Digital (kỹ THUẬT TRUYỀN số LIỆU SLIDE)

Digital Transmission Outline  Line coding  Encoding considerations  DC components in signals  Synchronization  Various line coding methods Line Coding  Process of converting binary data to digital signal Signal Levels vs Data Levels  Number of signal levels  Number of different voltage levels allowed in a signal  Number of data levels  Number of voltage levels that actually represent data values Signal vs Data Elements Pulse Rate vs Bit Rate +3 00 11 01 10 01 10 11 00 +1 t -1 -3 One pulse (one signal element) BitRate = PulseRate × b = PulseRate × log2L BitRate = PulseRate × b = PulseRate × log2L b – number of bits per pulse L – number of different signal elements Bit rate → Bits per second Bit rate → Bits per second Pulse rate → Baud (pulses or signals per second) Pulse rate → Baud (pulses or signals per second) Pulse Rate vs Bit Rate  Example: In Manchester Encoding, if the bit rate is 10 Mbps, what is the pulse rate? 0 1 t One bit One pulse (one signal element) Encoding Considerations  Signal spectrum  Lack of DC components  Lack of high frequency components  Clocking/synchronization  Error detection  Noise immunity  Cost and complexity DC Components  DC components in signals are not desirable  Cannot pass thru certain devices  Leave extra (useless) energy on the line 0 1 Signal with t 0 1 DC component Signal without t DC component Synchronization  To correctly decode a signal, receiver and sender must agree on bit interval 0 1 Sender sends: 01001101 t 0 1 1 Receiver sees: 0100011011 t 10 Other Schemes  mBnL  m data elements are substituted with n signal elements  E.g., 2B1Q (two binary, quaternary) +3 00 11 01 10 01 10 11 00 Bit sequence Voltage level 00 -3 01 -1 10 +3 11 +1 +1 t -1 -3 21 Multilevel: 8B6T  Eight binary, six ternary 22 Block Coding  Improves the performance of line coding  Provides  Synchronization  Error detection Division …01011010001… Line Substitution : Coding 0010 10110 1101 01011 0001 01010 : t : : 23 4B/5B Encoding Table Data Code Data Code Data Code 0000 11110 1000 10010 Q (Quiet) 00000 0001 01001 1001 10011 I (Idle) 11111 0010 10100 1010 10110 H (Halt) 00100 0011 10101 1011 10111 J (start delimiter) 11000 0100 01010 1100 11010 K (start delimiter) 10001 0101 01011 1101 11011 T (end delimiter) 01101 0110 01110 1110 11100 S (Set) 11001 0111 01111 1111 11101 R (Reset) 00111 24 Analog to Digital Conversion  Pulse Amplitude Modulation (PAM)  Converts an analog signal into a series of pulses by sampling PAM Analog signal PAM signal (Sampled analog data) 25 Pulse Code Modulation (PCM)  Converts an analog signal into a digital signal  PAM  Quantization  Binary encoding  Line coding 26 PCM: Quantization  Converts continuous values of data to a finite number of discrete values Output 2 Input 27 PCM: Quantization Quantization 28 Quantization Error  Assume sine-wave input and uniform quantization  Known as the dB/bit approximation See also: http://en.wikipedia.org/wiki/Quantization_error#Quantization_noise_model 29 Example: Quantization Error  A telephone subscriber line must have an SNR above 40 What dB is the minimum number of bits per sample? Solution We can calculate the number of bits as Telephone companies usually assign or bits per sample 30 PCM: Binary Encoding  Maps discrete values to binary digits 31 PCM: The Whole Process 32 Minimum Sampling Rate  Nyquist Theorem: Sampling Samplingrate ratemust mustbe begreater greaterthan than twice twicethe thehighest highestfrequency frequency Ex Find the maximum sampling interval for recording human voice (freq range 300Hz – 3000Hz) t sampling interval 33 Nyquist’s Sampling Theorem See also: Wagon-wheel effect Demo: sampling.py 34 Sampling and Bit Rate  Ex Calculate the minimum bit rate for recording human voice, if each sample requires 60 levels of precision 35 ... Synchronization  Various line coding methods Line Coding  Process of converting binary data to digital signal Signal Levels vs Data Levels  Number of signal levels  Number of different voltage... Uses three voltage levels (+, 0, and –) for data bits  Multilevel 12 Unipolar  Simplest form of digital encoding Rarely used  Only one polarity of voltage is used  E.g., polarity assigned to... delimiter) 01101 0110 01110 1110 11100 S (Set) 11001 0111 01111 1111 11101 R (Reset) 00111 24 Analog to Digital Conversion  Pulse Amplitude Modulation (PAM)  Converts an analog signal into a series