Transmission Media Based on Chapter 4 in Comer

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

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 CSIT 220 (Blum) 2

Source Transmitter Channel Receiver Destination CSIT 220 (Blum) 3

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. CSIT 220 (Blum) 4

Verbal Communication Example • Source: speaker’s brain • Transmitter: speaker’s vocal cords • Channel: air • Receiver: listener’s ears • Destination: listener’s brain CSIT 220 (Blum) 5

Source Transmitter Channel A repeater strengthens a weakening signal Repeater Channel Receiver CSIT 220 (Blum) Destination 6

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. CSIT 220 (Blum) 7

Wave = Moving Disturbance Equilibrium: No disturbance CSIT 220 (Blum) 8

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. CSIT 220 (Blum) 9

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 meter/second • 3 108 m/s – Electric signal? (30% to 70% of the speed of light) CSIT 220 (Blum) 10

Amplitude: how big is the disturbance (especially at its maximally disturbed positions) The two waves shown above have different amplitudes. CSIT 220 (Blum) 11

Frequency: how many cycles (one unit of repeated disturbance) go by in a second The two waves shown above have different frequencies. CSIT 220 (Blum) 12

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 -Giga. Hertz machine has a clock that goes through one billion such cycles every second. CSIT 220 (Blum) 13

CSIT 220 (Blum) 14

More Hertz • Another place where we see Hertz is in radio station carrier frequencies. AM Radio station carrier frequencies are in KHz (kilo. Hertz). FM Radio station carrier frequencies are in MHz (mega. Hertz). CSIT 220 (Blum) 15

Phase: what part of the cycle the wave is in at a particular time The above waves are “out of phase. ” CSIT 220 (Blum) 16

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. CSIT 220 (Blum) 17

Example: larger amplitude can represent a 1 CSIT 220 (Blum) 0 18

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). CSIT 220 (Blum) 19

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. CSIT 220 (Blum) 20

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) CSIT 220 (Blum) 21

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”. CSIT 220 (Blum) 22

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. CSIT 220 (Blum) 23

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. CSIT 220 (Blum) 24

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 CSIT 220 (Blum) 25

STP • Shielded Twisted Pair (STP) adds a layer of shielding around the twisted pair to provide additional protection from interference. CSIT 220 (Blum) 26

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 CSIT 220 (Blum) shield 27

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 CSIT 220 (Blum) 28

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 … CSIT 220 (Blum) 29

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) CSIT 220 (Blum) 30

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. CSIT 220 (Blum) 31

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. CSIT 220 (Blum) 32

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. CSIT 220 (Blum) 33

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. CSIT 220 (Blum) 34

Frequency range • These frequencies cover the portion of the electromagnetic spectrum, starting at around 9 k. Hz, and going up to thousands of gigahertz (GHz). • AM radio is between k. Hz and MHz • FM radio is in the MHz range – FM has higher bandwidth CSIT 220 (Blum) 35

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, CSIT 220 (Blum) 36

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. CSIT 220 (Blum) 37

RF Ranges Name Very low freq. Abbrev. Freq. VLF Wavelength 9 k. Hz-30 k. Hz 33 km - 10 km Low Freq. LF 30 k. Hz - 300 k. Hz 10 km - 1 km Medium Freq. MF 300 k. Hz - 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 CSIT 220 (Blum) 38

Infrared • Infrared is a higher-frequency range, often used for short-distance wireless connections • Ir. DA: Infrared Data Association, a group of device manufacturers that developed a standard for transmitting data via infrared light waves. • Ir. DA port where infrared signals enter or leave a computer. • There must be a clear “line of sight. ” CSIT 220 (Blum) 39

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. CSIT 220 (Blum) 40

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. CSIT 220 (Blum) 41

Fig. 4. 3 CSIT 220 (Blum) 42

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 CSIT 220 (Blum) 43

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. CSIT 220 (Blum) 44

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. CSIT 220 (Blum) 45

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. CSIT 220 (Blum) 46

Additional sources • • http: //www. webopedia. com http: //www. whatis. com Information Theory (Gordon Raisbeck) Physics (Paul Tipler) CSIT 220 (Blum) 47