Computer Networks encoding techniques for cse stdents
- 2. DIGITAL-TO-DIGITAL CONVERSION In this section, we see how we can represent digital data by using digital signals. The conversion involves three techniques: line coding, block coding, and scrambling. Line coding is always needed; block coding and scrambling may or may not be needed. Line Coding Line Coding Schemes Block Coding Topics discussed in this section:
- 3. DIGITAL-TO-DIGITAL CONVERSION Line Coding: • Converts sequence of bits to digital signal • Signal decode at the receiver end to get the original data
- 4. Figure 4.1 Line coding and decoding
- 5. Figure 4.2 Signal element versus data element Ratio r
- 6. Data rate Vs Signal rate
- 7. 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 Example 4.1
- 9. Figure 4.3 Effect of lack of synchronization
- 10. 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. Example 4.3 At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps.
- 11. Figure 4.4 Line coding schemes
- 13. Figure 4.5 Unipolar NRZ scheme Power needed to send 1 bit per unit line resistance
- 14. Figure 4.6 Polar NRZ-L(Level) and NRZ-I(invert) schemes
- 15. In NRZ-L the level of the voltage determines the value of the bit. In NRZ-I the inversion or the lack of inversion determines the value of the bit. Note
- 16. NRZ-L and NRZ-I both have an average signal rate of N/2 Bd. Note
- 17. NRZ-L and NRZ-I both have a DC component problem. Note
- 18. A system is using NRZ-I to transfer 10-Mbps data. What are the average signal rate and minimum bandwidth? Solution The average signal rate is S = N/2 = 500 kbaud. The minimum bandwidth for this average baud rate is Bmin = S = 500 kHz. Example 4.4
- 19. Figure 4.7 Polar RZ scheme Complex and uses greater bandwidth
- 20. Figure 4.8 Polar biphase: Manchester and differential Manchester schemes
- 21. In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization. Note
- 22. The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ. Note
- 23. In bipolar encoding, we use three levels: positive, zero, and negative. Note
- 24. Figure 4.9 Bipolar schemes: AMI and pseudoternary
- 25. In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln. Note
- 26. Figure 4.10 Multilevel: 2B1Q scheme
- 27. Table 4.1 Summary of line coding schemes
- 28. • Block coding is normally referred to as mB/nB coding • it replaces each m-bit group with an n-bit group. • Can give redundancy to ensure synchronization and improve performance of line coding Block Coding
- 29. Figure 4.14 Block coding concept
- 30. Figure 4.15 Using block coding 4B/5B with NRZ-I line coding scheme
- 31. Table 4.2 4B/5B mapping codes
- 32. Figure 4.16 Substitution in 4B/5B block coding
- 33. ◼ Redundancy bits add 20 percent more baud ◼ Does not solve the DC component of the NRZ-I baud.
- 34. Scrambling
- 35. Figure 4.9 revisiting Bipolar schemes: AMI and pseudoternary
- 36. Figure 4.18 AMI used with scrambling
- 37. B8ZS substitutes eight consecutive zeros with 000VB0VB. Note
- 38. Figure 4.19 Two cases of B8ZS scrambling technique
- 39. HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution. Note
- 40. Figure 4.20 Different situations in HDB3 scrambling technique
- 41. Figure 4.20 Different situations in HDB3 scrambling technique
- 42. ANALOG-TO-DIGITAL CONVERSION change an analog signal to digital data. In this section we describe two techniques, pulse code modulation and delta modulation. Pulse Code Modulation (PCM) Delta Modulation (DM) Topics discussed in this section:
- 43. Figure 4.21 Components of PCM encoder
- 44. Figure 4.22 Three different sampling methods for PCM
- 45. According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency contained in the signal. Note
- 47. Figure 4.26 Quantization and encoding of a sampled signal
- 48. We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample? Solution The human voice normally contains frequencies from 0 to 4000 Hz. So the sampling rate and bit rate are calculated as follows: Example 4.14
- 49. Figure 4.27 Components of a PCM decoder
- 50. Delta Modulation ◼ PCM finds the value of signal amplitude for each signal. ◼ DM finds the change from the previous sample
- 51. Figure 4.29 Delta modulation components
- 52. The process of delta modulation
- 53. Figure 4.30 Delta demodulation components