7-1 GUIDED MEDIA
Guided media, which are those that provide a conduit
قناةfrom one device to another, include twisted-pair
cable, coaxial cable, and fiber-optic cable.
Topics discussed in this section:
Figure 7.3 Twisted-pair cable
One to carry the signal and the other as a ground.
The receiver uses the difference between the two
Interference and crosstalk may affect the wires and create unwanted signals
Twisting makes it probable that both wires are equally affected by external
influences, it means that the receiver which calculate the difference between the
two receives no unwanted signals.
Figure 7.4 UTP and STP cables
The metal case prevent noise or crosstalk, quality is improved.
Table 7.1 Categories of unshielded twisted-pair cables by EIA
Figure 7.5 UTP connector
RJ ( registered jack)
It’s a key connector which means that it can can be inserted in only one way
Figure 7.6 UTP performance (Attenuation vs frequency & distance)
Gauge is a measure of the wire thickness
LAN 10 base T
Figure 7.7 Coaxial cable
It carries signals of higher frequency ranges
Table 7.2 Categories of coaxial cables
RG radio governments
Figure 7.8 BNC connectors
To prevent signal reflection
Figure 7.9 Coaxial cable performance
The attenuation is much higher than the twisted –pair cable, but it has a much
higher bandwidth but we need a frequent use of repeaters. Its used in analog tel.
It carry up to 10000 voice signal and in digital network to carry data up to 600 Mbps,
now it is replaced by the fiber optic.
Figure 7.10 Bending of light ray
Fiber optic cable is made of glass or plastic ,, transform signals in form of light
I (angle of incidence)
Light travel in a straight lines, as long as it moves through a single uniform substance, if more than one the
ray changes its direction( different density)
So the critical angle is defined as the angle of incidence which provides an angle of refraction of 90-degrees .
Figure 7.11 Optical fiber
A glass or plastic core is surrounded by a cladding of less dense glass or plastic
Figure 7.13 Modes
Multimodal : multiple beams
from a light source move
through the core in different
Step index : suddenness of
changing the beam angle at the
surface between the core and
cladding material, core
material of the same density.
Graded index : core of a
varying density, index refers to
refraction, density is high at
the center, and decreases
gradually at the edge
Single :Highly focused source
of light that limits beams to a
small range of angles.
Figure 7.14 Fiber construction
Figure 7.15 Fiber-optic cable connectors
Sc subscriber channel St straight-tip
Advantages : Higher bandwidth, less signal attenuation ( still up to 50 Km),
Immunity against electromagnetic interference, resistance to corrosive materials
Advantages & Disadvantages of Optical fibers
1- higher Bandwidth ( but limited due to the trans. & receive. Technology
2- Less signal Attenuation ( signal can run up to 50Km without regeneration
3- Immunity to Electromagnetic interference.
4- Resistance to corrosive materials.
5- Light weight.
6- Greater immunity to tapping.
1- Installation and maintenance
2- unidirectional light propagation
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
Topics discussed in this section:
Figure 7.17 Electromagnetic spectrum for wireless communication
Figure 7.19 Wireless transmission waves
Radio Waves: 3kHz – 1 GHz Omni directions, signal propagates in all
directions, travelling long distances AM&FM radios television
Microwave : 1GHz-3GHz uni-direction
Infrared : 3GHz7.27
Radio waves are used for multicast
communications, such as radio and
Figure 7.21 Unidirectional antennas
A Pair of antennas can be aligned without interfering with another pair.
Microwave propagation is a line of sight.
-wider sub bands are assigned ( wide band) , high data rate .
Disadvantages: can’t penetrate walls, certain portions of the band
requires permission fro authorities.
Microwaves are used for unicast (One to
one) communication such as cellular
telephones, satellite networks,
and wireless LANs.
Infrared signals can be used for shortrange communication in a closed area
using line-of-sight propagation.