2. Data Communications and Computer Networks Chapter 1 Network Architecture Models Logical and physical connections

3. Data Communications and Computer Networks Chapter 1 The Internet Model in Action Note the flow of data from user to web browser and back At each layer, information is either added or removed, depending on whether the data is leaving or arriving at a workstation The adding of information over pre-existing information is termed encapsulation

5. Data Communications and Computer Networks Chapter 2 Introduction Computer networks transmit signals Signals are the electomagnetic encoding of data Data and signals can be analog or digital

6. Data Communications and Computer Networks Chapter 2 Data and Signals Examples of data include: computer files movie on a DVD music on a compact disc collection of samples from a blood gas analysis device

7. Data Communications and Computer Networks Chapter 2 Data and Signals Examples of signals include: telephone conversation over a telephone line live television news interview from Europe Web page download over your telephone line via the Internet

8. Data Communications and Computer Networks Chapter 2 Analog versus Digital Analog is a continuous waveform, with examples such as music and video.

9. Data Communications and Computer Networks Chapter 2 Analog versus Digital Digital is a discrete or non-continuous waveform with fixed voltage levels that represent data 1s and 0s.

10. Data Communications and Computer Networks Chapter 2 Analog versus Digital It is harder to separate noise from an analog signal than it is to separate noise from a digital signal.

11. Data Communications and Computer Networks Chapter 2 Analog versus Digital Noise in a digital signal. You can still discern a high voltage from a low voltage. Regenerators are devices that automatically amplify and clean noise out of digital signals.

12. Data Communications and Computer Networks Chapter 2 Analog versus Digital Noise in a digital signal. Too much noise - you cannot discern a high voltage from a low voltage. Here data will be lost in transmission.

13. Data Communications and Computer Networks Chapter 2 Signals Have Three Components Amplitude Frequency Phase

14. Communicating Data Binary data (1s and 0s) are communicated by changing one or more of these components (amplitude, frequency, or phase) in predetermined ways at predetermined time intervals.

15. Data Communications and Computer Networks Chapter 2 Amplitude The amplitude of a signal is the height of the wave above or below a given reference point.

16. Data Communications and Computer Networks Chapter 2 Frequency The frequency is the number of times a signal makes a complete cycle within a given time frame. Spectrum - The range of frequencies that a signal spans from minimum to maximum. Bandwidth - The absolute value of the difference between the lowest and highest frequencies of a signal. Attenuation - Loss of signal strength.

17. Data Communications and Computer Networks Chapter 2

18. Data Communications and Computer Networks Chapter 2 Phase The phase of a signal is the position of the waveform relative to a given moment of time or relative to time zero. A change in phase can be any number of angles between 0 and 360 degrees. Phase changes often occur on common angles, such as 45, 90, 135, etc.

19. Data Communications and Computer Networks Chapter 2

20. Data Communications and Computer Networks Chapter 2 Loss of Signal Strength All signals experience power loss (attenuation) as they travel over a communications medium. Signals must be regenerated or amplified at regular intervals to prevent total signal loss. Attenuation is denoted as a decibel (dB) loss.

21. Data Communications and Computer Networks Chapter 2 Converting Digital Data into Digital Signals There are numerous techniques available to convert digital data into digital signals. Let’s examine four techniques: NRZ-L NRZ-I Manchester Differential Manchester

22. Data Communications and Computer Networks Chapter 2

23. Self-Clocking Codes Big difference between NRZ and Manchester codes: –For long strings of 0-bits, NRZ codes generate signal that does not change over long time period –Manchester codes always produce signal change during every bit transmission. –Manchester codes are called self-clocking codes, because they provide a guaranteed voltage change (a “clock signal”) in the middle of every bit received.

24. Data Communications and Computer Networks Chapter 2 Note how with a Differential Manchester code, every bit has at least one signal change. Some bits have two signal changes per bit (baud rate is twice the bps).

25. Self-Clocking Codes Why do we care about self-clocking codes? –Transmitter / receiver clocks are not perfectly synchronized to tick at same rate (too expensive). –NRZ-L or NRZ-I cannot be used at high data rates or long distances unless a separate clock signal is sent on another wire. –Manchester codes can be used at high data rates or long distances, because receiver continuously gets feedback on sender clock rate.

26. Self-Clocking Codes Any disadvantage to Manchester codes? –Manchester codes must transmit two signal changes per bit (2 baud per bit). –NRZ codes transmit only one signal change per bit (1 baud per bit). –Any transmission medium (copper wire, fiber optics, etc.) has a maximum baud capacity that it can support. NRZ can send more bits per second.

27. Data Communications and Computer Networks Chapter 2 4B/5B Digital Encoding Yet another encoding technique that converts four bits of data into five-bit quantities. The five-bit quantities are unique in that no five-bit code has more than 2 consecutive zeroes. The five-bit code is then transmitted using an NRZ-I encoded signal. This system is self-clocking, yet provides 4 bits for every 5 signal changes (more efficient than Manchester)

28. Data Communications and Computer Networks Chapter 2

29. Data Communications and Computer Networks Chapter 2 Converting Digital Data into Analog Signals Three basic techniques: Amplitude modulation Frequency modulation Phase modulation

30. Data Communications and Computer Networks Chapter 2 Amplitude Modulation One amplitude encodes a 0 while another amplitude encodes a 1.

31. Data Communications and Computer Networks Chapter 2 Amplitude Modulation Some systems use multiple amplitudes.

32. Data Communications and Computer Networks Chapter 2 Frequency Modulation One frequency encodes a 0, while another frequency encodes a 1.

33. Data Communications and Computer Networks Chapter 2 Phase Modulation One phase change encodes a 0, while another phase change encodes a 1.

34. Data Communications and Computer Networks Chapter 2 Quadrature Phase Modulation Four different phase angles are used: 45 degrees 135 degrees 225 degrees 315 degrees

35. Data Communications and Computer Networks Chapter 2

36. Data Communications and Computer Networks Chapter 2 Quadrature Amplitude Modulation In this technology, 12 different phases are combined with two different amplitudes. Since only 4 phase angles have 2 different amplitudes, there are a total of 16 combinations. With 16 signal combinations, each baud equals 4 bits of information. (2 ^ 4 = 16)

37. Data Communications and Computer Networks Chapter 2

38. Data Communications and Computer Networks Chapter 2 Converting Analog Data into Digital Signals To convert analog data into a digital signal, there are two basic techniques: Pulse code modulation Delta modulation

39. Data Communications and Computer Networks Chapter 2 Pulse Code Modulation The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values.

40. Data Communications and Computer Networks Chapter 2 Pulse Code Modulation When the binary values are later converted to an analog signal, a waveform similar to the original results is created, as long as enough samples are taken

41. Data Communications and Computer Networks Chapter 2 Pulse Code Modulation The more snapshots taken in the same amount of time, the better the resolution.

42. Data Communications and Computer Networks Chapter 2 Delta Modulation An analog waveform is tracked, using a binary 1 to represent a rise in voltage, and a 0 to represent a drop.

43. Data Communications and Computer Networks Chapter 2 Converting Analog Data into Analog Signals Many times it is necessary to modulate analog data onto a different set of analog frequencies. Broadcast radio and television are two very common examples of this. In this situation a data signal is modulated by a carrier signal to produce a composite signal that can be broadcast in a particular range of frequencies near the carrier frequency. For example: music (analog data signal) is modulated by a 91.5 MHz sine wave, which produces a composite signal with components between 91.4 MHz and 91.6 MHz. This is transmitted. You retrieve the original signal by demodulating (tuning into) frequency 91.5 MHz on your radio to get back the music

44. Data Communications and Computer Networks Chapter 2

45. Data Communications and Computer Networks Chapter 2 Spread Spectrum Technology A secure encoding technique that uses multiple frequencies or codes to transmit data. Two basic spread spectrum technologies: Frequency hopping spread spectrum Direct sequence spread spectrum

46. Data Communications and Computer Networks Chapter 2 Frequency Hopping Spread Spectrum

47. Data Communications and Computer Networks Chapter 2 Data Code The set of all textual characters or symbols and their corresponding binary patterns is called a data code. There are two basic data code sets plus a third code set that has interesting characteristics: ASCII EBCDIC Baudot Code

48. Data Communications and Computer Networks Chapter 2

49. Data Communications and Computer Networks Chapter 2

50. Data Communications and Computer Networks Chapter 2

51. Data Communications and Computer Networks Chapter 2 Data and Signal Conversions in Action Let us transmit the message “Sam, what time is the meeting with accounting? Hannah.” This message first leaves Hannah’s workstation and travels across a local area network.

52. Data Communications and Computer Networks Chapter 2 Data and Signal Conversions in Action

53. Data Communications and Computer Networks Chapter 2 Data and Signal Conversions in Action

54. Data Communications and Computer Networks Chapter 2 Data and Signal Conversions in Action