Error Detection - week 9

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  • Error Detection
    • Run-length limiting (RLL)
      • A 0 is added after a long run of 1's (for example after 5 1's)
        • Example: 0110111111100111110111111111100         Bit stuffed: 011011111{0}110011111{0}011111{0}11111{0}00
      • Limits errors occurring
    • The process of detecting errors caused by noise or interference during transmission
      • Error correction is then used in addition to reconstruct error-free data
        • Error detection and correction uses either parity bits (even/odd) or cyclic redundancy (CR)
    • Automatic repeat-request (ARQ)
      • The transmitter sends data with an error detection (ED) code to the receiver
        • The receiver either sends an acknowledgement (ACK) or requests for re-transmission if cyclic redundancy check (CRC) shows errors
          • If the transmitter doesn't receive an ACK after a certain period, data is re-transmitted
    • Forwards error correction (FEC)
      • The transmitter encodes data with an error-correction (EC) code and sends to the receiver
        • The receiver  decodes and unlike ARQ, doesn't send an ACK
    • Hybrid automatic repeat-request (HARQ)
      • This combines the two: ARQ and FEC. Minor errors are resolved with FEC and major errors are re-transmitted with ARQ


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