CODING.ppt
- 1. Linear Block and Cyclic Codes used for two error detection and single error correction -------------------------------- Anwar liwaa shakir MSC. Communication engineer Uok
- 2. Basic concepts Networks must be able to transfer data from one device to another with complete accuracy. Data can be corrupted during transmission. For reliable communication, errors must be detected and corrected. Error detection and correction are implemented either at the data link layer or the transport layer of the OSI model.
- 4. Error detection -Check if any error has occurred -Don't care the number of errors -Don't care the positions of errors Error correction -Need to know the number of errors -Need to know the positions of errors -More difficult
- 5. Single-bit error Multiple -bit error
- 6. Single bit errors are the least likely type of errors in serial data transmission because the noise must have a very short duration which is very rare. However this kind of errors can happen in parallel transmission. Example: If data is sent at 1Mbps then each bit lasts only 1/1,000,000 sec. or 1 μs. For a single-bit error to occur, the noise must have a duration of only 1 μs, which is very rare.
- 7. Burst error
- 8. The term burst error means that two or more bits in the data unit have changed from 1 to 0 or from 0 to 1. Burst errors does not necessarily mean that the errors occur in consecutive bits, the length of the burst is measured from the first corrupted bit to the last corrupted bit. Some bits in between may not have been corrupted.
- 9. Burst error is most likely to happen in serial transmission since the duration of noise is normally longer than the duration of a bit. The number of bits affected depends on the data rate and duration of noise. Example: If data is sent at rate = 1Kbps then a noise of 1/100 sec can affect 10 bits.(1/100*1000) If same data is sent at rate = 1Mbps then a noise of 1/100 sec can affect 10,000 bits.(1/100*106)
- 10. Error detection Error detection means to decide whether the received data is correct or not Error detection uses the concept of redundancy,
- 11. Redundancy
- 12. Parity check Linear block The simplest method available – it’s a linear ,systematic block code Single parity check Two dimensional parity check How to use parity methods? Parity generate-sender’s side Parity detect-receiver’s side
- 13. TWO-DIMENTIONAL PARITY CHECK (LRC) In two- dimensional parity check, a block of bits is divided into rows and redundant row of bits is added to the whole block. Even parity concept
- 14. Example: In transmitter : compute(m+n+1) parity bits
- 15. Drawbacks of 2D parity check Figure : Two-dimensional parity-check code
- 16. • 1 affect 3 ,2 affect 3,3 affect 4 ,4 affect 4 –so can not detect in this case • Disadvantage :too many check bits!! • Can not detect error, if bits in one data unit are changed and 2 bits in exactly the same position in another data unit is changed Figure Two-dimensional parity-check code
- 17. CYCLIC REDUNDANCY CHECK (CRC) • Most powerful of the redundancy checking • Parity is based on addition • CRC is based on binary division
- 18. Figure Division in CRC encoder
- 19. Figure Division in the CRC decoder for two cases
- 20. • In a cyclic code, • If s(x) ≠ 0, one or more bits is corrupted. • If s(x) = 0, either • a. No bit is corrupted. or • b. Some bits are corrupted, but the decoder failed to detect them.
- 21. ADVANTAGES OF CYCLIC CODES Cyclic codes have a very good performance in detecting single-bit errors, double errors, a n odd number of errors. They can easily be implemented in hardware and software. They are especially fast when implemented in hardware. This has made cyclic codes a good candidate for many networks
- 22. Thank you
Editor's Notes
- #11: Make sense of message. Make sense of message.