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4123702 Data Communications System

4123702 Data Communications System. By Ajarn Preecha Pangsuban. Part 2 – Physical Layer. Interacts with transmission media Creates a signal representing 0s and 1s Physical movement of data Determines direction of data flow

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4123702 Data Communications System

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  1. 4123702Data Communications System By Ajarn Preecha Pangsuban

  2. Part 2 – Physical Layer • Interacts with transmission media • Creates a signal representing 0s and 1s • Physical movement of data • Determines direction of data flow • Decides on the number of logical channelsfor transporting data coming from different source Coming up… 4123702 Data Communications System @YRU

  3. Position of the physical Layer 4123702 Data Communications System @YRU

  4. Physical layer services 4123702 Data Communications System @YRU

  5. Transmission Media • Guided • Twisted-pair • Coaxial cable • Fiber-optic • Unguided • Radio • Microwave 4123702 Data Communications System @YRU

  6. Networks and Technologies • Telephone network – circuit-switched network • High speed access • Modems • DSL • Cable 4123702 Data Communications System @YRU

  7. Part 2 Chapters Chapter 3 Signals Chapter 4 Digital Transmission Chapter 5 Analog Transmission Chapter 6 Multiplexing Chapter 7 Transmission Media Chapter 8 Circuit Switching and Telephone Network Chapter 9 High Speed Digital Access 4123702 Data Communications System @YRU

  8. Chapter 3: Signals 4123702 Data Communications System @YRU

  9. Note: To be transmitted, data must be transformed to electromagnetic signals. 4123702 Data Communications System @YRU

  10. Analog and Digital Signals • Signals can be analog or digital form • Analog signals can have an infinite number of values in a range • digital signals can have only a limited number of values. 4123702 Data Communications System @YRU

  11. Periodic and Aperiodic Signals • Periodic – completes a pattern within a measurable time frame, called a period • One full pattern is a cycle • Analog signals • Aperiodic – changes without exhibiting a pattern • Digital signals 4123702 Data Communications System @YRU

  12. PeriodicSignals 4123702 Data Communications System @YRU

  13. Aperiodic Signals 4123702 Data Communications System @YRU

  14. Note: In data communication, we commonly use periodic analog signals and aperiodic digital signals. 4123702 Data Communications System @YRU

  15. Analog Signals • Sine wave – most fundamental form of a periodic analog signal • Fully described by: Amplitude, Frequency and Phase 4123702 Data Communications System @YRU

  16. Analog Signaling • More susceptible to noise and less precise than a digital signal • Benefit - because they are more variable than digital signals, they can convey greater subtleties 4123702 Data Communications System @YRU

  17. Amplitude • Absolute value of a signal’s highest intensity • Normally measured in volts 4123702 Data Communications System @YRU

  18. Period and Frequency • Period - amount of time to complete one cycle, expressed in seconds (s) • Frequency – number of periods in one second, inverse of period 4123702 Data Communications System @YRU

  19. Frequency • Rate of change with respect to time, expressed in hertz (Hz) • Change in a short span of time means high frequency • Change over a long span of time means low frequency 4123702 Data Communications System @YRU

  20. Phase • Position of the waveform relative to time zero • Measured in degrees or radians 4123702 Data Communications System @YRU

  21. Table 3.1 Units of periods and frequencies 4123702 Data Communications System @YRU

  22. Example Sine Waves s(t) = A sin(2ft +) 4123702 Data Communications System @YRU

  23. 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 4123702 Data Communications System @YRU

  24. 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 2 /360 rad = 1.046 rad 4123702 Data Communications System @YRU

  25. Time and Frequency Domains • Time-domain plot – displays changes in signal amplitude with respect to time • Frequency-domain plot – relationship between amplitude and frequency • Best represents an analog signal 4123702 Data Communications System @YRU

  26. Figure 3.6 Sine wave examples 4123702 Data Communications System @YRU

  27. Figure 3.6 Sine wave examples (cont.) 4123702 Data Communications System @YRU

  28. Figure 3.6 Sine wave examples (cont.) 4123702 Data Communications System @YRU

  29. Figure 3.7 Time and frequency domains 4123702 Data Communications System @YRU

  30. Figure 3.7 Time and frequency domains (cont.) 4123702 Data Communications System @YRU

  31. Figure 3.7 Time and frequency domains (cont.) 4123702 Data Communications System @YRU

  32. Composite Signals • Composed of many simple sine waves of differing frequencies • Fourier – showed any composite signal is a sum of a set of sine waves of different frequencies, phases, and amplitudes (Harmonics) • Fourier analysis • Harmonics – components of digital signal, each having a different frequencies, phases, and amplitudes 4123702 Data Communications System @YRU

  33. Figure 3.8 Square wave 4123702 Data Communications System @YRU

  34. Figure 3.9 Three harmonics 4123702 Data Communications System @YRU

  35. Figure 3.10 Adding first three harmonics 4123702 Data Communications System @YRU

  36. Frequency Spectrum • Description of a signal using the frequency domain and containing all of its components • Dependent on medium used 4123702 Data Communications System @YRU

  37. Figure 3.11 Frequency spectrum comparison 4123702 Data Communications System @YRU

  38. Composite Signals and Transmission Medium • A medium’s characteristics may affect the signal • Some frequencies may be weakened or blocked • Signal corruption– when square wave is sent through a medium, other end which is not square wave at all Figure 3.12 Signal corruption 4123702 Data Communications System @YRU

  39. Bandwidth • Range of frequencies that a medium can pass without losing one-half of the power contained in the signal • Difference between the highest and the lowest frequencies that the medium can satisfactorily pass. • In this book, we use the term bandwidth to refer to the property of a medium or the width of a single spectrum. 4123702 Data Communications System @YRU

  40. Figure 3.13 Bandwidth 4123702 Data Communications System @YRU

  41. Frequency Spectrum 4123702 Data Communications System @YRU

  42. 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 ) 4123702 Data Communications System @YRU

  43. Figure 3.14 Example 3 4123702 Data Communications System @YRU

  44. 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 4123702 Data Communications System @YRU

  45. Figure 3.15 Example 4 4123702 Data Communications System @YRU

  46. Example 5 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? 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. 4123702 Data Communications System @YRU

  47. Quiz • In this picture, what is the bandwidth? when f=1MHz 4123702 Data Communications System @YRU

  48. Digital Signals • Use binary (0s and 1s) to encode information • Usually aperiodic; period and frequency are not appropriate • Less affected by interference (noise); fewer errors 4123702 Data Communications System @YRU

  49. Figure 3.16 A digital signal 4123702 Data Communications System @YRU

  50. Bit Interval and Bit Rate • Describe digital signals by • Bit interval – time required to send one single bit • Bit rate – number of bit intervals per second, usually expressed as bits per second (bps) Figure 3.17 Bit rate and bit interval 4123702 Data Communications System @YRU

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