1. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.1 Computer Networks and Internets with Internet Applications, 4e By Douglas E. Comer Lecture PowerPoints By Lami Kaya, LKaya@ieee.org

2. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.2 Chapter 4 Transmission Media

3. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.3 Topics Covered 4.1 Introduction 4.2 Copper Wires 4.3 Glass Fibers 4.4 Radio 4.5 Satellites 4.6 Geosynchronous Satellites 4.7 Low Earth Orbit Satellites 4.8 Low Earth Orbit Satellite Arrays 4.9 Microwave 4.10 Infrared 4.11 Light From A Laser

4. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.4 4.1 Introduction At the lowest level, all computer communication involves –encoding data in a form of energy –and sending the energy across a transmission medium HW devices attached to a computer perform the encoding and decoding of data –programmers and users do not need to know the details of data transmission This part covers the basics of data transmission: –The first chapter examines the media that are used for transmission in modern network systems –The next two chapters explain how data can be transferred across such media –Later sections explain how transmission forms the basis of data NW

5. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.5 4.2 Copper Wires (1) Conventional computer NW use wires as the primary medium –NW use copper wire almost exclusively because its low resistance The type of wiring used for NW is chosen to minimize interference Interference arises because an electrical signal traveling across a wire acts like a miniature radio station –the wire emits a small amount of electromagnetic (EM) energy, which can travel through the air Whenever it encounters another wire, an EM wave generates a small electric current in the wire. –The amount of current generated depends on the strength of the EM wave and the physical position of the wire When two wires are placed close together and in parallel, a strong signal sent on one wire will generate a similar signal on the other

6. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.6 4.2 Copper Wires (2) Problem of interference is severe –because wires that comprise a NW often are placed in parallel with many other wires To minimize interference, networks use one of three basic wiring types: –Unshielded Twisted Pair (UTP) –Coaxial Cable –Shielded Twisted Pair (STP)

7. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.7 4.2 Copper Wires (2) See Figure 4.1 for an illustration of twisted-pair cables Twists change the electrical properties of the wire: –First, they limit the EM energy the wire emits: So they help prevent electric currents on the wire from radiating energy that interferes with other wires –Second, they make the pair of wires less susceptible to EM energy: They help prevent signals on other wires from interfering with the pair

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9. 9 4.2 Copper Wires (3) Coaxial cables provides even more protection from interference than twisted pair –Instead of twisting wires around one another to limit interference, a coaxial cable consists of a single wire surrounded by a metal shield It’s shown in Figure 4.2 Metal shield in a coaxial cable forms a flexible cylinder around the inner wire to provide a barrier for EM radiation –The barrier isolates the inner wire in two ways: it protects the wire from incoming EM energy and keeps signals on the inner wire from radiating EM energy The cable can be placed parallel to other cables or bent and twisted around corners

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11. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.11 4.2 Copper Wires (4) Shielded Twisted Pair (STP) –A shielded twisted pair cable consists of a pair of wires surrounded by a metal shield Each wire is coated with an insulating material, –so the metal in one wire does not touch the metal in another The shield merely forms a barrier that prevents EM radiation from entering or escaping The additional shielding provided by coaxial or shielded twisted pair cabling is often used –when wires from a NW pass near equipment that generates strong electric or magnetic fields

12. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.12 4.3 Glass Fibers (1) NW also use flexible glass fibers to transmit data –known as an optical fiber Medium uses light to transport data The miniature glass fiber is encased in a plastic jacket –which allows the fiber to bend without breaking A transmitter at one end of a fiber uses –a light emitting diode (LED) or a laser to send pulses of light A receiver at the other end uses –a light sensitive transistor to detect the pulses

13. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.13 4.3 Glass Fibers (2) Main advantages fiber optics over wires: First, because they use light, –neither cause electrical interference in other cables –nor are they susceptible to electrical interference Second, because glass fibers can be manufactured to reflect most of the light inward –a fiber can carry a pulse of light much farther than a copper wire can carry a signal Third, because light can encode more information than electrical signals –an optical fiber can carry more information than a wire Fourth, unlike electricity, which always requires a pair of wires connected into a complete circuit, –light can travel from one computer to another over a single fiber

14. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.14 4.3 Glass Fibers (3) Optical fibers do have some disadvantages First, installing a fiber requires special equipment –that polishes the ends to allow light to pass through Second, if a fiber breaks inside the plastic jacket: –finding the location of the problem is difficult Third, repairing a broken fiber is difficult –special equipment is needed to join two fibers

15. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.15 4.4 Radio In Radio Frequency (RF) transmissions –each participating computer attaches to an antenna –Antenna can both transmit and receive RF Physically, the antennas used with RF networks may be large or small, depending on the range desired: –An antenna designed to propagate signals several miles may consist of a metal pole approximately two meters long that is mounted vertically on top of a building –An antenna designed to permit communication within a building may be small enough to fit inside a portable computer (e.g., less than twenty centimeters)

16. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.16 4.5 Satellites RF technology can be combined with satellites –to provide communication across longer distances Figure 4.3 illustrates a satellite in orbit The satellite contains a transponder –that consists of a radio receiver and transmitter The transponder –accepts an incoming radio transmission –amplifies it –and transmits the signal back toward the ground at a slightly different angle than it arrived A single satellite usually contains multiple transponders –Each transponder uses a different radio frequency (i.e., channel)

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18. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.18 4.6 Geosynchronous Satellites (1) Communication satellites can be grouped into categories according to the height at which they orbit: –The easiest is geosynchronous or geostationary satellites –The name arises because a geosynchronous satellite is placed in an orbit that is exactly synchronized with the rotation of the earth. –Such an orbit is classified as a Geostationary Earth Orbit (GEO) –When viewed from the ground, satellite appears to remain at exactly the same point in the sky at all times Laws of physics determine the exact distance from the earth that a satellite must orbit to remain synchronized with the earth's rotation –See Kepler's Law for details –The distance is 35,785 kilometers or 22,236 miles

19. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.19 4.6 Geosynchronous Satellites (2) GEO is about one tenth of the distance to the moon –Engineers refer the distance as “high earth orbit” There is a limited amount of ``space'' available in the GEO above the equator –because satellites using a given frequency must be separated from one another to avoid interference The minimum separation depends on the power of the transmitters

20. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.20 4.7 Low Earth Orbit Satellites (1) Second category of satellites operate in what is called Low Earth Orbit (LEO) –which means that they orbit a few hundred miles above the earth (typically 200 to 400 miles) The chief disadvantage of a LEO lies in the rate at which a satellite must travel –Their period of rotation is faster than the rotation of the earth LEOs do not stay above a single point on the earth's surface An observer, who stands on the earth looking upward through a telescope, sees LEOs move across the sky A single satellite can complete an entire orbit in approximately 1.5 hours

21. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.21 4.7 Low Earth Orbit Satellites (2) From a communication provider's point of view: having a satellite that does not appear to remain stationary causes problems: –First, the satellite can only be used during the time that its orbit passes between two ground stations –Second, maximal utilization requires complex control systems that continuously move the ground stations so they point directly at the satellite

22. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.22 4.8 Low Earth Orbit Satellite Arrays (1) Instead of focusing on one satellite, –the scheme requires a communication company to launch a set of satellites into low earth orbits Although a given satellite orbits quickly, –the set of orbits is chosen so that each point on the ground has at least one satellite overhead at any time –sixty-six (66) satellites are required to provide service over the entire surface of the earth From the point of view of an observer on earth, –it appears that a satellite emerges from a point on the horizon –flies overhead –and then disappears into a point on the opposite horizon The key to the scheme lies in the set of orbits –guarantees at least one satellite is available at any time

23. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.23 4.8 Low Earth Orbit Satellite Arrays (2) In addition to transponders used to communicate with ground stations –an array of satellites in low earth orbit contains radio equipment used to communicate with other satellites in the array As they move through their orbits –the satellites communicate with one another and agree to forward data

24. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.24 4.9 Microwave Many long-distance telephone companies use microwave (MW) to carry telephone conversations –A few large companies have also installed MW systems as part of the company's NW MW are merely a higher frequency version of radio waves, but they behave differently –Instead of broadcasting in all directions, a MW transmission can be aimed in a single direction, preventing others from intercepting –In addition, MW transmission can carry more information than lower frequency RF transmissions MW cannot penetrate metal structures: –transmission works best when a clear path exists between two parties –most MW installations consist of two towers that are taller than the surrounding buildings and vegetation –each MW transmitter aimed directly at a MW receiver on the other

25. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.25 4.10 Infrared Infrared is limited to a small area (e.g., a single room) Usually requires that the transmitter be pointed toward the receiver Infrared HW – is inexpensive compared to other mechanisms, –and does not require an antenna It is possible to equip a large room with a single infrared connection –that provides NW access to all computers –computers can remain in contact with the NW while they are moved within the room Infrared NW are especially convenient for small, portable computers

26. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.26 4.11 Light From A Laser A beam of light can also be used to carry data through the air A communication link that uses light consists of two sites that each have a transmitter and receiver –equipment is mounted in a fixed position, often on a tower –aligned so the transmitter at one location sends its beam of light directly to the receiver at the other The transmitter uses a laser to generate the beam of light –because a coherent laser beam will stay focused over a long distance Light from a laser must travel in a straight line and must not be blocked A laser beam cannot penetrate vegetation or weather conditions such as snow and fog: –Thus, laser transmission has limited use