Chapter 6: Errors, Error Detection, and Error Control - PowerPoint PPT Presentation

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Chapter 6: Errors, Error Detection, and Error Control
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Chapter 6: Errors, Error Detection, and Error Control

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  1. Chapter 6: Errors, Error Detection, and Error Control

  2. Objectives • After reading this chapter, you should be able to: • Identify the different types of noise commonly found in computer networks • Specify the different error-prevention techniques, and be able to apply an error-prevention technique to a type of noise • Compare the different error-detection techniques in terms of efficiency and efficacy

  3. Objectives (continued) • Perform simple parity and longitudinal parity calculations, and enumerate their strengths and weaknesses • Cite the advantages of cyclic redundancy checksum, and specify what types of errors cyclic redundancy checksum will detect • Differentiate between the three basic forms of error control, and describe the circumstances under which each may be used

  4. Objectives (continued) • Follow an example of Stop-and-wait ARQ, Go-back-N ARQ, and Selective-reject ARQ

  5. Introduction • Noise is always present • If a communications line experiences too much noise • Signal will be lost or corrupted • Communication systems should check for transmission errors • Once an error is detected, a system may perform some action • Some systems perform no error control, but simply let the data in error be discarded

  6. Noise and Errors – White Noise • Also known as thermal or Gaussian noise • Relatively constant • Can be reduced • If white noise gets to strong • Can completely disrupt signal

  7. White Noise (continued)

  8. Impulse Noise • One of the most disruptive forms of noise • Random spikes of power • Can destroy one or more bits of information • Difficult to remove from an analog signal • May be hard to distinguish from original signal • Impulse noise can damage more bits if the bits are closer together (transmitted at a faster rate)

  9. Impulse Noise (continued)

  10. Impulse Noise (continued)

  11. Crosstalk • Unwanted coupling between two different signal paths • For example, hearing another conversation while talking on the telephone • Relatively constant • Can be reduced with proper measures

  12. Crosstalk (continued)

  13. Echo • The reflective feedback of a transmitted signal as the signal moves through a medium • Most often occurs on coaxial cable • If echo bad enough, it could interfere with original signal • Relatively constant • Can be significantly reduced

  14. Echo (continued)

  15. Jitter • The result of small timing irregularities during transmission of digital signals • Occurs when a digital signal is repeated over and over • If serious enough, jitter forces systems to slow down their transmission • Steps can be taken to reduce jitter

  16. Jitter (continued)

  17. Delay Distortion and Attenuation • Delay Distortion - occurs because the velocity of propagation of a signal through a medium varies with the frequency of the signal • Can be reduced • Attenuation - the continuous loss of a signal’s strength as it travels through a medium

  18. Error Prevention • To prevent errors from happening, several techniques may be applied: • Proper shielding of cables to reduce interference • Telephone line conditioning or equalization • Replacing older media and equipment with new, possibly digital components • Proper use of digital repeaters and analog amplifiers • Observe the stated capacities of the media

  19. Error Prevention (continued)

  20. Error Detection • Despite best prevention techniques, errors may still occur • To detect an error, error detection code has to be added to the data/signal • Let’s examine two basic techniques for detecting errors: • Parity checking • Cyclic redundancy checksum

  21. Parity Checks • Simple parity - If performing even parity, add a parity bit such that an even number of 1s is maintained • If performing odd parity, add a parity bit such that an odd number of 1s is maintained • For example, send 1001010 using even parity • For example, send 1001011 using even parity

  22. Parity Checks (continued) • What happens if the character 10010101 is sent and the first two 0s accidentally become two 1s? • Thus, the following character is received: 11110101 • Will there be a parity error? • Problem: Simple parity only detects odd numbers of bits in error

  23. Longitudinal Parity • Longitudinal parity • Adds parity bit to each character • Then adds row of parity bits after a block of characters • Row of parity bits is actually a parity bit for each “column” of characters • Row parity bits plus column parity bits add a great amount of redundancy to a block of characters

  24. Longitudinal Parity (continued)

  25. Longitudinal Parity (continued)

  26. Parity Checks (continued) • Both simple parity and longitudinal parity do not catch all errors • Simple parity only catches odd numbers of bit errors • Longitudinal parity is better at catching errors • But requires too many check bits added to a block of data • We need a better error detection method • What about cyclic redundancy checksum?

  27. 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

  28. 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

  29. Cyclic Redundancy Checksum (continued)

  30. Error Control • Once an error is detected, what is the receiver going to do? • Do nothing • Return an error message to the transmitter • Fix the error with no further help from the transmitter

  31. Error Control (continued) • Do nothing • Seems like a strange way to control errors • Some newer systems such as frame relay perform this type of error control • Return a message has three basic formats: • Stop-and-wait ARQ • Go-back-N ARQ • Selective-reject ARQ

  32. Stop-and-wait ARQ • Simplest error control protocol • A transmitter sends a frame then stops and waits for an acknowledgment • If a positive acknowledgment (ACK) is received, the next frame is sent • If a negative acknowledgment (NAK) is received, the same frame is transmitted again

  33. Stop-and-wait ARQ (continued)

  34. Go-back-N ARQ • Go-back-N ARQ and selective reject are more efficient protocols • They assume that multiple frames are in transmission at one time (sliding window) • A sliding window protocol allows transmitter to send up to the window size frames before receiving any acknowledgments • When a receiver does acknowledge receipt, the returned pack contains the number of the frame expected next

  35. Sliding Window Protocol

  36. Go-back-N ARQ (continued) • Using the go-back-N ARQ protocol, if a frame arrives in error, the receiver can ask the transmitter to go back to the Nth frame and retransmit it • After the Nth frame is retransmitted, the sender resends all subsequent frames

  37. Selective-reject ARQ • Most efficient error control protocol • If a frame is received in error, the receiver asks transmitter to resend ONLY the frame that was in error • Subsequent frames following the Nth frame are not retransmitted

  38. Selective-reject ARQ (continued)

  39. Selective-reject ARQ (continued)

  40. Selective-reject ARQ (continued)

  41. Selective-reject ARQ (continued)

  42. Correct the Error • For a receiver to correct the error with no further help from the transmitter requires a large amount of redundant information accompanying original data • This redundant information allows the receiver to determine the error and make corrections • This type of error control is often called forward error correction

  43. Error Detection in Action • Asynchronous transfer mode (ATM) incorporates many types of error detection and error control • ATM inserts a CRC into the data frame (the cell), which checks only the header and not the data • This CRC is also powerful enough to perform simple error correction on the header • A second layer of ATM applies a CRC to the data, with varying degrees of error control

  44. Summary • Noise in computer networks • Error-prevention techniques • Simple parity and longitudinal parity calculations • Cyclic redundancy checksum • Three forms of error control • Stop-and-wait ARQ, Go-back-N ARQ and Selective-reject ARQ