1. Chapter 4
Digital Transmission

2. Components of Data Communication
Analog: Continuous value data (sound, light, temperature)
Digital: Discrete value (text, integers, symbols)
Signal
Analog: Continuously varying electromagnetic wave
Digital: Series of voltage pulses (square wave)

3. Analog Data-->Signal Options
Analog data to analog signal
Inexpensive, easy conversion (eg telephone)
Used in traditional analog telephony
Analog data to digital signal
Requires a codec (encoder/decoder)
Allows use of digital telephony, voice mail

4. Digital Data-->Signal Options
Digital data to analog signal
Requires modem (modulator/demodulator)
Necessary when analog transmission is used
Digital data to digital signal
Less expensive when large amounts of data are involved
More reliable because no conversion is involved

5. Topics discussed in this section:
4-1 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.
Topics discussed in this section:
Line Coding
Line Coding Schemes Block Coding
Scrambling

6. Figure 4.1 Line coding and decoding

7. Figure 4.2 Signal element versus data element
r = number of data elements / number of signal elements

8. Data Rate Vs. Signal Rate
Data rate: the number of data elements (bits) sent in 1s (bps). It’s also called the bit rate
Signal rate: the number of signal elements sent in 1s (baud). It’s also called the pulse rate, the modulation rate, or the baud rate.
We wish to:
1. increase the data rate (increase the speed of transmission)
2. decrease the signal rate (decrease the bandwidth requirement)
Worst case, best case, and average case of r
S = c * N / r baud

9. Baseline: running average of the received signal power
Baseline wandering
Baseline: running average of the received signal power
DC Components
Constant digital signal creates low frequencies
Self-synchronization
Receiver Setting the clock matching the sender’s

10. Figure 4.3 Effect of lack of synchronization

11. Figure 4.4 Line coding schemes

12. Figure 4.5 Unipolar NRZ scheme

13. Digital Encoding of Digital Data
Most common, easiest method is different voltage levels for the two binary digits
Typically, negative=1 and positive=0
Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the voltage during a bit transmission is level

14. Differential NRZ Differential version is NRZI (NRZ, invert on ones)
Change=1, no change=0
Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level

15. Problems With NRZ
Difficult to determine where one bit ends and the next begins
In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses
Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

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

17. Figure 4.7 Polar RZ scheme

18. Manchester Code Transition in the middle of each bit period
Transition provides clocking and data
Low-to-high=1 , high-to-low=0
Used in Ethernet

19. Differential Manchester
Midbit transition is only for clocking
Transition at beginning of bit period=0
Transition absent at beginning=1
Has added advantage of differential encoding
Used in token-ring

20. Figure 4.8 Polar biphase: Manchester and differential Manchester schemes

21. High=0, Low=1
No change at begin=0, Change at begin=1
H-to-L=0, L-to-H=1
Change at begin=0, No change at begin=1

22. Bipolar schemes: AMI (Alternate Mark Inversion) and pseudoternary

23. Multilevel Schemes
In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln
m: the length of the binary pattern
B: binary data
n: the length of the signal pattern
L: number of levels in the signaling
B for l=2 binary
T for l=3 ternary
Q for l=4 quaternary

24. Figure 4.10 Multilevel: 2B1Q scheme
Used in DSL

25. Figure 4.11 Multilevel: 8B6T scheme

26. Figure 4.13 Multitransition: MLT-3 scheme

27. Table 4.1 Summary of line coding schemes
Polar

28. Block Coding
Redundancy is needed to ensure synchronization and to provide error detecting
Block coding is normally referred to as mB/nB coding
it replaces each m-bit group with an n-bit group
m < n

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. Figure 4.17 8B/10B block encoding

34. Scrambling
It modifies the bipolar AMI encoding (no DC component, but having the problem of synchronization)
It does not increase the number of bits
It provides synchronization
It uses some specific form of bits to replace a sequence of 0s

35. B8ZS substitutes eight consecutive zeros with 000VB0VB
Figure Two cases of B8ZS scrambling technique
B8ZS substitutes eight consecutive zeros with 000VB0VB

36. HDB3 substitutes four consecutive zeros with 000V or B00V depending
Figure Different situations in HDB3 scrambling technique
HDB3 substitutes four consecutive zeros with 000V or B00V depending
on the number of nonzero pulses after the last substitution.

37. 4-2 ANALOG-TO-DIGITAL CONVERSION
The tendency today is to change an analog signal to digital data.
In this section we describe two techniques,
pulse code modulation and delta modulation.

38. Figure 4.21 Components of PCM encoder

39. What can we get from this:
According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency contained in the signal.
What can we get from this:
1. we can sample a signal only if the signal is
band-limited
2. the sampling rate must be at least 2 times the highest frequency, not the bandwidth

40. Figure 4.23 Nyquist sampling rate for low-pass and bandpass signals

41. Figure 4.24 Recovery of a sampled sine wave for different sampling rates

42. Figure 4.25 Sampling of a clock with only one hand

43. Example
An example related is the seemingly backward rotation of the wheels of a forward-moving car in a movie.
This can be explained by under-sampling. A movie is filmed at 24 frames per second. If a wheel is rotating more than 12 times per second, the under-sampling creates the impression of a backward rotation.

44. Example
A complex low-pass signal has a bandwidth of 200 kHz. What is the minimum sampling rate for this signal?
Solution
The bandwidth of a low-pass signal is between 0 and f, where f is the maximum frequency in the signal. Therefore, we can sample this signal at 2 times the highest frequency (200 kHz). The sampling rate is therefore 400,000 samples per second.

45. Example
A complex bandpass signal has a bandwidth of 200 kHz. What is the minimum sampling rate for this signal?
Solution
We cannot find the minimum sampling rate in this case because we do not know where the bandwidth starts or ends. We do not know the maximum frequency in the signal.

46. Figure 4.26 Quantization and encoding of a sampled signal

47. Contribution of the quantization error to SNRdb
SNRdb= 6.02nb dB
nb: bits per sample (related to the number of level L)
What is the SNRdB in the example of Figure 4.26?
Solution
We have eight levels and 3 bits per sample, so
SNRdB = 6.02 x = dB
Increasing the number of levels increases the SNR.

48. Example
A telephone subscriber line must have an SNRdB above 40. What is the minimum number of bits per sample?
Solution
We can calculate the number of bits as
Telephone companies usually assign 7 or 8 bits per sample.

49. PCM decoder: recovers the original signal

50. The minimum bandwidth of the digital signal is nb times greater than the bandwidth of the analog signal.
Bmin= nb x Banalog
We have a low-pass analog signal of 4 kHz. If we send the analog signal, we need a channel with a minimum bandwidth of 4 kHz. If we digitize the signal and send 8 bits per sample, we need a channel with a minimum bandwidth of 8 × 4 kHz = 32 kHz.

51. DM (delta modulation) finds the change from the previous sample
Next bit is 1, if amplitude of the analog signal is larger
Next bit is 0, if amplitude of the analog signal is smaller

52. Figure 4.29 Delta modulation components

53. Figure 4.30 Delta demodulation components

54. 4-3 TRANSMISSION MODES
1. The transmission of binary data across a link can be accomplished in either parallel or serial mode.
2. In parallel mode, multiple bits are sent with each clock tick.
3. In serial mode, 1 bit is sent with each clock tick.
4. there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.

55. Figure 4.31 Data transmission and modes

56. Figure 4.32 Parallel transmission

57. Figure 4.33 Serial transmission

58. Asynchronous transmission
1. We send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte.
2. There may be a gap between each byte.
3. Extra bits and gaps are used to alert the receiver, and allow it to synchronize with the data stream.
4. Asynchronous here means “asynchronous at the byte level,”
but the bits are still synchronized, their durations are the same.

59. Synchronous transmission
In synchronous transmission, we send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits.