EDITED BY MARINA MD ARSHAD, CSC FSKSM UTM JB

7.1

Chapter 7

Transmission Media

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Transmission media are actually located below the physical layer and are directly controlled by the physical layer.

A transmission medium can be broadly defined as anything that carry information from a source to a destination.

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Figure 7.1 Transmission medium and physical layer

7.3

Figure 7.2 Classes of transmission media

7.4

7-1 GUIDED MEDIA

Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable that use metallic (copper) conductors, and fiber-optic cable that accepts and transports signals in the form of light.

Twisted-Pair CableCoaxial CableFiber-Optic Cable

Topics discussed in this section:

Consists of two conductors (normally copper), each with its own plastic insulation, twisted together.

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.

Interference (noise) and crosstalk may affect both wires and create unwanted signals.

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Figure 7.3 Twisted-pair cable

7.6

Figure 7.4 UTP and STP cables

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Table 7.1 Categories of unshielded twisted-pair cables

7.8

Figure 7.5 UTP connector

7.9

Figure 7.6 UTP performance

Applications

Twisted-pair cables are used in telephone lines to provide voice and data channels.

The DSL lines that are used by the telephone companies to privide high-data-rate connections also use the high-bandwidth capability of UTP cables.

LAN such as 10Base-T and 100Base-T, also use twisted-pair cables.7.10

Coax carries signals of higher frequency ranges than twisted-pair cable.

Outer metallic wrapping serves both as a shield against noise and as the second conductor that completes the circuit.

7.11

Figure 7.7 Coaxial cable

Coaxial Cable Standards

Categorized by radio government (RG) ratings.

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Table 7.2 Categories of coaxial cables

Used to connect coax with devices. Most common: bayone-Neill-Concelman (BNC) BNC connector is used to connect end of cable to a

device such as TV set. BNC T is used in Ethernet networks to branch out to

a connection to a computer or other device. BNC terminator is used at the end of cable to

prevent the reflection of the signal.

7.13

Figure 7.8 BNC connectors

7.14

Figure 7.9 Coaxial cable performance

7.15

FIBER-OPTIC CABLE Figure 7.11 Optical fiber

A fibre optic cable is made of glass or plastic and transmits signals in the form of light.

7.16

Figure 7.14 Fiber construction

Subscriber channel (SC) connector is used for cable TV.

Straight-tip (ST) connector is used for connecting cable to networking devices.

MT-RJ is the same size as RJ45

7.17

Figure 7.15 Fiber-optic cable connectors

7.18

Figure 7.16 Optical fiber performance

Advantages and DisadvantagesAdvantages Higher bandwidth Less signal

attenuation Immunity to

electromagnetic interference

Resistance to corrosive materials

Light weight Greater immunity to

tapping.

Disadvantages

Installation and maintenance.

Unidirectional light propagation.

Cost.

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7.20

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.

Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them.

Radio WavesMicrowavesInfrared

Topics discussed in this section:

7.21

Figure 7.17 Electromagnetic spectrum for wireless communication

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Figure 7.18 Propagation methods

7.23

Table 7.4 Bands

7.24

Figure 7.19 Wireless transmission waves

Frequencies between 3kHz and 1 Ghz.

When an antenna transmits radio waves, they are propagated in all directions (omnidirectional).

It is susceptible to interference by another antenna.

Advantage: can penetrate walls. Disadvantage: cannot isolate inside or

outside communication.

Used for multicast communications, such as radio and television, and paging systems.

7.25

RADIO WAVES: Figure 7.20 Omnidirectional antenna

Microwaves Frequencies between 1 and 300 GHz.

Unidirectional. Advantage: not interfering with another pair of aligned antennas.

Characteristics: Line-of-sight propagation. Very high-frequency microwaves cannot penetrate walls. The bad is relatively wide, almost 299 GHz. Use of certain portions of the band requires permission

from authorities.

Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

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7.27

Figure 7.21 Unidirectional antennas

Infrared Frequencies from 300 GHz to 400 THz.

Used for short-range communication in a closed area using line-of-sight propagation that prevents interference between systems. useless for long-range communications.

Cannot be used outside a building because the sun’s rays contain infrared waves that can interfere with communications.

Application: IrDA (Infrared Data Association) port that allows a wireless keyboard to communicate with a PC

7.28