7.1

Chapter 7

Transmission Media

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7.2

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, and fiber-optic cable.

Twisted-Pair Cable

Coaxial Cable

Fiber-Optic Cable

Topics discussed in this section:

7.5

Figure 7.3 Twisted-pair cable

7.6

Effect of Noise on Parallel Lines

7.7

Noise on Twisted-Pair Lines

7.8

Figure 7.4 UTP and STP cables

7.9

Table 7.1 Categories of unshielded twisted-pair cables

UTP ConnectorsFigure 7-9

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

7.11

Figure 7.5 UTP connector

7.12

Figure 7.6 UTP performance

7.13

Figure 7.7 Coaxial cable

7.14

Table 7.2 Categories of coaxial cables

7.15

Figure 7.8 BNC connectors

7.16

Figure 7.9 Coaxial cable performance

7.17

Figure 7.10 Bending of light ray

How is light guided down the fiber optic cable in the core? This occurs because the core and cladding have different indices

of refraction with the index of the core, , always being greater than the index of the cladding, . If a light ray is injected and

strikes the core-to-cladding interface at an angle greater than the critical angle then it is reflected back into the core. Since the

angle of incidence is always equal to the angle of reflection the reflected light will again be reflected. The light ray will then

continue this bouncing path down the length of the fiber optic cable. If the light ray strikes the core-to-cladding interface at

an angle less than the critical angle then it passes into the cladding where it is attenuated very rapidly with propagation

distance.

7.18

Figure 7.11 Optical fiber

Basically, a fiber optic cable is composed of two concentric layers termed the core

and the cladding. The core and cladding have different indices of refraction with the

core having and the cladding . Light is piped through the core

7.19

Figure 7.12 Propagation modes

7.20

Figure 7.13 Modes

7.21

Table 7.3 Fiber types

7.22

Figure 7.14 Fiber construction

7.23

Figure 7.15 Fiber-optic cable connectors

7.24

Figure 7.16 Optical fiber performance

7.25

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.

Radio Waves

Microwaves

Infrared

Topics discussed in this section:

7.26

Figure 7.17 Electromagnetic spectrum for wireless communication

7.27

Figure 7.18 Propagation methods

7.28

Table 7.4 Bands

7.29

Figure 7.19 Wireless transmission waves

7.30

Figure 7.20 Omnidirectional antenna

7.31

Radio waves are used for multicast

communications, such as radio and

television, and paging systems.

Note

7.32

Figure 7.21 Unidirectional antennas

7.33

Microwaves are used for unicast

communication such as cellular

telephones, satellite networks,

and wireless LANs.

Note

7.34

Infrared signals can be used for short-

range communication in a closed area

using line-of-sight propagation.

Note

7.35

Transmission Media

* Transmission media lie below the physical layer.

* A guided medium provides a physical conduit from one device to another.

* Twisted-pair cable, coaxial cable, and optical fiber are the most popular types of guided media.

* Twisted-pair cable consists of two insulated copper wires twisted together. Twisting allows each wire to have

approximately the same noise environment.

* Twisted-pair cable is used in telephone lines for voice and data communications.

* Coaxial cable has the following layers (starting from the center): a metallic rod-shaped inner conductor, an insulator

covering the rod, a metallic outer conductor (shield), an insulator covering the shield, and a plastic cover.

* Coaxial cable can carry signals of higher frequency ranges than twisted-pair cable.

* Coaxial cable is used in cable TV networks and traditional Ethernet LANs.

* Fiber-optic cables are composed of a glass or plastic inner core surrounded by cladding, all encased in an outside

jacket.

* Fiber-optic cables carry data signals in the form of light. The signal is propagated along the inner core by reflection.

* Fiber-optic transmission is becoming increasingly popular due to its noise resistance, low attenuation, and high-

bandwith capabilities.

* Signal propagation in optical fibers can be multimode (multiple beams from a light source) or single-mode (essentially

one beam from a light source).

* In multimode step-index propagation, the core density is constant and the light beam changes direction suddenly at

the interface between the core and the cladding.

* In multimode graded-index propagation, the core density decreases with distance from the center. This causes a

curving of the light beams.

* Fiber-optic cable is used in backbone networks, cable TV networks, and Fast Ethernet networks.

* Unguided media (usually air) transport electromagnetic waves without the use of a physical conductor.

* Wireless data is transmitted through ground propagation, sky propagation, and line-of-sight propagation.

* Wireless data can be classifed as radio waves, microwaves, or infrared waves.

* Radio waves are omnidirectional. The radio wave band is under government regulation.

* Microwaves are unidirectional; propagation is line of sight. Microwaves are used for cellular phone, satellite, and

wireless LAN communications.

* The parabolic dish antenna and the horn antenna are used for transmission and reception of microwaves.