7.1

Chapter 7Transmission Media

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7.2

Figure 7.1 Transmission medium and physical layer

7.3

Figure 7.2 Classes of transmission media

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7-1 GUIDED MEDIA7-1 GUIDED MEDIA

Guided media, which are those that provide a conduit Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable.coaxial cable, and fiber-optic cable.

Twisted-Pair CableCoaxial CableFiber-Optic Cable

Topics discussed in this section:Topics discussed in this section:

7.5

Figure 7.3 Twisted-pair cable

Why twisted?To make unwanted signals interferencecancel out each other.

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Figure 7.4 UTP (unshielded twisted pair) and STP (shielded twisted pair) cables

Example: Ethernet, phone line

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UTP cables

1 – Jacket2 – Solid twisted pair3 – Spacer

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SSTP cables

1 – Jacket2 – Shield-braid3 – Shield-foil4 – Stranded twisted pair

7.9

Figure 7.5 UTP connector

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Figure 7.7 Coaxial cable

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Figure 7.8 BNC connectors

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Twisted-pair cable vs. coaxial cableTwisted-pair cable vs. coaxial cable

Bandwidth: coaxial > twisted-pairBandwidth: coaxial > twisted-pair

Transmission distance: twisted-pair > coaxialTransmission distance: twisted-pair > coaxialThus cable needs frequent use of repeaters.Thus cable needs frequent use of repeaters.

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Figure 7.10 Bending of light ray

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Figure 7.11 Optical fiber

This is the reason why This is the reason why optical fiber cannot be optical fiber cannot be bended arbitrarily.bended arbitrarily.

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Figure 7.14 Fiber construction

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Figure 7.15 Fiber-optic cable connectors

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Pros and Cons for Optical Fiber CablePros and Cons for Optical Fiber Cable

Pros:Pros:•Higher bandwidthHigher bandwidth•Less signal attenuation (50km without repeater; Less signal attenuation (50km without repeater; twisted pair and coaxial requires 5km per repeater)twisted pair and coaxial requires 5km per repeater)•Immune to electromagnetic interferenceImmune to electromagnetic interference•Resistance to corrosive materialsResistance to corrosive materials•Light weightLight weight•Good resist to tappingGood resist to tappingCons:Cons:•Installation and maintanceInstallation and maintance•One direction communication for one line (not duplex)One direction communication for one line (not duplex)•CostCost

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7-2 UNGUIDED MEDIA: WIRELESS7-2 UNGUIDED MEDIA: WIRELESS

Unguided media transport electromagnetic waves Unguided media transport electromagnetic waves without using a physical conductor. This type of without using a physical conductor. This type of communication is often referred to as wireless communication is often referred to as wireless communication.communication.

Radio WavesMicrowavesInfrared

Topics discussed in this section:Topics discussed in this section:

7.19

Figure 7.17 Electromagnetic spectrum for wireless communication

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Electromagnetic spectrum for wireless communication

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Figure 7.18 Propagation methods

FM radio: 87.5 to 108.0 MHz

AM radio: long wave:: 148.5 kHz–283.5 kHzMedium wave:: 520 kHz–1,610 kHzShort wave:: 2.3 MHz–26.1 MHz

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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

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Example: Omnidirectional antenna

Use:Wireless router

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Radio waves are used for multicast communications, such as radio and

television, and paging systems.

Note

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Figure 7.21 Unidirectional antennas

7.28

Microwaves are used for unicast communication such as cellular telephones, satellite networks,

and wireless LANs.

Note

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Infrared signals can be used for short-range communication in a closed area

using line-of-sight propagation.

Note

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Example: Unidirectional Anntena

Use:Wireless link connecting two remote WLANs

7.31

Example: Homemade Unidirectional Antenna - Can antenna

7.32

Example: Cellular Tower

Use:Cellular Anntena Tower

Cellular Tower

7.33

More details: http://en.wikipedia.org/wiki/Cellular_tower

In suburban areas, masts are commonly spaced 1–2 miles (2–3 km) apart and in dense urban areas, masts may be as close as ¼-½ mile (400–800 m) apart.

The maximum range of a mast:it is possible to get between 50 to 70 km (30–45 miles).

Check map of cell tower in your area:http://www.gotreception.com/

Advantages and Disadvantages Wireless Communication Advantages

User Mobility Easy to install Reduced cost Scalability

Disadvantages High data error rate Lower transmission data rates Security Battery of Mobile Devices Health Issues