7.1

Chapter 7

Transmission Media

7.2

Figure 7.1 Transmission medium and physical layer

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

7.3

Figure 7.2 Classes of transmission media

signals: electromagnetic

7.4

7-1 GUIDED MEDIA7-1 GUIDED MEDIA

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

1. Uses a conductor such as a wire or a fiber optic cable to move the signal from sender to receiver.

2. Transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint

3. A signal traveling along any of these media is directed and contained by the physical limits of the medium.

4. 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.

7.5

Figure 7.3 Twisted-pair cable

1. Telephone line in your home; Low frequency transmission medium2. Consists of two conductors (normally copper), each with its own plastic

insulation, twisted together. 3. 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. 4. Interference (noise) and crosstalk may affect both wires and create unwanted

signals. 5. What if the two wires are parallel?6. Why twisted? To minimize the electromagnetic interference between adjacent

pairs. 7. 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.

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

7.10

Twisted Pair

Advantages Inexpensive and readily available Flexible and light weight Easy to work with and installDisadvantages Susceptibility to interference and noise Attenuation problem

For analog, amps needed every 5-6km For digital, repeaters needed every 2-3km

Relatively low bandwidth

7.11

Figure 7.7 Coaxial cable

Used for cable television, LANs, etc Conductors share a common center

axial, hence the term “co-axial”

7.12

Table 7.2 Categories of coaxial cables

7.13

Figure 7.9 Coaxial cable performance

7.14

Coax

Advantages Higher bandwidth Can be tapped easily Much less susceptible to interference than

twisted pairDisadvantages High attenuation rate makes it expensive

over long distance Bulky

7.15

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

7.16

Figure 7.11 Optical fiber

7.17

Figure 7.12 Propagation modes

7.18

Figure 7.13 Modes

7.19

Table 7.3 Fiber types

7.20

Figure 7.16 Optical fiber performance

7.21

Fiber OpticAdvantages greater capacity smaller size and lighter weight lower attenuation immunity to environmental

interference highly secure due to tap difficulty and

lack of signal radiationDisadvantages expensive over short distance requires highly skilled installers adding additional nodes is difficult

7.22

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.

7.23

Figure 7.17 Electromagnetic spectrum for wireless communication

7.24

Figure 7.18 Propagation methods

7.25

Table 7.4 Bands

7.26

Figure 7.19 Wireless transmission waves

7.27

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

7.28

Figure 7.21 Unidirectional antennas

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

and wireless LANs. Ranged from 1-300GHz

7.29

1. Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation.

2. Ranged from 300 GHz to 400 THz. 3. Cannot penetrate walls: prevents

interference between two systems.4. Useless for long-range

communication5. Cannot be used outside of a building6. Applications?

Infrared