. The FCC licenses broadcasters and regulates the location and transmitting strength, or range, stations have in an effort to prevent interference from nearby signals. The FCC and the U.S. government have also assumed roles in limiting the types of business practices in which telecommunications companies can engage. The U.S. Department of Justice filed an antitrust lawsuit against AT&T Corp., arguing that the company used its monopoly position to stifle competition, particularly through its control over local telephone service facilities. The lawsuit was settled in 1982, and AT&T agreed to disperse its local telephone companies, thereby creating seven new independent companies. In 1996 the U.S. government enacted the Telecommunications Reform Act to further encourage competition in the telecommunications marketplace. This legislation removed government rules preventing local and long-distance phone companies, cable television operators, broadcasters, and wireless services from directly competing with one another. The act spurred consolidation in the industry, as regional companies joined forces to create telecommunications giants that provided telephone, wireless, cable, and Internet services. Deregulation, however, also led to overproduction of fiber optic cable and a steep decline in the fortunes of the telecommunications industry beginning in 2000. The increased competition provided the backdrop for the bankruptcy of a leading telecommunications company, WorldCom, Inc., in 2002, when it admitted to the largest accounting fraud in the history of U.S. business. E International Telecommunications Networks In order to provide overseas telecommunications, people had to develop networks that could link widely separated nations. The first networks to provide such linkage were telegraph networks that used undersea cables, but these networks could provide channels for only a few simultaneous communications. Shortwave radio also made it possible for wireless transmissions of both telegraphy and voice over very long distances. To take advantage of the wideband capability of satellites to provide telecommunications service, companies from all over the world pooled resources and shared risks by creating a cooperative known as the International Telecommunications Satellite Organization, or Intelsat, in 1964. Transoceanic satellite telecommunications first became possible in 1965 with the successful launch of Early Bird, also known as Intelsat 1. Intelsat 1 provided the first international television transmission and had the capacity to handle one television channel or 240 simultaneous telephone calls. Intelsat later expanded and diversified to meet the global and regional satellite requirements of more than 200 nations and territories. In response to private satellite ventures entering the market, the managers of Intelsat converted the cooperative into a private corporation better able to compete with these emerging companies. The International Mobile Satellite Organization (Inmarsat) primarily provided service to oceangoing vessels when it first formed as a cooperative in 1979, but it later expanded operations to include service to airplanes and users in remote land areas not served by cellular radio or wireline services. Inmarsat became a privatized, commercial venture in 1999. F Current Developments Personal computers have pushed the limits of the telephone system as more and more complex computer messages are being sent over telephone lines, and at rapidly increasing speeds. This need for speed has encouraged the development of digital transmission technology. The growing use of personal computers for telecommunications has increased the need for innovations in fiber-optic technology. Telecommunications and information technologies are merging and converging. This means that many of the devices now associated with only one function may evolve into more versatile equipment. This convergence is already happening in various fields. Some telephones and pagers are able to store not only phone numbers but also names and personal information about callers. Wireless phones with keyboards and small screens can access the Internet and send and receive e-mail messages. Personal computers can now access information and video entertainment and are in effect becoming a combined television set and computer terminal. Television sets can access the Internet through add-on appliances. Future modifications and technology innovations may blur the distinctions between appliances even more. Convergence of telecommunications technologies may also trigger a change in the kind of content available. Both television and personal computers are likely to incorporate new multimedia, interactive, and digital features. However, in the near term, before the actualization of a fully digital telecommunications world, devices such as modems will still be necessary to provide an essential link between the old analog world and the upcoming digital one. Contributed By: Robert Frieden Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.