Chapter 7 Transmission Media PDF

CHAPTER 7 Transmission Media W e discussed many issues related to the physical layer in Chapters 3 through 6. In this chapter, we discuss transmission media. We definitely need transmission media to conduct signals from the source to the destination....

CHAPTER 7 Transmission Media W e discussed many issues related to the physical layer in Chapters 3 through 6. In this chapter, we discuss transmission media. We definitely need transmission media to conduct signals from the source to the destination. However, the media can be wired or wireless. This chapter is divided into three sections: ❑ The first section introduces the transmission media and defines its position in the Internet model. It shows that we can classify transmission media into two broad categories: guided and unguided media. ❑ The second section discusses guided media. The first part describes twisted-pair cables and their characteristics and applications. The second part describes coaxial cables and their characteristics and applications. Finally, the third part describes fiber-optic cables and their characteristics and applications. ❑ The third section discusses unguided media. The first part describes radio waves and their characteristics and applications. The second part describes microwaves and their characteristics and applications. Finally, the third part describes infrared waves and their characteristics and applications. 185 186 PART II PHYSICAL LAYER 7.1 INTRODUCTION Transmission media are actually located below the physical layer and are directly con- trolled by the physical layer. We could say that transmission media belong to layer zero. Figure 7.1 shows the position of transmission media in relation to the physical layer. Figure 7.1 Transmission medium and physical layer Sender Physical layer Physical layer Receiver Transmission medium Cable or air A transmission medium can be broadly defined as anything that can carry infor- mation from a source to a destination. For example, the transmission medium for two people having a dinner conversation is the air. The air can also be used to convey the message in a smoke signal or semaphore. For a written message, the transmission medium might be a mail carrier, a truck, or an airplane. In data communications the definition of the information and the transmission medium is more specific. The transmission medium is usually free space, metallic cable, or fiber-optic cable. The information is usually a signal that is the result of a conversion of data from another form. The use of long-distance communication using electric signals started with the invention of the telegraph by Morse in the 19th century. Communication by telegraph was slow and dependent on a metallic medium. Extending the range of the human voice became possible when the telephone was invented in 1869. Telephone communication at that time also needed a metallic medium to carry the electric signals that were the result of a conversion from the human voice. The communication was, however, unreliable due to the poor quality of the wires. The lines were often noisy and the technology was unsophisticated. Wireless communication started in 1895 when Hertz was able to send high- frequency signals. Later, Marconi devised a method to send telegraph-type messages over the Atlantic Ocean. We have come a long way. Better metallic media have been invented (twisted-pair and coaxial cables, for example). The use of optical fibers has increased the data rate incredibly. Free space (air, vacuum, and water) is used more efficiently, in part due to the technologies (such as modulation and multiplexing) discussed in the previous chapters. As discussed in Chapter 3, computers and other telecommunication devices use signals to represent data. These signals are transmitted from one device to another in the form of electromagnetic energy, which is propagated through transmission media. Electromagnetic energy, a combination of electric and magnetic fields vibrating in relation to each other, includes power, radio waves, infrared light, visible light, ultraviolet  CHAPTER 7 TRANSMISSION MEDIA 187 light, and X, gamma, and cosmic rays. Each of these constitutes a portion of the elec- tromagnetic spectrum. Not all portions of the spectrum are currently usable for tele- communications, however. The media to harness those that are usable are also limited to a few types. In telecommunications, transmission media can be divided into two broad catego- ries: guided and unguided. Guided media include twisted-pair cable, coaxial cable, and fiber-optic cable. Unguided medium is free space. Figure 7.2 shows this taxonomy. Figure 7.2 Classes of transmission media Transmission media Guided Unguided (wired) (wireless) Twisted-pair Coaxial Fiber-optic Radio wave Microwave Infrared cable cable cable 7.2 GUIDED MEDIA Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a cable that accepts and transports signals in the form of light. 7.2.1 Twisted-Pair Cable A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together, as shown in Figure 7.3. Figure 7.3 Twisted-pair cable Insulator Conductor One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. The receiver uses the difference between the two. In addition to the signal sent by the sender on one of the wires, interference (noise) and crosstalk may affect both wires and create unwanted signals. If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because they are at different locations relative to the noise or crosstalk sources (e.g., one is closer and the other is farther). This results in a difference at the receiver. 188 PART II PHYSICAL LAYER By twisting the pairs, a balance is maintained. For example, suppose in one twist, one wire is closer to the noise source and the other is farther; in the next twist, the reverse is true. Twisting makes it probable that both wires are equally affected by external influ- ences (noise or crosstalk). This means that the receiver, which calculates the difference between the two, receives no unwanted signals. The unwanted signals are mostly can- celed out. From the above discussion, it is clear that the number of twists per unit of length (e.g., inch) has some effect on the quality of the cable. Unshielded Versus Shielded Twisted-Pair Cable The most common twisted-pair cable used in communications is referred to as unshielded twisted-pair (UTP). IBM has also produced a version of twisted-pair cable for its use, called shielded twisted-pair (STP). STP cable has a metal foil or braided- mesh covering that encases each pair of insulated conductors. Although metal casing improves the quality of cable by preventing the penetration of noise or crosstalk, it is bulkier and more expensive. Figure 7.4 shows the difference between UTP and STP. Our discussion focuses primarily on UTP because STP is seldom used outside of IBM. Figure 7.4 UTP and STP cables Metal shield Plastic cover Plastic cover a. UTP b. STP Categories The Electronic Industries Association (EIA) has developed standards to classify unshielded twisted-pair cable into seven categories. Categories are determined by cable quality, with 1 as the lowest and 7 as the highest. Each EIA category is suitable for specific uses. Table 7.1 shows these categories. Table 7.1 Categories of unshielded twisted-pair cables Data Rate Category Specification (Mbps) Use 1 Unshielded twisted-pair used in telephone < 0.1 Telephone 2 Unshielded twisted-pair originally used in 2 T-1 lines T lines 3 Improved CAT 2 used in LANs 10 LANs 4 Improved CAT 3 used in Token Ring networks 20 LANs 5 Cable wire is normally 24 AWG with a jacket 100 LANs and outside sheath  CHAPTER 7 TRANSMISSION MEDIA 189 Table 7.1 Categories of unshielded twisted-pair cables (continued) Data Rate Category Specification (Mbps) Use 5E An extension to category 5 that includes 125 LANs extra features to minimize the crosstalk and electromagnetic interference 6 A new category with matched components 200 LANs coming from the same manufacturer. The cable must be tested at a 200-Mbps data rate. 7 Sometimes called SSTP (shielded screen 600 LANs twisted-pair). Each pair is individually wrapped in a helical metallic foil followed by a metallic foil shield in addition to the outside sheath. The shield decreases the effect of crosstalk and increases the data rate. Connectors The most common UTP connector is RJ45 (RJ stands for registered jack), as shown in Figure 7.5. The RJ45 is a keyed connector, meaning the connector can be inserted in only one way. Figure 7.5 UTP connector 12345678 5678 1234 RJ-45 Female RJ-45 Male Performance One way to measure the performance of twisted-pair cable is to compare attenuation versus frequency and distance. A twisted-pair cable can pass a wide range of frequencies. However, Figure 7.6 shows that with increasing frequency, the attenuation, measured in decibels per kilometer (dB/km), sharply increases with frequencies above 100 kHz. Note that gauge is a measure of the thickness of the wire. Applications Twisted-pair cables are used in telephone lines to provide voice and data channels. The local loop—the line that connects subscribers to the central telephone office— commonly consists of unshielded twisted-pair cables. We discuss telephone networks in Chapter 14. The DSL lines that are used by the telephone companies to provide high-data-rate connections also use the high-bandwidth capability of unshielded twisted-pair cables. We discuss DSL technology in Chapter 14. 190 PART II PHYSICAL LAYER Figure 7.6 UTP performance Gauge Diameter (inches) 26 gauge 20 18 0.0403 24 gauge 18 22 0.02320 16 24 0.02010 Attenuation (dB/km) 26 0.0159 22 gauge 14 12 18 gauge 10 8 6 4 2 1 10 100 1000 f (kHz) Local-area networks, such as 10Base-T and 100Base-T, also use twisted-pair cables. We discuss these networks in Chapter 13. 7.2.2 Coaxial Cable Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted- pair cable, in part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover (see Figure 7.7). Figure 7.7 Coaxial cable Insulator Insulator Inner conductor Outer conductor Plastic cover (shield) Coaxial Cable Standards Coaxial cables are categorized by their Radio Government (RG) ratings. Each RG number denotes a unique set of physical specifications, including the wire gauge of the  CHAPTER 7 TRANSMISSION MEDIA 191 inner conductor, the thickness and type of the inner insulator, the construction of the shield, and the size and type of the outer casing. Each cable defined by an RG rating is adapted for a specialized function, as shown in Table 7.2. Table 7.2 Categories of coaxial cables Category Impedance Use RG-59 75 Ω Cable TV RG-58 50 Ω Thin Ethernet RG-11 50 Ω Thick Ethernet Coaxial Cable Connectors To connect coaxial cable to devices, we need coaxial connectors. The most common type of connector used today is the Bayonet Neill-Concelman (BNC) connector. Figure 7.8 shows three popular types of these connectors: the BNC connector, the BNC T connector, and the BNC terminator. Figure 7.8 BNC connectors BNC T Cable BNC connector 50-W Ground BNC terminator wire The BNC connector is used to connect the end of the cable to a device, such as a TV set. The BNC T connector is used in Ethernet networks (see Chapter 13) to branch out to a connection to a computer or other device. The BNC terminator is used at the end of the cable to prevent the reflection of the signal. Performance As we did with twisted-pair cable, we can measure the performance of a coaxial cable. We notice in Figure 7.9 that the attenuation is much higher in coaxial cable than in twisted-pair cable. In other words, although coaxial cable has a much higher bandwidth, the signal weakens rapidly and requires the frequent use of repeaters. Applications Coaxial cable was widely used in analog telephone networks where a single coaxial network could carry 10,000 voice signals. Later it was used in digital telephone networks where a single coaxial cable could carry digital data up to 600 Mbps. How- ever, coaxial cable in telephone networks has largely been replaced today with fiber- optic cable. 192 PART II PHYSICAL LAYER Figure 7.9 Coaxial cable performance 35 0.7/2.9 mm Attenuation (dB/km) 30 25 1.2/4.4 mm 20 2.6/9.5 mm 15 10 5 0.01 0.1 1.0 10 100 f (MHz) Cable TV networks (see Chapter 14) also use coaxial cables. In the traditional cable TV network, the entire network used coaxial cable. Later, however, cable TV providers replaced most of the media with fiber-optic cable; hybrid networks use coaxial cable only at the network boundaries, near the consumer premises. Cable TV uses RG-59 coaxial cable. Another common application of coaxial cable is in traditional Ethernet LANs (see Chapter 13). Because of its high bandwidth, and consequently high data rate, coaxial cable was chosen for digital transmission in early Ethernet LANs. The 10Base-2, or Thin Ethernet, uses RG-58 coaxial cable with BNC connectors to transmit data at 10 Mbps with a range of 185 m. The 10Base5, or Thick Ethernet, uses RG-11 (thick coaxial cable) to transmit 10 Mbps with a range of 5000 m. Thick Ethernet has specialized connectors. 7.2.3 Fiber-Optic Cable A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. To understand optical fiber, we first need to explore several aspects of the nature of light. Light travels in a straight line as long as it is moving through a single uniform sub- stance. If a ray of light traveling through one substance suddenly enters another substance (of a different density), the ray changes direction. Figure 7.10 shows how a ray of light changes direction when going from a more dense to a less dense substance. As the figure shows, if the angle of incidence I (the angle the ray makes with the line perpendicular to the interface between the two substances) is less than the critical angle, the ray refracts and moves closer to the surface. If the angle of incidence is equal to the critical angle, the light bends along the interface. If the angle is greater than the critical angle, the ray reflects (makes a turn) and travels again in the denser  CHAPTER 7 TRANSMISSION MEDIA 193 Figure 7.10 Bending of light ray Less Less Less dense dense dense More More More dense dense dense I I I I < critical angle, I = critical angle, I > critical angle, refraction refraction reflection substance. Note that the critical angle is a property of the substance, and its value differs from one substance to another. Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. See Figure 7.11. Figure 7.11 Optical fiber Cladding Sender Core Receiver Cladding Propagation Modes Current technology supports two modes (multimode and single mode) for propagating light along optical channels, each requiring fiber with different physical characteristics. Multi- mode can be implemented in two forms: step-index or graded-index (see Figure 7.12). Figure 7.12 Propagation modes Mode Multimode Single mode Step index Graded index Multimode Multimode is so named because multiple beams from a light source move through the core in different paths. How these beams move within the cable depends on the struc- ture of the core, as shown in Figure 7.13. 194 PART II PHYSICAL LAYER Figure 7.13 Modes Source Destination a. Multimode, step index Source Destination b. Multimode, graded index Source Destination c. Single mode In multimode step-index fiber, the density of the core remains constant from the center to the edges. A beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding. At the interface, there is an abrupt change due to a lower density; this alters the angle of the beam’s motion. The term step-index refers to the suddenness of this change, which contributes to the distor- tion of the signal as it passes through the fiber. A second type of fiber, called multimode graded-index fiber, decreases this dis- tortion of the signal through the cable. The word index here refers to the index of refraction. As we saw above, the index of refraction is related to density. A graded- index fiber, therefore, is one with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge. Figure 7.13 shows the impact of this variable density on the propagation of light beams. Single-Mode Single-mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. The single-mode fiber itself is manufactured with a much smaller diameter than that of multimode fiber, and with substantially lower density (index of refraction). The decrease in density results in a critical angle that is close enough to 90° to make the propagation of beams almost horizontal. In this case, propagation of different beams is almost identical, and delays are negligible. All the beams arrive at the destination “together” and can be recombined with little distortion to the signal (see Figure 7.13).  CHAPTER 7 TRANSMISSION MEDIA 195 Fiber Sizes Optical fibers are defined by the ratio of the diameter of their core to the diameter of their cladding, both expressed in micrometers. The common sizes are shown in Table 7.3. Note that the last size listed is for single-mode only. Table 7.3 Fiber types Type Core (μm) Cladding (μm) Mode 50/125 50.0 125 Multimode, graded index 62.5/125 62.5 125 Multimode, graded index 100/125 100.0 125 Multimode, graded index 7/125 7.0 125 Single mode Cable Composition Figure 7.14 shows the composition of a typical fiber-optic cable. The outer jacket is Figure 7.14 Fiber construction DuPont Kevlar Outer jacket for strength Cladding Plastic buffer Glass or plastic core made of either PVC or Teflon. Inside the jacket are Kevlar strands to strengthen the cable. Kevlar is a strong material used in the fabrication of bulletproof vests. Below the Kevlar is another plastic coating to cushion the fiber. The fiber is at the center of the cable, and it consists of cladding and core. Fiber-Optic Cable Connectors There are three types of connectors for fiber-optic cables, as shown in Figure 7.15. The subscriber channel (SC) connector is used for cable TV. It uses a push/pull locking system. The straight-tip (ST) connector is used for connecting cable to networking devices. It uses a bayonet locking system and is more reliable than SC. MT-RJ is a connector that is the same size as RJ45. Performance The plot of attenuation versus wavelength in Figure 7.16 shows a very interesting phenomenon in fiber-optic cable. Attenuation is flatter than in the case of twisted-pair cable and coaxial cable. The performance is such that we need fewer (actually one- tenth as many) repeaters when we use fiber-optic cable. 196 PART II PHYSICAL LAYER Figure 7.15 Fiber-optic cable connectors SC connector ST connector RX TX MT-RJ connector Figure 7.16 Optical fiber performance 100 50 10 Loss (dB/km) 5 1 0.5 0.1 0.05 0.01 800 1000 1200 1400 1600 1800 Wavelength (nm) Applications Fiber-optic cable is often found in backbone networks because its wide bandwidth is cost-effective. Today, with wavelength-division multiplexing (WDM), we can transfer data at a rate of 1600 Gbps. The SONET network that we discuss in Chapter 14 provides such a backbone. Some cable TV companies use a combination of optical fiber and coaxial cable, thus creating a hybrid network. Optical fiber provides the backbone structure while coaxial cable provides the connection to the user premises. This is a cost-effective con- figuration since the narrow bandwidth requirement at the user end does not justify the use of optical fiber.  CHAPTER 7 TRANSMISSION MEDIA 197 Local-area networks such as 100Base-FX network (Fast Ethernet) and 1000Base-X also use fiber-optic cable. Advantages and Disadvantages of Optical Fiber Advantages Fiber-optic cable has several advantages over metallic cable (twisted-pair or coaxial). ❑ Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and hence data rates) than either twisted-pair or coaxial cable. Currently, data rates and bandwidth utilization over fiber-optic cable are limited not by the medium but by the signal generation and reception technology available. ❑ Less signal attenuation. Fiber-optic transmission distance is significantly greater than that of other guided media. A signal can run for 50 km without requiring regeneration. We need repeaters every 5 km for coaxial or twisted-pair cable. ❑ Immunity to electromagnetic interference. Electromagnetic noise cannot affect fiber-optic cables. ❑ Resistance to corrosive materials. Glass is more resistant to corrosive materials than copper. ❑ Light weight. Fiber-optic cables are much lighter than copper cables. ❑ Greater immunity to tapping. Fiber-optic cables are more immune to tapping than copper cables. Copper cables create antenna effects that can easily be tapped. Disadvantages There are some disadvantages in the use of optical fiber. ❑ Installation and maintenance. Fiber-optic cable is a relatively new technology. Its installation and maintenance require expertise that is not yet available everywhere. ❑ Unidirectional light propagation. Propagation of light is unidirectional. If we need bidirectional communication, two fibers are needed. ❑ Cost. The cable and the interfaces are relatively more expensive than those of other guided media. If the demand for bandwidth is not high, often the use of optical fiber cannot be justified. 7.3 UNGUIDED MEDIA: WIRELESS Unguided medium transport electromagnetic waves without using a physical conduc- tor. This type of communication is often referred to as wireless communication. Sig- nals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them. Figure 7.17 shows the part of the electromagnetic spectrum, ranging from 3 kHz to 900 THz, used for wireless communication. Unguided signals can travel from the source to the destination in several ways: ground propagation, sky propagation, and line-of-sight propagation, as shown in Figure 7.18. 198 PART II PHYSICAL LAYER Figure 7.17 Electromagnetic spectrum for wireless communication Light wave Radio wave and microwave Infrared 3 300 400 900 kHz GHz THz THz Figure 7.18 Propagation methods Ionosphere Ionosphere Ionosphere Ground propagation Sky propagation Line-of-sight propagation (below 2 MHz) (2–30 MHz) (above 30 MHz) In ground propagation, radio waves travel through the lowest portion of the atmo- sphere, hugging the earth. These low-frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the planet. Distance depends on the amount of power in the signal: The greater the power, the greater the distance. In sky propagation, higher-frequency radio waves radiate upward into the ionosphere (the layer of atmosphere where particles exist as ions) where they are reflected back to earth. This type of transmission allows for greater distances with lower output power. In line-of-sight propagation, very high-frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other, and either tall enough or close enough together not to be affected by the curvature of the earth. Line-of- sight propagation is tricky because radio transmissions cannot be completely focused. The section of the electromagnetic spectrum defined as radio waves and microwaves is divided into eight ranges, called bands, each regulated by government authorities. These bands are rated from very low frequency (VLF) to extremely high frequency (EHF). Table 7.4 lists these bands, their ranges, propagation methods, and some applications. Table 7.4 Bands Band Range Propagation Application very low frequency (VLF) 3–30 kHz Ground Long-range radio navigation low frequency (LF) 30–300 kHz Ground Radio beacons and navigational locators  CHAPTER 7 TRANSMISSION MEDIA 199 Table 7.4 Bands (continued) Band Range Propagation Application middle frequency (MF) 300 kHz–3 MHz Sky AM radio high frequency (HF) 3–30 MHz Sky Citizens band (CB), ship/aircraft very high frequency (VHF) 30–300 MHz Sky and VHF TV, FM radio line-of-sight ultrahigh frequency (UHF) 300 MHz–3 GHz Line-of-sight UHF TV, cellular phones, paging, satellite superhigh frequency (SF) 3–30 GHz Line-of-sight Satellite extremely high frequency (EHF) 30–300 GHz Line-of-sight Radar, satellite We can divide wireless transmission into three broad groups: radio waves, micro- waves, and infrared waves. 7.3.1 Radio Waves Although there is no clear-cut demarcation between radio waves and microwaves, elec- tromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are normally called radio waves; waves ranging in frequencies between 1 and 300 GHz are called micro- waves. However, the behavior of the waves, rather than the frequencies, is a better criterion for classification. Radio waves, for the most part, are omnidirectional. When an antenna transmits radio waves, they are propagated in all directions. This means that the sending and receiving antennas do not have to be aligned. A sending antenna sends waves that can be received by any receiving antenna. The omnidirectional property has a disadvantage, too. The radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signals using the same frequency or band. Radio waves, particularly those waves that propagate in the sky mode, can travel long distances. This makes radio waves a good candidate for long-distance broadcast- ing such as AM radio. Radio waves, particularly those of low and medium frequencies, can penetrate walls. This characteristic can be both an advantage and a disadvantage. It is an advan- tage because, for example, an AM radio can receive signals inside a building. It is a dis- advantage because we cannot isolate a communication to just inside or outside a building. The radio wave band is relatively narrow, just under 1 GHz, compared to the microwave band. When this band is divided into subbands, the subbands are also narrow, leading to a low data rate for digital communications. Almost the entire band is regulated by authorities (e.g., the FCC in the United States). Using any part of the band requires permission from the authorities. Omnidirectional Antenna Radio waves use omnidirectional antennas that send out signals in all directions. Based on the wavelength, strength, and the purpose of transmission, we can have sev- eral types of antennas. Figure 7.19 shows an omnidirectional antenna. 200 PART II PHYSICAL LAYER Figure 7.19 Omnidirectional antenna Applications The omnidirectional characteristics of radio waves make them useful for multicasting, in which there is one sender but many receivers. AM and FM radio, television, mari- time radio, cordless phones, and paging are examples of multicasting. Radio waves are used for multicast communications, such as radio and television, and paging systems. 7.3.2 Microwaves Electromagnetic waves having frequencies between 1 and 300 GHz are called micro- waves. Microwaves are unidirectional. When an antenna transmits microwaves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. The unidirectional property has an obvious advantage. A pair of antennas can be aligned without interfering with another pair of aligned antennas. The following describes some characteristics of microwave propagation: ❑ Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to be in direct sight of each other, towers that are far apart need to be very tall. The curvature of the earth as well as other blocking obstacles do not allow two short towers to communicate by using microwaves. Repeaters are often needed for long- distance communication. ❑ Very high-frequency microwaves cannot penetrate walls. This characteristic can be a disadvantage if receivers are inside buildings. ❑ The microwave band is relatively wide, almost 299 GHz. Therefore wider subbands can be assigned, and a high data rate is possible. ❑ Use of certain portions of the band requires permission from authorities. Unidirectional Antenna Microwaves need unidirectional antennas that send out signals in one direction. Two types of antennas are used for microwave communications: the parabolic dish and the horn (see Figure 7.20).  CHAPTER 7 TRANSMISSION MEDIA 201 Figure 7.20 Unidirectional antennas Focus Waveguide a. Parabolic dish antenna b. Horn antenna A parabolic dish antenna is based on the geometry of a parabola: Every line parallel to the line of symmetry (line of sight) reflects off the curve at angles such that all the lines intersect in a common point called the focus. The parabolic dish works as a funnel, catching a wide range of waves and directing them to a common point. In this way, more of the signal is recovered than would be possible with a single-point receiver. Outgoing transmissions are broadcast through a horn aimed at the dish. The micro- waves hit the dish and are deflected outward in a reversal of the receipt path. A horn antenna looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem (resembling a handle) and deflected outward in a series of narrow parallel beams by the curved head. Received transmissions are collected by the scooped shape of the horn, in a manner similar to the parabolic dish, and are deflected down into the stem. Applications Microwaves, due to their unidirectional properties, are very useful when unicast (one- to-one) communication is needed between the sender and the receiver. They are used in cellular phones (Chapter 16), satellite networks (Chapter 16), and wireless LANs (Chapter 15). Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs. 7.3.3 Infrared Infrared waves, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm), can be used for short-range communication. Infrared waves, having high frequencies, cannot penetrate walls. This advantageous characteristic prevents interfer- ence between one system and another; a short-range communication system in one room cannot be affected by another system in the next room. When we use our infrared remote control, we do not interfere with the use of the remote by our neighbors. How- ever, this same characteristic makes infrared signals useless for long-range communica- tion. In addition, we cannot use infrared waves outside a building because the sun’s rays contain infrared waves that can interfere with the communication. 202 PART II PHYSICAL LAYER Applications The infrared band, almost 400 THz, has an excellent potential for data transmission. Such a wide bandwidth can be used to transmit digital data with a very high data rate. The Infrared Data Association (IrDA), an association for sponsoring the use of infrared waves, has established standards for using these signals for communication between devices such as keyboards, mice, PCs, and printers. For example, some manufacturers provide a special port called the IrDA port that allows a wireless keyboard to commnicate with a PC. The standard originally defined a data rate of 75 kbps for a distance up to 8 m. The recent standard defines a data rate of 4 Mbps. Infrared signals defined by IrDA transmit through line of sight; the IrDA port on the keyboard needs to point to the PC for transmission to occur. Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation 7.4 END-CHAPTER MATERIALS 7.4.1 Recommended Reading For more details about subjects discussed in this chapter, we recommend the following books. The items in brackets [...] refer to the reference list at the end of the text. Books Transmission media is discussed in [GW04], [Sta04], and [Tan03]. [SSS05] gives full coverage of transmission media. 7.4.2 Key Terms angle of incidence omnidirectional antenna Bayonet Neill-Concelman (BNC) optical fiber cladding parabolic dish antenna coaxial cable Radio Government (RG) rating core radio wave critical angle reflection electromagnetic spectrum refraction fiber-optic cable RJ45 gauge shielded twisted-pair (STP) ground propagation single-mode fiber guided media sky propagation horn antenna straight-tip (ST) connector infrared wave subscriber channel (SC) connector IrDA port transmission medium line-of-sight propagation twisted-pair cable microwave unguided medium MT-RJ unidirectional antenna multimode graded-index fiber unshielded twisted-pair (UTP) multimode step-index fiber wireless communication