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Lecture 4. Review of Lecture 3 Transmission medium Signal encoding techniques Digital data into digital signals Digital data into analog signals Analog data into digital signals . Line Encoding. Encoding data into signals Digital data into digital signal

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lecture 4
Lecture 4
  • Review of Lecture 3
  • Transmission medium
  • Signal encoding techniques
    • Digital data into digital signals
    • Digital data into analog signals
    • Analog data into digital signals

ECS 152A Computer Networks

line encoding
Line Encoding
  • Encoding data into signals
    • Digital data into digital signal
    • Analog data into digital signal
    • Digital data into analog signal
    • Analog data into analog signal

ECS 152A Computer Networks

analog and digital data transmission
Analog and Digital Data Transmission
  • Data
    • Entities that convey meaning
  • Signals
    • Electric or electromagnetic representations of data
  • Transmission
    • Communication of data by propagation and processing of signals

ECS 152A Computer Networks

slide4
Data
  • Analog
    • Continuous values within some interval
    • e.g. sound, video
  • Digital
    • Discrete values
    • e.g. text, integers

ECS 152A Computer Networks

signals
Signals
  • Means by which data are propagated
  • Analog
    • Continuously variable
    • Various media
      • wire, fiber optic, space
    • Speech bandwidth 100Hz to 7kHz
    • Telephone bandwidth 300Hz to 3400Hz
    • Video bandwidth 4MHz
  • Digital
    • Use two DC components

ECS 152A Computer Networks

data and signals
Data and Signals
  • Usually use digital signals for digital data and analog signals for analog data
  • Can use analog signal to carry digital data
    • Modem
  • Can use digital signal to carry analog data
    • Compact Disc audio

ECS 152A Computer Networks

encoding digital data into digital signal

0

1

0

1

1

1

1

0

0

Unipolar

NRZ

Polar NRZ

NRZ-Inverted

(Differential

Encoding)

Bipolar

Encoding

Manchester

Encoding

Differential

Manchester

Encoding

Encoding digital data into digital signal

ECS 152A Computer Networks

key design issues
Key Design Issues
  • Bandwidth
  • Timing information
  • Immunity to noise
  • Signal power
  • Error detection

ECS 152A Computer Networks

power spectrum of different encoding schemes
Power spectrum of different encoding schemes
  • NRZ has spectrum that is concentrated at the lower frequencies
  • Bipolar codes are designed for bandpass channels
  • Manchester encoding requires higher bandwidth

ECS 152A Computer Networks

differential encoding
Differential Encoding
  • Detecting transitions is more immune to noise
  • Differential encoding
    • The signal level or the transition depends on the preceding sequence of bits
    • Error occur in pairs

ECS 152A Computer Networks

bit error rate and eb no
Bit Error Rate and Eb/No
  • Different encoding schemes need different amount signal power for a given probability of bit error.

ECS 152A Computer Networks

modulating a signal

1

0

1

1

0

1

(a) Information

+A

+A

(b) Baseband

Signal Xi(t)

(c) Modulated

Signal Yi(t)

t

6T

2T

4T

5T

T

3T

0

6T

2T

4T

5T

3T

T

0

-A

-A

t

6T

2T

4T

5T

3T

T

0

+2A

(d) 2Yi(t) cos(2fct)

t

-2A

Modulating a signal

ECS 152A Computer Networks

multilevel psk

Bk

Bk

Ak

Ak

16 “levels”/ pulse

4 bits / pulse

4W bits per second

4 “levels”/ pulse

2 bits / pulse

2W bits per second

Multilevel PSK

ECS 152A Computer Networks

other constellation

Bk

2-D signal

2-D signal

Ak

Bk

Ak

16 “levels”/ pulse

4 bits / pulse

4W bits per second

4 “levels”/ pulse

2 bits / pulse

2W bits per second

Other constellation

ECS 152A Computer Networks

analog data to digital signals
Analog Data to Digital Signals
  • Analog data is digitized to digital data – codec (coder/decoder)
  • Digital data can be encoded to digital signals
  • Digital data can also be encoded into analog signal

ECS 152A Computer Networks

pulse amplitude modulation
Pulse Amplitude Modulation
  • The signal is sampled at a rate higher than twice the maximum frequency – Nyquist Sampling Theorem

ECS 152A Computer Networks

pulse code modulation
Pulse Code Modulation
  • Pulse amplitude modulation
  • Quantization
    • Quantization error for n bit quantized sample
    • SNR(db) = 6.02n + 1.76bB

ECS 152A Computer Networks

nonlinear encoding
Nonlinear Encoding
  • In linear encoding the quantization levels are equally spaced
  • Lower amplitude values have higher errors
  • Companding (Compressing-expanding)

ECS 152A Computer Networks

delta modulation
Delta Modulation

ECS 152A Computer Networks