ECP2056 chapter 4 part A - ECP 2056 : DATACOMMUNICATIONS...

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FACULTY OF ENGINEERING, CYBERJAYA YEAR 2008 ECP 2056 : DATACOMMUNICATIONS AND COMPUTER NETWORKING Saiful Jumaat Osman, SUPELEC France  4.1) Error detection and correction CHAPTER 4 : Reliable Data Communications
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CHAPTER 4 : Reliable Data Communications Table Of Contents (1)  Error detection and correction (2)  Framing, flow and error control (3) Stop-and-Wait protocol, Automatic Repeat  (4)  HDLC
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Error Detection If  transmission  lines  are  in  electrically  noisy environment  such  as  PSTN  (Public  switched telephone network), the electrical signal representing the  transmitted  bit  stream  may  be  changed (corrupted) [affected by EMI, distortion, thermal noise etc] Altered signal(s) incorrectly interpreted at receiver -transmitted ‘1’ received as a ‘0’ and vice versa Error detection uses the concept of redundancy, which means  adding  extra  bits  for  detecting  errors  and  its destination CHAPTER 4 : Reliable Data Communications Error detection and correction
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CHAPTER 4 : Reliable Data Communications Error detection and correction Error Detection Error Detection
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Error Detection There are 2 ways to alleviate the problem a) Forward error control – receiver has the ability to  detect and correct errors b) Feedback (backward) error control – the receiver  only able to detect errors since it does not know the  exact location of the errors; hence a retransmission  control scheme is used to request that another copy  of the information to be sent CHAPTER 4 : Reliable Data Communications Error detection and correction
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Error Detection – Forward Error  Control Forward Error Control (FEC) - transmitted data contains additional redundant data  (in  addition to the information bits)  for receiver - receiver can detect when corruption is present - determine which bits are corrupt - correct corruption – inverting incorrect bits  - number of bits required for FEC increases rapidly as  frame size increases  - high overhead, used only if retransmission is  impractical, such as simplex transmission, long delay,  high error rate CHAPTER 4 : Reliable Data Communications Error detection and correction
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Error Detection – Feedback Error  Control Feedback (Backward) Error Control (FBC) - transmitted data contains redundant data for  receiver - receiver can detect when corruption is  present - retransmission control scheme used to  request another - for low error rate in data communication CHAPTER 4 : Reliable Data Communications Error detection and correction
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Error Detection Disadvantages of FEC i The number of additional bits required to achieve  reliable forward error control increase rapidly with the  number of information bits
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ECP2056 chapter 4 part A - ECP 2056 : DATACOMMUNICATIONS...

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