1. Chapter 4 Digital Transmission
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2. 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

3. Figure 4.1 Line coding and decoding

4. Figure 4.2 Signal element versus data element

5. Figure 4.3 Effect of lack of synchronization

6. Figure 4.4 Line coding schemes

7. Figure 4.5 Unipolar NRZ scheme

8. Nonreturn to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bits
Voltage constant during bit interval
no transition I.e. no return to zero voltage
e.g. Absence of voltage for zero, constant positive voltage for one
More often, negative voltage for one value and positive for the other
This is NRZ-L

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

10. Nonreturn to Zero Inverted
Nonreturn to zero inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signal transition at beginning of bit time
Transition (low to high or high to low) denotes a binary 1
No transition denotes binary 0
An example of differential encoding

11. Note
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.

12. Differential Encoding
Data represented by changes rather than levels
More reliable detection of transition rather than level
In complex transmission layouts it is easy to lose sense of polarity

13. NRZ pros and cons Pros Cons Used for magnetic recording
Easy to engineer
Make good use of bandwidth
Cons
dc component
Lack of synchronization capability
Used for magnetic recording
Not often used for signal transmission

14. Multilevel Binary Use more than two levels Bipolar-AMI
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a problem)
No net dc component
Lower bandwidth
Easy error detection

15. Pseudoternary One represented by absence of line signal
Zero represented by alternating positive and negative
No advantage or disadvantage over bipolar-AMI

16. Figure 4.9 Bipolar schemes: AMI and pseudoternary

17. Trade Off for Multilevel Binary
Not as efficient as NRZ
Each signal element only represents one bit
In a 3 level system could represent log23 = 1.58 bits
Receiver must distinguish between three levels (+A, -A, 0)
Requires approx. 3dB more signal power for same probability of bit error

18. Biphase Manchester Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3(ethernet) standard for baseband coaxial cable and twisted-pair CSMA/CD bus LANs
Differential Manchester
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE token ring LAN, using shielded twisted pair

19. Manchester Encoding

20. Differential Manchester Encoding

21. Biphase Pros and Cons Con Pros
At least one transition per bit time and possibly two
Maximum modulation rate is twice NRZ
Requires more bandwidth
Pros
Synchronization on mid bit transition (self clocking)
No dc component
Error detection
Absence of expected transition

22. Figure 4.7 Polar RZ scheme

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

24. In Manchester and differential Manchester encoding, the transition
Note
In Manchester and differential Manchester encoding, the transition
at the middle of the bit is used for synchronization.

25. There is another factor that can be used to improve performance, and that is the encoding scheme. The encoding scheme is simply the mapping from data bits to signal elements. A variety of approaches have been tried. In what follows, we describe some of the more common ones; they are defined in Table 5.2

26. Figure 5.2 shows the signal encoding for the binary sequence
using six different signal encoding schemes.

27. Figure 4.18 AMI used with scrambling

28. Figure 4.19 Two cases of B8ZS scrambling technique

29. B8ZS substitutes eight consecutive zeros with 000VB0VB.
Note
B8ZS substitutes eight consecutive zeros with 000VB0VB.

30. Figure 4.20 Different situations in HDB3 scrambling technique

31. HDB3 substitutes four consecutive zeros with 000V or B00V depending
Note
HDB3 substitutes four consecutive zeros with 000V or B00V depending
on the number of nonzero pulses after the last substitution.

32. Topics discussed in this section:
4-2 ANALOG-TO-DIGITAL CONVERSION
We have seen in Chapter 3 that a digital signal is superior to an analog signal. 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.
Topics discussed in this section:
Pulse Code Modulation (PCM) Delta Modulation (DM)

33. Figure 4.21 Components of PCM encoder

34. According to the Nyquist theorem, the sampling rate must be
Note
According to the Nyquist theorem, the sampling rate must be
at least 2 times the highest frequency contained in the signal.

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

36. Example 4.9
Telephone companies digitize voice by assuming a maximum frequency of 4000 Hz. The sampling rate therefore is 8000 samples per second.

37. Figure 4.26 Quantization and encoding of a sampled signal

38. Example 4.12 What is the SNRdB in the example of Figure 4.26? Solution
We can use the formula to find the quantization. We have eight levels and 3 bits per sample, so
SNRdB = 6.02(3) = dB
Increasing the number of levels increases the SNR.

39. Example 4.13
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.

40. Topics discussed in this section:
4-3 TRANSMISSION MODES
The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.
Topics discussed in this section:
Parallel Transmission Serial Transmission

41. Figure 4.31 Data transmission and modes

42. Figure 4.32 Parallel transmission

43. Figure 4.33 Serial transmission

44. Note
In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte.

45. Asynchronous here means “asynchronous at the byte level,”
Note
Asynchronous here means “asynchronous at the byte level,”
but the bits are still synchronized; their durations are the same.

46. Figure 4.34 Asynchronous transmission

47. Note
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.

48. Figure 4.35 Synchronous transmission

49. Asynchronous Transmission
Data transmitted 1 character at a time
Character format is 1 start & 1+ stop bit, plus data of 5-8 bits
Character may include parity bit
Timing needed only within each character
Resynchronization each start bit
Uses simple, cheap technology
Wastes 20-30% of bandwidth 3

51. Example
An asynchronous transmission scheme uses 8 data bits, an even parity bit, and a stop element of length 2 bits. What percentage of clock inaccuracy can be tolerated at the receiver with respect to the framing error? Assume that the bit samples are taken at the middle of the clock period. Also assume that at the beginning of the start bit the clock and incoming bits are in phase.

52. Synchronous Transmission
Large blocks of bits transmitted without start/stop codes
Synchronized by clock signal or clocking data
Data framed by preamble and postamble bit patterns
More efficient than asynchronous
Overhead typically below 5%
Used at higher speeds than asynchronous
Requires error checking, usually provided by HDLC 4

53. Synchronization Choices
Low-speed terminals and PCs commonly use asynchronous transmission
inexpensive
“burst” tendency of communication reduces impact of inefficiency
Large systems and networks commonly use synchronous transmission
overhead too expensive; efficiency necessary
error-checking more important