CHAPTER 5
Transmission Medium
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.
Topics discussed in this section:
• Twisted-Pair Cable
• Coaxial Cable
Twisted-pair cable
A twisted pair consists of two conductors
Fiber-Optic Cable
Fiber optic cable is made of glass or
plastic & transmits signals in the form of light.
Light travels in a straight line as long as
it is moving through a single uniform substance.
If a ray of light traveling through one
substance suddenly enters another
Bending of light ray
Optical fibers use reflection to guide light
through a channel.
A glass or plastic core is surrounded by a
cladding of less dense glass or plastic.
The difference in density of the two
materials must be such that a beam of light moving through the core is
Multimode
Multiple beams from a light source move through the core in different paths.
In multimode step-index fiber, the density of the core remains constant from the center to the edges. A beam of light moves through this constant density
in a straight line until it reaches the interface of the core and the cladding.
At the interface, there is an abrupt change to a lower density that alters the angle of the beam’s motion. The term step index refers to the suddenness of this
Multimode
Multimode graded-index fiber
Decrease the distortion of the signal
through the cable.
Index refers to the index of refraction.
Index of refraction is related to density.
Density is highest at the center of the
Single Mode
Uses step-index fiber & a highly focused source of light that limits beams to a small range of angles, all close to the horizontal.
Manufactured with a much smaller diameter than that of multimode fiber, & with substantially lower density (index of refraction).
Decrease in density results in a critical angle that is close enough to 90° to make the propagation of
beams almost horizontal.
Fiber-optic cable connectors
Fiber optic cable has several advantages over metallic cable(twisted-pair or coaxial)
Advantages
Higher bandwidth ( higher data rates)
Less signal attenuation (a signal can run
for 50km without regeneration. Repeater is need every 5km for coaxial or twisted-pair cable).
Immunity to electromagnetic
interference
Resistance to corrosive materials
Disadvantages
Installation/maintenance (need
expertise)
Unidirectional (propagation of lights is
unidirectional. If we need bidirectional communication, two fibers are needed.
Applications
Often found in backbone networks
because its wide bandwidth is cost-effective.
With WDM, data can be transfer at a rate
of 1600 Gbps.
Waveguide
Waveguides are used to transfer
electromagnetic power efficiently from one point in space to another.
Some common guiding structures are
shown in the figure below.
These include the typical coaxial cable,
Microstrip
Microstrip is a type of electrical transmission
line which can be fabricated using printed circuit board technology, and is used to convey
microwave-frequency signals.
It consists of a conducting strip separated from
a ground plane by a dielectric layer known as the substrate.
Microwave components such as antennas,
couplers, filters, power dividers etc. can be
Microstrip
Cross-section of microstrip geometry.
Conductor (A) is separated from ground plane (D) by dielectric substrate (C).
Upper dielectric (B) is typically air.
Microstrip
Advantage
Microstrip is thus much less expensive than
traditional waveguide technology, as well as being far lighter and more compact.
Disadvantage
The disadvantages of microstrip compared with
waveguide are the generally lower power handling capacity, and higher losses. Also, unlike waveguide, microstrip is not enclosed, and is therefore
UNGUIDED MEDIUM
Unguided media transport electromagnetic
waves without using a physical conductor.
Radio Wave Transmission (Ground Wave)
There are two principal ways in which
electromagnetic (radio) energy travels from a transmitting antenna to a
receiving antenna.
One way is by GROUND WAVES and the
other is by SKY WAVES.
Ground waves are radio waves that
travel near the surface of the Earth (surface and space waves).
Sky waves are radio waves that are
Ground Waves
The ground wave is actually composed of two
separate component waves.
These are known as the SURFACE WAVE and the
SPACE WAVE. The determining factor in whether a ground wave component is classified as a
space wave or a surface wave is simple.
A surface wave travels along the surface of the
Earth.
Surface Wave
The surface wave reaches the receiving
site by traveling along the surface of the ground.
A surface wave can follow the contours
of the Earth because of the process of diffraction. When a surface wave meets an object and the dimensions of the
Space Wave
The space wave follows two distinct paths from the transmitting antenna to the receiving antenna - one through the air directly to the receiving antenna, the other reflected from the ground to the receiving
antenna.
The primary path of the space wave is directly from the transmitting antenna to the receiving antenna. So, the receiving antenna must be located within the radio
Sky Wave Propagation
Sky-wave propagation allows
communication over great distances
with simple equipment and
reasonable power levels : 100 W to a
few kW.
The sky wave, often called the
ionospheric wave, is radiated in an
upward direction and returned to Earth at some distant location because of
Satellite Communications
A communications
satellite (sometimes abbreviated to COMSAT) is an artificial satellite stationed in space for the purpose of telecommunications. Modern
Satellite Orbit
Seminar National Communication Satellite User Requirements, 29 April 2010, K.
HISTORICAL -The First in
Malaysia
Satellite First Launched
Measat-1 First Malaysian communication satellite Jan 1996
Measat-2 Malaysian communication satellite Nov 1996 Tiungsat-1 First Malaysian microsatellite in Low
Earth Orbit (LEO) 26 Sept. 2000
Razaksat First world remote sensing satellite launched into Near Equatorial Orbit (NEqO)
Overview
Satellite is a microwave repeater in the
space.
There are about 750 satellite in the
space, most of them are used for communication.
They are:
Wide area coverage of the earth’s surface. Transmission delay is about 0.3 sec.
Satellite up links and down links can operate in
different frequency bands:
The up-link is a highly directional, point to point link
The down-link can have a footprint providing coverage for a substantial area "spot beam“.
Band Up-Link
(Ghz) Down-link (Ghz) ISSUES
C 4 6 Interference with ground links.
Ku 11 14 Attenuation due to rain
Satellite Advantages
Satellite versus terrestrial link (Mitra, Communication, 2005)
Capable of transmitting signal long distances without using relay with higher capacity
Point to multipoint
Satellite circuit can be installed rapidly
Flexibility in interconnecting mobile vehicle
Cost independent of terrain
Digital Satellite (Keesee, Satellite Communication, 2009) Less distortion and interference, easy to regenerate,
Satellite Limitation
Long signal delay
High propagation loss
Repairing and maintenance after launching is
difficult
High cost and high risks on launching
Congestion of frequencies and orbit
Active communications satellite systems are
limited by two things
Antenna
An antenna (or aerial) is an electrical
device which converts electric
currents into radio waves, and vice versa.
An antenna can be used for both
Antenna
An antenna is an electrical conductor or
system of conductors
o Transmission - radiates electromagnetic energy
into space
o Reception - collects electromagnetic energy from space
In two-way communication, the same
Types of Antennas
Isotropic antenna (idealized)
o Radiates power equally in all directions
Dipole antennas
o Half-wave dipole antenna (or Hertz antenna)
o Quarter-wave vertical antenna (or Marconi antenna)
Parabolic Reflective Antenna
o Used for terrestrial microwave and satellite applications
o Larger the diameter, the more tightly directional is the
Radio waves are used for multicast communications,
such as radio and television, and paging systems.
They can penetrate through walls. Highly
regulated. Use omni directional antennas
Unidirectional antennas
Microwaves are used for unicast
communication such as cellular telephones, satellite networks, and wireless LANs.
Higher frequency ranges cannot
penetrate walls.
Use directional antennas - point to point
