Размещено на http://www.allbest.ru

Размещено на http://www.allbest.ru

Communicating over long distances has been a challenge throughout history. In ancient times, runners were used to carry important between messages 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 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.

1. The Telegraph

Beginning in the mid-1800s, the telegraph delivered the first inter-city, 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.

2. 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 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 (the 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 Corp.), which was a regulated monopoly.

3. The Emergence of Broadcasting

Telephones and telegraphs are point-to-point systems of telecommunications, but with the invention of the radio, point-to-multipoint signals could be sent through a central transmitter to be received by anyone possessing a receiver. Italian inventor and electrical engineer Guillermo 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.

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.

4. 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 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 along with 240 simultaneous telephone calls.

Intelsat has expanded and diversified to meet the global and regional satellite requirements of over 200 nations and territories. In response to private satellite ventures entering the market, the managers of Intelsat have sought to convert the cooperative into a corporation better able to compete with these emerging companies. A separate cooperative known as the International Mobile Satellite Organization (Inmarsat) primarily provides service to oceangoing vessels, but it has expanded operations to include service to airplanes and users in remote land areas not served by cellular radio or wireline services. Inmarsat also seeks to become a private corporation, because of competition from private satellite ventures.

5. 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. Innovations in fiber-optic technology will hopefully keep up with the growing use of personal computers for telecommunications. The next generation of cellular telephones, pagers, and televisions will also benefit from the speed and clarity of digital telecommunications.

Telecommunications and information technologies are merging and converging. This means that many of the devices that we associate 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. Advanced phones with keyboards and small screens are now in development that can access the Internet and send and receive e-mail. Personal computers can now access information and video entertainment and are in effect becoming a combined television set and computer terminal. Television sets, which we currently associate with broadcast and cable-delivered video programming, are able to gain access to the Internet through add-on appliances. Future modifications and technology innovations may blur the distinctions between appliances even more.

Convergence of telecommunications technologies will also trigger a change in the content available and the composition of the content provider. Both television and personal computers will be incorporating new multimedia, interactive, and digital features. For example, an entertainment program might have on-screen pointers to World Wide Web pages containing more information about the actors. In the near term, before the actualization of a fully digital telecommunications world, devices like modems will still be necessary to provide an essential link between the old analog world and the upcoming digital one.

Wireless Communications.

1. Introduction.

Wireless сommunications are various telecommunications systems that use radio waves to carry signals and messages across distances. Wireless communications systems use devices called transmitters to generate radio waves. A microphone or other mechanism converts messages, like sounds or other data, into electronic impulses. The transmitters change, or modulate, the radio waves so they can carry the impulses, and then transmit the modulated radio signals across distances. Radio receivers pick up these signals and decode them back into original messages. Commercial radio and television are also wireless telecommunications system, but radio and television are mainly public broadcast services rather than personal communications systems.

Wireless communications allow people greater flexibility while communicating, because they do not need to remain at a fixed location, such as a home or office. Wireless technologies make communications services more readily available than traditional wire-based services (such as ordinary telephones), which require the installation of wires. This is useful in places where only temporary communications services are needed, such as at outdoor festivals or large sporting events. These technologies are also useful for communicating in remote locations, such as mountains, jungles, or deserts, where telephone service might not exist. Wireless services allow people to communicate while in a car, airplane, or other moving vehicle. Police, fire, and other emergency departments use two-way radio to communicate information between vehicles that are already responding to emergency calls, which saves valuable time. Construction and utility workers frequently use hand-held radios for short-range communication and coordination. Many businesspeople use wireless communications, particularly cellular radio telephones, to stay in contact with colleagues and clients while traveling.

All wireless communications devices use radio waves to transmit and receive signals. These devices operate on different radio frequencies so that signals from one device will not overlap and interfere with nearby transmissions from other devices.

2. Principles of Wireless Communications.

Wireless communications begin with a message that is converted into an electronic signal by a device called a transmitter. The transmitter uses an oscillator to generate radio waves. The transmitter modulates the radio wave to carry the electronic signal and then sends the modified radio signal out through space, where it is picked up by a receiver. The receiver decodes, or demodulates, the radio wave and plays the decoded message over a speaker. Wireless communications provide more flexibility than wire-based means of communication. However, there are some drawbacks. Wireless communications are limited by the range of the transmitter (how far a signal can be sent), and since radio waves travel through the atmosphere, they can be disturbed by electrical interferences (such as lightning) that cause static.

Wireless communications systems involve either one-way transmissions, in which a person merely receives notice of a message, or two-way transmissions, such as a telephone conversation between two people. An example of a device that sends one-way transmission is a pager, which is a radio receiver. When a person dials a pager number, the pager company sends a radio signal to the desired pager. The encoded signal triggers the pager circuitry and notifies the customer carrying the pager of the incoming call with a tone or a vibration, and often the telephone number of the caller. Advanced pagers can display short messages from the caller, or provide news updates or sports scores.

Two-way transmissions require both a transmitter and a receiver for sending and receiving signals. A device that functions as both a transmitter and a receiver is called a transceiver. Cellular radio telephones and two-way radios use transceivers, so that back-and-forth communication between two people can be maintained. Early transceivers were very large, but they have decreased in size due to advances in technology. Fixed-base transceivers, such as those used at police stations, can fit on a desktop, and hand-held transceivers have shrunk in size as well. Several current models of hand-held transceivers weigh less than 0.2 kg (0.5 lb).

3. Modes of Wireless Communications.

Wireless communications systems have grown and changed as technology has improved. Several different systems are used today, all of which operate on different radio frequencies. New technologies are being developed to provide greater service and reliability.

A. Air Transceivers.

Radio operators still monitor distress channels, but maritime and aviation telecommunications systems now use high-frequency radios and satellites capable of transmitting speech, rather than wireless telegraphy, to send messages. Aircraft pilots use radios to communicate with air traffic controllers at airports and also to communicate with other pilots. Navigation beacons are equipped with transmitters that send automated signals to help ships and aircraft in distress determine their positions. While high-frequency radio can transmit signals over long distances, the quality of these signals can be diminished by bad weather or by electrical interference in the atmosphere, which is often caused by radiation from the sun.

B. Hand-Held Radio Transceivers.

Police, fire, and other emergency organizations, as well as the military, have used two-way wireless radio communication since the 1930s. Early vehicle-based radios were large, heavy units. After the invention of the transistor in 1948, radios shrank in size to small hand-held radio transceivers, which civil authorities now use to communicate with each other directly. Public two-way radios with several frequency options are widely available as well. Usually limited in range to a few miles, these units are great aids for such mobile professionals as construction workers, film crews, event planners, and security personnel. Simpler two-way radios, called walkie-talkies, have been popular children's toys for years.

C. Shortwave.

Long-range broadcast services and frequencies, in what is known as the shortwave radio band (with frequencies of 3 to 30 megahertz), are available for amateur or ham radio operators. Shortwave radio broadcasts can travel long distances because of the concentration of ionized, or electrically charged, particles in the layer of the atmosphere known as the ionosphere. This layer reflects radio signals, sending signals that are transmitted upward back to earth. This skipping of waves against the ionosphere can greatly increase the range of the transmitter. The degree of reflectivity of the ionosphere depends on the time of day.

D. Cellular Radio Telephones.

Cellular radio telephones, or cell phones, combine their portable radio capability with the wired, or wireline, telephone network to provide mobile users with access to the rest of the public telephone system used by non-mobile callers. Modern cellular telephones use a network of several short-range antennas that connect to the telephone system. Because the antennas have a shorter range, frequencies can be reused a short distance away without interference.

E. Satellite Communications.

Satellite communications services connect users directly to the telephone network from almost anywhere in the world. Special telephones are available to consumers that communicate directly with communications satellites orbiting the earth. The satellites transmit these signals to ground stations that are connected to the telephone system. These satellite services, while more expensive than cellular or other wireless services, give users access to the telephone network in areas of the world where no telephone service exists.

The number of companies offering wireless communications services has grown steadily in recent years. In 1988 about 500 companies offered cellular radio telephone (cell phone) services. By 1995 that number had grown to over 1500 companies serving millions of subscribers. Wireless communication is becoming increasingly popular because of the convenience and mobility it affords, the expanded availability of radio frequencies for transmitting, and improvements in technology.

Communications Satellites.

1. Introduction.

A. communications satellite is any earth-orbiting spacecraft that provides communication over long distances by reflecting or relaying radio-frequency signals.

2. History and Development.

Some of the first communications satellites were designed to operate in a passive mode. Instead of actively transmitting radio signals, they served merely to reflect signals that were beamed up to them by transmitting stations on the ground. Signals were reflected in all directions, so they could be picked up by receiving stations around the world. Echo 1, launched by the United States in 1960, consisted of an aluminized plastic balloon 30 m (100 ft) in diameter. Launched in 1964, Echo 2, was 41 m (135 ft) in diameter. The capacity of such systems was severely limited by the need for powerful transmitters and large ground antennas.

Satellite communications currently make exclusive use of active systems, in which each satellite carries it own equipment for reception and transmission. Score, launched by the United States in 1958, was the first active communications satellite. It was equipped with a tape recorder that stored messages received while passing over a transmitting ground station. These messages were retransmitted when the satellite passed over a receiving station. Telstar 1, launched by American Telephone and Telegraph Company in 1962, provided direct television transmission between the United States, Europe, and Japan and could also relay several hundred voice channels. Launched into an elliptical orbit inclined 45 ° to the equatorial plane, Telstar could only relay signals between two ground stations for a short period during each revolution, when both stations were in its line of sight.

Hundreds of active communications satellites are now in orbit. They receive signals from one ground station, amplify them, and then retransmit them at a different frequency to another station. One frequency band used, 500 MHZ wide, is divided into repeater channels of various bandwidths (located at 6 GHZ for upward, or uplink, transmission and 4 GHZ for downward, or downlink, transmission). A band at 14 GHZ (uplink) and 11 or 12 GHZ (downlink) is also much in use, mostly with fixed (non-mobile) ground stations. An 80-MH Z-wide band at about 1.5 GHZ (up-and downlink) is used with small, mobile ground stations (ships, land vehicles, and aircraft). Solar energy cells mounted on large panels attached to the satellite provide power for reception and transmission.

3. Geosynchronous Orbit.

A satellite in a geosynchronous orbit follows a circular orbit over the equator at an altitude of 35,800 km (22,300 mi) completing one orbit every 24 hours, in the time that it takes the earth to rotate once. Moving in the same direction as the earth's rotation, the satellite remains in a fixed position over a point on the equator, thereby providing uninterrupted contact between ground stations in its line of sight. The first communications satellite to be placed in this type of orbit was Syncom 2, launched by the National Aeronautics and Space Administration (NASA) in 1963. Most of those that followed were also placed in geosynchronous orbit.

4. Commercial Communications Satellites.

Deployment and operation of communications satellites on a commercial basis began with the founding of the Communications Satellite Corporation (COMSAT) in 1963. When the International Telecommunications Satellite Organization (INTELSAT) was formed in 1964, COMSAT became the U.S. member. Based in Washington, D.C., INTELSAT is owned by more than 120 nations. Intelsat 1, known as Early Bird, launched in 1965, provided either 2400 voice circuits or one two-way television channel between the United States and Europe. During the 1960s and 1970s, message capacity and transmission power of the Intelsat 2, 3, and 4 generations were progressively increased by beaming the satellite power only to the earth and segmenting the broadcast spectrum into transponder units of a certain bandwidth. The first of the Intelsat 4s, launched in 1971, provided 4000 voice circuits. With the Intelsat 5 series (1980), introduction of multiple beam operation resulted in additional increases in capacity. A satellite's power could now be concentrated on small regions of the earth, making possible smaller-aperture, lower-cost ground stations. An Intelsat 5 satellite can typically carry 12,000 voice circuits. The Intelsat 6 satellites, which entered service in 1989, can carry 24,000 circuits and feature dynamic on-board switching of telephone capacity among six beams, using a technique called SS-TDMA (satellite-switched time division multiple access). By the early 1990s, Intelsat had 15 satellites in orbit, providing the world's most extensive telecommunications system. Other systems also provide international service in competition with Intelsat. By 1997, all regulatory restraints to such competition will have been lifted. The growth of international systems has been paralleled by domestic and regional systems, such as the U.S. Telstar, Galaxy, and Spacenet programs and Europe's Eutalsat and Telecom.

5. Services.

Commercial satellites provide a wide range of communications services. Television programs are relayed internationally, giving rise to the phenomenon known as the "global village." Satellites also relay programs to cable television systems as well as to homes equipped with dish antennas. In addition, very small aperture terminals (VSATs) relay digital data for a multitude of business services. Intelsat satellites now carry over 100,000 telephone circuits, with growing use of digital transmission. Digital source coding methods (see Telecommunications) have resulted in a ten-fold reduction in the transmission rate needed to carry a voice channel, thus enhancing the capacity of existing facilities and reducing the size of ground stations that provide telephone service.

The International Mobile Satellite Organization (INMARSAT), founded in 1979 as the International Maritime Satellite Organization, is a mobile telecommunications network, providing digital data links, telephone, and facsimile transmission, or fax, service between ships, offshore facilities, and shore-based stations throughout the world. It is also now extending satellite links for voice and fax transmission to aircraft on international routes.

6. Recent Technical Advances.

Communications satellite systems have entered a period of transition from point-to-point high-capacity trunk communications between large, costly ground terminals to multipoint-to-multipoint communications between small, low-cost stations. The development of multiple access methods has both hastened and facilitated this transition. With TDMA, each ground station is assigned a time slot on the same channel for use in transmitting its communications; all other stations monitor these slots and select the communications directed to them. By amplifying a single carrier frequency in each satellite repeater, TDMA ensures the most efficient use of the satellite's onboard power supply.

A technique called frequency reuse allows satellites to communicate with a number of ground stations using the same frequency by transmitting in narrow beams pointed toward each of the stations. Beam widths can be adjusted to cover areas as large as the entire United States or as small as a state like Maryland. Two stations far enough apart can receive different messages transmitted on the same frequency. Satellite antennas have been designed to transmit several beams in different directions, using the same reflector.

A new method for interconnecting many ground stations spread over great distances is scheduled to be tested in 1993, with the launch of NASA's ACTS (Advanced Communications Technology Satellite). Known as the hopping spot beam technique, it combines the advantages of frequency reuse, spot beams, and TDMA. By concentrating the energy of the satellite's transmitted signal, ACTS can use ground stations that have smaller antennas and reduced power requirements.

The concept of multiple spot beam communications was successfully demonstrated in 1991 with the launch of Italsat, developed by the Italian Research Council. With six spot beams operating at 30 GHZ (uplink) and 20 GHZ (downlink), the satellite interconnects TDMA transmissions between ground stations in all the major economic centers of Italy. It does this by demodulating uplink signals routing them between up- and downlink beams, and combining and remodulating them for downlink transmission.

The application of laser technology to satellite communications has been studied for over a decade. Laser beams can be used to transmit signals between a satellite and earth, but the rate of transmission is limited because of absorption and scattering by the atmosphere. Lasers operating in the blue-green wavelength, which penetrates water, have been used for communication between satellites and submarines. communicating broadcasting network satellite

Размещено на Allbest.ru