Error detection.ppt

Jan 22, 20230 likes63 views

This document discusses error detection and correction in data transmission. It explains that networks must transfer data accurately and that errors can occur during transmission. It then describes different types of errors like single-bit errors and burst errors. It also discusses various techniques for error detection using redundancy like vertical redundancy check, longitudinal redundancy check, cyclic redundancy check, and checksums. Finally, it covers error correction methods like having the sender retransmit data or using error-correcting codes like Hamming codes to automatically correct certain errors.

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.

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.

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.

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.

Error detection
Error detection means to decide whether the
received data is correct or not without having a
copy of the original message.
Error detection uses the concept of redundancy,
which means adding extra bits for detecting
errors at the destination.

Four types of redundancy checks are used
in data communications

Performance
It can detect burst errors only if the total
number of errors is odd.

Performance
LCR increases the likelihood of detecting
burst errors.
If two bits in one data units are damaged
and two bits in exactly the same positions in
another data unit are also damaged, the LRC
checker will not detect an error.

Cyclic Redundancy Check
• Given a k-bit frame or message, the transmitter generates an n-bit
sequence, known as a frame check sequence (FCS), so that the
resulting frame, consisting of (k+n) bits, is exactly divisible by some
predetermined number.
• The receiver then divides the incoming frame by the same number and, if
there is no remainder, assumes that there was no error.

At the sender
The unit is divided into k sections, each of n
bits.
All sections are added together using one’s
complement to get the sum.
The sum is complemented and becomes the
checksum.
The checksum is sent with the data

At the receiver
The unit is divided into k sections, each of n
bits.
All sections are added together using one’s
complement to get the sum.
The sum is complemented.
If the result is zero, the data are accepted:
otherwise, they are rejected.

Performance
The checksum detects all errors involving an
odd number of bits.
It detects most errors involving an even number
of bits.
If one or more bits of a segment are damaged
and the corresponding bit or bits of opposite
value in a second segment are also damaged, the
sums of those columns will not change and the
receiver will not detect a problem.

Error Correction
It can be handled in two ways:
1) receiver can have the sender retransmit the
entire data unit.
2) The receiver can use an error-correcting
code, which automatically corrects certain
errors.

Single-bit error correction
To correct an error, the receiver reverses the value
of the altered bit. To do so, it must know which bit
is in error.
Number of redundancy bits needed
• Let data bits = m
• Redundancy bits = r
Total message sent = m+r
The value of r must satisfy the following relation:
2r ≥ m+r+1

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Error detection.ppt

1. 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.

2. Types of Errors

3. Single-bit error

4. 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.

5. Burst error

7. 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.

8. 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.

9. Error detection Error detection means to decide whether the received data is correct or not without having a copy of the original message. Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.

10. Redundancy

11. Four types of redundancy checks are used in data communications

12. Vertical Redundancy Check VRC

14. Performance It can detect burst errors only if the total number of errors is odd.

15. Longitudinal Redundancy Check LRC

16. Performance LCR increases the likelihood of detecting burst errors. If two bits in one data units are damaged and two bits in exactly the same positions in another data unit are also damaged, the LRC checker will not detect an error.

18. Cyclic Redundancy Check CRC

19. Cyclic Redundancy Check • Given a k-bit frame or message, the transmitter generates an n-bit sequence, known as a frame check sequence (FCS), so that the resulting frame, consisting of (k+n) bits, is exactly divisible by some predetermined number. • The receiver then divides the incoming frame by the same number and, if there is no remainder, assumes that there was no error.

20. Binary Division

21. Polynomial

22. Polynomial and Divisor

23. Standard Polynomials

25. Checksum

28. At the sender The unit is divided into k sections, each of n bits. All sections are added together using one’s complement to get the sum. The sum is complemented and becomes the checksum. The checksum is sent with the data

29. At the receiver The unit is divided into k sections, each of n bits. All sections are added together using one’s complement to get the sum. The sum is complemented. If the result is zero, the data are accepted: otherwise, they are rejected.

30. Performance The checksum detects all errors involving an odd number of bits. It detects most errors involving an even number of bits. If one or more bits of a segment are damaged and the corresponding bit or bits of opposite value in a second segment are also damaged, the sums of those columns will not change and the receiver will not detect a problem.

31. Error Correction It can be handled in two ways: 1) receiver can have the sender retransmit the entire data unit. 2) The receiver can use an error-correcting code, which automatically corrects certain errors.

32. Single-bit error correction To correct an error, the receiver reverses the value of the altered bit. To do so, it must know which bit is in error. Number of redundancy bits needed • Let data bits = m • Redundancy bits = r Total message sent = m+r The value of r must satisfy the following relation: 2r ≥ m+r+1

33. Error Correction

34. Hamming Code

35. Hamming Code

36. Hamming Code

37. Example of Hamming Code

38. Single-bit error

39. Error Detection

Editor's Notes

#10: Make sense of message. Make sense of message.

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