1. CE 4228 Data Communications and Networking
Transmission Techniques

2. Transmission Techniques – Outline
Digital data/digital signals
Analog data/analog signals
Digital data/analog signals
Analog data/digital signals

3. Data and Signals Data and Signals Analog Signals Digital Signals
Analog Data
Baseband/Modulation
Codec
Digital Data
Modem
Encoding

4. Digital Transmissions
Treatment of Signals
Treatment of Signals
Analog Transmissions
Digital Transmissions
Analog Signals
Amplifiers
May use repeaters
Digital Signals
Not used
Repeaters

5. Digital Data/Digital Signals
Discrete, discontinuous voltage pulses
Each pulse is a signal element
Binary data encoded into signal elements

6. Coding Schemes – Terminologies
Unipolar
All signal elements have same sign
Polar
One logic state represented by positive voltage, the other by negative voltage
Data rate
Rate of data transmission in bits per second
Duration or length of a bit
Time taken for a transmitter to emit a bit

7. Coding Schemes – Terminologies
Modulation rate
Rate at which the signal level changes
Measured in baud
Signal elements per second
Mark and space
Binary 1 and binary 0 respectively

8. Coding Schemes – Interpretations
Need to know
Timing of bits
When they start and end
Signal levels
Factors affecting successful signal interpretations
Signal to noise ratio
Data rate
Bandwidth

9. Coding Schemes – Comparison
Bandwidth
Time and frequency reciprocal relationship
The smaller the pulse width, the larger the bandwidth of the signal pulse
Since a digital signal is made up of signal pulses of the same duration, its bandwidth is the same as the bandwidth of a single signal pulse
Therefore, the bandwidth of a digital signal is completely determined by the signal pulse it uses
The pulse width and pulse shape

10. Coding Schemes – Comparison
Signal spectrum
Lack of high frequencies reduces required bandwidth
Lack of DC component allows AC coupling via transformer, providing isolation
Concentrate power in the middle of the bandwidth
Clocking
Synchronizing transmitters and receivers
External clock
Sync mechanism based on signals

11. Coding Schemes – Comparison
Error detection
Can be built into signal encoding
Signal interference and noise immunity
Some codes are better than others
Cost and complexity
Higher signal rate leads to higher costs
Some codes require signal rate greater than data rate

12. Coding Schemes Unipolar Biphase Multilevel binary Scrambling NRZ-L
Non-return-to-zero level
NRZI
Non-return-to-zero inverted
Biphase
Manchester
Differential Manchester
Multilevel binary
Bipolar-AMI
Pseudoternary
Scrambling
B8ZS
HDB3

13. Coding Schemes – NRZ-L Non-return-to-zero level
Two different voltages represent 0 and 1
Voltage constant during bit interval
No transition, no return to zero voltage
Absence of voltage for zero, constant positive voltage for one
More often, negative voltage for one value and positive for the other

14. Coding Schemes – NRZI Non-return-to-zero inverted
Invert on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signal transition at beginning of bit time
Transition, from low to high or high to low, denotes a binary 1
No transition denotes binary 0
An example of differential encoding

15. Coding Schemes – Differential
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

16. Coding Schemes – NRZ

17. Coding Schemes – NRZ 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 transmissions

18. Coding Schemes – Bipolar-AMI
Bipolar alternate mark inversion
Multilevel binary
Use more than two levels
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

19. Coding Schemes – Pseudoternary
Multilevel binary
One represented by absence of line signal
Zero represented by alternating positive and negative
No advantage or disadvantage over bipolar-AMI

20. Coding Schemes – Bipolar

21. Coding Schemes – Bipolar
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
Require approximately 3dB more signal power for same probability of bit error

22. Coding Schemes – Manchester
Biphase
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

23. Coding Schemes – Manchester

24. Coding Schemes – Diff Manchester
Differential Manchester
Mid-bit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Differential encoding scheme
Used by IEEE 802.5

25. Coding Schemes – Diff Manchester

26. Coding Schemes – Biphase
Cons
At least one transition per bit time and possibly two
Maximum modulation rate is twice of NRZ
Require more bandwidth
Pros
Synchronization on mid-bit transition
Self clocking
No DC component
Error detection
Absence of expected transition

27. Coding Schemes – Modulation Rate

28. Coding Schemes – Scrambling
Use scrambling to replace sequences that would produce constant voltage
Filling sequence
Must produce enough transitions to sync
Must be recognized by receiver and replaced with original
Same length as original
No DC component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability

29. Coding Schemes – B8ZS Bipolar with 8 zeros substitution
Based on bipolar-AMI
If octet of all zeros and last voltage pulse preceding was positive encode as 000+−0−+
If octet of all zeros and last voltage pulse preceding was negative encode as 000−+0+−
Cause two violations of AMI code
Unlikely to occur by noise
Receiver detects and interprets as octet of all zeros

30. Coding Schemes – HDB3 High density bipolar 3 zeros
Based on bipolar-AMI
String of four zeros replaced with one or two pulses

31. Coding Schemes – Scrambling

32. Coding Schemes – Spectral Density

33. Coding Schemes – Summary

34. Analog Data/Analog Signal
Baseband or modulation
Analog signals may need modulation
Higher frequency could be transmitted more efficiently
Permit frequency division multiplexing
Spectrum relocation
Types of modulation
Amplitude
Frequency
Phase

35. Modulation – AM Amplitude modulation
DC component added at baseband to prevent loss of information
Modulation index nA < 1
x(t) ≤ 1 → nAx(t) = m(t)
Bandwidth = 2B
B = bandwidth of m(t)
DSB, SSB, VSB

36. Modulation – PM/FM Angle modulation φ(t) = nPm(t) for PM
φ'(t) = nFm(t) for FM
Carson’s rule
Bandwidth = 2(β+1)B
FM typical bandwidth ≈ 14B
Less susceptible to noise

37. Modulation

38. Digital Data/Analog Signal
Wireless
Public telephone system
300 Hz to 3400 Hz
Use modem
Amplitude shift keying
Frequency shift keying
Phase shift keying

39. Modulation

40. Modulation – ASK Amplitude shift keying
Values represented by different amplitudes of carrier
Presence and absence of carrier is used
OOK
Susceptible to sudden gain changes
Inefficient
Up to 1200 bps on voice grade lines
Used over optical fiber as IM

41. Modulation – FSK Frequency shift keying
Most common form is binary FSK or BFSK
Two binary values represented by two different frequencies near carrier
Less susceptible to error than ASK
Up to 1200 bps on voice grade lines
High frequency radio
Even higher frequency on LANs using co-ax

42. Modulation – FSK

43. Modulation – Multiple FSK
More than two frequencies used
More bandwidth efficient
More prone to error
Each signaling element represents more than one bit

44. Modulation – PSK Phase shift keying Binary PSK Differential PSK
Phase of carrier signal is shifted to represent data
Binary PSK
Two phases represent two binary digits
Differential PSK
Phase shifted relative to previous transmission rather than some reference signal

45. Modulation – Differential PSK

46. Modulation – Quadrature PSK
More efficient use by each signal element representing more than one bit
Shifts of /2 or 90o
Each element represents two bits
Offset QPSK
Orthogonal QPSK
Delay in Q stream

47. Modulation – OQPSK Modulator

48. Modulation – OQPSK Waveforms

49. Modulation – Multilevel PSK
Can use 8 phase angles or more and have more than one amplitude
Modulation rate D in baud
Data rate R in bps
D = R/log2M
For 8-ary PSK, 1 symbol represents 3 bits
M = 8
1 symbol/second = 1 baud = 3 bps

50. Modulation – QAM Quadrature amplitude modulation
Combination of ASK and PSK
Logical extension of QPSK
Used on asymmetric digital subscriber line or ADSL and some wireless
Send two different signals simultaneously on same carrier frequency
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output

51. Modulation – QAM Modulator

52. Modulation – QAM Levels
Two level ASK
Each of two streams in one of two states
Four state system
Essentially QPSK
Four level ASK
Combined stream in one of 16 states
64 and 256 state systems have been implemented
Improved data rate for given bandwidth
Increased potential error rate

53. Modulation – Constellations
Show amplitudes and phases of modulation
Similar to phasor
ASK
BPSK
QPSK
8-ary PSK
16-ary QAM
Minimize BER by maximizing the spacing between signals points

54. Modulation – Modems V.32 V.32 bis QAM with a 32-point constellation
4 data bits and 1 parity bit per symbol
2400 baud
9600 bps
V.32 bis
QAM with a 128-point constellation
6 data bits and 1 parity bit per symbol
14.4 kbps

55. Modulation – Modems V.34 V.34 bis
QAM with a 960-point constellation
28.8 kbps
V.34 bis
QAM with a 1664-point constellation
33.6 kbps
Shannon limit for the telephone system is about 35 kbps
V.90 use different technique

56. Modulation – Coherent Systems
Receiver must synchronize the phase of carrier in order to retrieve data
Non-coherent systems
Synchronization not required
Coherent systems are much more costly, but offer much better performance
Lower BER
ASK and FSK can be either one
PSK and QAM must be coherent since data are carried by the carrier phase

57. Modulation – Bandwidth
BWASK = (1+r)R
BWFSK = (1+r)R + ΔF
BWMPSK = (1+r)R/log2M
0 < r < 1
ΔF = f1 − f0
ASK and PSK bandwidth directly related to bit rate
FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies
In the presence of noise, bit error rate of PSK and QPSK are about 3 dB superior to ASK and FSK

58. Modulation – Performance
Comparison of data rate to bandwidth ratio
Bandwidth efficiency

59. Analog Data/Digital Signal
Digitization
Conversion of analog data into digital data
Digital data can then be transmitted using NRZ-L or other codes
Digital data can also be converted to analog signal
Analog to digital conversion done using a codec
Pulse code modulation
Delta modulation

60. Digitization

61. PCM Pulse code modulation 3 steps Sampling Quantizing Encoding
Discretize time
Quantizing
Discretize value
Encoding
Digitize value

62. PCM – Sampling Periodical sampling
Taking uniformly spaced samples from an analog signal waveform
Intuitively, if the sampling rate is high enough, or T is small enough, the information contained in the analog signal is well represented by its samples  T

63. PCM – Sampling How high is high enough Sampling theorem
Nyquist, 1928
An analog signal of bandwidth B Hz can be completely reconstructed from its samples by filtering if the sampling rate is larger than 2B samples per second
fS > 2B, T < 1/2B
2B is called Nyquist rate

64. PCM – Sampling
If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal
Voice data limited to below 4000 Hz
Require 8000 samples per second
Analog samples called PAM
Pulse amplitude modulation

65. PCM – Quantizing Each sample is assigned to discrete value
Quantizing resembles rounding, which rounds each sample to pre-specified discrete values dubbed quantum levels
Number of levels related to number of bits used in encoding
8-bit sample gives 256 levels

66. PCM – Quantizing Quantization error or noise
Approximations mean it is impossible to recover the exact original signal
The larger the step size, the worse the quantization noise, but the fewer the steps needed to cover a given input range
Bandwidth implications
Quality comparable with analog transmission

67. PCM – Quantizing

68. PCM – Encoding
After being quantized, each sample can take on only a finite number of quantum levels each can be regarded as a logical symbol
Assign bits for each quantized sample
An encoding scheme is used to translate each of these symbols to a binary codeword consisting of a number of binary digits
8000 samples per second of 8 bits each gives 64 kbps

69. PCM – Encoding

70. PCM – D/A Process 2 steps Decoding Filtering
Quantizing cannot be reversed and thus quantization noise has to be tolerated

71. PCM – Performance Good voice reproduction
128 levels PCM
7 bits/sample
Voice bandwidth 4 kHz
Should be 8000 × 7 = 56 kbps for PCM
Data compression can improve on this
Interframe coding techniques for video
ADPCM
Adaptive differential PCM
Used in digital TV

72. PCM – Nonlinear Encoding
Quantization levels not evenly spaced
Reduce overall signal distortion
Can also be done by companding

73. PCM – Nonlinear Encoding

74. PCM – Companding Channel Output signal Input signal Compress Expand SiSx Sy So Sy Sx Sy So Si Sx m m
Compressor
Expander
Channel

75. PCM – Companding
Each of these plots shows output signal strength versus input signal strength
Slope = gain
The compressor ensures that signal entering the channel within the linear operating range of the channel
The expander is the inverse function of the compressor and thus restores the original signal strength contrast

76. PCM – Companding

77. PCM – Companding Rules -law compressor A-law compressor
y = ln(1+ x)/ln(1+)
 is degree of compression
Used in North America telephone systems
Soft voices actually get amplified while strong voices are suppressed
A-law compressor
y = Ax/(1 + lnA) for 0  x  1/A
y = (1 + lnAx)/(1+ lnA) for 1/A  x  1
Used in Europe and followers

78. Delta Modulation Analog input is approximated by a staircase function
Move up or down one  level at each sample interval
Binary behavior
Function moves up or down at each sample interval

79. Delta Modulation – Example

80. Delta Modulation – Operation

81. Transmission Techniques – Conclusion
Various types of modulation and encoding
Many other variations not discussed
Understanding the basic properties