Data Communication Error_Detection_n_Correction.ppt
- 1. TRANSMISSION IMPAIRMENT DCS1203: Data Communication & Computer Networks @SPU
- 2. Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise.
- 3. ATTENUATION Means loss of energy -> weaker signal When a signal travels through a medium it loses energy overcoming the resistance of the medium Amplifiers are used to compensate for this loss of energy by amplifying the signal Measuring Attenuation To show the loss or gain of energy the unit “decibel” is used. P1 - input signal P2 - output signal
- 5. EX.1 Suppose a signal travels through a transmission medium and its power is reduced to one-half. This means that P2 is (1/2)P1. In this case, the attenuation (loss of power) can be calculated as A loss of 3 dB (–3 dB) is equivalent to losing one- half the power.
- 6. EX. 2 A signal travels through an amplifier, and its power is increased 10 times. This means that P2 = 10P1 . In this case, the amplification (gain of power) can be calculated as
- 7. One benefit of using the decibel to measure the changes in the strength of a signal is that decibel numbers can be added (or subtracted) when we are measuring several points (cascading) instead of just two. In the figure below a signal travels from point 1 to point 4. In this case, the decibel value can be calculated as
- 8. EX. 3 Sometimes the decibel is used to measure signal power in milliwatts. In this case, it is referred to as dBm and is calculated as dBm = 10 log10Pm , where Pm is the power in milliwatts. Calculate the power of a signal with dBm = −30. Solution: We can calculate the power in the signal as
- 9. DISTORTION Means that the signal changes its form or shape Distortion occurs in composite signals Each frequency component has its own propagation speed traveling through a medium. The different components therefore arrive with different delays at the receiver. That means that the signals have different phases at the receiver than they did at the source.
- 10. NOISE There are different types of noise Thermal - random noise of electrons in the wire creates an extra signal Induced - from motors and appliances, devices act are transmitter antenna and medium as receiving antenna. Crosstalk - same as above but between two wires. Impulse - Spikes that result from power lines, lighning, etc.
- 11. SIGNAL TO NOISE RATIO (SNR) To measure the quality of a system the SNR is often used. It indicates the strength of the signal wrt the noise power in the system. It is the ratio between two powers. It is usually given in dB and referred to as SNRdB. Ex.1 : The power of a signal is 10 mW and the power of the noise is 1 μW; what are the values of SNR and SNRdB ? Solution: The values of SNR and SNRdB can be calculated as follows:
- 12. The values of SNR and SNRdB for a noiseless channel are This is an ideal ratio which can never be achieved in real life.
- 13. ERROR DETECTION AND CORRECTION BCS2106: Data Communication SPU
- 14. Basic concepts Basic concepts o Networks must be able to transfer data from one device to another with complete accuracy. o Data can be corrupted during transmission. o For reliable communication, errors must be detected and corrected. o Error detection and correction are implemented either at the data link layer or the transport layer of the OSI model.
- 15. Types of Errors
- 16. Single-bit error
- 17. 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: 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.
- 18. Burst error
- 20. The term burst error 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.
- 21. 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. Example: Example: If data is sent at rate = 1Kbps then a noise of 1/100 sec can affect 10 bits.(1/100*1000) If same data is sent at rate = 1Mbps then a noise of 1/100 sec can affect 10,000 bits. (1/100*106 )
- 22. ERROR DETECTION 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.
- 23. Redundancy
- 24. Four types of redundancy checks are used Four types of redundancy checks are used in data communications in data communications
- 25. Vertical Redundancy Check VRC Performance: It can detect single bit error It can detect burst errors only if the total number of errors is odd.
- 26. Longitudinal Redundancy Check LRC 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.
- 27. VRC and LRC
- 29. CYCLIC REDUNDANCY CHECK 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.
- 30. Binary Division
- 31. Polynomial
- 34. Checksum
- 35. AT THE SENDER 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
- 36. AT THE RECEIVER 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.
- 37. PERFORMANCE 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.
- 38. ERROR CORRECTION 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.
- 39. SINGLE-BIT ERROR CORRECTION 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: 2 2r r ≥ m+r+1 ≥ m+r+1
- 40. Error Correction
- 41. Hamming Code
- 42. Hamming Code
- 43. Hamming Code
- 44. Example of Hamming Code
- 45. Single-bit error
- 46. Error Detection
Editor's Notes
- #22: Make sense of message. Make sense of message.