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. Noise is always present
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. One of the most disruptive forms of noise Random spikes of power
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)
The bottom figure should show much more distortion, completely blowing out one or two bits of information.

11. Unwanted coupling between two different signal paths
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. To prevent errors from happening, several techniques may be applied:
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. Despite best prevention techniques, errors may still occur
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. For example, send 1001010 using even parity
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 using even parity
For example, send using even parity

22. Parity Checks (continued)
What happens if the character is sent and the first two 0s accidentally become two 1s?
Thus, the following character is received:
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. (continued) Cyclic Redundancy Checksum
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 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