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MODULE 7

MODULE 7. ERROR DETECTION. TRANSMISSION ERRORS. Transmission errors are caused by: thermal noise {Shannon} impulse noise (e..g, arcing relays) signal distortion during transmission (attenuation) crosstalk voice amplitude signal compression (companding) quantization noise (PCM)

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MODULE 7

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  1. MODULE 7 ERROR DETECTION PREPARED BY: ENGR. JO-ANN C. VIÑAS

  2. TRANSMISSION ERRORS • Transmission errors are caused by: • thermal noise {Shannon} • impulse noise (e..g, arcing relays) • signal distortion during transmission (attenuation) • crosstalk • voice amplitude signal compression (companding) • quantization noise (PCM) • jitter (variations in signal timings) • receiver and transmitter out of synch PREPARED BY: ENGR. JO-ANN C. VIÑAS

  3. OBJECTIVES: 1. Introduce Data Communications 2. Define Data Communication Codes 3. Discuss the Types of Data Transmission and Types of Errors 4. Explain Error Detection Schemes PREPARED BY: ENGR. JO-ANN C. VIÑAS

  4. DATA COMMUNICATIONS - It is the process of transferring digital information (usually binary form) between two or more points PREPARED BY: ENGR. JO-ANN C. VIÑAS

  5. FORMS OF DATA a. Alphabetical information b. Numeric information c. Symbolic information PREPARED BY: ENGR. JO-ANN C. VIÑAS

  6. DATA CODE - is a set of rules that translates alphanumeric characters into binary numbers - also called character codes, character sets, character languages or symbol codes PREPARED BY: ENGR. JO-ANN C. VIÑAS

  7. DATA COMMUNICATION CODES 1. Baudot Code - 1st fixed-length character - developed for machine rather than people - 5-bit character code primarily used for low- speed teletype system equipment PREPARED BY: ENGR. JO-ANN C. VIÑAS

  8. DATA COMMUNICATION CODES 2. ASCII Code (American Standard for Information Interchange) - 7-bit fixed length character set - is the standard character set for source coding the alphanumeric character set that humans understand but computers do not. PREPARED BY: ENGR. JO-ANN C. VIÑAS

  9. DATA COMMUNICATION CODES 3. EBCDIC Code (Extended Binary-Coded Decimal Interchange Code) - 8-bit fixed length character set developed in 1962 by IBM - used almost exclusively with IBM mainframe computers and peripheral equipment PREPARED BY: ENGR. JO-ANN C. VIÑAS

  10. DATA COMMUNICATION CODES 4. Bar Codes - is a series of vertical black bars separated by vertical white bars (called spaces) PREPARED BY: ENGR. JO-ANN C. VIÑAS

  11. SERIAL and PARALLEL DATA TRANSMISSION 1. Parallel by bit or Serial by character 2. Serial by bit PREPARED BY: ENGR. JO-ANN C. VIÑAS

  12. SERIAL DATA TRANSMISSION PREPARED BY: ENGR. JO-ANN C. VIÑAS

  13. PARALLEL DATA TRANSMISSION PREPARED BY: ENGR. JO-ANN C. VIÑAS

  14. TYPES OR ERRORS 1. Single bit error - 1 bit of error has occurred 2. Burst error - 2 or more bits in the data unit have changed from 1 to 0 or from 0 to 1 PREPARED BY: ENGR. JO-ANN C. VIÑAS

  15. SINGLE BIT ERROR PREPARED BY: ENGR. JO-ANN C. VIÑAS

  16. BURST ERROR PREPARED BY: ENGR. JO-ANN C. VIÑAS

  17. “Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.” PREPARED BY: ENGR. JO-ANN C. VIÑAS

  18. ERROR DETECTION - Is the process of monitoring data transmission and determining when errors have occurred. Purpose: “ Not to prevent error from occurring but to prevent UNDETECTED ERROR.” PREPARED BY: ENGR. JO-ANN C. VIÑAS

  19. GENERAL ERROR DETECTION SYSTEM PREPARED BY: ENGR. JO-ANN C. VIÑAS

  20. ERROR DETECTION METHODS 1. REDUNDANCY - Transmitting each character twice 2. EXACT COUNT ENCODING - The number of 1’s in each character is the same PREPARED BY: ENGR. JO-ANN C. VIÑAS

  21. REDUNDANCY PREPARED BY: ENGR. JO-ANN C. VIÑAS

  22. PARITY BIT - Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity) PREPARED BY: ENGR. JO-ANN C. VIÑAS

  23. EVEN PARITY CONCEPT PREPARED BY: ENGR. JO-ANN C. VIÑAS

  24. “Simple parity check can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd.” PREPARED BY: ENGR. JO-ANN C. VIÑAS

  25. ERROR DETECTION METHODS 3. PARITY CHECKING - adds 1 additional bit to each byte in the message A) Odd Parity B) Even Parity PREPARED BY: ENGR. JO-ANN C. VIÑAS

  26. PARITY GENERATORS I. Serial Transmission Parity Generator II. Parallel Transmission Parity Generator PREPARED BY: ENGR. JO-ANN C. VIÑAS

  27. SERIAL TRANSMISSION PARITY GENERATOR PREPARED BY: ENGR. JO-ANN C. VIÑAS

  28. PARALLEL TRANSMISSION PARITY GENERATOR PREPARED BY: ENGR. JO-ANN C. VIÑAS

  29. PARITY CHECKING Advantage: 1. Simple Disadvantages: 1. If even number of errors has occurred it can not be detected 2. 50% efficiency PREPARED BY: ENGR. JO-ANN C. VIÑAS

  30. ERROR DETECTION METHODS 4. VRC/LRC A) LRC - message parity B) VRC - character parity PREPARED BY: ENGR. JO-ANN C. VIÑAS

  31. VERTICAL REDUNDANCY CHECKING VRC entails the appending of a parity bit at the end of each transmitted character of value to create an odd or even total mathematical bit value. PREPARED BY: ENGR. JO-ANN C. VIÑAS

  32. LONGITUDINAL REDUNDANCY CHECKING OR BLOCK CHECKING CHARACTER LRC adds another level of reliability, as data is viewed in a block or data set, as though the receiving device were viewing data set in a matrix format. LRC adds a significant measure of reliability. Also known as checksum, the LRC is sent as an extra character at the end of each data block. PREPARED BY: ENGR. JO-ANN C. VIÑAS

  33. VRC/LRC Advantage: 1. Simple 2. 98% reliability PREPARED BY: ENGR. JO-ANN C. VIÑAS

  34. EXAMPLE Determine the VRC and LRC for the message “SANTINO”. Use ASCII Character, and also use odd parity for VRC and even parity for LRC. Determine What ASCII character is to be transmitted for checking the message. PREPARED BY: ENGR. JO-ANN C. VIÑAS

  35. “In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is added to the whole block.” PREPARED BY: ENGR. JO-ANN C. VIÑAS

  36. ERROR DETECTION METHODS 5. CRC - Is generally used with 8-bit codes such as EBCDIC. - CRC 16: most common used CRC code in US and is identical to CCITT V.41 PREPARED BY: ENGR. JO-ANN C. VIÑAS

  37. CRC GENERATING CIRCUIT PREPARED BY: ENGR. JO-ANN C. VIÑAS

  38. CRC GENERATOR PREPARED BY: ENGR. JO-ANN C. VIÑAS

  39. ERROR DETECTION METHODS 5. CRC - CRC Character is the remainder of a division process. G(x) - data message P(x) - generator polynomial function PREPARED BY: ENGR. JO-ANN C. VIÑAS

  40. ERROR DETECTION METHODS CRC 16 P(x) = x16 + x12 + x5 + x0 Advantage: 1. 99.95% efficiency PREPARED BY: ENGR. JO-ANN C. VIÑAS

  41. CRC ALGORITHM 1. Multiply G(x) by xn-k (puts zeros in (n-k) low order positions) 2. Divide xn-k G(x) by P(x) 3. Add remainder B(x) to xn-k G(x) (puts check bits in the n-k low order positions): PREPARED BY: ENGR. JO-ANN C. VIÑAS

  42. CRC CHECKING 1. Add the CRC to the end of G(x) 2. Divide the product obtained in Step 1 by P(x) “The remainder of the process should be equal to 0, otherwise error has occurred.” PREPARED BY: ENGR. JO-ANN C. VIÑAS

  43. EXAMPLE Determine the BCS for the ff data and CRC generating polynomials. G(x) = x7 + x5 + x4 + x2 + x1 + x0 P(x) = x5 + x4 + x1 + x0 PREPARED BY: ENGR. JO-ANN C. VIÑAS

  44. MOST COMMONLY USED CYCLICCODES GENERATOR POLYNOMIAL CRC-12 : x12 + X11 + X3 + X2 + X + 1 CRC-16: x16 + x 15 + x2 + 1 CRC-CCITT: x16 + x12 + x5 + 1 PREPARED BY: ENGR. JO-ANN C. VIÑAS

  45. DATA UNIT AND CHECKSUM PREPARED BY: ENGR. JO-ANN C. VIÑAS

  46. 6. CHECKSUM PREPARED BY: ENGR. JO-ANN C. VIÑAS

  47. CHECKSUM ALGORITHM AT THE SENDER The sender follows these steps: The unit is divided into k sections, each of n bits. All sections are added using one’s complement to get the sum. The sum is complemented and becomes the checksum. The checksum is sent with the data. PREPARED BY: ENGR. JO-ANN C. VIÑAS

  48. CHECKSUM ALGORITHM AT THE RECEIVER The receiver follows these steps: The unit is divided into k sections, each of n bits. All sections are added using one’s complement to get the sum. The sum is complemented. If the result is zero, the data are accepted: otherwise, rejected. PREPARED BY: ENGR. JO-ANN C. VIÑAS

  49. EXAMPLE Suppose the following block of 16 bits is to be sent using a checksum of 8 bits. 10101001 00111001 Determine the checksum. Prove that the receiver can detect the error. PREPARED BY: ENGR. JO-ANN C. VIÑAS

  50. EXAMPLE Now suppose there is a burst error of length 5 that affects 4 bits. 10101111 11111001 00011101 Prove that the receiver can detect the error. PREPARED BY: ENGR. JO-ANN C. VIÑAS

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