Data Transmission

1. Data Transmission Lesson 3 NETS2150/2850

2. Lesson Outline • Understand the properties a signal • Explain the difference of Data vs Signal • Understand the influence of attenuation, delay distortion andnoise on signal propagation • Appreciation of unit of decibel

3. Position of the physical layer McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

4. To be transmitted, data must be transformed to electromagnetic signals Signals can be analogue or digital. Analogue signals can have an infinite number of values in a range; Digital signals can have only a limited number of values. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

5. Signals • Analogue signal • Varies in a smooth way over time • Digital signal • Maintains a constant level then changes to another constant level • Periodic signal • Pattern repeated over time • Aperiodic signal • Pattern not repeated over time

6. Analogue & Digital Signals

7. PeriodicSignals

8. In data communication, we commonly use periodic analogue signals and aperiodic digital signals. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

9. A Sine Wave s(t) = A sin(2ft + ) McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

10. Sine Wave • Peak Amplitude - A • maximum strength of signal • In volts (V) • Frequency - f • Rate of change of signal • Hertz (Hz) or cycles per second • Period = time for one repetition (T) • T = 1/f • Phase -  (in degree or radian) • the position of the waveform relative to time zero • How far from origin when voltage change from -ve to +ve

11. Amplitude McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

12. Frequency is the rate of change with respect to time. Change in a short span of time means high frequency. Change over a long span of time means low frequency.

13. Period and frequency McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

14. Frequency and period are inverses of each other

15. Table 3.1 Units of periods and frequencies McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

16. Solution We make the following substitutions: 100 ms = 100  10-3 s = 100  10-3 106ms = 105ms Now we use the inverse relationship to find the frequency, changing hertz to kilohertz 100 ms = 10-1 s f = 1/10-1 Hz = 10 Hz = 10  10-3 KHz = 10-2 KHz Example Express a period of 100 ms in microseconds, and express the corresponding frequency in kilohertz

17. If a signal does not change at all, its frequency is zero If a signal changes instantaneously, its frequency is infinite McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

18. Relationships between different phases McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

19. Solution We know that one complete cycle is 360 degrees. Therefore, 1/6 cycle is (1/6) 360 = 60 degrees = 60 x 2p /360 rad = 1.046 rad Example A sine wave is offset one-sixth of a cycle with respect to time zero. What is its phase in degrees and radians? McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

20. Sine wave examples McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

21. Sine wave examples (continued) McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

22. Wavelength • Distance occupied by one cycle (in meters) •  • Assuming signal velocity v •  = vT • f = v • c = 3*108 ms-1 (speed of light in free space)

23. An analogue signal is best represented in the frequency domain McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

24. Time and frequency domains McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

25. A single-frequency sine wave is not useful in data communications; we need to change one or more of its characteristics to make it useful. When we change one or more characteristics of a single-frequency signal, it becomes a composite signal made of many frequencies. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

26. According to Fourier analysis, any composite signal can be represented as a combination of simple sine waves with different frequencies, phases, and amplitudes. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

27. Three odd harmonics

28. Adding first three harmonics

29. Frequency spectrum comparison

30. Analogue and Digital Data Transmission • Data • Entities that convey meaning • Signals • Electric or electromagnetic (EM) representations of data • Transmission • Communication of data by propagation and processing of signals

31. Analogue and Digital Data • Analogue • Continuous values within some interval • e.g. sound • Digital • Discrete values • e.g. text, integers

32. Analogue and Digital Signals • Means by which data are propagated • Analogue • Continuously variable • Speech range 100Hz to 7kHz • Telephone range 300Hz to 3400Hz • Video bandwidth 4MHz • Digital • Use two DC components

33. Advantages & Disadvantages of Digital • Pro: • Cheaper • Less susceptible to noise • Con: • Greater attenuation • Pulses become rounded and smaller • Leads to loss of information

34. Attenuation of Digital Signals

35. Data vs Signal Analogue Analogue Analogue Analogue

36. Analogue Transmission • Analogue signal transmitted without regard to content • May be analogue or digital data • Attenuated over distance • Use amplifiers to boost signal • But this also amplifies noise

37. Digital Transmission • Concerned with content • Integrity endangered by noise, attenuation etc. • Repeaters used • Repeater extracts bit pattern from received signal and retransmits • Attenuation is overcome • Noise is not amplified

38. Advantages of Digital Transmission • Digital technology • Low cost LSI/VLSI technology (smaller) • Data integrity • Longer distances over lower quality lines • Capacity utilization • High bandwidth links economical • High degree of multiplexing easier with digital techniques • Security & Privacy • Encryption

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

40. Attenuation and Dispersion (Delay Distortion)

41. Attenuation • Signal strength falls off with distance • Depends on type of medium • Received signal strength: • must be enough to be detected • must be sufficiently higher than noise to be received without error • Attenuation is an increasing function of frequency

42. Signal corruption McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

43. Delay Distortion • Propagation velocity varies with frequency • Different signal component travel at different rate resulting in distortion

44. Noise • Additional unwanted signals inserted between transmitter and receiver • e.g. thermal noise, crosstalk etc.

45. Spectrum & Bandwidth • Spectrum • range of frequencies contained in signal • Bandwidth • width of spectrum • band of frequencies containing most of the energy

46. Bandwidth McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

47. Solution B = fh-fl = 900 - 100 = 800 Hz The spectrum has only five spikes, at 100, 300, 500, 700, and 900 Example If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is the bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

48. McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

49. Solution B = fh- fl 20 = 60 - fl fl = 60 - 20 = 40 Hz Example A signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? McGraw-Hill • The McGraw-Hill Companies, Inc., 2004

50. Solution The answer is definitely no. Although the signal can have the same bandwidth (1000 Hz), the range does not overlap. The medium can only pass the frequencies between 3000 and 4000 Hz; the signal is totally lost. Example A signal has a spectrum with frequencies between 1000 and 2000 Hz (bandwidth of 1000 Hz). A medium can pass frequencies from 3000 to 4000 Hz (a bandwidth of 1000 Hz). Can this signal faithfully pass through this medium? McGraw-Hill • The McGraw-Hill Companies, Inc., 2004