Communications and Connectivity 269
larger cable. This type of communications line has become very popular because of its capacity and reduced need for signals to he refreshed (every 2 to 4 miles). Coaxial cables are most often used as the primary communications medium for locally connected networks in which all computer communication is within a limited geographic area, such as in the same building. Computers connected by coaxial cable do not need to use modems. Coaxial cable is also used for undersea telephone lines.
Point-to-Point and Multipoint Lines When devices are connected to a telephone line or coaxial cable line, one of two principal configurations is generally used: point-to-point and multipoint. • Point-to-point: A point-to-point line is a single line that directly connects the sending and receiving devices, such as a terminal V p. 431 and a central computer. There is no intermediate computer. This arrangement is. appropriate for a private line whose sole purpose is to keep data secure by transmitting it only from one particular device to another. • Mu ltipoint: A multipoint line (also called a multidrop line) is a single line that interconnects several communications devices to one computer. Often on a multipoint line only one communications device, such as a terminal, can transmit at any given time.
Electromagnetic Waves: Microwave and Satellite Systems
Microwave Systems Instead of using wire or cable, microwave systems use the earth’s atmosphere as the medium through which to transmit signals. These systems are extensively used for high-volume as well as long-distance communication of electromagnetic waves both data and voice in the form of electromagnetic waves. These waves are similar to radio waves but are in a higher frequency range. Microwave signals are often referred to as “line of sight” signals because they cannot bend around the curvature of the earth; instead, they must be relayed from point to point by microwave towers, or relay stations, placed 20 to 30 miles apart. (See Figure 8.6.) The distance between the towers depends on the curvature of the surface terrain in the vicinity. The surface of the earth typically curves about 8 inches every mile. The towers have either a dish- or a horn-shaped antenna. The size of the antenna varies according to the distance the signals must cover. A long-distance antenna could easily be 10 feet or larger in size; a disk of 2 to 4 feet in diameter, which you often see on city buildings, is large enough for small distances. Each tower facility receives incoming traffic, boosts the signal strength, and sends the signal to the next station. The primary advantage of using microwave systems for voice and data communications is that direct physical cabling is not required. (Obviously, telephone lines and other types of cable must physically connect all communications system points that can’t receive atmospheric signals.) More than half of the telephone system now uses microwave transmission. However, in some areas, the saturation of the airwaves with microwave transmissions has reached the point where future needs will have to be satisfied by other communications methods, such as satellite systems.
Satellite Systems Satellite communications systems transmit signals in the gigahertz range— billions of cycles per second. The satellite must be placed in a geosynchronous orbit. 22.300 miles above the earth’s surface so it revolves once a day
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Figure 8.6 Microwave systems. Microwaves cannot bend around corners or around the curvature of the earth. Therefore, microwave antennas must be in “line of sight” of each other—that is, unobstructed. Microwave dishes and relay towers are usually situated atop high places, such as mountains or tall buildings, so that signals can be beamed over uneven terrain.
Line-of-sight signal Microwave relay station A( ov
with the earth. (See Figure 8.7.) To an observer, it appears to be fixed over one region at all times. A satellite is a solar-powered electronic device that has up to 100 transponders (a transponder is a small, specialized radio) that receive, amplify, and retransmit signals; the satellite acts as a relay station between satellite transmission stations on the ground (called earth stations). Although establishing satellite systems is costly (owing to the cost of a satellite and the problems associated with getting it into orbit above the earth’s surface and compensating for failures), satellite communications systems have become the most popular and cost-effective method for moving large quantities of data over long distances. The primary advantage of satellite communications is the vast area that can be covered by a single satellite. Three satellites placed in particular orbits can cover the entire surface of the earth, with some overlap. However, satellite transmission does have some problems:
1. The signals can weaken over long distances, and weather conditions and solar activity can cause noise interference.
2. A satellite is useful for only seven to ten years, after which it loses its orbit.
3. Anyone can listen in on satellite signals, so sensitive data must be sent in a secret, or encrypted, form.
4. Depending on the satellite’s transmission frequency, microwave stations on earth can jam, or prevent, transmission by operating at the same frequency.
5. Signal transmission may be slow if the signals must travel over very long distances.
Companies must lease satellite communications time from suppliers such as Intelsat, Comsat, Inmarsat, Utelsat, and Telstar (AT&T). Large companies that have offices around the world benefit the most from satellite communications. Fig Sate (a) 1 the
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Figure 8.7 Satellite commurications. (a) The satellite orbiting the earth has solarpowered transponders that receive microwave signals from the earth’s surface, amplify the signals, and retransmit them to the earth’s surface (b). Part (c) illustrates how various communications media can work together as communications links. —Control antenna Transmission antenna (a) Concentrated beam antenna
Solar cells Satellite earth station Microwave
Light Pulses: Fiber Optics Communications and Connectivity 271
1u/ (4 (4 Microwave !at tower A1A AFA ’11(
Fiber Optics Although satellite systems are expected to be the dominant communications medium for long Oistance during the rest of the 90s, fiber-optics technology is revolutionizing the communications industry because of its low cost, high transmission volume, 19w error rate, and message security. Fiber-optic cables are replacing copper wire as the major communications medium in buildings and cities; major communications companies are currently investing huge Telephone (c)
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sums of money in fiber-optics communications networks that can carry digital signals, thus increasing communications capacity. (Computers connected by fiber-optic cables do not need to use modems.) In fiber-optics communications, signals are converted to light form and fired by laser in bursts through insulated, very thin (2/1000 of an inch) glass or plastic fibers. (See Figure 8.8.) The pulses of light represent the on state in electronic data transmission and can occur nearly 1 billion times per second—up to 80 gigabytes of digital data per second can be sent through a fiber-optic cable. Equally important, fiber-optic cables aren’t cumbersome in size: a fiber-optic cable (insulated fibers bound together) that is only 0.12 inch thick is capable of supporting nearly 250,000 voice conversations at the same time (soon to be doubled to 500,000). However, since the data is communicated in the form of pulses of light, specialized communications equipment must be used. Fiber-optic cables are not susceptible to electronic noise and so have much lower error rates than normal telephone wire and cable. In addition, their potential speed for data cOmmunications is up to 10,000 times faster than that of microwave and satellite systems. Fiber-optic communications is also resistant to illegal data theft, because it is almost impossible to tap into it to listen to the data being transmitted or to change the data without being detected; in fact, it is currently being used by the Central Intelligence Agency. Another advantage to fiber-optic transmission is that electrical signals don’t escape from the cables—in other x‘ ords, the cables don’t interfere with sensitive electrical equipment that may be nearby. Given its significant advantages, it is not surprising that fiber-optic cable is much more expensive than telephone wire and cable. (A twisted-pair telephone wire of 4 megahertz might send only 1 kilobyte of data in a second. A coaxial cable of 100 megahertz might send 10 megabytes. And a fiber-optic cable of 2 billion megahertz might send 1 gigabyte.) AT&T has developed undersea optical fiber cables for transatlantic use in the belief that fiber optics will eventually replace satellite communications in terms of cost-effectiveness and efficiency. Japan has already laid an underwater fiber-optic cable. Sprint uses a fiber-optic communications network laid along railroad rights-of-way in the United States that carries digital signals (analog voice signals are converted to digital signals at company switching stations). Most of the “information superhighway” covered so much in ‘,he mass media today involves fiber-optic connections. (See The Clipboard: Digital Convergence and the Information Superhighway.)
Figure 8.8 Fiber-optic cable. Thin glass strands transmit pulsating light instead of electricity. These strands can carry computer and voice data over long distances. ….. ir
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rergence and Irmation ighway
The essence of all revolution, stated philosopher Hannah Arendt, is the start of a new story in human experience. For us, the “new story” is the fusion of several important industries in a unification called digital convergence. Digital convergence is the technological merger of several industries through various devices that exchange information in the electronic, or digital, format used by computers. The industries are computers, communications, consumer electronics, entertainment, and mass media/publishing. Called “the mother of all industries,” digital convergence has tremendous significance. It means that, from a common electronic base, information can be communicated in all the ways we are accustomed to receiving it. These include the media of newspapers, photographs, films, recordings, radio, and television. However, they also include newer technology—satellite, fiber-optic cable, cellular phone, fax machine, compact disk. More important, as time goes on, the same information may be exchanged among many kinds of equipment, all using the language of computers—Os and 1s. This development signifies a shift from single, isolated technologies to a unified digital technology. This shift has given rise to the use of the term information superhighway The information superhighway, which will be constructed mostly of fiber-optic cable, is a metaphor for a fusion of the two-way wired and wireless capabilities of telephones and networked computers with cable TV’s capacity to transmit hundreds of programs. The resulting interacLive [i p. 27] digitized traffic would include movies, TV shows, phone calls, databases, shopping services, and online services. This superhighway, it is hoped, would link all homes, schools, businesses, and government organizations. (See Figure 8.9.)
At present, this electronic highway remains a vision, much as today’s U.S. interstate highway system was a vision in the 1950s. It is as though we still had old-fashioned Highway 40s and Route 66s, along with networks of one-lane secondary and gravel back roads. These, of course, have largely been replaced by high-speed blacktop and eight-lane freeways. In 40 years, will the world be as changed by the electronic highway as North America has been by the interstate highways of the past four decades? However, before the new digital world is to be realized, much cooperation will be needed between hardware, software, and communications companies, along with agreement on standards for digitizing, integrating, storing, and manipulating all the information around us. In addition, it will take time to convert enough existing, nondigital materials for a broad base of information and to put in place the necessary fiber-optic networks and other equipment.
Wireless Transmission: Infrared, Spread Spectrum, and Standard Radio Waves
A new generation of wireless data communications devices is rapidly gaining attention. These devices use three basic technologies: infrared, spread spectrum, and standard radio transmission. Infrared (IR) technology uses the same method as TV remote control units (invisible radiation at a p;Irticular frequency). However, transmission devices that use infrared must be within line of sight of one another and so cannot be used for mobile computing (objects may come between the transmission units). Spread spectrum radio was developed by the U.S. Army during World War II for jam-proof and interception-proof transmissions. This technology is being used in small networks within individual buildings. Standard radio technology is also being employed for data transmission in networks, but it involves some licensing difficulties. (Standard radio frequencies cannot be used without government licenses.)
Communications Hardware, Software, and Protocols
Communications hardware commmly used in business includes modems, fax modems, multiplexers, concentrators, and front-end processors. Communications software manages data transmission and controls error correction, data compression, remote control, and terminal emulation. Protocols set the standards for data transmission.
Much of the hardware and software used in data communications is operated by technical professionals and is rarely of immediate consequence to the user unless it stops working—when you’re calling from New York and can’t reach your division office in London, for example. However, you should be familiar with certain types of communications hardware and the software that makes it run.
To access the transmission channels described in the last section, you must use communications hardware and software that adheres to certain standards for communicating, called protocols.
First, let’s discuss the communications hardware commonly used in business.
• Fax modems
• Multiplexers, concentrators, and front-end processors Modems
Modems are probably the most widely used data communications hardware. They are certainly the most familiar to microcomputer users who communicate with one another or with a larger computer. As you learned earlier in this chapter, the word modem is a contraction of modulate and demodulate. A modem’s basic purpose is to convert digital computer signals to analog signals for transmission over phone lines, then to receive these signals and convert them back to digital signals. A modem allows the user to directly connect the computer to the telephone line. (See Figure 8.10.) Transmission speed is measured in bits per second (bps). Modems commonly transmit and receive data at 1,200 and
Communications and Connectivity 277
Figure 8.10 External versus internal modem. An external modem is a box that is outside the computer. An internal modem is a circuit board installed in an expansion slot inside the system cabinet. Telephone outlet
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2,400 bps (considered slow); 4,800 and 9,600 bps (moderately fast); 14,400 and 19,200 bps (high speed); and 28,800 bps (very high speed). A 10-page single-spaced letter can be transmitted by a 2,400-bps modem in 21/2 minutes. It can be transmitted by a 9,600-bps modem in 38 seconds and by a 19,200bps modem in 19 seconds. (Note that high-speed modems Will automatically adjust to run at slower speeds, but slower modems cannot adjust to run faster.) Modems are either internal or external. An internal modem is located on a circuit board that is placed inside a microcomputer (plugged into an expansion slot) [4,/ p. 961. (See Figure 8.10.) The internal modem draws its power directly from the computer’s power supply. No special cable is required to connect the modem to the computer. An external direct-connect modem is an independent hardware component—that is, it is outside the computer—and uses its own power supply. (See Figure 8.10.) The modem is
connected to the computer via a serial cable plugged into a port [/ p. 991. / outlet Telephone External modem
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IBM-type computers have four serial ports: COM1, COM2, COM3, and COM4. Since the mouse is connected to COM1, an external modem would be connected to COM2. The communications software (discussed shortly) that is used with the modem must also be set up to send transmitted data to COM2. (Macintosh computers have a serial modem port identified by an icon of a modem.)
A fax modem, which is installed as a circuit board inside the computer’s system Cabinet, is a modem with fax capability. It enables you to send data and scanned-in images directly from your computer to someone else’s fax machine or fax modem. Fax modems are installed inside portable computers, including pocket PCs V p. 731, as well as in desktop computers. Note: Users who purchase modems and communications software should check the documentation V p. 51 or ask the seller about compatibility. The modem and the software must be compatible with each other and with the user’s computer.
Multiplexers, Concentrators, and Front-End Processors When an organization’s data comMunications needs grow, the lines available for that purpose often become overtaxed, even if the company has leased one or more private telephone lines—called dedicated lines—used only for data communications. Multiplexing optimizes the use of communications lines by allowing multiple users or devices to share one high-speed line, thereby reducing communications costs. Multiplexing can be done by multiplexers, ti concentrators, or front-end processors. multiplexer A multiplexer is a device that merges several low-speed transmissions into one high-speed transmission. Depending on the multiplexer model,
32 or more devices may share a single communications line. Messages sent by a multiplexer must be received by a multiplexer of thesame type. The receiving multiplexer sorts out the individual messages and directs them to the proper recipient. High-speed multiplexers, called Ti multiplexers, which use high-speed digital lines, can carry as many messages, both voice and data, as 24 analog telephone lines. (See Figure 8.11.) Like a multiplexer, a concentrator is a piece of hardware that enables several devices to share a single communications line. However, unlike a multiplexer, a concentrator collects data in a temporary storage area. It then forwards the data when enough has been accumulated to be sent economically. Often a concentrator is a .minicomputer. • The most sophisticated of these communications-management devices is the front-end processor, a computer that handles communications for mainframes. A front-end processor is a smaller computer that is connected to a larger computer and assists with communications functions. The front-end processor is itself. a minicomputer or even a mainfrante. It transmits and receives messages over the communications channels, corrects errors, and relieves the larger computer of routine computational tasks. Sometimes the term front-end processor is used synonymously with the term communications controller, although this latter device is usually less sophisticated. (A communications controller handles communications between a computer and peripheral devices such as terminals and printer.)