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Concepts and Theory

Concepts and Theory. Information can be transmitted by varying some physical property of a signal. Voltage Current Intensity Magnetic Orientation Usually the information is transmitted as an electric signal. Direct Current.

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Concepts and Theory

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  1. Concepts and Theory • Information can be transmitted by varying some physical property of a signal. • Voltage • Current • Intensity • Magnetic Orientation • Usually the information is transmitted as an electric signal. ICSS420/740 - Communication Theory

  2. Direct Current • An electric current is called direct current if the electrons are always flowing in the same direction. • Direct current can be either continuous or intermittent 5v 0v ICSS420/740 - Communication Theory

  3. Alternating Current • An alternating voltage reverses its polarity, which produces current that reverses its direction. • In the US, house current is 120 volts AC. ICSS420/740 - Communication Theory

  4. Periodic Waves Amplitude Cycle Cycle Cycle • The frequency of a periodic wave, measured in hertz (Hz), is the number of cycles that occur per second ICSS420/740 - Communication Theory

  5. Phase • Two identical waves that begin at different points in time are said to differ in their phase. ICSS420/740 - Communication Theory

  6. Fourier Analysis • Any reasonably behaved periodic function, g(t), with period T can be constructed by summing a (possibly infinite) number of sines and cosines: Where f=1/T is the fundamental frequency and an and bn are the sine and cosine amplitudes of the nth harmonics (terms) ICSS420/740 - Communication Theory

  7. Fourier in Practice • Say you wanted to send the following signal 1 0 1 1 0 0 0 1 0 1 ICSS420/740 - Communication Theory

  8. Components of a Digital Signal ICSS420/740 - Communication Theory

  9. Components of a Digital Signal ICSS420/740 - Communication Theory

  10. Components of a Digital Signal ICSS420/740 - Communication Theory

  11. Components of a Digital Signal ICSS420/740 - Communication Theory

  12. Components of a Digital Signal ICSS420/740 - Communication Theory

  13. How does this affect communication? • No transmission facility can transmit signals without loosing some power in the process. • If all Fourier components were equally diminished, the resulting signal would be reduced in amplitude but not distorted. • Usually, the amplitudes are transmitted undiminished from 0 up to some frequency fc with all frequencies above the cutoff strongly attenuated. ICSS420/740 - Communication Theory

  14. Communication over a telephone • An ordinary telephone line has an artificially introduced cutoff frequency near 3000Hz. • If we transmit digital data at a rate of b bits/sec, the time required to send 8 bits is 8/b seconds, so the frequency of the first harmonic is b/8Hz. • This means on a telephone line the number of the highest harmonic passed through is 3000/(b/8) or 24,000/b. ICSS420/740 - Communication Theory

  15. Data Rates and Harmonics ICSS420/740 - Communication Theory

  16. Baud Rate • The speed at which analog transmissions take place is usually measured in terms of BAUD. • The BAUD rate is the rate of signaling changes per second on a channel. The BAUD rate does not have to equal the bit rate. • Based on the Fourier analysis of a voice grade telephone line the highest BAUD rate that can be used is 2400 BAUD. ICSS420/740 - Communication Theory

  17. Multi-Level Encoding • Multi-level encoding sends several bits in a single signal unit. Red 00 Green 01 Blue 10 White 11 ICSS420/740 - Communication Theory

  18. Multi-Level Encoding Bit Stream 10 00 11 01 01 00 10 11 Signal Changes ICSS420/740 - Communication Theory

  19. Multi-Level Encoding ICSS420/740 - Communication Theory

  20. Bandwidth • Bandwidth: the information carrying capability of a channel. • The bandwidth for a given transmission medium is fixed. • Different mediums have different bandwidths. • “never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.” ICSS420/740 - Communication Theory

  21. Bandwidth LOW SPEED HIGH SPEED Ball size represents the Data Rate Box size represents the Bandwidth ICSS420/740 - Communication Theory

  22. Bandwidth ICSS420/740 - Communication Theory

  23. Analog Transmission • Analog transmission has dominated all communication for the past 100 years. • Even though long-distance trunks are now digital, the local loop is still analog and will probably stay that way for a long time. • So when a computer uses a telephone to send data, the data must be converted to analog form for transmission. ICSS420/740 - Communication Theory

  24. Modems • A device that converts digital data into a modulated analog carrier signal that can be sent over analog transmission lines is called a modem. • MODEM stands for MOdulator/DEmodulator. • Modulation refers to the process of superimposing digital data onto an analog carrier signal. • Demodulation refers to the process of recovering the digital data from the modulated carrier. ICSS420/740 - Communication Theory

  25. THE DTE/DCE Interface DCE DTE ICSS420/740 - Communication Theory

  26. The DTE/DCE Interface • DTE - Data Terminal Equipment • typically an end-user device • supports end-user applications • DCE - Data Communications Equipment • connects the DTE into the communications circuit • The data communications path is the physical path between the DCEs. ICSS420/740 - Communication Theory

  27. Digital to Analog Conversion • Three basic methods of digital-to-analog modulation exist • amplitude • frequency • phase • The modulation techniques superimpose data onto a carrier signal • a carrier signal is a signal with known properties ICSS420/740 - Communication Theory

  28. Amplitude Modulation • AM modems alter the carrier signal’s amplitude. • In its simplest form, the carrier is switched on and off to represent the binary state. • AM modulation is not often used because of power and signal distortion problems. ICSS420/740 - Communication Theory

  29. Frequency Modulation • FM changes the frequency of the carrier in accordance with the digital bit stream. • The most common form of FM modulation is frequency shift key (FSK) which uses 4 frequencies within the telephone line bandwidth: • 1070/1270Hz represent 0 (space) • 2025/2225Hz represent 1 (mark) • FSK was widely used in low-speed modems. ICSS420/740 - Communication Theory

  30. Phase Modulation • Phase modulation is used almost exclusively on high speed modems • The phase modulation method is also called phase shift keying (PSK). • PSK can be used to provide multi-level encoding: ICSS420/740 - Communication Theory

  31. PSK 90 90 135 45 135 45 0 0 180 180 225 315 225 315 270 270 4PSK 4800bps 4PSK 4800 bps 8PSK 7200bps ICSS420/740 - Communication Theory

  32. Quadrature Amplitude Modulation • QAM uses phase and amplitude modulation to achieve higher bit rates 8 combinations 7200bps 16 combinations 9600bps ICSS420/740 - Communication Theory

  33. Speed, I want speed!!! • So we are now up to 9600bps, can we go faster? • The next step up is 14400bps. This can be achieved by transmitting 6 bits per sample. Its constellation pattern has 64 points. • At these speeds small amounts of noise can cause big problems. • Many modems add a parity bit, the points are then coded to minimize error. This is called trellis coding. ICSS420/740 - Communication Theory

  34. Transmission Media • Transmission media refers to the way in which two computers are connected. • A wide variety of media exists, but they fall into two main classes: • Bounded: the data is confined to specific pathways. Common examples include wire and fiber optic cable. • Unbounded: transmit the data carrying signal through space. Broadcast radio and television are examples of unbounded media. ICSS420/740 - Communication Theory

  35. Bounded Media • Some important characteristics of cable include: • Resistance to EMI • Bandwidth • Attenuation (how the cable reduces the strength of the signal with distance) • Cost • Installation • Maintenance ICSS420/740 - Communication Theory

  36. Twisted Pair • A twisted pair consists of two insulated wires twisted together in a helical form. • The purpose of twisting the wires is to reduce electrical interference from similar pairs close by. • The most common application of twisted pair is the telephone system. • The bandwidth of the pair depends on the thickness of the wire and the distance traveled. ICSS420/740 - Communication Theory

  37. Types of Twisted Pair Cabling • Twisted pair comes in several varieties, two of which are important for networking • Category 3 consists of two insulated wires twisted together. Four such pairs are typically grouped together in a plastic sheath. • Category 5 cabling has more twists per centimeter and uses Teflon insulation. This results in less crosstalk and a better quality signal over long distances. • Both of these wiring types are usually called UTP (unshielded twisted pair). ICSS420/740 - Communication Theory

  38. Twisted Pair Summary • Advantages • telephone cable standards are mature • Materials are plentiful and easy to work with. • It may be possible to use existing wiring • UTP is the lowest cost cabling • Disadvantages • STP is expensive and hard to work with • TP is sensitive to EMI • International standards for some uses do not exist. ICSS420/740 - Communication Theory

  39. Coaxial Cable • Has better shielding that TP, so it can span longer distances at higher speeds. • Two kinds of coaxial are widely used: • 50 ohm and 75 • The construction and shielding of coax gives it high bandwidth and excellent noise immunity. • For 1km cables data rates of 1-2 Gbps are feasible. ICSS420/740 - Communication Theory

  40. Types of Coaxial Cable • A wide variety of coaxial cable is available: • RG-8 and RG-11 are 50 ohm cables used for thickwire Ethernet (10base5) • RG-58 is a smaller 50 ohm cable used with thinwire (cheaper-net) Ethernet (10base2). • RG-59 is a 75 ohm cable used to wire cable TV and is used in some token ring networks. • RG-62 is a 93 ohm cable used for ARCnet. ICSS420/740 - Communication Theory

  41. Types of Coaxial Cable • In addition to impedance the cables may differ in the type of center conductor: • solid copper • twisted • The coating used on most coaxial cables is PVC which cannot be used in some situations. It is possible to get Teflon coated coaxial cable which is referred to as plenum. ICSS420/740 - Communication Theory

  42. Broadband Coaxial Cable • The broadband coaxial cable system uses analog transmission on standard cable television cabling. • Broadband: • Telephone world: anything with a bandwidth greater than 4Khz. • Computer world: any cable using analog transmission. • Because analog transmission is being used bandwidths up to 300Mhz that can run for nearly 100km. ICSS420/740 - Communication Theory

  43. Broadband Coaxial Cable • Broadband systems are divided into multiple channels, frequently the 6Mhz channels used for television broadcasting. • Broadband systems typically need analog amplifiers to boost the signal periodically. The amplifiers can transmit in one direction only. • This results in two types of broadband systems: • dual cable • single cable ICSS420/740 - Communication Theory

  44. Coaxial Cable Summary • Advantages: • Good resistance to EMI • High Bandwidth • Capable of withstanding harsh environments • Very mature technology • Easy to install • Disadvantages • Bulky • Expensive ICSS420/740 - Communication Theory

  45. Fiber Optics • An optical transmission system consists of three components: • light source • transmission medium • detector • Conventionally, a pulse of light indicates a 1-bit and the absence of light indicates a 0-bit. ICSS420/740 - Communication Theory

  46. Fiber Cable Summary • Advantages • High bandwidth • Immune to EMI • No RF emissions • Disadvantages • Expensive ICSS420/740 - Communication Theory

  47. Cable Characteristics ICSS420/740 - Communication Theory

  48. Unbounded Media • Broadcasting data signals through space. • Advantages • Easy to relocate equipment • Mobility • Cost effective in certain circumstances • Disadvantages • Security • Interference • Reception Quality ICSS420/740 - Communication Theory

  49. The Electromagnetic Spectrum • When electrons move they create electromagnetic waves. • The fundamental relationship between frequency (f), wavelength (), and the speed of light (c) is  f = c So 1Mhz waves are about 300 meters long and 1cm waves have a frequency of about 30Ghz ICSS420/740 - Communication Theory

  50. The Electromagnetic Spectrum 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 TP Satellite Coaxial Fiber Optics AM FM Terrestrial Microwave Maritime Radio TV ICSS420/740 - Communication Theory

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