Transmission Media - WordPress.com · 2 July 2008 Transmission Media Beulah A. 2 Guided Transmission Media Magnetic Media A tape can hold 7 gigabytes. A box can hold about 1000 tapes

2 July 2008 Transmission Media Beulah A. 1

Transmission Media

Beulah A

L/CSE


Guided Transmission Media

Magnetic Media

A tape can hold 7 gigabytes. A box can hold about 1000 tapes.

Assume a box can be delivered in 24 hours. The effective

bandwidth=7*1000*8/86400=648 Mbps

Cost of 1000 tapes=5000. If a tape can be reused 10 times and

the shipping cost is 200, we have a cost of 700 to ship 7000

gigabytes or 10 cents per gigabytes. No network carrier on

earth can compete with that.

Never underestimate the bandwidth of a station wagon full of

tapes hurtling down the highway.


Twisted Pair

Although the bandwidth characteristics of magnetic tape are

excellent, the delay characteristics are poor.

Twisted Pair: used in local loop in telephone systems

The purpose of twisting the wires is to reduced electrical

interference from similar pairs close by.


Twisted Pair Cont…

Unshielded Twisted Pair (UTP)

(a) Category 3 UTP. (16MHz)

(b) Category 5 UTP. (100MHz)

More twists per centimeter, less crosstalk


Coaxial Cable

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

2 Gbps is feasible.


Fiber Optics

Computing speed: a factor of 10 improvement per decade

Communication speed: a gain of more than a factor of 100 per

decade

In the race between computing and communication,

communication won. The new conventional wisdom should be

that all computers are hopelessly slow, and networks should

try to avoid computation at all costs, no matter how much

bandwidth that wastes.


Fiber Optics Cont…

An optical transmission system has three components: the light

source, the transmission medium, and the detector.

Light source: LED (Light Emitting Diode) or Laser (Light

Amplification by Simulated Emission of Radiation)

Transmission Media: ultra-thin fiber of glass

Detector: using light-electricity effect, generate an electrical

pulse when light falls on it




refraction and reflection

incident

rayreflected ray

refracted ray



perpendicular light partially reflected total reflection

critical angle

θα



Multimode fiber

cross section

core

cladding

protective coatingtwo propagation modes

η2

η1

η1 η2>



Multimode fiber



Electromagnetic Waves

speed=frequency × wavelength

m/s=cycles/s × m/cycles

one cycle

Hz(hertz)

speed of light (in vacuum)=

3 108× m / s

The Electromagnetic Spectrum

cf =•λ

Unguided Transmission Media


The Electromagnetic Spectrum Cont…


To prevent total chaos, there are national and international

agreements about who gets to use which frequencies. Since

everyone wants a higher data rate, everyone wants more

spectrum.

Therefore, we have to share.

FDMA: Frequency Division Multiple Access

TDMA: Time Division Multiple Access

CDMA: Code Division Multiple Access

(using spread spectrum technique)



Many transmissions use a narrow frequency band to get

the best reception.

However, in some cases, a wide band is used with two variations.

Frequency hopping spread spectrum

Direct sequence spread spectrum



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.


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

curvature of the earth.

Radio Transmission Cont…


In the HF they bounce off the ionosphere.

At height 100 to 500km

Radio Transmission Cont…


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 a parabolic 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.


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

Microwave Transmission Cont…


ISM (Industrial/Scientific/Medical) Band

The ISM bands in the United States.

Microwave Transmission


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 use infrared 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.


Affected by fog or rain

Lightwave Transmission


Communication Satellites

downlink channeluplink channel

Contain several transponders.

Properties:

1. Longer delay

2. Broadcast in nature

3. Bad security

4. Deployment is fast

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Transmission Media - WordPress.com · 2 July 2008 Transmission Media Beulah A. 2 Guided Transmission Media Magnetic Media A tape can hold 7 gigabytes. A box can hold about 1000 tapes