4 signal encodingtechniques

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4 signal encodingtechniques

  1. 1. Data Communication - Signal Encoding Techniques
  2. 2. Encoding Techniques <ul><li>Digital data, digital signal </li></ul><ul><li>Analog data, digital signal </li></ul><ul><li>Digital data, analog signal </li></ul><ul><li>Analog data, analog signal </li></ul>
  3. 3. Terms <ul><li>Digital signaling </li></ul><ul><ul><li>Data source (digital/analog) encoded into digital signal </li></ul></ul><ul><li>Analog signaling </li></ul><ul><ul><li>Data source (digital/analog) encoded into constant-frequency signal (carrier signal) </li></ul></ul><ul><li>Modulation </li></ul><ul><ul><li>Process of encoding source data onto a carrier signal with frequency f c </li></ul></ul><ul><li>Modulating signal/ baseband signal </li></ul><ul><ul><li>Input signal (analog/digital)  modulator </li></ul></ul><ul><li>Modulated signal/ bandlimited(bandpass) signal </li></ul><ul><ul><li>Result of modulating the carrier signal </li></ul></ul>
  4. 4. Digital Data, Digital Signal <ul><li>In general, the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment. </li></ul><ul><li>Digital signal </li></ul><ul><ul><li>Discrete, discontinuous voltage pulses </li></ul></ul><ul><ul><li>Each pulse is a signal element </li></ul></ul><ul><ul><li>Binary data encoded into signal elements </li></ul></ul>
  5. 5. Types of Digital to Digital Encoding
  6. 6. Types of Polar Encoding <ul><li>Nonreturn to Zero-Level (NRZ-L) </li></ul><ul><li>Nonreturn to Zero Inverted (NRZI) </li></ul><ul><li>Bipolar –AMI -Altenate Mark Inversion </li></ul><ul><li>Pseudoternary </li></ul><ul><li>Manchester </li></ul><ul><li>Differential Manchester </li></ul><ul><li>B8ZS (Commonly used in North America) </li></ul><ul><li>HDB3 (Commonly used in Europe and Japan) </li></ul>
  7. 7. Types of Bipolar Encoding PSEUDOTERNARY
  8. 8. Definition of Digital Signal Encoding Format <ul><li>i. Nonreturn to Zero-Level(NRZ-L) </li></ul><ul><li>0 = high level </li></ul><ul><li>1 = low level </li></ul><ul><li>ii. Nonreturn to Zero Inverted (NRZ-I) </li></ul><ul><li>0 = no transition at beginning of interval </li></ul><ul><li>1 = transition at beginning of interval </li></ul><ul><li>iii. Bipolar –AMI </li></ul><ul><li>0 = no signal </li></ul><ul><li>1 = +ve or –ve, alternating for successive ones </li></ul><ul><li>iv. Pseudoternary </li></ul><ul><li>0 = +ve or –ve level, alternating for successive zeros </li></ul><ul><li>1 = no line signal </li></ul><ul><li>v. Manchester </li></ul><ul><li>0 = transition from high to low in middle of interval </li></ul><ul><li>1 = transition from low to high in middle of interval </li></ul>
  9. 9. Terms (1) <ul><li>Unipolar </li></ul><ul><ul><li>All signal elements have same sign </li></ul></ul><ul><ul><li>(positive and negative) </li></ul></ul><ul><li>Polar </li></ul><ul><ul><li>One logic state represented by positive voltage the other by negative voltage </li></ul></ul><ul><li>Data rate </li></ul><ul><ul><li>Rate of data transmission in bits per second </li></ul></ul><ul><li>Duration or length of a bit </li></ul><ul><ul><li>Time taken for transmitter to emit the bit </li></ul></ul>
  10. 10. Terms (2) <ul><li>Modulation rate </li></ul><ul><ul><li>Rate at which the signal level changes </li></ul></ul><ul><ul><li>Measured in baud = signal elements per second </li></ul></ul><ul><li>Mark and Space </li></ul><ul><ul><li>Binary 1 and Binary 0 respectively </li></ul></ul>
  11. 11. Digital Data, Analog Signal <ul><li>Some transmission media, such as optical fiber and unguided media , will only propagate analog signals. </li></ul><ul><li>Public telephone system </li></ul><ul><ul><li>300Hz to 3400Hz (voice frequency range) </li></ul></ul><ul><ul><li>Use modem (modulator-demodulator) </li></ul></ul><ul><li>Three basic encoding (modulation techniques) for transforming digital data into analog signals: </li></ul><ul><ul><li>Amplitude shift keying (ASK) </li></ul></ul><ul><ul><li>Frequency shift keying (FSK) </li></ul></ul><ul><ul><li>Phase shift keying (PSK) </li></ul></ul>
  12. 12. Modulation Techniques
  13. 13. Amplitude Shift Keying (ASK) <ul><li>Values represented by different amplitudes of carrier </li></ul><ul><li>Usually, one amplitude is zero </li></ul><ul><ul><li>i.e. presence and absence of carrier is used </li></ul></ul><ul><li>Susceptible to sudden gain changes </li></ul><ul><li>Inefficient </li></ul><ul><li>Up to 1200bps on voice grade lines </li></ul><ul><li>Used over optical fiber </li></ul>ASK
  14. 14. Binary Frequency Shift Keying (BFSK) <ul><li>Most common form is binary FSK (BFSK) </li></ul><ul><li>Two binary values represented by two different frequencies (near carrier) </li></ul><ul><li>Less susceptible to error than ASK </li></ul><ul><li>Up to 1200bps on voice grade lines </li></ul><ul><li>High frequency-radio (3 – 30 MHz) </li></ul><ul><li>Even higher frequency on LANs using coaxial cable </li></ul>BFSK
  15. 15. Phase Shift Keying (PSK) <ul><li>Phase of carrier signal is shifted to represent data </li></ul><ul><li>Binary PSK (Two-level PSK) </li></ul><ul><ul><li>Two phases represent two binary digits </li></ul></ul><ul><li>An alternative form of Two-Level PSK </li></ul><ul><ul><li>Differential PSK </li></ul></ul><ul><ul><li>Phase shifted relative to previous transmission rather than some reference signal </li></ul></ul>BPSK
  16. 16. Differential PSK 0 = signal burst of the same phase as previous signal burst 1 = signal burst of the opposite phase as previous signal burst
  17. 17. Analog Data, Digital Signal <ul><li>Analog data, such as voice and video, are often digitized to be able to use digital transmission facilities. </li></ul><ul><li>Digitization </li></ul><ul><ul><li>Conversion of analog data into digital data </li></ul></ul><ul><ul><li>Digital data can then be transmitted using NRZ-L (coding scheme) </li></ul></ul><ul><ul><li>Digital data can then be transmitted using code other than </li></ul></ul><ul><ul><li>NRZ-L </li></ul></ul><ul><ul><li>Digital data can then be converted to analog signal </li></ul></ul><ul><ul><li>Analog to digital conversion done using a codec </li></ul></ul><ul><ul><li>Two principal techniques used in codec: </li></ul></ul><ul><ul><ul><li>Pulse code modulation (PCM) </li></ul></ul></ul><ul><ul><ul><li>Delta modulation (DM) </li></ul></ul></ul>
  18. 18. Digitizing Analog Data
  19. 19. Pulse Code Modulation(PCM) (1) <ul><li>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 </li></ul><ul><li>Voice data limited to below 4000Hz </li></ul><ul><li>Require 8000 sample per second </li></ul><ul><li>Analog samples (Pulse Amplitude Modulation, PAM) </li></ul><ul><li>Each sample assigned digital value </li></ul>
  20. 20. Pulse Code Modulation(PCM) (2) <ul><li>4 bit system gives 16 levels </li></ul><ul><li>Quantized: Quantizing error or noise </li></ul><ul><ul><li>Approximations mean it is impossible to recover original exactly </li></ul></ul><ul><li>8 bit sample gives 256 levels </li></ul><ul><li>Quality comparable with analog transmission </li></ul><ul><li>8000 samples per second of 8 bits each gives 64kbps </li></ul><ul><ul><li>8000 samples per second X 8 bits per sample </li></ul></ul><ul><ul><li>= 64 kbps </li></ul></ul>PAM = pulse amplitude modulation PAM Value 1.1 9.2 15.2 10.8 5.6 2.8 2.7 Quantized code # 1 9 15 10 5 2 2 PCM code 0001 1001 1111 1010 0101 0010 0010
  21. 21.
  22. 22. Analog Data, Analog Signals <ul><li>Analog data are modulated by a carrier frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission system. </li></ul><ul><li>Why modulate analog signals? </li></ul><ul><ul><li>Higher frequency can give more efficient transmission </li></ul></ul><ul><ul><li>Permits frequency division multiplexing </li></ul></ul><ul><li>Types of modulation </li></ul><ul><ul><li>Amplitude (AM) </li></ul></ul><ul><ul><li>Frequency (FM) </li></ul></ul><ul><ul><li>Phase (PM) </li></ul></ul>
  23. 23. Analog Modulation STARTING
  24. 24. Key Point <ul><li>Synchronization </li></ul><ul><li>The receiver must know the rate at which bits are being received so that it can sample the line at appropriate intervals to determine the value of each received bit. </li></ul>

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