Chapter 7
Transmission Media
7.1
Figure 7.1 Transmission medium and physical layer
Transmission media are located below the physical layer and
are directly controlled by the physical layer.
7.2
Figure 7.2 Classes of transmission media
signals: electromagnetic
7.3
7-1 GUIDED MEDIA
Guided media, which are those that provide a
conduit from one device to another, include twistedpair 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.
7.4
Figure 7.3 Twisted-pair cable
1.
2.
3.
4.
5.
6.
7.
7.5
Telephone line in your home; Low frequency transmission medium
Consists of two conductors (normally copper), each with its own plastic
insulation, twisted together.
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.
Interference (noise) and crosstalk may affect both wires and create unwanted
signals.
What if the two wires are parallel?
Why twisted? To minimize the electromagnetic interference between adjacent
pairs.
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.
Figure 7.4 UTP and STP cables
7.6
Table 7.1 Categories of unshielded twisted-pair cables
7.7
Figure 7.5 UTP connector
7.8
Figure 7.6 UTP performance
7.9
Twisted Pair
Advantages
 Inexpensive and readily available
 Flexible and light weight
 Easy to work with and install
Disadvantages
 Susceptibility to interference and noise
 Attenuation problem
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

7.10
For analog, amps needed every 5-6km
For digital, repeaters needed every 2-3km
Relatively low bandwidth
Figure 7.7 Coaxial cable


7.11
Used for cable television, LANs, etc
Conductors share a common center axial,
hence the term “co-axial”
Table 7.2 Categories of coaxial cables
7.12
Figure 7.9 Coaxial cable performance
7.13
Coax
Advantages
 Higher bandwidth
 Can be tapped easily
 Much less susceptible to interference than
twisted pair
Disadvantages
 High attenuation rate makes it expensive over
long distance
 Bulky
7.14
Figure 7.10 Bending of light ray
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
7.15
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
Figure 7.11 Optical fiber
7.16
Figure 7.12 Propagation modes
7.17
Figure 7.13 Modes
7.18
Table 7.3 Fiber types
7.19
Figure 7.16 Optical fiber performance
7.20
Fiber Optic
Advantages
 greater capacity
 smaller size and lighter weight
 lower attenuation
 immunity to environmental interference
 highly secure due to tap difficulty and lack
of signal radiation
Disadvantages
 expensive over short distance
 requires highly skilled installers
 adding additional nodes is difficult
7.21
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.
7.22
Figure 7.17 Electromagnetic spectrum for wireless communication
7.23
Figure 7.18 Propagation methods
7.24
Table 7.4 Bands
7.25
Figure 7.19 Wireless transmission waves
7.26
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
7.27
Figure 7.21 Unidirectional antennas
Microwaves are used for unicast communication such
as cellular telephones, satellite networks,
and wireless LANs. Ranged from 1-300GHz
7.28
Infrared
1. Infrared signals can be used for shortrange 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
communication
5. Cannot be used outside of a building
6. Applications?
7.29