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.
Friday, December 24, 2010
2.4.7. Personal Computers and Links
Personal computers use telecommunications to provide a transmission link for the deliverance 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.
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.
2.4.6. Global Positioning and Navigation System
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.
2.4.5. 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.
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.
2.4.4. 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.
In a broadcast system, the central high-powered broadcast tower transmits a high-frequency electromagnetic wave to numerous low-powered receivers. The high-frequency wave sent by the tower is modulated with a signal containing visual or audio information. The receiver is then tuned so as to pick up the high-frequency wave and a demodulator is used to retrieve the signal containing the visual or audio information. The broadcast signal can be either analogue (signal is varied continuously with respect to the information) or digital (information is encoded as a set of discrete values).
The broadcast media industry is at a critical turning point in its development, with many countries moving from analogue to digital broadcasts. This move is made possible by the production of cheaper, faster and more capable integrated circuits. The chief advantage of digital broadcasts is that they prevent a number of complaints with traditional analogue broadcasts. For television, this includes the elimination of problems such as snowy pictures, ghosting and other distortion. These occur because of the nature of analogue transmission, which means that perturbations due to noise will be evident in the final output. Digital transmission overcomes this problem because digital signals are reduced to discrete values upon reception and hence small perturbations do not affect the final output. In a simplified example, if a binary message 1011 was transmitted with signal amplitudes [1.0 0.0 1.0 1.0] and received with signal amplitudes [0.9 0.2 1.1 0.9] it would still decode to the binary message 1011 — a perfect reproduction of what was sent. From this example, a problem with digital transmissions can also be seen in that if the noise is great enough it can significantly alter the decoded message. Using forward error correction a receiver can correct a handful of bit errors in the resulting message but too much noise will lead to incomprehensible output and hence a breakdown of the transmission.
2.4.3. 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.
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
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
2.4.2. Telephone
The telephone network also uses both wire line 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.
In an analogue telephone network, the caller is connected to the person he wants to talk to by switches at various telephone exchanges. The switches form an electrical connection between the two users and the setting of these switches is determined electronically when the caller dials the number. Once the connection is made, the caller's voice is transformed to an electrical signal using a small microphone in the caller's handset. This electrical signal is then sent through the network to the user at the other end where it is transformed back into sound by a small speaker in that person's handset. There is a separate electrical connection that works in reverse, allowing the users to converse.
The fixed-line telephones in most residential homes are analogue — that is, the speaker's voice directly determines the signal's voltage. Although short-distance calls may be handled from end-to-end as analogue signals, increasingly telephone service providers are transparently converting the signals to digital for transmission before converting them back to analogue for reception. The advantage of this is that digitized voice data can travel side-by-side with data from the Internet and can be perfectly reproduced in long distance communication (as opposed to analogue signals that are inevitably impacted by noise).
Mobile phones have had a significant impact on telephone networks. Mobile phone subscriptions now outnumber fixed-line subscriptions in many markets. Sales of mobile phones in 2005 totaled 816.6 million with that figure being almost equally shared amongst the markets of Asia/Pacific (204 m), Western Europe (164 m), CEMEA (Central Europe, the Middle East and Africa) (153.5 m), North America (148 m) and Latin America (102 m). In terms of new subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth. Increasingly these phones are being serviced by systems where the voice content is transmitted digitally such as GSM or W-CDMA with many markets choosing to depreciate analogue systems such as AMPS.
In an analogue telephone network, the caller is connected to the person he wants to talk to by switches at various telephone exchanges. The switches form an electrical connection between the two users and the setting of these switches is determined electronically when the caller dials the number. Once the connection is made, the caller's voice is transformed to an electrical signal using a small microphone in the caller's handset. This electrical signal is then sent through the network to the user at the other end where it is transformed back into sound by a small speaker in that person's handset. There is a separate electrical connection that works in reverse, allowing the users to converse.
The fixed-line telephones in most residential homes are analogue — that is, the speaker's voice directly determines the signal's voltage. Although short-distance calls may be handled from end-to-end as analogue signals, increasingly telephone service providers are transparently converting the signals to digital for transmission before converting them back to analogue for reception. The advantage of this is that digitized voice data can travel side-by-side with data from the Internet and can be perfectly reproduced in long distance communication (as opposed to analogue signals that are inevitably impacted by noise).
Mobile phones have had a significant impact on telephone networks. Mobile phone subscriptions now outnumber fixed-line subscriptions in many markets. Sales of mobile phones in 2005 totaled 816.6 million with that figure being almost equally shared amongst the markets of Asia/Pacific (204 m), Western Europe (164 m), CEMEA (Central Europe, the Middle East and Africa) (153.5 m), North America (148 m) and Latin America (102 m). In terms of new subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth. Increasingly these phones are being serviced by systems where the voice content is transmitted digitally such as GSM or W-CDMA with many markets choosing to depreciate analogue systems such as AMPS.
2.4.1. Telegraph
Telegraph services use both wire line 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.
2.4. Some Telecommunication Systems, Operations and Applications
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.
2.3. Telecommuting
Telecommuting, e-commuting, e-work, telework, working from home (WFH), or working at home (WAH) is a work arrangement in which employees enjoy flexibility in working location and hours. In other words, the daily commute to a central place of work is replaced by telecommunication links. Many work from home, while others, occasionally also referred to as nomad workers or web commuters utilize mobile telecommunications technology to work from coffee shops or myriad other locations. Telework is a broader term, referring to substituting telecommunications for any form of work-related travel, thereby eliminating the distance restrictions of telecommuting. All telecommuters are teleworkers but not all teleworkers are telecommuters. A frequently repeated motto is that "work is something you do, not something you travel to”. A successful telecommuting program requires a management style which is based on results and not on close scrutiny of individual employees. This is referred to as management by objectives as opposed to management by observation. The terms telecommuting and telework were coined by Jack Nilles in 1973.
All of the electronic links among the people in a modern office can be extended beyond the building walls to workers at home or in satellite offices. This capability has led to a sharp increase in telecommuting. In 1991 an estimated 5.5 million U.S. workers worked at least part of the time outside the main office, a 38 percent increase over 1990. Managers and professional employees were the major participants in this trend. Early reports of increased productivity among people who no longer spent hours traveling from home to office indicated that further increases in telecommuting were likely.
All of the electronic links among the people in a modern office can be extended beyond the building walls to workers at home or in satellite offices. This capability has led to a sharp increase in telecommuting. In 1991 an estimated 5.5 million U.S. workers worked at least part of the time outside the main office, a 38 percent increase over 1990. Managers and professional employees were the major participants in this trend. Early reports of increased productivity among people who no longer spent hours traveling from home to office indicated that further increases in telecommuting were likely.
iv. Satellites
Communications Satellite is any earth-orbiting spacecraft that provides communication over long distances by reflecting or relaying radio-frequency signals. Communication satellites are a kind of Line of Sight communication. Communications satellites provide a means of transmitting telecommunications all over the globe, without the need for a network of wires and cables. The communication is carried through uplinks and downlinks and downlinks. Uplinks and downlinks are also called earth stations because they are located on the earth. 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.
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. Satellites use ranges of different frequencies, measured in hertz (Hz) or cycles per second, for receiving and transmitting signals. Many satellites use a band of frequencies of about 6 billion hertz, or 6 gigahertz (GHz) for upward, or uplink, transmission and 4 GHZ for downward, or downlink, transmission. Another band at 14 GHZ (uplink) and 11 or 12 GHZ (downlink) is also much in use, mostly with fixed (non mobile) ground stations. A band at about 1.5 GHZ (for both uplink 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.
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 method for interconnecting many ground stations spread over great distances was demonstrated in 1993 with the launch of NASA's ACTS (Advanced Communications Technology Satellite). The satellite uses what is known as the hopping spot beam technique to combine 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. Laser beams can also be used to transmit signals between a satellite and the 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.
The latest development in satellites is the use of networks of small satellites in low earth orbit (2,000 km (1,200 mi) or less) to provide global telephone communication. The Iridium system uses 66 satellites in low earth orbit, while other groups have or are developing similar systems. Special telephones that communicate with these satellites allow users to access the regular telephone network and place calls from anywhere on the globe. Anticipated customers of these systems include international business travelers and people living or working in remote areas.
iii. Infrared
Infrared offers a great unbound photonic solution. Like fiber optic cabling, infrared communication uses light, and not being bound by the limitations of electricity. The wavelengths of infrared radiation are shorter than those of radio waves and longer than those of light waves. They range between approximately 10-6 and 10-3 (about 0.0004 and 0.04 in). Infrared radiation may be detected as heat, and instruments such as bolometer are used to detect it.
ii. Microwave
Microwaves are high-frequency radio waves lying roughly between very-high-frequency (infrared) waves and conventional radio waves. Microwaves thus range in length from about 1 mm to 30 cm (about 0.04 to 12 in). They are generated in special electron tubes, such as the klystron and the magnetron, with built-in resonators to control the frequency or by special oscillators or solid-state devices. Microwaves have many applications: in radio and television, radar, meteorology, satellite communications, distance measuring, and research into the properties of matter. Microwave Communications, in short, is a system to use of a high-frequency electromagnetic wave to transmit information. Microwave transmission can be described as microwave radio or microwave telegraphy. It involves receiving and resending microwave signals between relay stations. They are line of sight transmission.
i. Radio wave
Radio waves can operate on single or multiple frequency bands. In this case, the signals are carried aver carrier waves which have frequencies in the range of radio frequency spectrum. There are three main types of Radio Frequency (RF), namely, ground wave, ionospheric and line of sight. 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.
b) Unbound transmission media or Wireless communication
Wireless communication extends beyond the limiting confines of cabling. They provide an excellent communication alternative for WANs. The lack of physical restrictions provides larger bandwidth as well as wide area capabilities. Wireless communications begin with a message that is converted into an electronic signal by a device called a transmitter. There are two types of transmitters: analog and digital. An analog transmitter sends electronic signals as modulated radio waves. The analog transmitter modulates the radio wave to carry the electronic signal and then sends the modified radio signal through space. A digital transmitter encodes electronic signals by converting messages into a binary code, the series of zeros and ones that are the basis of all computer programming. The encoded electronic signal is then sent as a radio wave. Devices known as receivers decode or demodulate the radio waves and reproduce the original 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 only receives one-way transmission is a pager, which is a high-frequency 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’s 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 handheld transceivers weigh less than 0.2 kg (0.5 lb). Some pagers also use transceivers to provide limited response options. These brief return-communication opportunities allow paging users to acknowledge reception of a page and to respond using a limited menu of options. Some unbound medias are discussed below:
iv. Optical Fiber
Optical Fiber plays a complete different set of rules than other bounded media. You won’t find any electricity here. Instead, fiber-optic cabling uses pulses of light (photons) for network communications, i.e. it relies on photonics instead of electronics. As a result, it is completely immune to EMI (Electro Magnetic Interference) and is extremely fast. The benefit of communicating with optic fibers is that they offer a drastic increase in data capacity. This increase in data capacity is due to several factors: First, optic fibers are physically much smaller than competing technologies. Second, they do not suffer from crosstalk which means several hundred of them can be easily bundled together in a single cable. Lastly, improvements in multiplexing have led to an exponential growth in the data capacity of a single fiber. Assisting communication across many modern optic fiber networks is a protocol known as Asynchronous Transfer Mode.
Fiber Optical cabling pertains the transmission of light through hair-thin, transparent fibers. As mentioned earlier, data travel in the form of light. Light signals that enter at one end of a fiber travel through the fiber with very low loss of light, even if the fiber are curved. A basic fiber-optic system consists of a transmitting device (which generates the light signal), an optical-fiber cable (which carries the light), and a receiver (which accepts the transmitted light signal and converts it to an electrical signal). A principle called total internal reflection allows optical fibers to retain the light they carry. When light passes from a dense substance into a less dense substance, there is an angle, called the critical angle, beyond which 100 percent of the light is reflected from the surface between substances. Total internal reflection occurs when light strikes the boundary between substances at an angle greater than the critical angle. An optical-fiber core is clad (coated) by a lower density glass layer. Light traveling inside the core of an optical fiber strikes the outside surface at an angle of incidence greater than the critical angle so that all the light is reflected toward the inside of the fiber without loss. As long as the fiber is not curved too sharply, light traveling inside cannot strike the outer surface at less than the critical angle. Thus, light can be transmitted over long distances by being reflected inward thousands of times with no loss.
Use of fiber optics in communications is growing. Fiber-optic communications systems have key advantages over older types of communication. They offer vastly increased bandwidths, allowing tremendous amounts of information to be carried quickly from place to place. They also allow signals to travel for long distances without repeaters, which are needed to compensate for reductions in signal strength. Fiber-optic repeaters are currently about 100 km (about 62 mi) apart, compared to about 1.5 km (about 1 mi) for electrical systems. Many long-distance fiber-optic communications networks for both transcontinental connections and undersea fiber cables for international connections are in operation. Companies such as AT&T, MCI WorldCom, and Sprint have virtually replaced their long-distance copper lines with optical-fiber cables. Local telephone service providers use fiber-optic cables between central office switches and sometimes extend it into neighborhoods and even individual homes. Cable television companies transmit high-bandwidth TV signals to subscribers via fiber-optic cable.
Fiber Optical cabling pertains the transmission of light through hair-thin, transparent fibers. As mentioned earlier, data travel in the form of light. Light signals that enter at one end of a fiber travel through the fiber with very low loss of light, even if the fiber are curved. A basic fiber-optic system consists of a transmitting device (which generates the light signal), an optical-fiber cable (which carries the light), and a receiver (which accepts the transmitted light signal and converts it to an electrical signal). A principle called total internal reflection allows optical fibers to retain the light they carry. When light passes from a dense substance into a less dense substance, there is an angle, called the critical angle, beyond which 100 percent of the light is reflected from the surface between substances. Total internal reflection occurs when light strikes the boundary between substances at an angle greater than the critical angle. An optical-fiber core is clad (coated) by a lower density glass layer. Light traveling inside the core of an optical fiber strikes the outside surface at an angle of incidence greater than the critical angle so that all the light is reflected toward the inside of the fiber without loss. As long as the fiber is not curved too sharply, light traveling inside cannot strike the outer surface at less than the critical angle. Thus, light can be transmitted over long distances by being reflected inward thousands of times with no loss.
Use of fiber optics in communications is growing. Fiber-optic communications systems have key advantages over older types of communication. They offer vastly increased bandwidths, allowing tremendous amounts of information to be carried quickly from place to place. They also allow signals to travel for long distances without repeaters, which are needed to compensate for reductions in signal strength. Fiber-optic repeaters are currently about 100 km (about 62 mi) apart, compared to about 1.5 km (about 1 mi) for electrical systems. Many long-distance fiber-optic communications networks for both transcontinental connections and undersea fiber cables for international connections are in operation. Companies such as AT&T, MCI WorldCom, and Sprint have virtually replaced their long-distance copper lines with optical-fiber cables. Local telephone service providers use fiber-optic cables between central office switches and sometimes extend it into neighborhoods and even individual homes. Cable television companies transmit high-bandwidth TV signals to subscribers via fiber-optic cable.
iii. Co-axial Cable
In communications systems, cables commonly consist of numerous pairs of paper-insulated wire, encased in a lead sheath; the individual pairs of wire are intertwined to minimize induced interference with other circuits in the same cable. To avoid electrical interference from external circuits, cables used in radio broadcasting are often shielded with a winding of metal braid, which is grounded. The development of the coaxial cable was an important advance in the communications field. This type of cable consists of several copper tubes; each tube contains a wire conductor that extends along its center. The entire cable is sheathed in lead and is generally filled with nitrogen under pressure to prevent corrosion. It has a single central conductor made of solid wire and is surrounded by a jacket made of Polyvinyl chloride (PVC). Its more frequency spectrum offers stronger transmission. It has multiplexing and broadband facilities. Because the coaxial cable has a broad frequency range, it is valuable in the transmission of carrier-current telephony.
ii. Shielded Twisted Pair (STP)
STP cable has a metal foil or braided-mesh covering that covers each pair of insulated conductors. The metal foil is used to prevent infiltration of electromagnetic noise. This shield also helps to eliminate crosstalk during telephone conversation. STP provides protection against EMI (Electro Magnetic Interference), which UTP doesn’t. Twisted pairs have limited frequency spectrum and thus require repeaters at short distances.
i. Unshielded Twisted Pair (UTP)
UTP is the most common type of telecommunication media these days. It is most suited for data and voice communication. It consists of two metal conductor hats that are insulated separately with their own colored plastic insulation. It can transmit up to 96000 bps. UTP usually is intended for analog communications, not digital.
a) Bound Transmission Media
As said earlier, bound transmission media consists of a central conductor surrounded by a physical jacket. This property offers advantage in security, reliability, and speed. Bound media are ideal for LANs, but distance limitations can be a problem for WANs. The bounded medias are:
2.2.2. Transmission of data through Communication Media
Transmission media provide the physical path through which electrons flow. At the physical layer of OSI model of networking, electrons represent network data as binary 0s and 1s. Transmission media provides these electrons with a bound or unbound communication path. Telephone lines are well proven and commonly used communication media.
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 wire line) 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.
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 wire line) 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.
2.2.1. Creating and Receiving Signals
Telegraphs, telephones, radio, and televisions 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.
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.
2.2. How Telecommunications Work
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.
2.1. Introduction
The term “Telecommunications” represents the 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 (point-to-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, usually involve point-to-point transmission. Point-to-multipoint telecommunications, often called broadcasts, provide the basis for commercial radio and television programming.
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 (point-to-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, usually involve point-to-point transmission. Point-to-multipoint telecommunications, often called broadcasts, provide the basis for commercial radio and television programming.
1.3.7. Bandwidth
Bandwidth, in computer science, refers to the amount of information that can be sent through a connection between two computers in a given amount of time. Computers may be connected by telephone wires, by coaxial cable, or through radio waves or microwaves. A connection that can transmit more data in a shorter period of time is said to have more bandwidth than another, slower connection.
The term bandwidth originated with radio broadcasting and in that context refers to the amount of the electromagnetic spectrum (i.e. the range of frequencies) that is allocated for a specific use. A band consists of a range of frequencies, and its width is determined by the difference between its highest and lowest frequencies. For instance, FM radio stations are allotted 200 kilohertz (200,000 cycles per second) of bandwidth. Since FM broadcasting relies on changes in frequency to transmit information, a station at 102.5 on the FM dial is using frequencies between 102.4 MHz and 102.6 MHz, with a small buffer at either end, to broadcast its information. In contrast, AM radio stations, which use changes in amplitude rather than frequency for information transmission, do not require as much bandwidth and are allotted only 10kHz.
Bandwidth in computers is measured not in cycles per second but by the number of bits per second (bps) that can be sent over a connection. A bit is the smallest unit of information a computer uses and it may have a value of either 0 or 1. Bits are usually combined into groups of eight to form bytes that can represent visual information, such as a letter, number, punctuation mark, or symbol. Computers manipulate bits by turning small switches, called transistors, off and on. When a transistor is on, bits can pass through the computer connection. When a transistor is off, the bits stop traveling through the connection.
Early telecommunication systems had low bandwidths that sent information at a relatively slow speed. The first teletype machines, used by newspapers to send news stories, operated at just 110 bps, or about 11 characters per second. At this speed it took a full second to transmit the word characters and the space that follows it. That is extremely slow by modern standards—today information can be sent at speeds of millions or even billions of bits per second. The text of this article would reach your computer in less than a second using some of today’s high-speed data connections. To simplify the expression of these larger bandwidth speeds, bandwidth is usually expressed in units of kbps (where k stands for kilo, the metric term for thousand), mbps (where m stands for mega, a million), or gbps (in which g stands for giga, a billion). Using this shorthand, you can shorten 56,000 bps to 56 kbps and 1,500,000 bps to 1.5 mbps.
Bandwidth directly affects the quality of transmitted information. For example, when a caller telephones into a radio show, the caller’s voice is not as clear to a listener as the radio host’s voice because the bandwidth of a telephone connection is smaller than the bandwidth of radio signals. The larger bandwidth of radio signals can carry a broader range of sound frequencies, and this improves the sound quality of the human voice. The telecommunications industry, including telephone companies, cable companies, and Internet service providers, continually seeks new technologies that will increase the amount of bandwidth it can provide in order to improve the quality of information flow and attract more customers.
1.3.6. Multiplexing
Multiplexing, in computer science, is a technique used in communications and input/output operations for transmitting a number of separate signals simultaneously over a single channel or line. Multiplexing or Muxing refers to the process of funneling multiple data connections into one circuit for transport across a single medium. To maintain the integrity of each signal on the channel, multiplexing can separate the signals by time, space, or frequency. The device used to combine the signals is a multiplexer. The most common instance would be the availability of numerous channels or stations through a single piece of coaxial cabling.
c. Phase Modulation
It is a method of transmitting a voice or other signal in which the phase of a radio carrier wave is varied in accordance with the signal. In such system of modulation, the phase of the carrier wave is changed to encode the digital information.
b. Frequency Modulation
Frequency Modulation represents a system of radio transmission in which the carrier wave is modulated so that its frequency varies with the audio signals being transmitted. The first workable system for radio communication was described by the American inventor Edwin H. Armstrong in 1936.
Frequency modulation has several advantages over the system of amplitude modulation (AM) used in the alternate form of radio broadcasting. The most important of these advantages is that an FM system has greater freedom from interference and static. Various electrical disturbances, such as those caused by thunderstorms and automobile ignition systems; create amplitude modulated radio signals that are received as noise by AM receivers. A well-designed FM receiver is not sensitive to such disturbances when it is tuned to an FM signal of sufficient strength. Also, the signal-to-noise ratio in an FM system is much higher than that of an AM system. Finally, FM broadcasting stations can be operated in the very-high-frequency bands at which AM interference is frequently severe; commercial FM radio stations are assigned frequencies between 88 and 108 MHz The range of transmission on these bands is limited so that stations operating on the same frequency can be located within a few hundred miles of one another without mutual interference.
Frequency modulation has several advantages over the system of amplitude modulation (AM) used in the alternate form of radio broadcasting. The most important of these advantages is that an FM system has greater freedom from interference and static. Various electrical disturbances, such as those caused by thunderstorms and automobile ignition systems; create amplitude modulated radio signals that are received as noise by AM receivers. A well-designed FM receiver is not sensitive to such disturbances when it is tuned to an FM signal of sufficient strength. Also, the signal-to-noise ratio in an FM system is much higher than that of an AM system. Finally, FM broadcasting stations can be operated in the very-high-frequency bands at which AM interference is frequently severe; commercial FM radio stations are assigned frequencies between 88 and 108 MHz The range of transmission on these bands is limited so that stations operating on the same frequency can be located within a few hundred miles of one another without mutual interference.
a. Amplitude Modulation
Modulation of the carrier wave so that it may carry impulses is performed either at low level or high level. In the former case the audio-frequency signal from the microphone, with little or no amplification, is used to modulate the output of the oscillator, and the modulated carrier frequency is then amplified before it is passed to the antenna; in the latter case the radio-frequency oscillations and the audio-frequency signal are independently amplified, and modulation takes place immediately before the oscillations are passed to the antenna. The signal may be impressed on the carrier either by frequency modulation (FM) or amplitude modulation (AM).
The carrier wave may also be modulated by varying the amplitude, or strength, of the wave in accordance with the variations of frequency and intensity of a sound signal, such as a musical note. This form of modulation, AM, is used in many radiotelephony services including standard radiobroadcasts. AM is also employed for carrier current telephony, in which the modulated carrier is transmitted by wire, and in the transmission of still pictures by wire or radio.
The carrier wave may also be modulated by varying the amplitude, or strength, of the wave in accordance with the variations of frequency and intensity of a sound signal, such as a musical note. This form of modulation, AM, is used in many radiotelephony services including standard radiobroadcasts. AM is also employed for carrier current telephony, in which the modulated carrier is transmitted by wire, and in the transmission of still pictures by wire or radio.
Radio Modulation
Audio-frequency waves must be combined with carrier waves in order to be transmitted over the radio. Either the frequency (rate of oscillation) or the amplitude (height) of the waves may be modified in a process called modulation. This accounts for the option on the radio dial for AM or FM stations; the signals are very different, so both kinds may not be received simultaneously.
The shaping of a signal to convey information is known as modulation. Modulation can be used to represent a digital message as an analogue waveform. This is known as keying and several keying techniques exist (these include phase-shift keying, frequency-shift keying and amplitude-shift keying). Bluetooth, for example, uses phase-shift keying to exchange information between devices. The simplest form of modulation is keying, interrupting the carrier wave at intervals with a key or switch used to form the dots and dashes in continuous-wave radiotelegraphy.
Modulation can also be used to transmit the information of analogue signals at higher frequencies. This is helpful because low-frequency analogue signals cannot be effectively transmitted over free space. Hence the information from a low-frequency analogue signal must be superimposed on a higher-frequency signal (known as the carrier wave) before transmission. There are several different modulation schemes available to achieve this (two of the most basic being amplitude modulation and frequency modulation). An example of this process is a DJ's voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel "96 FM").
The shaping of a signal to convey information is known as modulation. Modulation can be used to represent a digital message as an analogue waveform. This is known as keying and several keying techniques exist (these include phase-shift keying, frequency-shift keying and amplitude-shift keying). Bluetooth, for example, uses phase-shift keying to exchange information between devices. The simplest form of modulation is keying, interrupting the carrier wave at intervals with a key or switch used to form the dots and dashes in continuous-wave radiotelegraphy.
Modulation can also be used to transmit the information of analogue signals at higher frequencies. This is helpful because low-frequency analogue signals cannot be effectively transmitted over free space. Hence the information from a low-frequency analogue signal must be superimposed on a higher-frequency signal (known as the carrier wave) before transmission. There are several different modulation schemes available to achieve this (two of the most basic being amplitude modulation and frequency modulation). An example of this process is a DJ's voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel "96 FM").
1.3.4. Carrier wave
1.3.3. Channels
A channel is a division in a transmission medium so that it can be used to send multiple streams of information. For example, a radio station may broadcast at 96.1 MHz while another radio station may broadcast at 94.5 MHz In this case, the medium has been divided by frequency and each channel has received a separate frequency to broadcast on. Alternatively, one could allocate each channel a recurring segment of time over which to broadcast—this is known as time-division multiplexing and is used in optic fiber communication.
The channel is the medium that is used to transmit the signal. The channel is often noisy, in the sense that when the signal arrives at the receiver, it may contain noise or static, or it may be slightly garbled. For example, the channel could be the millions of kilometers of empty space between Jupiter and Earth, with noise arising because the received signal is so weak. Or it could be the surface of a CD, with noise occurring because of fingerprints, dust, or scratches on the surface.
The channel is the medium that is used to transmit the signal. The channel is often noisy, in the sense that when the signal arrives at the receiver, it may contain noise or static, or it may be slightly garbled. For example, the channel could be the millions of kilometers of empty space between Jupiter and Earth, with noise arising because the received signal is so weak. Or it could be the surface of a CD, with noise occurring because of fingerprints, dust, or scratches on the surface.
Computer Networking
Networks are connections between groups of computers and associated devices that allow users to transfer information electronically. The local area network shown on the left is representative of the setup used in many offices and companies. Individual computers are called work stations (W.S.), and communicate to each other via cable or telephone line linking to servers. Servers are computers exactly like the W.S., except that they have administrative functions and are devoted entirely to monitoring and controlling W.S. access to part or all of the network and to any shared resources (such as printers). The red line represents the larger network connection between servers, called the backbone; the blue line shows local connections. A modem (modulator/demodulator) allows computers to transfer information across standard telephone lines. Modems convert digital signals into analog signals and back again, making it possible for computers to communicate, or network, across thousands of miles.
Computers can communicate with other computers through a series of connections and associated hardware called a network. The advantage of a network is that data can be exchanged rapidly, and software and hardware resources, such as hard-disk space or printers, can be shared. Networks also allow remote use of a computer by a user who cannot physically access the computer. One type of network, a local area network (LAN), consists of several PCs or workstations connected to a special computer called a server, often within the same building or office complex. The server stores and manages programs and data. A server often contains all of a networked group’s data and enables LAN workstations or PCs to be set up without large storage capabilities. In this scenario, each PC may have “local” memory (for example, a hard drive) specific to itself, but the bulk of storage resides on the server. This reduces the cost of the workstation or PC because less expensive computers can be purchased, and it simplifies the maintenance of software because the software resides only on the server rather than on each individual workstation or PC.
Mainframe computers and supercomputers commonly are networked. They may be connected to PCs, workstations, or terminals that have no computational abilities of their own. These “dumb” terminals are used only to enter data into, or receive output from, the central computer. Wide area networks (WANs) are networks that span large geographical areas. Computers can connect to these networks to use facilities in another city or country. For example, a person in Los Angeles can browse through the computerized archives of the Library of Congress in Washington, D.C. The largest WAN is the Internet, a global consortium of networks linked by common communication programs and protocols (a set of established standards that enable computers to communicate with each other). The Internet is a mammoth resource of data, programs, and utilities. American computer scientist Vinton Cerf was largely responsible for creating the Internet in 1973 as part of the United States Department of Defense Advanced Research Projects Agency (DARPA). In 1984 the development of Internet technology was turned over to private, government, and scientific agencies. The World Wide Web, developed in the 1980s by British physicist Timothy Berners-Lee, is a system of information resources accessed primarily through the Internet. Users can obtain a variety of information in the form of text, graphics, sounds, or video. These data are extensively cross-indexed, enabling users to browse (transfer their attention from one information site to another) via buttons, highlighted text, or sophisticated searching software known as search engines.
Computers can communicate with other computers through a series of connections and associated hardware called a network. The advantage of a network is that data can be exchanged rapidly, and software and hardware resources, such as hard-disk space or printers, can be shared. Networks also allow remote use of a computer by a user who cannot physically access the computer. One type of network, a local area network (LAN), consists of several PCs or workstations connected to a special computer called a server, often within the same building or office complex. The server stores and manages programs and data. A server often contains all of a networked group’s data and enables LAN workstations or PCs to be set up without large storage capabilities. In this scenario, each PC may have “local” memory (for example, a hard drive) specific to itself, but the bulk of storage resides on the server. This reduces the cost of the workstation or PC because less expensive computers can be purchased, and it simplifies the maintenance of software because the software resides only on the server rather than on each individual workstation or PC.
Mainframe computers and supercomputers commonly are networked. They may be connected to PCs, workstations, or terminals that have no computational abilities of their own. These “dumb” terminals are used only to enter data into, or receive output from, the central computer. Wide area networks (WANs) are networks that span large geographical areas. Computers can connect to these networks to use facilities in another city or country. For example, a person in Los Angeles can browse through the computerized archives of the Library of Congress in Washington, D.C. The largest WAN is the Internet, a global consortium of networks linked by common communication programs and protocols (a set of established standards that enable computers to communicate with each other). The Internet is a mammoth resource of data, programs, and utilities. American computer scientist Vinton Cerf was largely responsible for creating the Internet in 1973 as part of the United States Department of Defense Advanced Research Projects Agency (DARPA). In 1984 the development of Internet technology was turned over to private, government, and scientific agencies. The World Wide Web, developed in the 1980s by British physicist Timothy Berners-Lee, is a system of information resources accessed primarily through the Internet. Users can obtain a variety of information in the form of text, graphics, sounds, or video. These data are extensively cross-indexed, enabling users to browse (transfer their attention from one information site to another) via buttons, highlighted text, or sophisticated searching software known as search engines.
1.3.2. Networks
A network is a collection of transmitters, receivers and transceivers that communicate with each other. It is a collection of distributed, intelligent machines that share data and information interconnected lines of communication being either bound or unbound. Digital networks consist of one or more routers that work together to transmit information to the correct user. An analogue network consists of one or more switches that establish a connection between two or more users. For both types of network, repeaters may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combat attenuation that can render the signal indistinguishable from noise.
b. Digital signals
Digital data is the data stored in the form of 0s and 1s. When the signal is at a high point, its value is 1 and when it is low, its value is 0. A signal in digital format has precise voltages that are not affected by noise or attenuation compared to analog signals, which are very prone to noise. Digital data represent two states either 0 or 1.
a. Analog signals
An analog signal is a continuous waveform that changes smoothly over time. The analog signals represent the continuous variation in the data. When data are plotted on the Y-axis with time, signal representing curves are obtained. During data transmission, the signals are modulated or changed in analog form.
1.3.1. Analogue or digital
Signals can be either analogue or digital. In an analogue signal, the signal is varied continuously with respect to the information. In a digital signal, the information is encoded as a set of discrete values (for example ones and zeros). During transmission the information contained in analogue signals will be degraded by noise. Conversely, unless the noise exceeds a certain threshold, the information contained in digital signals will remain intact. Noise resistance represents a key advantage of digital signals over analogue signals.
1.3. Basic elements
A basic telecommunication system consists of three elements:
A transmitter that takes information and converts it to a signal;
A transmission medium that carries the signal; and,
Receivers that receive the signal and convert it back into usable information.
For example, in a radio broadcast the broadcast tower is the transmitter, free space is the transmission medium and the radio is the receiver. Often telecommunication systems are two-way with a single device acting as both a transmitter and receiver or transceiver. For example, a mobile phone is a transceiver. Telecommunication over a telephone line is called point-to-point communication because it is between one transmitter and one receiver. Telecommunication through radio broadcasts is called broadcast communication because it is between one powerful transmitter and numerous receivers.
A transmitter that takes information and converts it to a signal;
A transmission medium that carries the signal; and,
Receivers that receive the signal and convert it back into usable information.
For example, in a radio broadcast the broadcast tower is the transmitter, free space is the transmission medium and the radio is the receiver. Often telecommunication systems are two-way with a single device acting as both a transmitter and receiver or transceiver. For example, a mobile phone is a transceiver. Telecommunication over a telephone line is called point-to-point communication because it is between one transmitter and one receiver. Telecommunication through radio broadcasts is called broadcast communication because it is between one powerful transmitter and numerous receivers.
1.2.5. International Telecommunication System
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 wire line services. Inmarsat became a privatized, commercial venture in 1999.
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 wire line services. Inmarsat became a privatized, commercial venture in 1999.
1.2.4. Computer networks and the Internet
On 11 September 1940, George Stibitz was able to transmit problems using teletype to his Complex Number Calculator in New York and receive the computed results back at Dartmouth College in New Hampshire. This configuration of a centralized computer or mainframe with remote dumb terminals remained popular throughout the 1950s. However, it was not until the 1960s that researchers started to investigate packet switching — a technology that would allow chunks of data to be sent to different computers without first passing through a centralized mainframe. A four-node network emerged on 5 December 1969; this network would become ARPANET, which by 1981 would consist of 213 nodes. ARPANET's development centered on the Request for Comment process and on 7 April 1969, RFC 1 was published. This process is important because ARPANET would eventually merge with other networks to form the Internet and many of the protocols the Internet relies upon today were specified through the Request for Comment process. In September 1981, RFC 791 introduced the Internet Protocol v4 (IPv4) and RFC 793 introduced the Transmission Control Protocol (TCP) — thus creating the TCP/IP protocol that much of the Internet relies upon today. However, not all important developments were made through the Request for Comment process. Two popular link protocols for local area networks (LANs) also appeared in the 1970s. A patent for the token ring protocol was filed by Olof Soderblom on 29 October 1974 and a paper on the Ethernet protocol was published by Robert Metcalfe and David Boggs in the July 1976 issue of Communications of the ACM.
1.2.3. Radio and television
In 1832, James Lindsay gave a classroom demonstration of wireless telegraphy to his students. By 1854, he was able to demonstrate a transmission across the Firth of Tay from Dundee, Scotland to Woodhaven, a distance of two miles (3 km), using water as the transmission medium. In December 1901, Guglielmo Marconi established wireless communication between St. John's, Newfoundland (Canada) and Poldhu, Cornwall (England), earning him the 1909 Nobel Prize in physics (which he shared with Karl Braun). However small-scale radio communication had already been demonstrated in 1893 by Nikola Tesla in a presentation to the National Electric Light Association.
On 25 March 1925, John Logie Baird was able to demonstrate the transmission of moving pictures at the London department store Selfridges. Baird's device relied upon the Nipkow disk and thus became known as the mechanical television. It formed the basis of experimental broadcasts done by the British Broadcasting Corporation beginning 30 September 1929. However, for most of the twentieth century televisions depended upon the cathode ray tube invented by Karl Braun. The first version of such a television to show promise was produced by Philo Farnsworth and demonstrated to his family on 7 September 1927.
On 25 March 1925, John Logie Baird was able to demonstrate the transmission of moving pictures at the London department store Selfridges. Baird's device relied upon the Nipkow disk and thus became known as the mechanical television. It formed the basis of experimental broadcasts done by the British Broadcasting Corporation beginning 30 September 1929. However, for most of the twentieth century televisions depended upon the cathode ray tube invented by Karl Braun. The first version of such a television to show promise was produced by Philo Farnsworth and demonstrated to his family on 7 September 1927.
1.2.2. Telegraph and telephone
The first commercial electrical telegraph was constructed by Sir Charles Wheatstone and Sir William Fothergill Cooke and opened on 9 April 1839. Both Wheatstone and Cooke viewed their device as "an improvement to the electromagnetic telegraph" not as a new device.
Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837. His code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was successfully completed on 27 July 1866, allowing transatlantic telecommunication for the first time.
The conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876.Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849. However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London.
Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837. His code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was successfully completed on 27 July 1866, allowing transatlantic telecommunication for the first time.
The conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876.Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849. However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London.
1.2.1Evolution of Early telecommunications
In the middle Ages, chains of beacons were commonly used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London signaling the arrival of Spanish ships.
In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system (or semaphore line) between Lille and Paris. However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometers (six to nineteen miles). As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880.
In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system (or semaphore line) between Lille and Paris. However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometers (six to nineteen miles). As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880.
1.2. Background
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.
1.1. Introduction
The word telecommunication was adapted from the French word télécommunication. It is a compound of the Greek prefix tele, meaning 'far off', and the Latin communicare, meaning 'to share'. The French word télécommunication was coined in 1904 by French engineer and novelist Édouard Estaunié. Telecommunication is the transmission of signals over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, drums, semaphore, flags or heliograph. In modern times, telecommunication typically involves the use of electronic devices such as telephones, television, radio or computers. Early inventors in the field of telecommunication include Alexander Graham Bell, Guglielmo Marconi and John Logie Baird. Telecommunication is an important part of the world economy and the telecommunication industry's revenue was estimated to be $1.2 trillion in 2006.
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