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all about how data is error is detected and corrected through transmission

all about how data is error is detected and corrected through transmission

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  • 1. Digital Data Communications Techniques
  • 2. Transmission Mode • How many bits can be sent in one clock cycle? • Serial mode —Only one bit is sent in one clock cycle • Parallel mode —n bits are sent in one clock cycle —Uses n wires to send n bits
  • 3. Parallel Transmission
  • 4. Serial Transmission • Used for long distance transmission. Why? • Two types: Asynchronous and synchronous • Asynchronous • One byte of data sent serially • Start bit = 0, Stop bit=1  In total, ten bits are sent • Next byte may not follow previous byte immediately – i.e. there may be idle time between bytes • It is asynchronous only at the byte level. Individual bits are still synchronized.
  • 5. Asynchronous Serial
  • 6. Asynchronous … • Data transmitted on character at a time —5 to 8 bits • Timing only needs maintaining within each character • Resynchronize with each character
  • 7. Asynchronous (diagram)
  • 8. Asynchronous - Behavior • In a steady stream, interval between characters is uniform (length of stop element) • In idle state, receiver looks for transition 1 to 0 • Then samples next seven intervals (char length) • Then looks for next 1 to 0 for next char • Simple • Cheap • Overhead of 2 or 3 bits per char (~20%) • Good for data with large gaps (keyboard)
  • 9. Synchronous Serial • Bits are sent one after another without gap —No start and stop bits • Receiver must organize bits into bytes • Synchronous sends more data bits than asynchronous • Synchronous: used for high speed transmission • Asynchronous: for low speed transmission
  • 10. Synchronous
  • 11. Types of Error • An error occurs when a bit is altered between transmission and reception • Single bit errors —One bit altered —Adjacent bits not affected —White noise • Burst errors —Length B —Contiguous sequence of B bits in which first last and any number of intermediate bits in error —Impulse noise —Fading in wireless —Effect greater at higher data rates
  • 12. Error Detection Process
  • 13. Error Detection • Additional bits added by transmitter for error detection code • Parity —Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones —Even number of bit errors goes undetected
  • 14. Cyclic Redundancy Check • For a block of k bits transmitter generates n bit sequence • Transmit k+n bits which is exactly divisible by some number • Receiver divides frame by that number —If no remainder, assume no error
  • 15. Error Correction • Correction of detected errors usually requires data block to be retransmitted • Not appropriate for wireless applications —Bit error rate is high • Lots of retransmissions —Propagation delay can be long (satellite) compared with frame transmission time • Would result in retransmission of frame in error plus many subsequent frames • Need to correct errors on basis of bits received
  • 16. Error Correction Process Diagram
  • 17. The End