1. DATA COMMUNICATION
Lecture-33

2. Recap of Lecture 32 T-Lines in Analog Transmission Fractional T-Lines
E-Lines
Fiber To The Curb (FTTC)
Error Detection and Correction

3. Overview of Lecture 33 Types of Errors Error Detection Techniques
Redundancy
Types of Redundancy Checks

4. Types of Errors

5. Single-Bit Error Only one bit of a given data unit is changed
The least likely type of error in serial transmission
Single-bit error can happen in parallel transmission

6. Single-Bit Error

7. Burst Error Two or more bits in the data unit are changed
Most likely to happen in a serial transmission
Number of bits affected depends on the data rate and duration of noise

8. Burst Error

9. Redundancy
Redundancy means adding extra bits for detecting errors at the destination

10. Redundancy

11. Detection Methods Four types of redundancy checks
VRC, LRC, and CRC: For use in the data link layer
Checksum: Used by upper layers

12. Detection Methods

13. Vertical Redundancy Check
Often called a Parity Check
A parity bit is added to every data unit
Even-parity checking or odd-parity checking

14. Vertical Redundancy Check

15. Example 9.1
Sender wants to send “world” In ASCII, the five characters are coded as: w o r l d

16. Example 9.2
Suppose “world” is received by the receiver without being corrupted:
Receiver counts the 1’s and passes it w o r l d

17. Example 9.3 Suppose “world” is received by the receiver corrupted:
Receiver counts the 1’s and rejects it w o r l d

18. Performance of VRC VRC can check all single-bit errors
In case of even-parity checking
It can detect burst errors if the total number of errors in each data unit is odd
It can not detect errors if the total number of errors is even

19. Performance of VRC
We have an Even Parity data unit where the total number of 1’s including the parity bit is ‘6’ :
If 3 bits change value resulting parity will be odd and an error will be detected: : 1’s = 9

20. Performance of VRC
If 2 bits change value resulting parity will still be even and error will not be detected: : 1’s = 8
VRC cannot detect errors when the total number of bits changed are even

21. Summary Types of Errors Error Detection Techniques Redundancy
Types of Redundancy Checks

22. Suggested Reading Section 9.1, 9.2, 9.3
“Data Communications and Networking” 2nd Edition by Behrouz A. Forouzan