1 / 61

Data Transmission

Data Transmission. Chapter 3. Key points. Voice, data, image, video can be represented by electromagnetic signals Analog or digital signals Any electromagnetic signal is made up of a number of constituent frequencies Bw: width of the range of these frequencies

kyra-palmer
Download Presentation

Data Transmission

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Data Transmission Chapter 3 Data Communication

  2. Key points • Voice, data, image, video can be represented by electromagnetic signals • Analog or digital signals • Any electromagnetic signal is made up of a number of constituent frequencies • Bw: width of the range of these frequencies • Major problem: transmission impairments • Attenuation • Noise • Distortion Data Communication

  3. Terminology (1) • Transmitter • Receiver • Medium • Guided medium • Medium in which the waves are guided along a physical path • e.g. twisted pair, optical fiber • Unguided medium • Medium in which the electromagnetic waves are transmitted freely • e.g. air, water, vacuum Data Communication

  4. Terminology (2) • Direct link • No intermediate devices (other than amplifier or repeater) • Both guided and unguided medium • Point-to-point • Direct link • Guided medium • Only 2 devices share medium • Multi-point • More than two devices share the same medium Data Communication

  5. Terminology (3) • Simplex • One direction • e.g. Television • Half duplex • Either direction, but only one way at a time • e.g. police radio • Full duplex • Both directions at the same time • e.g. telephone Data Communication

  6. Frequency, Spectrum and Bandwidth • The signal is • generated by a transmitter and transmitted over a medium • a function of time • but can also be expressed as a function of frequency Data Communication

  7. Frequency, Spectrum and Bandwidth • Time domain concepts • Continuous signal • Varies in a smooth way over time • Lim ta s(t) = s(a)  a • Discrete signal • Maintains a constant level then changes to another constant level • Periodic signal • Pattern repeated over time • There exists a constant T such that s(t) = s(t+T) t. T is called the period of the signal. • Aperiodic signal • Pattern not repeated over time Data Communication

  8. Analog & Digital Signals Ex. speech Ex. Binary 0s and 1s Data Communication

  9. PeriodicSignals Data Communication

  10. Sine Wave s(t) = A sin (2ft +) • Peak Amplitude (A) • maximum strength of signal • Measured in volts • Frequency (f) • Rate at which the signal repeats • Hertz (Hz) or cycles per second • Period (T) • time for one repetition, T = 1 / f • Phase () • Relative position in time Data Communication

  11. Varying Sine Wavess(t) = A sin(2ft +) Data Communication

  12. Varying Sine Waves (in distance) • In the figures, horizontal axes is time • The sinusoidal waveform can also be drawn with horizontal axis representing distance in space • The shape of the waveform is the same however, scale of the horizontal axis is different • i.e. at a particular instant of time, the intensity of the signal varies in a sinusoidal way as a function of distance from the source. Data Communication

  13. Wavelength  • Distance occupied by one cycle or • Distance between two points of corresponding phase in two consecutive cycles • Denoted by  • Assuming signal velocity v •  = v T • f = v • c = 3*108 m / s(speed of light in free space) Data Communication

  14. Frequency Domain Concepts • Signal usually made up of many frequencies • E.g s(t)=(4/)  [ sin(2ft) + (1/3) sin (2(3f)t)] • Fundamental frequency fo • All the frequency components are integer multiples of fundamental frequency • Period T = 1 / f0 • Components are sine waves • Can be shown (Fourier analysis) that any signal is made up of components at various frequencies Data Communication

  15. Addition of FrequencyComponents(T=1/f) Data Communication

  16. Fourier Analysis The mathematical tool that lets us compute the above-mentioned infinite sum is called Fourier Analysis. For every signal: • the time function s(t) specifies the amplitude at any instant of time. • The frequency function S(f) specifies the coefficient of the sinusoidal at any frequency. • Periodic signals  Fourier Series • Aperiodic signals  Fourier Transform Data Communication

  17. FrequencyDomainRepresentations Data Communication

  18. Spectrum & Bandwidth • Spectrum • range of frequencies contained in signal • Absolute bandwidth • width of spectrum (2f in previous figure) • Effective bandwidth • Often just bandwidth • Narrow band of frequencies containing most of the energy • DC Component • Component of zero frequency • With no dc component, a signal has a avg amplitute of zero Data Communication

  19. Signal with DC Component Data Communication

  20. Data Rate vs Bandwidth • Data Rate: The data rate is the number of bits transmitted in one second (bits per second, bps) • any transmission system has a limited band of frequencies • this limits the data rate that can be carried • have a direct relationship between data rate & bandwidth • the higher the data rate of a signal, the greater is its required effective bandwidth Data Communication

  21. Approximating Signals A typical square wave s(t) with amplitudes A and –A can be expressed as  s (t) = A (4/)  (1/k) sin (2kft). k=1 k odd • square have infinite components and hence bandwidth • but most energy in first few components Question: How many terms in the infinite sum above shall we consider so that the original signal is reasonably approximated? Answer: The number of terms that we take into account is dependent on the ability of the receiver to discern the difference between 0 and 1. This, in turn, is affected from the noise (and transmission impairments) of the channel Data Communication

  22. Data Rate vs Bandwidth Question: How do we relate the data rate to the bandwidth of the signal. • Suppose a digital transmission system is capable of transmitting signals with a bw 4 MHz • If the frequency of a square wave is 1 MHz (106 cps), and if three terms in the summation (f, 3f, 5f) are sufficient, then the bw is • (5f-f)=4MHz • T = 1 s • A bitstring of 0s and1s: one bit occurs every 0.5 s • the data rate that can be achieved is 2 Mbps. Data Communication

  23. Examples • Example 1: • 1 MHz square wave • 3 terms in the sum, frequencies f, 3f, 5f • Data rate 2 Mbps, Bandwidth 4 MHz • Example 2: • 2 MHz square wave • 3 terms in the sum, frequencies f, 3f, 5f • Data rate 4 Mbps, Bandwidth 8 MHz • Example 3: • 2 MHz square wave • 2 terms in the sum • Data rate 4 Mbps, Bandwidth 4 MHz • The given bw can support various data rate Data Communication

  24. Addition of more components Data Communication

  25. Analog and Digital Data Transmission • Data • Entities that convey meaning • Signals • Electric or electromagnetic representations of data • Transmission • Communication of data by propagation and processing of signals Data Communication

  26. Analog and Digital Data • Analog • Continuous values within some interval • e.g. sound, video • Digital • Discrete values • e.g. text, integers Data Communication

  27. Acoustic Spectrum (Analog) Data Communication

  28. Video • Amount of illumination produced is proportional to the intensity of the beam • Beam takes on an analog value of intensity to produce desired brightness • The screen is scanned from left to right, top to bottom. • 30 complete scans per second is adequate but the motion is flickering • To solve flickering problem, interlacing is used • Scan the screen with half of the lines • And then scan the other half • Screen is refreshed 60 times Data Communication

  29. TV Picture Production 1 Data Communication

  30. TV Picture Production 2 Data Communication

  31. Analog and Digital Signals • Means by which data are propagated • Analog • Continuously variable • Various media • wire, fiber optic, space • Speech bandwidth 100Hz to 7kHz • Telephone bandwidth 300Hz to 3400Hz • Video bandwidth 4MHz • Digital • Use two voltage levels Data Communication

  32. Advantages & Disadvantages of Digital • Cheaper • Less susceptible to noise interference • Greater attenuation • Pulses become rounded and smaller • Leads to loss of information Data Communication

  33. Attenuation of Digital Signals Data Communication

  34. Components of Speech • Frequency range (of hearing) 20Hz-20kHz • Speech 100Hz-7kHz • Easily converted into electromagnetic signal for transmission • Sound frequencies with varying volume converted into electromagnetic frequencies with varying voltage • Limit frequency range for voice channel • 300-3400Hz Data Communication

  35. Conversion of Voice Input into Analog Signal Data Communication

  36. Video Components • USA - 483 lines scanned per frame at 30 frames per second • 525 lines but 42 lost during vertical retrace • So 525 lines x 30 scans = 15750 lines per second • 63.5s per line • 11s for retrace, so 52.5 s per video line • Max frequency if line alternates black and white • Horizontal resolution is about 450 lines giving 225 cycles of wave in 52.5 s • Max frequency of 4.2MHz Data Communication

  37. Binary Digital Data • From computer terminals etc. • Converted into digital voltage pulses for transmission • Two voltage levels • Bandwidth depends on data rate Data Communication

  38. Conversion of PC Input to Digital Signal Data Communication

  39. Data and Signals • Usually use digital signals for digital data and analog signals for analog data • Can use analog signal to carry digital data • Modem • Can use digital signal to carry analog data • Compact Disc audio Data Communication

  40. Analog Signals Carrying Analog and Digital Data Data Communication

  41. Digital Signals Carrying Analog and Digital Data Data Communication

  42. Analog Transmission • Analog signal transmitted without regard to content • May be analog or digital data • Attenuated over distance • Use amplifiers to boost signal • Also amplifies noise Data Communication

  43. Digital Transmission • Concerned with content • Integrity endangered by noise, attenuation etc. • Repeaters used • Repeater receives signal • Extracts bit pattern • Retransmits • Attenuation is overcome • Noise is not amplified Data Communication

  44. Advantages of Digital Transmission • Digital technology • Low cost LSI/VLSI technology • Data integrity • Longer distances over lower quality lines • Capacity utilization • High bandwidth links economical • High degree of multiplexing easier with digital techniques (time divison) • Security & Privacy • Encryption • Integration • Can treat analog and digital data similarly Data Communication

  45. Transmission Impairments • Signal received may differ from signal transmitted • Analog - degradation of signal quality • Digital - bit errors • Caused by • Attenuation and attenuation distortion • Delay distortion • Noise Data Communication

  46. Attenuation • Signal strength falls off with distance • Guided medium: logarithmic • Unguided medium: distance and atmosphere structure • Depends on medium • Received signal strength: • must be enough to be detected • must be sufficiently higher than noise to be received without error • Use amplifiers or repeaters • Attenuation is an increasing function of frequency • Equalization techniques are developed to reduce the effect of attenuation Data Communication

  47. Equalization: Nf = -10 log10 Pf / P1000 Data Communication

  48. Delay Distortion 1 • Only in guided media • Propagation velocity varies with frequency • Some of the signal components of one bit position will split over into other bit positions • Causes problems in digital data • Intersymbol interference • Equalization techniques are developed Data Communication

  49. Delay Distortion 2 Data Communication

  50. Noise (1) • Additional signals inserted between transmitter and receiver • Thermal • Due to thermal agitation of electrons • Uniformly distributed • White noise • Intermodulation • Crosstalk • Impulse noise Data Communication

More Related