Physical Layer

1. REVIEW Physical Layer

2. Position of the physical layer

3. Services

4. Signals

5. Note: To be transmitted, data must be transformed to electromagnetic signals.

6. 4.1 Analog and Digital Analog and Digital Data Analog and Digital Signals Periodic and A periodic Signals

7. Data – Entities that convey meanings, or information Signals- Electric or electromagnetic representations of data Signaling – Physical propagation of the signal along a suitable medium Transmission – Communication of data by the propagation and processing of signals Basic Context

8. Analog data Take on continuous values in some interval e.g. sound, video Digital data Take on discrete values e.g. text, integers Analog and Digital Data

9. Note: Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital signals can have only a limited number of values.

10. Figure 4.1Comparison of analog and digital signals

11. Analog Signal An continuously varying electromagnetic wave that may be propagated over a variety of media (e.g., twisted pair or coaxial cable, atmosphere), depending on spectrum. Digital Signal A sequence of voltage pulses that may be transmitted over a wire medium, e.g., a constant positive voltage level may represent binary 0 and a constant negative voltage level may represent binary 1. Advantages of digital signal over analog signal Cheaper in price Less susceptible to noise interference Disadvantages of digital signal over analog signal Suffer more from attenuation Pulses become rounded and smaller Leads to loss of information Analog and Digital Signals

12. Note: In data communication, we commonly use periodic analog signals and aperiodic digital signals.

13. Conversion of Voice Input to Analog Signal

14. Conversion of PC Input to Digital Signal

15. 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 and Signals

16. Analog Signals Carrying Analog and Digital Data

17. Digital Signals Carrying Analog and Digital Data

18. 4.2 Analog Signals Sine Wave Phase Examples of Sine Waves Time and Frequency Domains Composite Signals Bandwidth

19. Figure 4.2A sine wave

20. Figure 4.3Amplitude

21. Note: Frequency and period are inverses of each other.

22. Figure 4.4Period and frequency

23. Table 4.1 Units of periods and frequencies

24. Example 1 Express a period of 100 ms in microseconds, and express the corresponding frequency in kilohertz. Solution From Table 3.1 we find the equivalent of 1 ms.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 = 100  10-3 s = 10-1 s f = 1/10-1 Hz = 10  10-3 KHz = 10-2 KHz

25. Note: 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.

26. Note: If a signal does not change at all, its frequency is zero. If a signal changes instantaneously, its frequency is infinite.

27. Note: Phase describes the position of the waveform relative to time zero.

28. Figure 4.5Relationships between different phases

29. Example 2 A sine wave is offset one-sixth of a cycle with respect to time zero. What is its phase in degrees and radians? 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

30. Figure 4.6Sine wave examples

31. Figure 4.6Sine wave examples (continued)

32. Figure4.6Sine wave examples (continued)

33. Note: An analog signal is best represented in the frequency domain.

34. Figure 4.7Time and frequency domains

35. Figure 4.7Time and frequency domains (continued)

36. Figure 4.7Time and frequency domains (continued)

37. Note: 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.

38. Note: When we change one or more characteristics of a single-frequency signal, it becomes a composite signal made of many frequencies.

39. Note: According to Fourier analysis, any composite signal can be represented as a combination of simple sine waves with different frequencies, phases, and amplitudes.

40. Figure 4.8Square wave

41. Figure 4.9Three harmonics

42. Figure 4.10Adding first three harmonics

43. Figure 4.11Frequency spectrum comparison

44. Figure 4.12Signal corruption

45. Note: The bandwidth is a property of a medium: It is the difference between the highest and the lowest frequencies that the medium can satisfactorily pass.

46. Note: In this book, we use the term bandwidth to refer to the property of a medium or the width of a single spectrum.

47. Figure 4.13Bandwidth

48. Example 3 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. Solution B = fh-fl = 900 - 100 = 800 Hz The spectrum has only five spikes, at 100, 300, 500, 700, and 900 (see Figure 13.4 )

49. Figure 4.14Example 3

50. Example 4 A signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? Draw the spectrum if the signal contains all integral frequencies of the same amplitude. Solution B = fh- fl 20 = 60 - fl fl = 60 - 20 = 40 Hz