Chapter 7

Transmission Media

7.1

EDITED BY MARINA MD ARSHAD, CSC FSKSM UTM JB

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

7.2

Figure 7.1

Transmission medium and physical layer

 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.

7.3

Figure 7.2

Classes of transmission media

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.

Topics discussed in this section:

Twisted-Pair Cable Coaxial Cable Fiber-Optic Cable 7.4

Figure 7.3

Twisted-pair cable

7.5

 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.

7.6

Figure 7.4

UTP and STP cables

7.7

Table 7.1

Categories of unshielded twisted-pair cables

7.8

Figure 7.5

UTP connector

7.9

Figure 7.6

UTP performance

Applications

7.10

 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.

Figure 7.7

Coaxial cable

  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

7.12

Coaxial Cable Standards

 Categorized by radio government (RG) ratings.

Table 7.2

Categories of coaxial cables

Figure 7.8

BNC connectors

     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 7.9

Coaxial cable performance

7.14

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.

7.15

Figure 7.14

Fiber construction

7.16

Figure 7.15

Fiber-optic cable connectors

   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 7.16

Optical fiber performance

7.18

Advantages and Disadvantages

7.19

Advantages

      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.

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.

Topics discussed in this section:

Radio Waves Microwaves Infrared 7.20

Figure 7.17

Electromagnetic spectrum for wireless communication

7.21

Figure 7.18

Propagation methods

7.22

7.23

Table 7.4

Bands

Figure 7.19

Wireless transmission waves

7.24

RADIO WAVES: Figure 7.20

Omnidirectional antenna

 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

7.26

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.

Figure 7.21

Unidirectional antennas

7.27

7.28



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