Guided_unguided Transmission Media

8/3/2019 Guided_unguided Transmission Media

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2.2 GUIDED TRANSMISSION MEDIA

The physical layer is to transport a raw bit stream from one machineto another.

Various physical media used for the actual transmission.

Each one has its own niche in terms of bandwidth, delay, cost, and

ease of installation and maintenance.

Media

Guided media

(such as copper wire and

fiber optics)

Unguided media

(such as radio and

lasers through the air)


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2.2.1 Magnetic Media

to write data onto magnetic tape or removable media (e.g.,

recordable DVDs),

physically transport the tape or disks to the destination

machine, and read them back again.

Not as sophisticated as using a geosynchronous

communication satellite.

It is often more cost effective, especially for applications in

which high bandwidth or cost per bit transported is the key

factor.

Although the bandwidth characteristics of magnetic tape areexcellent, the delay characteristics are poor. Transmission

time is measured in minutes or hours, not milliseconds


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In computer networks, bandwidth is often used as a synonym

fordata transfer rate - the amount of data that can be carried

from one point to another in a given time period (usually asecond). This kind of bandwidth is usually expressed in bits

(of data) per second (bps).

In general, a link with a high bandwidth is one that may be

able to carry enough information to sustain the succession of

images in a video presentation.

In electronic communication, bandwidth is the width of the

range (orband) of frequencies that an electronic signal uses

on a given transmission medium. the frequency of a signal is

measured in hertz. A typical voice signal has a bandwidth of

approximately three kilohertz (3 kHz); an analog television

(TV) broadcast video signal has a bandwidth of six megahertz

(6 MHz) -- some 2,000 times as wide as the voice signal.


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2.2.2. Twisted Pair

It is the oldest and still most common transmission

medium.

A pair consists of two insulated copper wires (about 1

mm thick each).

Its most common application is the telephone system.

Twisted pairs can run several km without amplification,

but for longer distances, repeaters are needed.

It can transmit either analog or digital information.

The bandwidth depends on the thickness of the wire and

the distance traveled. A few Mbps can be achieved for a

few km.

Main advantages: adequate performance and low cost.


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2.2.2 Twisted Pair

Twisting is done because two parallel wires constitute a fine antenna.

When the wires are twisted, the waves from different twists cancel out, so

the wire radiates less effectively.

Amplifier- a natural or artificial device intended to make a signal

stronger.

A repeateris an electronic device that receives a signal and

retransmits it at a higher level and/or higher power, or onto the

other side of an obstruction, so that the signal can cover longer

distances. Telephone towers are example of wireless repeater


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2.2.2 Twisted Pair

Unshielded Twisted Pair (UTP)

Category 3 twisted pairs consist of two insulated wires gently twisted

together. Four pairs are grouped together in a plastic sheath to protect

the wires and to keep them together.

Category 5: Similar to category 3 pair, but more twists per cm which

results in less crosstalk and better quality signal over longer distance

making them more suitable for high-speed computer communication

Upcoming categories are 6 and 7, which are capable of handling

signals with bandwidth of 250 MHz and 600 MHz respectively


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Coaxial cable is the type of transmission media. Two types of coaxialcables widely used are a 50-Ohm cable (Baseband coaxial cable) is

used for digital transmission and 75-ohm cable (Broadband) is used for

analog transmission and cable television

A coaxial cable consists of a stiff copper wire at core surrounded by an

insulating material and encased in a cylinder of closely-woven braided

mesh and covered in a protective plastic sheath (outer conductor).

The construction and shielding gives it a good combination of high

bandwidth and excellent noise immunity.

Bandwidth depends on cable length, quality and signal-noise ratio of

data

Modern cables have a bandwidth of close to 1GHZ.

It is widely used for LANs and cable TV.

Coaxial Cable


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2.2 Transmission Media

2.2.3 Baseband Coaxial Cable

Use digital transmission. For 1-km cables, a data rate of 1 to

2 Gbps is feasible.

2. Physical Layer


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2.2.4. Broadband coaxial cable

This is another kind of coaxial cable (75-ohm) which is usedforanalog transmission.

In the telephone world, ``broadband cable'' refers to anything

wider than 4 kHz.

In the computer networking world, this term means any

cable network using analog transmission.

The bandwidth is 300-450 MHz for nearly 100 km.

This bandwidth is divided up into multiple channels,

frequently the 6-MHz channels for TV broadcasting.


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2.2.4. Broadband coaxial cable

To transmit digital signals on an analog network, each

interface must contain devices to convert the outgoing bit

stream to an analog signal, and the incoming analog

signal to a bit stream.

1 bps may occupy roughly 1 Hz of bandwidth.

At higher frequencies, many bits per Hz are possibleusing advanced modulation techniques.


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2.2.4 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.

The transmission medium is an ultra-thin fiber of glass.

The detector generates an electrical pulse when light falls on

it.

By attaching a light source to one end of an optical fiber and a

detector to the other, a unidirectional data transmission systemthat accepts an electrical signal, converts and transmits it by light

pulses, and then reconverts the output to an electrical signal at the

receiving end.


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When a light ray passes from one medium to another, for

example, from fused silica to air, the ray is refracted (bent) at

the silica/air boundary, as shown in Fig.2-5(a).Here we see a light ray incident on the boundary at an angle

1 emerging at an angle 1.

The amount of refracting depends on the properties of the two

media (in particular, their indices of refraction).For angles of incidence above a certain critical value, the light

is refracted back into the silica;

none of its escapes into the air.

a light ray incident at or above the critical angle is trappedinside the fiber, as shown in Fig 2-5(b), and can propagate for

many kilometers with virtually no loss.

But since any light ray incident on the boundary above the

critical angle will be reflected internally, many different rays willbe bouncing around at different angles.


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Each ray is said to have a different mode, so a fiber having this

property is called a multimode fiber.

If the fibers diameter is reduced to a few wavelengths of light,the fiber acts like a wave guide,

and the light can propagate only in a straight line, without

bouncing, yielding a single-mode fiber.

Single-mode fibers are more expensive but are widely usedfor longer distances.


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Fiber Optics

(a) Three examples of a light ray from inside asilica fiber impinging on the air/silica

boundary at different angles.

(b) Light trapped by total internal reflection.


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Fiber Optic Cables Fiber optic cables are similar to coaxial cable except the braid.

At the center is the glass core center through which light propagates

In multimode fibers, core is typically 50 microns in diameter and in a

single mode 8 to 10 microns

Core is surrounded by a glass cladding with a lower index of

refraction than the core.

The cladding is protected by a thin plastic jacket.

Fibers generally grouped in bundles protected by an outer sheath.

FiberCabling - Where Found?

Terrestrial fiber sheath Laid in the ground. Near shore trans oceanic fiber sheaths buried in trenches by a

kind of sea plow

Deep water just lie at the bottom


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2.2 Transmission Media

2.2.5 Fiber Optics

Multimode fiber

FiberCables (2)


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Fiber Optic Cables

Fibers can be connected in 3 ways They can terminate in connectors and be plugged into

fiber sockets. Connectors lose 10 to 20 % of light but

make it easy to reconfigure systems.

They can be spliced mechanically. Mechanical splicestake trained personnel about 5minutes and result in 10%

light loss

Two pieces of fiber can be melted/fused to form a solid

connections and small amount of attenuation occurs.

Two kinds oflight sources are used for signaling - LED

(Light Emitting Diodes) and semiconductor lasers. Both

differ in properties.


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FiberCables (2)

A comparison of semiconductor diodes and

LEDs as light sources.


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Fiber Optic Networks

A fiber optic ring with active repeaters.


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Fiber Optic Networks (2)

A passive star connection in a fiber optics

network.


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Comparison of fiber optics to the copper wire.

Positive side:

Extremely high bandwidth with little power loss.

Not affected by power line surges, electromagnetic interference, orcorrosive chemicals in the air can be used in harsh environments

unsuitable for coaxial cable.

Very thin, a big plus for companies with thousands of cables and

bulging cable ducts.

Minus side:

An unfamiliar technology requiring skills most network engineers do

not have.

Difficult to splice and even more difficult to tap (how about

security ?).

Inherently unidirectional, and interfaces are considerably moreexpensive than electrical interfaces.

The future of all fixed data communication for distances of more

than a few meters is clearly with fiber.


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The Electromagnetic Spectrum

The electromagnetic spectrum and its uses

for communication.


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2.3 Wireless Transmission

2.3.2 Radio Transmission

Radio waves are easy to generate, can travel long distance, and

penetrate buildings easily, so they are widely used for

communication, both indoors and outdoors.

Radio waves are also omnidirectional, meaning that they travel

in all directions from the source, so that the transmitter and

receiver do not have to be carefully aligned physically.

Omnidirectional waves sometimes can have undesired side effects.

2. Physical Layer


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In the VLF, LF, and MF bands, radio waves follow the

curvature of the earth.

2. Physical Layer


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In the HF they bounce off the ionosphere.

At height 100 to 500km

2. Physical Layer


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2.3.3 Microwave Transmission

Above 100 MHz, the waves travel in straight lines and can

therefore be narrowly focused.

Concentrating all the energy into a small beam using aparabolic antenna gives a much higher signal to noise ratio.

Since the microwaves travel in a straight line, if the

towers are too far apart, the earth will get in the way.

Consequently, repeaters are needed periodically.

2. Physical Layer


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Disadvantages:

do not pass through buildings well

multipath fading problem (the delayed waves cancel the signal)absorption by rain above 8 GHz

severe shortage of spectrum

Advantages:

no right way is needed (compared to wired media)relatively inexpensive

simple to install

2. Physical Layer


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ISM (Industrial/Scientific/Medical) Band

Transmitters using these bands do not require government licensing.

One band is allocated worldwide: 2.400-2.484 GHz.

In addition, in the US and Canada, bands also exist from 902-928

MHz and from 5.725-5.850 GHz.

These bands are used for cordless telephones, garage door openers,

wireless hi-fi speakers, security gates, etc.

2. Physical Layer


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2.3.4 Infrared and Millimeter Waves

Unguided infrared and millimeter waves are widely used for short-range

communication.

The remote controls used on televisions, VCRs, and stereos all useinfrared communication.

They are relatively directional, cheap, and easy to build, but have a

major drawback: they do not pass through solid objects.

This property is also a plus. It means that an infrared system in one

room will not interfere with a similar system in adjacent room. It is

more secure against eavesdropping.

2. Physical Layer


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Affected by fog or rain


2.3.5 Lightwave Transmission

2. Physical Layer

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Guided_unguided Transmission Media