Lecture 4

1. 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

2. 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

3. 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

4. Data • Analog • Continuous values within some interval • e.g. sound, video • Digital • Discrete values • e.g. text, integers ECS 152A Computer Networks

5. 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

6. 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

7. 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

8. Key Design Issues • Bandwidth • Timing information • Immunity to noise • Signal power • Error detection ECS 152A Computer Networks

9. 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

10. 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

11. 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

12. Analog Signals Carrying Analog and Digital Data ECS 152A Computer Networks

13. Encoding Digital Data into Analog Signals ECS 152A Computer Networks

14. 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

15. 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

16. 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

17. 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

18. Pulse Amplitude Modulation • The signal is sampled at a rate higher than twice the maximum frequency – Nyquist Sampling Theorem ECS 152A Computer Networks

19. Pulse Code Modulation • Pulse amplitude modulation • Quantization • Quantization error for n bit quantized sample • SNR(db) = 6.02n + 1.76bB ECS 152A Computer Networks

20. Nonlinear Encoding • In linear encoding the quantization levels are equally spaced • Lower amplitude values have higher errors • Companding (Compressing-expanding) ECS 152A Computer Networks

21. Delta Modulation ECS 152A Computer Networks