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Chapter 7 Transmission Media
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Figure 7.1 Transmission medium and physical layer
Transmission media are located below the physical layer and are directly controlled by the physical layer.
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signals: electromagnetic
Figure 7.2 Classes of transmission media signals: electromagnetic
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7-1 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. Uses a conductor such as a wire or a fiber optic cable to move the signal from sender to receiver. Transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint 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.
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Figure 7.3 Twisted-pair cable
Telephone line in your home; Low frequency transmission medium Consists of two conductors (normally copper), each with its own plastic insulation, twisted together. One is used to carry signals to the receiver, the other is used only for ground reference. The receiver uses the difference between the two. Interference (noise) and crosstalk may affect both wires and create unwanted signals. What if the two wires are parallel? Why twisted? To minimize the electromagnetic interference between adjacent pairs. Twising the pairs, a balance is maintained. One twist one is closer, then next twist, the other is closer. So both wires are equally affected by external influences. The unwanted signals are mostly canceled out.
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Figure 7.4 UTP and STP cables
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Table 7.1 Categories of unshielded twisted-pair cables
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Figure 7.5 UTP connector
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Figure 7.6 UTP performance
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Twisted Pair Advantages Inexpensive and readily available
Flexible and light weight Easy to work with and install Disadvantages Susceptibility to interference and noise Attenuation problem For analog, amps needed every 5-6km For digital, repeaters needed every 2-3km Relatively low bandwidth
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Figure 7.7 Coaxial cable Used for cable television, LANs, etc Conductors share a common center axial, hence the term “co-axial”
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Table 7.2 Categories of coaxial cables
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Figure 7.9 Coaxial cable performance
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Coax Advantages Higher bandwidth Can be tapped easily
Much less susceptible to interference than twisted pair Disadvantages High attenuation rate makes it expensive over long distance Bulky
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Figure 7.10 Bending of light ray
Relatively new transmission medium used by telephone companies in place of long-distance trunk lines Also used by private companies in implementing local data communications networks
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Figure Optical fiber
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Figure 7.12 Propagation modes
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Figure Modes
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Table 7.3 Fiber types
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Figure 7.16 Optical fiber performance
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Fiber Optic Advantages greater capacity
smaller size and lighter weight lower attenuation immunity to environmental interference highly secure due to tap difficulty and lack of signal radiation Disadvantages expensive over short distance requires highly skilled installers adding additional nodes is difficult
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7-2 UNGUIDED MEDIA: WIRELESS
Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication.
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Figure 7.17 Electromagnetic spectrum for wireless communication
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Figure 7.18 Propagation methods
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Table 7.4 Bands
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Figure 7.19 Wireless transmission waves
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Figure 7.20 Omnidirectional antenna
Radio waves are used for multicast communications, such as AM, FM, maritime radio, cordless phones, television, and paging systems. Ranged from 3kHz – 1GHz
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Figure 7.21 Unidirectional antennas
Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs. Ranged from 1-300GHz
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Infrared Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation. Ranged from 300 GHz to 400 THz. Cannot penetrate walls: prevents interference between two systems. Useless for long-range communication Cannot be used outside of a building Applications?
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