Understanding Signals in the Physical Layer: Analog versus Digital Signals

1. Chapter 3 Signals

2. Physical layer To be transmitted, data must be transformed to electromagnetic signals.

3. Figure 3.1Comparison of analog and digital signals

4. Figure 3.2A sine wave

5. Figure 3.3Amplitude

6. Figure 3.4Period and frequency

7. Table 3.1 Units of periods and frequencies

8. 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

9. Two signals with the same amplitude,but different frequencies

10. Frequency 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. • If a signal does not change at all, its frequency is zero • If a signal changes instantaneously, its frequency is infinite.

11. Figure 3.5Relationships between different phases

12. Phase Phase describes the position of the waveform relative to time 0 Three sine waves with the same amplitude and frequency,but different phases

13. Example A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians? Solution We know that 1 complete cycle is 360°. Therefore, 1/6 cycle is ° ° °

14. Wavelength and period Wavelength = Propagation speed x Period = Propagation speed / Frequency

15. Figure 3.6Sine wave examples

16. Figure 3.6Sine wave examples (continued)

17. Figure 3.6Sine wave examples (continued)

18. Figure 3.7Time and frequency domains

19. Figure 3.7Time and frequency domains (continued)

20. Figure 3.7Time and frequency domains (continued)

21. Composite Signal • A single sine wave can carry electric energy from one place to another • Power company sends a single sine wave with a frequency of say 60Hz to distribute electric energy to our houses. • Use composite signal made of many simple frequencies to send data.

22. Frequency Domain • The frequency domain is more compact and useful when we are dealing with more than one sine wave.

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

24. Harmonics • Let fundamental frequency be 25 Hz = 1st harmonic • 2nd harmonic = 50 Hz • 3rd harmonic = 70 Hz • If the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc.

25. Figure 3.9Three harmonics

26. Figure 3.10Adding first three harmonics

27. Another composite periodic signal Decomposition of the composite periodic signal in the time and frequency domains

28. Time and frequency domains of a nonperiodic signal • A nonperiodic composite signal • It can be a signal created by a microphone or a telephone set when a word or two is pronounced. • In this case, the composite signal cannot be periodic • because that implies that we are repeating the same word or words with exactly the same tone.

29. Bandwidth The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal.

30. Example A nonperiodic composite signal has a bandwidth of 200 kHz, with a middle frequency of 140 kHz and peak amplitude of 20 V. The two extreme frequencies have an amplitude of 0. Draw the frequency domain of the signal. Solution The lowest frequency must be at 40 kHz and the highest at 240 kHz.

31. 3 Meanings of Bandwidth • 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. • The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. • Generally, the width of a single spectrum. 5: DataLink Layer

32. Figure 3.13Bandwidth

33. 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

34. Figure 3.14Example 3

35. 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

36. Figure 3.15Example 4

37. 3-3 DIGITAL SIGNALS • In addition to being represented by an analog signal, information can also be represented by a digital signal. • For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. • A digital signal can have more than two levels. • In this case, we can send more than 1 bit for each level.

38. Figure 3.17Bit rate and bit interval

39. Two digital signals: one with two signal levels and the other with four signal levels

40. Example 6 A digital signal has a bit rate of 2000 bps. What is the duration of each bit (bit interval) Solution The bit interval is the inverse of the bit rate. Bit interval = 1/ 2000 s = 0.000500 s = 0.000500 x 106ms = 500 ms

41. Examples A digital signal has 8 levels. How many bits are needed per level? We calculate the number of bits from the formula Each signal level is represented by 3 bits. A digital signal has 9 levels. How many bits are needed per level? Each signal level is represented by 3.17 bits. The number of bits sent per level needs to be an integer as well as a power of 2. Hence, 4 bits can represent one level.

42. Example 3.18 Assume we need to download files at a rate of 100 pages per second. A page is an average of 24 lines with 80 characters in each line where one character requires 8 bits. What is the required bit rate of the channel? 5

43. Example 3.19 A digitized voice channel is made by digitizing a 4-kHz bandwidth analog voice signal. We sample the signal at twice the highest frequency (two samples per hertz). Assume that each sample contains 8 bits. What is the bit rate?

44. Example 3.20 HDTV uses digital signals to broadcast high quality video signals. There are 1920 by 1080 pixels per screen, and the screen is renewed 30 times per second. Also, 24 bits represents one color pixel. What is the bit rate for high-definition TV (HDTV)? The TV stations reduce this rate to 20 to 40 Mbps through data compression.

45. Representing digital signal Decomposition of the composite periodic signal in the time and frequency domains

46. The time and frequency domains of periodic and nonperiodic digital signals

47. Digital Signal as a Composite Analog Signal • Vertical: If a signal changes instantaneously, its frequency is infinite. • Horizontal: If a signal does not change at all, its frequency is zero. • Range of a digital signal goes from 0 to infinite. • Bandwidth of a digital signal is infinite.

48. Figure 3.18 Baseband transmission Baseband transmission: sending a digital signal over a channel without changing the digital signal to an analog signal.

49. Low pass channel has a bandwidth with frequencies between 0 and f. Lower limit is 0, the upper limit can be any frequency (including infinity) Band-pass channel has bandwidth with frequencies between f1 and f2. The analog bandwidth of a medium is expressed in hertz; the digital bandwidth, in bits per second Low-pass channel is suitable for digital transmission because of 0 frequency for square wave. Cannot use band-pass to transmit digital signal. Analog transmission uses a band-pass channel to avoid low frequency interference. Figure 3.19Low-pass and band-pass

50. Figure 3.20 Baseband transmission using a dedicated medium In practice, the medium has finite bandwidth. The received signal is an approximation of the sent signal.