network ch 2

7/31/2019 Network Ch 2

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Physical Layer

Transmission MediaEmad Al-Shalabi


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Magnetic Media

One of the most common ways to

transport data from one computer to

another is to write them onto magnetic

tape or removable media


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Twisted Pair

One of the oldest and still most common

transmission media is twisted pair

A twisted pair consists of two insulatedcopper wires, typically about 1 mm thick.

The wires are twisted together in a helical

form, just like a DNA molecule.

When the wires are twisted, the waves

from different twists cancel out


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Twisted pairs can be used for transmitting

either analog or digital signals.

The bandwidth depends on the thicknessof the wire and the distance traveled


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Twisted pair cabling comes in several

varieties

Category 3

Category 5

Category 6

All of these wiring types are often referred

to as UTP (Unshielded Twisted Pair),(a) Category 3 UTP. (b) Category 5 UTP.


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Coaxial Cable

better shielding than twisted pairs

it can span longer distances at higher

speeds.

Two kinds of coaxial cable

50-ohm cable

75-ohm cable


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Coaxial Cable

consists of a stiff copper wire as the core

surrounded by an insulating material

insulator is encased by a cylindricalconductor


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Fiber Optics

An optical transmission system has three

key components

the light source,

the transmission medium,

and the detector

a pulse of light indicates a 1 bit and the

absence of light indicates a 0 bit.


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Fiber Cable

multimode fiber. many different rays will

be bouncing around at different angles.

single-mode fiber. light can propagate

only in a straight line, without bouncing


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Fiber Cables

(a) Side view of a single fiber.

(b) End view of a sheath with three fibers.


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Fiber Cables (2)

A comparison of semiconductor diodes and

LEDs as light sources.


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Fiber optics can be used for LANs as well

as for long-haul transmission


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Fiber Optic Networks

A fiber optic ring with active repeaters.


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Comparison of Fiber Optics and

Copper Wire

Fiber has many advantages it can handle much higher bandwidths than

copper

Fiber also has the advantage of not beingaffected by power surges, electromagneticinterference, or power failures

Fiber thin and lightweight

fibers do not leak light and are quite difficult totap. These properties gives fiber excellentsecurity against potential wire trappers


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Comparison of Fiber Optics and

Copper Wire

On the downside,

fiber is a less familiar technology requiring skills not all

engineers have,

and fibers can be damaged easily by being bent toomuch.

Since optical transmission is inherently unidirectional,

two-way communication requires either two fibers or

two frequency bands on one fiber. Finally, fiber interfaces cost more than electrical

interfaces.


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Wireless Transmission

The Electromagnetic Spectrum

Radio Transmission


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Wireless Transmission

Some people believe that the future holds

only two kinds of communication:

fiber and wireless.

Wireless has advantages for even fixed

devices if running a fiber to a building is difficult due to the terrain (mountains,jungles, swamps, etc.), wireless may be better


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When electrons move, they createelectromagnetic waves that can propagatethrough space (even in a vacuum).

The number of oscillations per second of awave is called its frequency and is measured in Hz

The distance between two consecutivemaxima (or minima) is called thewavelength


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When an antenna of the appropriate size

is attached to an electrical circuit, the

electromagnetic waves can be broadcast

efficiently and received by a receiver somedistance away. All wireless communication

is based on this principle.


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Radio Transmission

Radio waves are easy to generate,

can travel long distances,

can penetrate buildings easily,

they are widely used for communication, both

indoors and outdoors.

Radio waves also are omni directional, meaning

that they travel in all directions from the source,so the transmitter and receiver do not have to be

carefully aligned physically.


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Radio Transmission

(a) In the VLF, LF, and MF bands, radio waves follow thecurvature of the earth.

(b) In the HF band, they bounce off the ionosphere.


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radio waves tend to travel in straight linesand bounce off obstacles.

They are also absorbed by rain.

At all frequencies, radio waves are subjectto interference from motors and otherelectrical equipment.

Due to radio's ability to travel longdistances, interference between users is aproblem.


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Communication Satellites

Geostationary Satellites

Medium-Earth Orbit Satellites

Low-Earth Orbit Satellites

Satellites versus Fiber


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Communication satellites properties

a communication satellite can be thought of

as a big microwave repeater in the sky. It

contains several transponders, each of whichlistens to some portion of the spectrum,

amplifies the incoming signal, and then

rebroadcasts it at another frequency to avoid

interference with the incoming signal.


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The downward beams can be broad,

covering a substantial fraction of the

earth's surface, or narrow, covering an

area only hundreds of kilometers indiameter. This mode of operation is known

as a bent pipe.


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Communication satellites and some of their properties,

including altitude above the earth, round-trip delay

time and number of satellites needed for global

coverage.


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Assignment

Satellites versus Fiber

network ch 2

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