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Computer Networks - Error Detection & Error Correction

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Error Detection & Error Correction

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Computer Networks - Error Detection & Error Correction

  1. 1. Error Detection & Error Correction Computer Networks
  2. 2. Team Members Group-I Assignment Topic : Error Detection & Error Correction Group's representative: TANGUTURU SAI KRISHNA S.No. BITS ID NAME Official Email ID Personal Email ID 1 2011HW69898 TANGUTURU SAI KRISHNA saikrishna.tanguturu@wipro.com sai.tsk2008@gmail.com 2 2011HW69900 RAYAPU MOSES rayapu.moses@wipro.com stalinkvd001@gmail.com 3 2011HW69932 SHENBAGAMOORTHY A shenbagamoorthy.a83@wipro.com moorthy2626@gmail.com 4 2011HW69913 ANURUPA K C anurupa.c85@wipro.com anu.rupa30@gmail.com 5 2011HW69909 ARUNJUNAISELVAM P arunjunaiselvam.p95@wipro.com arunjunai.carrer@gmail.com 6 2011HW69569 PRANOB JYOTI KALITA pranob.kalita@wipro.com pranob.kalita90@gmail.com 7 2011HW69893 TINNALURI V N PRASANTH prasanth.tinnaluri@wipro.com naga.prasanth985@gmail.com 8 2011HW69904 KONDALA SUMATHI sumathi.kondala@wipro.com sumathi.kondala@gmail.com 9 2011HW69896 DASIKA KRISHNA dasika.krishna@wipro.com dasikakrishnas@gmail.com 10 2011HW69907 SHEIK SANAVULLA sheik.sanavulla@wipro.com sanavulla.sms@gmail.com 11 2011HW70163 K ASWINI PRIYANKA kamma.priyanka@wipro.com ashupriya.priyanka6@gmail.com 12 2011HW70828 BODDU MADHAVI REDDY boddu.reddy@wipro.com mreddy786@gmail.com
  3. 3. 3 Data Link Layer
  4. 4. Definition of Error  Networks must be able to transform data from once device to another with complete accuracy. While the transmission data can be corrupted, for reliable communication errors must be detected and corrected.
  5. 5. 5 Types of Errors  Single-bit errors  Burst errors
  6. 6. 6 Redundancy  To detect or correct errors, redundant bits of data must be added
  7. 7. Detection/Correction Techniques  Parity Checks  Checksumming methods  Cyclic redundancy checks
  8. 8. Parity Checks Parity Bit (PB) One additional bit per character Even parity Odd Parity
  9. 9. How many bit errors can PB detect ? 10001110 --- 10101110 => error ! 10001110 --- 10100110 => No error detected !!! Conclusion – 1 PB can only detect an odd number of errors !
  10. 10. Single Bit Error Correction Parity for each character(byte=line) + parity for each column (set of data bytes sent)
  11. 11. Example - Single Bit Error Correction Hamming - Correctable single bit error
  12. 12. 12 Cyclic Redundancy Checksum (CRC) •CRC error detection method treats packet of data to be transmitted as a large polynomial •Transmitter •Using polynomial arithmetic, divides polynomial by a given generating polynomial •Quotient is discarded •Remainder is “attached” to the end of message
  13. 13. 13 Cyclic Redundancy Checksum (continued) •Message (with the remainder) is transmitted to the receiver •Receiver divides the message and remainder by same generating polynomial •If a remainder not equal to zero results  error during transmission •If a remainder of zero results  error during transmission
  14. 14. 14 CRC Encoder/Decoder
  15. 15. CRC - Example Frame – 1101011011 G(x)=x4+x+1 Transmitted frame: 11010110110000 – 00000000001110 ---------------------- 11010110111110
  16. 16. Checksums  • A checksum “adds” together “chunks” of data  – The “add” operation may not be normal integer addition  – The chunk size is typically 8, 16, or 32 bits  • We’ll discuss:  – Integer addition “checksum”  – One’s complement “checksum”  – Fletcher Checksum  – Adler Checksum  – ATN Checksum (AN/466)
  17. 17. 17 Error Correction  Two methods  Retransmission after detecting error  Forward error correction (FEC)
  18. 18. 18 Forward Error Correction  Consider only a single-bit error in k bits of data  k possibilities for an error  One possibility for no error  #possibilities = k + 1  Add r redundant bits to distinguish these possibilities; we need 2r  k+1  But the r bits are also transmitted along with data; hence 2r  k+r+1
  19. 19. 19 Number of Redundant Bits Number of data bits k Number of redundancy bits r Total bits k + r 1 2 3 2 3 5 3 3 6 4 3 7 5 4 9 6 4 10 7 4 11
  20. 20. 20 Hamming Code  Simple, powerful FEC  Widely used in computer memory  Known as ECC memory error-correcting bits
  21. 21. 21 Redundant Bit Calculation
  22. 22. 22 Example: Hamming Code
  23. 23. Thank You Saikrishna Tanguturu

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