CSIT 220 (Blum)1 Transmission Media Based on Chapter 4 in Comer.

CSIT 220 (Blum) 1

Transmission Media

Based on Chapter 4 in Comer

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Communication

• Communication consists of the transmission –(moving from one location to another – of information (data)

• A generalized communication system consists of – Information source

– Transmitter

– Channel

– Receiver

– Destination

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Source

Transmitter

Channel

Receiver

Destination

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Signal• Information in motion is often referred to as a signal.• The transmitter takes information from the source

and puts it into a form that can be carried away.• The channel is the medium through which the signal

moves. – In the case of an electromagnetic signal, it can be just

empty space (which is strictly speaking not a medium).

• The receiver detects the signal and puts the information into a form “understood” by the destination.

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Verbal Communication Example

• Source: speaker’s brain

• Transmitter: speaker’s vocal cords

• Channel: air

• Receiver: listener’s ears

• Destination: listener’s brain

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Source

Transmitter

Channel

Receiver

Destination

Channel

RepeaterA repeater strengthens a weakening signal

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Waves

• If you strip away the “information content” and concentrate solely on the physical phenomena, you would have a wave.

• A wave is the transmission of a disturbance. – In the sound example, it is a pressure wave.

– The disturbance is that the air has more or less than its usual pressure.

– The region of unusual pressure changes location.

– That is, the disturbance moves.

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Wave = Moving Disturbance

Equilibrium: No disturbance

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A periodic wave

• The characteristics of waves play an important role in understanding the physical aspects of communication.

• A simple case to look at is when the same disturbance is repeated over and over again.– Such a wave is said to be periodic.

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Wave speed

• How fast does the disturbance move from location to location - it depends on the medium.– Sound waves move at approximately 340

meter/second through air.– Light waves move at 300,000,000 meter/second

• 3 108 m/s

– Electric signal? (30% to 70% of the speed of light)

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Amplitude: how big is the disturbance (especially at its

maximally disturbed positions)

The two waves shown above have different amplitudes.

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Frequency: how many cycles (one unit of repeated disturbance) go by in a second

The two waves shown above have different frequencies.

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Hertz

• Frequency is measured in Hertz (cycles per second).

• A computer’s clock is a crystal that repeatedly vibrates back and forth (goes through a cycle).

• A 1-GigaHertz machine has a clock that goes through one billion such cycles every second.

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More Hertz

• Another place where we see Hertz is in radio station carrier frequencies.

AM Radio station carrier frequencies are in KHz (kiloHertz).

FM Radio station carrier frequencies are in MHz (megaHertz).

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Phase: what part of the cycle the wave is in at a particular time

The above waves are “out of phase.”

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Information content• There is little to no information in a purely periodic

wave, after all it is characterized by only a few quantities (amplitude, frequency, phase).

• But one can modulate (change) the wave as time goes by to send information.– Amplitude modulation (AM) changes the peak value.– Frequency modulation (FM) changes the frequency.– Phase modulation changes the phase.– Combinations thereof.

• The original un-modulated wave is called the carrier.

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Example: larger amplitude can represent a 1

0 1 0

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Bandwidth

• Bandwidth is the rate at which information flows.

• Digital: measured in bits per second (bps) or bytes per second (Bps) – Related to but distinct from baud rate

• Analog: measured in Hertz (Hz).

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Bandwidth (Cont.)

• Typically there is a limit to the information that can be conveyed in one cycle.

• Therefore, the rate of information (bandwidth) is increased if the frequency (number of cycles per second) is increased.

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Media• Wired: connected by some physical

medium.– Twisted-pair– Coaxial cable – Fiber-optic cable

• Wireless– Electromagnetic radiation (radio, microwave,

infrared, visible light, ultraviolet, x-rays, gamma rays)

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Circuit

• With electric signals, one needs a closed circuit, a loop.

• In addition to the wire going from source to destination, there must be a second wire (the return) from the destination back to the source.

• If there is not a complete circuit, it is said that there is an “open” or a “break”.

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UTP• Unshielded Twisted Pair, a type of wire/cable

consisting of two unshielded wires twisted around each other.

• Used for phones and some LAN connections.• Compared to the alternatives, it’s cheap and easy

to work with. • But it is limited in bandwidth and offers less

protection from interference.

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Interference

• When two waves occupy the same space, they combine to form a new wave.

• When one of the waves is carrying information, the interference can result in a corruption or loss of that information.

• Interference is sometimes called crosstalk.

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Why is it twisted?• The twisting is to reduce the effects of

interference. • One source of interference is stray magnetic fields. • The size of the interference is determined by the

size of the magnetic field (over which we have little control) and the size of the area surrounded by the circuit (loop) made by the wire.

• Twisted the wire reduces the area, reducing the interference.

• demo

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STP

• Shielded Twisted Pair (STP) adds a layer of shielding around the twisted pair to provide additional protection from interference.

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Coaxial-cable• A type of wire that consists of a center wire

surrounded by insulation and then a grounded shield of braided wire. The shield minimizes electrical and radio frequency interference.

• The shield is also the return

Center wire

insulation

shield

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Coax (Cont.)

• Used for cable television and some computer networks (especially for connections over longer distances)

• More expensive than UTP

• Better defense against interference (hence it can be longer)

• Higher bandwidth

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Fiber optic cable

• Cable consists of a bundle of glass (or plastic) threads

• The signal is sent by modulating laser light – No return required

• Expensive and more difficult to work with, repair, but …

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Fiber optic cable (cont.)

• Significantly greater bandwidth

• Significant reduction in interference

• Cables are thinner and lighter

• Signals are more naturally digital (as opposed to analog)

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Audio frequency

• AC (alternating current) between 20 and 20,000 Hz is known as Audio frequency (AF)

• If such current is fed into a speaker, it will produce sound waves within the range of human hearing.

• All telephone circuits operate with AF signals in a restricted range of approximately 300 Hz to 3300 Hz.

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Modem

• Modems (Modulator-DEModulator) convert data that is in binary form into an analog signal in the AF range.

• That signal can be transmitted over the telephone wire.

• Modems also receive the AF signals and convert them back into binary form.

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Limitations

• The phone system was designed to work over a limited range of frequencies (300-3300 Hz) suitable to human voices.

• This limitation in frequencies puts limitations on bandwidth which is related to the rate of information flow.

• The baud rate of a modem is tied to the frequencies phone lines were designed to handle.

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Radio Frequencies

• Radio frequency (RF) refers to an AC voltage that if applied to an antenna would produce an electromagnetic wave of the sort used in radio and other wireless communications.

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Frequency range

• These frequencies cover the portion of the electromagnetic spectrum, starting at around 9 kHz, and going up to thousands of gigahertz (GHz).

• AM radio is between kHz and MHz

• FM radio is in the MHz range – FM has higher bandwidth

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Wireless • Many wireless devices make use of RF fields.

– Cordless and cellular telephone

– radio and television broadcast stations,

– satellite communications systems

– two-way radio services

• Some wireless devices operate at higher frequencies (Infrared IR or visible-light) frequencies – most television-set remote-control boxes

– some cordless computer keyboards and mice,

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Ranges of RF

• The RF spectrum is divided into several ranges, or bands.

• The table shows the eight bands in the RF spectrum, along with their frequency and corresponding wavelengths.

• The SHF and EHF bands are often referred to as the microwave spectrum.

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RF RangesName Abbrev. Freq. WavelengthVery low freq. VLF 9kHz-30kHz 33 km - 10 km

Low Freq. LF 30 kHz - 300 kHz

10 km - 1 km

Medium Freq. MF 300 kHz - 3 MHz 1 km - 100 m

High Freq. HF 3 MHz - 30 MHz 100 m - 10 m

Very High Freq. VHF 30 MHz - 300 MHz

10 m - 1 m

Ultra High Freq. UHF 300 MHz - 3 GHz

1 m - 100 mm

Super High Freq.

SHF 3 GHz - 30 GHz 100 mm - 10 mm

Extremely High Freq.

EHF 30 GHz - 300 GHz

10 mm - 1 mm

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Infrared

• Infrared is a higher-frequency range, often used for short-distance wireless connections

• IrDA: Infrared Data Association, a group of device manufacturers that developed a standard for transmitting data via infrared light waves.

• IrDA port where infrared signals enter or leave a computer.

• There must be a clear “line of sight.”

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The World is Round

• Wireless signals travel in a straight or nearly straight line.

• A straight line connecting two distant locations on the earth’s surface alas goes through the earth.

• The signal needs to be relayed.• A way to cover large distances without a

large number of relays is to use a satellite.

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Satellite

• A satellite is an object that orbits around another object. For example, the moon orbits a round the earth – so the moon is a satellite. – Not to be confused with a satellite dish.

• A satellite is a special case of a wireless transmitter-receiver.

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Fig. 4.3

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Geosynchronous

• A satellite must revolve around the earth, otherwise they’d come crashing down, so satellites are always on the move

• But the earth also rotates once a day and thus it is possible to have a satellite that takes exactly one day to complete its revolution

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Geosynchronous (Cont.)

• Relative to the Earth’s surface, the satellite remains in a fixed position.

• Such an orbit is called geosynchronous (it is “in sync with the earth”) or geostationary.

• Geosynchronous satellites must have a special height – approximately 22,000 miles up.

• E.g. weather satellites.

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Low Earth Orbit Satellites • A geosynchronous orbit must have a particular

height, and there’s only so much room up there at that height.

• But satellites at other heights move with respect to the earth’s surface (making them intermittent).

• This problem is solved by have an array of satellites, so that one is always overhead.

• Low earth orbit (LEO) are a few hundred miles up.• Satellites at a height approximately a thousand

miles up are Medium earth orbit (MEO) satellites.

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GPS

• A set of 24 satellites make up what is called the global positioning system (GPS).

• A typical point on the earth can receive signals from four of them.

• From the precise information the satellites broadcast about their position and a very accurate clock they have on board, the position of a GPS device can be determined within a 100 meters or better.

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Additional sources

• http://www.webopedia.com

• http://www.whatis.com

• Information Theory (Gordon Raisbeck)

• Physics (Paul Tipler)