Chapter 04 Transmission Media in networking.ppt

Media that is used to transfer data from one device to another

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Computer Networks and Internets with
Internet Applications, 4e
By Douglas E. Comer
Lecture PowerPoints
By Lami Kaya, [email protected]

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Chapter 4
Transmission Media

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Topics Covered
•4.1 Introduction
•4.2 Copper Wires
•4.3 Glass Fibers
•4.4 Radio
•4.5 Satellites
•4.6 Geosynchronous Satellites
•4.7 Low Earth Orbit Satellites
•4.8 Low Earth Orbit Satellite Arrays
•4.9 Microwave
•4.10 Infrared
•4.11 Light From A Laser

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4.1 Introduction
•At the lowest level, all computer communication involves
–encoding data in a form of energy
–and sending the energy across a transmission medium
•HW devices attached to a computer perform the encoding and
decoding of data
–programmers and users do not need to know the details of data
transmission
•This part covers the basics of data transmission:
–The first chapter examines the media that are used for transmission in
modern network systems
–The next two chapters explain how data can be transferred across such
media
–Later sections explain how transmission forms the basis of data NW

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4.2 Copper Wires (1)
•Conventional computer NW use wires as the primary medium
–NW use copper wire almost exclusively because its low resistance
•The type of wiring used for NW is chosen to minimize interference
•Interference arises because an electrical signal traveling across a
wire acts like a miniature radio station
–the wire emits a small amount of electromagnetic (EM) energy, which
can travel through the air
•Whenever it encounters another wire, an EM wave generates a
small electric current in the wire.
–The amount of current generated depends on the strength of the EM
wave and the physical position of the wire
•When two wires are placed close together and in parallel, a strong
signal sent on one wire will generate a similar signal on the other

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4.2 Copper Wires (2)
•Problem of interference is severe
–because wires that comprise a NW often are placed in parallel
with many other wires
•To minimize interference, networks use one of three
basic wiring types:
–Unshielded Twisted Pair (UTP)
–Coaxial Cable
–Shielded Twisted Pair (STP)

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4.2 Copper Wires (2)
•See Figure 4.1 for an illustration of twisted-pair cables
•Twists change the electrical properties of the wire:
–First, they limit the EM energy the wire emits:
•So they help prevent electric currents on the wire from radiating
energy that interferes with other wires
–Second, they make the pair of wires less susceptible to EM
energy:
•They help prevent signals on other wires from interfering with the pair

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4.2 Copper Wires (3)
•Coaxial cables provides even more protection from
interference than twisted pair
–Instead of twisting wires around one another to limit interference,
•a coaxial cable consists of a single wire surrounded by a metal shield
• It’s shown in Figure 4.2
•Metal shield in a coaxial cable forms a flexible cylinder
around the inner wire to provide a barrier for EM radiation
–The barrier isolates the inner wire in two ways:
•it protects the wire from incoming EM energy
•and keeps signals on the inner wire from radiating EM energy
•The cable can be placed parallel to other cables or bent
and twisted around corners

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4.2 Copper Wires (4)
Shielded Twisted Pair (STP)
–A shielded twisted pair cable consists of a pair of wires
surrounded by a metal shield
•Each wire is coated with an insulating material,
–so the metal in one wire does not touch the metal in another
•The shield merely forms a barrier that prevents EM
radiation from entering or escaping
•The additional shielding provided by coaxial or shielded
twisted pair cabling is often used
–when wires from a NW pass near equipment that generates
strong electric or magnetic fields

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4.3 Glass Fibers (1)
•NW also use flexible glass fibers to transmit data
–known as an optical fiber
•Medium uses light to transport data
•The miniature glass fiber is encased in a plastic jacket
–which allows the fiber to bend without breaking
•A transmitter at one end of a fiber uses
–a light emitting diode (LED) or a laser to send pulses of light
•A receiver at the other end uses
–a light sensitive transistor to detect the pulses

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4.3 Glass Fibers (2)
Main advantages fiber optics over wires:
•First, because they use light,
–neither cause electrical interference in other cables
–nor are they susceptible to electrical interference
•Second, because glass fibers can be manufactured to reflect most
of the light inward
–a fiber can carry a pulse of light much farther than a copper wire can
carry a signal
•Third, because light can encode more information than electrical
signals
–an optical fiber can carry more information than a wire
•Fourth, unlike electricity, which always requires a pair of wires
connected into a complete circuit,
–light can travel from one computer to another over a single fiber

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4.3 Glass Fibers (3)
Optical fibers do have some disadvantages
•First, installing a fiber requires special equipment
–that polishes the ends to allow light to pass through
•Second, if a fiber breaks inside the plastic jacket:
–finding the location of the problem is difficult
•Third, repairing a broken fiber is difficult
–special equipment is needed to join two fibers

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4.4 Radio
•In Radio Frequency (RF) transmissions
–each participating computer attaches to an antenna
–Antenna can both transmit and receive RF
•Physically, the antennas used with RF networks may be
large or small, depending on the range desired:
–An antenna designed to propagate signals several miles
•may consist of a metal pole approximately two meters long that is
mounted vertically on top of a building
–An antenna designed to permit communication within a building
•may be small enough to fit inside a portable computer (e.g., less
than twenty centimeters)

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4.5 Satellites
•RF technology can be combined with satellites
–to provide communication across longer distances
•Figure 4.3 illustrates a satellite in orbit
•The satellite contains a transponder
–that consists of a radio receiver and transmitter
•The transponder
–accepts an incoming radio transmission
–amplifies it
–and transmits the signal back toward the ground at a slightly
different angle than it arrived
•A single satellite usually contains multiple transponders
–Each transponder uses a different radio frequency (i.e., channel)

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4.6 Geosynchronous Satellites (1)
•Communication satellites can be grouped into categories according
to the height at which they orbit:
–The easiest is geosynchronous or geostationary satellites
–The name arises because a geosynchronous satellite is placed in an
orbit that is exactly synchronized with the rotation of the earth.
–Such an orbit is classified as a Geostationary Earth Orbit (GEO)
–When viewed from the ground,
•satellite appears to remain at exactly the same point in the sky at all times
•Laws of physics determine the exact distance from the earth that a
satellite must orbit to remain synchronized with the earth's rotation
–See Kepler's Law for details
–The distance is 35,785 kilometers or 22,236 miles

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4.6 Geosynchronous Satellites (2)
•GEO is about one tenth of the distance to the moon
–Engineers refer the distance as “high earth orbit”
•There is a limited amount of ``space'' available in the
GEO above the equator
–because satellites using a given frequency must be separated
from one another to avoid interference
•The minimum separation depends on the power of the transmitters

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4.7 Low Earth Orbit Satellites (1)
•Second category of satellites operate in what is called
Low Earth Orbit (LEO)
–which means that they orbit a few hundred miles above the earth
(typically 200 to 400 miles)
•The chief disadvantage of a LEO lies in the rate at which
a satellite must travel
–Their period of rotation is faster than the rotation of the earth
•LEOs do not stay above a single point on the earth's surface
•An observer, who stands on the earth looking upward through a
telescope, sees LEOs move across the sky
•A single satellite can complete an entire orbit in
approximately 1.5 hours

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4.7 Low Earth Orbit Satellites (2)
From a communication provider's point of view:
•having a satellite that does not appear to remain
stationary causes problems:
–First, the satellite can only be used during the time
•that its orbit passes between two ground stations
–Second, maximal utilization requires complex control systems
•that continuously move the ground stations so they point directly at
the satellite

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4.8 Low Earth Orbit Satellite Arrays (1)
•Instead of focusing on one satellite,
–the scheme requires a communication company to launch a set of
satellites into low earth orbits
•Although a given satellite orbits quickly,
–the set of orbits is chosen so that each point on the ground has at least
one satellite overhead at any time
–sixty-six (66) satellites are required to provide service over the entire
surface of the earth
•From the point of view of an observer on earth,
–it appears that a satellite emerges from a point on the horizon
–flies overhead
–and then disappears into a point on the opposite horizon
•The key to the scheme lies in the set of orbits
–guarantees at least one satellite is available at any time

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4.8 Low Earth Orbit Satellite Arrays (2)
•In addition to transponders used to communicate with
ground stations
–an array of satellites in low earth orbit contains radio equipment
used to communicate with other satellites in the array
•As they move through their orbits
–the satellites communicate with one another and agree to
forward data

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4.9 Microwave
•Many long-distance telephone companies use microwave (MW) to
carry telephone conversations
–A few large companies have also installed MW systems as part of the
company's NW
•MW are merely a higher frequency version of radio waves, but they
behave differently
–Instead of broadcasting in all directions,
•a MW transmission can be aimed in a single direction, preventing others
from intercepting
–In addition, MW transmission can carry more information than lower
frequency RF transmissions
•MW cannot penetrate metal structures:
–transmission works best when a clear path exists between two parties
–most MW installations consist of two towers
•that are taller than the surrounding buildings and vegetation
–each MW transmitter aimed directly at a MW receiver on the other

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4.10 Infrared
•Infrared is limited to a small area (e.g., a single room)
•Usually requires that the transmitter be pointed toward
the receiver
•Infrared HW
– is inexpensive compared to other mechanisms,
–and does not require an antenna
•It is possible to equip a large room with a single infrared
connection
–that provides NW access to all computers
–computers can remain in contact with the NW while they are
moved within the room
•Infrared NW are especially convenient for small, portable
computers

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4.11 Light From A Laser
•A beam of light can also be used to carry data through the air
•A communication link that uses light consists of two sites that each
have a transmitter and receiver
–equipment is mounted in a fixed position, often on a tower
–aligned so the transmitter at one location sends its beam of light directly
to the receiver at the other
•The transmitter uses a laser to generate the beam of light
–because a coherent laser beam will stay focused over a long distance
•Light from a laser must travel in a straight line and must not be
blocked
•A laser beam cannot penetrate vegetation or weather conditions
such as snow and fog:
–Thus, laser transmission has limited use