Data Communication Networks

1. Data Communication Networks Lec8 and 9

2. Physical Layer and Media • Bottom-most layer. • Interacts with transmission media. • Physical part of the network. • Provides services to data link layer. • 0s and 1s ready to be send on transmission medium. • Must be converted into another entity- SIGNALS

3. Data and Signals • Move data in the form of electromagnetic signals across transmission medium.

4. Analog and Digital • Analog is the data that is continuous. • Digital is the data that is discrete. • Sounds by human voice • Digital in the form of 1s and 0s

5. Analog and Digital Signals

6. Analog and Digital Signals • 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.

7. Periodic signal • That completes a pattern within a measurable time frame • Repeats the pattern over subsequent identical periods. • The completion of one full pattern is called a cycle.

8. Periodic Analog Signal • Simple or composite. • Sine wave, a simple oscillating curve , changes over course of time is smooth and consistent , a continuous , rolling flow.

9. A Sine Wave

10. Peak Amplitude • Absolute value of its highest intensity. • Energy it carries. • Electrical signals , measured in volts.

11. Peak Amplitude

12. Period and frequency • Period is the amount of time , in seconds , a signal need to complete one cycle. • Frequency refer to number of periods in 1s • F = 1/T, T=1/F

13. Period and Frequency

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

15. 3-4 TRANSMISSION IMPAIRMENT Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise. Topics discussed in this section: AttenuationDistortionNoise

16. Figure 3.25 Causes of impairment

17. Figure 3.26 Attenuation

18. Example 3.26 Suppose a signal travels through a transmission medium and its power is reduced to one-half. This means that P2 is (1/2)P1. In this case, the attenuation (loss of power) can be calculated as A loss of 3 dB (–3 dB) is equivalent to losing one-half the power.

19. Example 3.27 A signal travels through an amplifier, and its power is increased 10 times. This means that P2 = 10P1 . In this case, the amplification (gain of power) can be calculated as

20. Example 3.28 One reason that engineers use the decibel to measure the changes in the strength of a signal is that decibel numbers can be added (or subtracted) when we are measuring several points (cascading) instead of just two. In Figure 3.27 a signal travels from point 1 to point 4. In this case, the decibel value can be calculated as

21. Figure 3.27 Decibels for Example 3.28

22. Example 3.29 Sometimes the decibel is used to measure signal power in milliwatts. In this case, it is referred to as dBm and is calculated as dBm = 10 log10 Pm , where Pm is the power in milliwatts. Calculate the power of a signal with dBm = −30. Solution We can calculate the power in the signal as

23. Example 3.30 The loss in a cable is usually defined in decibels per kilometer (dB/km). If the signal at the beginning of a cable with −0.3 dB/km has a power of 2 mW, what is the power of the signal at 5 km? Solution The loss in the cable in decibels is 5 × (−0.3) = −1.5 dB. We can calculate the power as

24. Figure 3.28 Distortion

25. Figure 3.29 Noise

26. Example 3.31 The power of a signal is 10 mW and the power of the noise is 1 μW; what are the values of SNR and SNRdB ? Solution The values of SNR and SNRdB can be calculated as follows:

27. Example 3.32 The values of SNR and SNRdB for a noiseless channel are We can never achieve this ratio in real life; it is an ideal.

28. Figure 3.30 Two cases of SNR: a high SNR and a low SNR

29. Digital Transmission • To represent digital data by using digital signals. • Involve three techniques • Line coding • Block coding • Scrambling

30. Line coding • Process of converting digital data to digital signals. • Data in the form of text, numbers, audio, images are stored in computer memory as sequence of bits. • Line coding convert sequence of bits to a digital signal. • At the sender, digital data are encoded into digital signal; • At the receiver, the digital data are recreated by decoding the digital signal.

31. Line coding and decoding

32. Difference between Data Element and Signal Element??????

33. Signal element versus data element

34. Data Rate Vs Signal Rate • The data rate defines the number of data elements (bits) sent in 1s. • Unit is bits per second. • The signal rate is the number of signal elements sent in 1s. • Unit is baud. • Data rate is sometimes called bit rate, signal rate is sometimes called pulse rate, modulation rate and baud rate.

35. Data Rate Vs Signal Rate • Goal in DC is to increase data rate , while decreasing signal rate. • Increase will increase speed of transmission. • Decrease will decrease bandwidth requirements.

36. Relationship B/w Bit rate and baud rate • Depend on the value of r. • Also depend on data pattern. • Three cases ; worst, best, and average. • Worst case is the maximum signal rate ; best case is the minimum. • S=cxNx1/r baud

37. Example A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1? Solution We assume that the average value of c is 1/2 . The baud rate is then

38. Note Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite.

39. Baseline Wandering • In decoding a digital signal, the receiver calculates a running average of the received signal power; called baseline. • A long string of 0s and 1s can cause a drift in the baseline(baseline wandering) • Difficult for receiver to decode correctly. • Good line coding scheme needs to prevent baseline wandering.

40. DC Components • Constant voltage level , creates frequencies around zero. • Called DC components. • Telephone lines cannot pass frequency below 200 hz • Need scheme with no DC component.

41. Figure 4.3 Effect of lack of synchronization

42. Example 4.3 In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 kbps? How many if the data rate is 1 Mbps? Solution At 1 kbps, the receiver receives 1001 bps instead of 1000 bps. At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps.

43. Built-in Error Detection • Capability to detect some or all errors that occurred in transmission.

44. Immunity to Noise and interference • Should have capability of immunity to noise and interference.

45. Complexity • Four level more costly than two levels.

46. Figure 4.4 Line coding schemes

47. Unipolar Scheme • All signal levels are on the one side of the time axis, either above or below. • NRZ(non-Return-to-zero): in which the positive voltage defines bit 1 and the zero voltage defines bit 0. • Costly, power required is double than polar NRZ. • Not used in data communication.

48. Figure 4.5 Unipolar NRZ scheme

49. Polar Scheme • The voltage is at both side of the time axis. • Non-Return-to-Zero(NRZ):uses two levels of the voltage amplitude. • NRZ-L:the level of the voltage determine the value of the bit. • NRZ-I: the change or lack of change in the level of the voltage determine the value of the bit. • If no change the bit is 0 ; If there is change the bit is 1.

50. Figure 4.6 Polar NRZ-L and NRZ-I schemes