Error detection and correction
- 1. DATA COMMUNICATION AND NETWORKING ERROR CORRECTION AND DETECTION
- 2. OBJECTIVES To identify the types of errors commonly found in communication networks. To Specify the different error prevention techniques. To perform the simple different types of parity checking To understand the error correction process
- 3. Transmission Errors Single bit errors ◦ only one bit altered ◦ caused by white noise Burst errors ◦ contiguous sequence of B bits in which first last and any number of intermediate bits in error ◦ caused by impulse noise or by fading in wireless ◦ effect greater at higher data rates
- 4. Other errors ◦ Impulse noise – electrical interference ◦ White noise – movement of electrons ◦ Attenuation – loss of signal ◦ Crosstalk – signals interfere Inter-modulation noise – combination of frequencies ◦ Delay distortion – data arrives at different times ◦ Line failure – loss of line
- 5. ERROR DETECTION Errors can result when data is lost, corrupted, or damaged, making error control critical Error control has two components: ◦ Error Detection ◦ Error Correction Common error detection methods include; ◦ Parity checking 50% probability of detection ◦ Longitudinal redundancy checking 98% probability of detection ◦ Checksum checking 99.6% probability of detection
- 6. Parity Bits and Parity Checking Parity checking requires the sender ◦ to compute an additional bit ◦ called a parity bit ◦ and to attach it to each character before sending The receiver ◦ removes the parity bit ◦ performs the same computation as the sender ◦ and verifies that the result agrees with the
- 7. Parity bits and parity checking The parity computation is chosen such that ◦ if one of the bits in the character is damaged in transit, the receiver's computation will not agree with the parity bit ◦ the receiver will report that an error occurred. There are two forms of parity ◦ even or odd ◦ both the sender and receiver must agree
- 8. Parity Checking An extra parity bit is added to the byte Assuming even parity: ◦ 10000010 – data sent 10000110 - data received Error detected on receiver side (single bit) ◦ 10000010 – data sent 10011010 – data received No error detected on receiver side (multiple bit) Simple parity detects only single bit errors
- 9. Parity checking Parity mechanism discussed above ◦ works well to detect a single bit error, ◦ but it cannot detect all possible errors There are several alternatives ◦ In each mechanism, the sender transmits additional information along with the data ◦ and the receiver uses the information to verify Differences among the mechanisms arise in three ways: ◦ the size of the additional information ◦ the computational complexity of the algorithm ◦ and the number of bit errors that can be detected
- 10. Longitudinal Redundancy checking Longitudinal literally means “lengthwise” The sender, for each byte in the message, calculates a parity value, creating an additional block check character or BCC ◦ As with parity checking, the parity value is odd or even The BCC is added to the end of the message block The receiver performs the same lengthwise LRC computation If the receiver’s calculated BCC does not equal the sender’s calculated BCC, the receiver assumes a
- 11. LRC 01000010 01011001 01010100 01000101 – Before BCC
- 12. LRC 01000010 01011001 01010100 01000101 00001010 – After BCC The BCC added to the end of the data block.
- 13. Checksum checking- CC The message sender: ◦ Evaluates each binary byte in the message to its decimal value ◦ Totals the decimal values of all bytes ◦ Divides the total by 255, creating a remainder ◦ Using the remainder for the CC, adds the CC to the end of the message block The message receiver: ◦ Performs the same byte-by-byte calculation and creates his own CC ◦ Compares his calculated CC to the sender’s ◦ Assumes a transmission error if the two CC values differ
- 14. Checksum checking - CC
- 15. ERROR CORRECTION Correction of detected errors usually requires data block to be retransmitted Not appropriate for wireless applications ◦ bit error rate is high causing lots of retransmissions ◦ when propagation delay long (satellite) compared with frame transmission time, resulting in retransmission of frame in error plus many subsequent frames Instead need to correct errors on basis of bits
- 16. FLOW CONTROL Prevents a sender from overwhelming a receiver with traffic: ◦ A sender and receiver each have a memory (buffer) area in which they can store frames ◦ A sender can overwhelm, or overflow, a receiver’s memory buffer without proper flow control ◦ If an overflow occurs, data would likely be lost Protocols with flow control mechanism allow multiple PDUs (protocol data units) in transit at the same time PDUs arrive in same order they’re sent Two common forms of flow control are: ◦ Stop-and-wait Each single frame sent requires receipt of one acknowledgement ◦ Sliding windows The sending of multiple frames requires a single acknowledgement returned
- 17. Flow control Reasons for breaking up a block of data before transmitting: ◦ Limited buffer size of receiver ◦ Retransmission of PDU due to error requires smaller amounts of data to be retransmitted ◦ On shared medium, larger PDUs occupy medium for extended period, causing
- 18. Error control requirements Error detection ◦ Receiver detects errors and discards PDUs Positive acknowledgement ◦ Destination returns acknowledgment of received, error-free PDUs Retransmission after timeout ◦ Source retransmits unacknowledged PDU Negative acknowledgement and retransmission ◦ Destination returns negative acknowledgment to PDUs in error
- 19. Model of frame transmission
- 20. Stop and wait Source transmits single frame Wait for ACK If received frame damaged, discard it ◦ transmitter has timeout ◦ if no ACK within timeout, retransmit If ACK damaged, transmitter will not recognize it ◦ transmitter will retransmit ◦ receive gets two copies of frame ◦ use alternate numbering and ACK0 / ACK1
- 21. Stop and wait Most efficient for messages containing a few large frames that traverse short links. Requires one acknowledgement for each frame sent pros and cons ◦ Simple ◦ inefficient
- 22. Sliding window flow control Sender maintains window of frames it can send ◦ Needs to buffer them for possible retransmission ◦ Window advances with next acknowledgements Receiver maintains window of frames it can receive ◦ Needs to keep buffer space for arrivals ◦ Window advances with in-order arrivals Allows multiple numbered frames to be in transit
- 23. Sliding window flow control If two stations exchange data, each needs to maintain two windows one for transmit and one for receive Each side needs to send the data and acknowledgments to the other.
- 24. Sliding window flow control • Most efficient for messages containing many small frames that traverse long links. Allows for one acknowledgement for multiple frames
- 25. Automatic Repeat Request (ARQ) Collective name for such error control mechanisms, including: ◦ Go back N ◦ Selective reject (selective retransmission)
- 26. Go Back N Based on sliding window If no error, ACK as usual use window to control number of outstanding frames Receiver only accepts/acks frames that arrive in order: ◦ Discard that frame and all future frames until error frame received correctly ◦ Sender times out and resends all outstanding frames
- 27. Go Back N Tradeoff made for Go-Back-N: ◦ Simple strategy for receiver; needs only 1 frame ◦ Wastes link bandwidth for errors with large windows; entire window is retransmitted Damaged Frame ◦ error in frame i so receiver rejects frame i ◦ transmitter retransmits frames from i
- 28. Selective reject Only rejected frames are retransmitted (Selective repeat retransmission).Subsequent frames are accepted by the receiver and buffered. This minimizes retransmission. Receiver must maintain large enough buffer more complex logic in transmitter and hence less widely used. But useful for satellite links with long propagation delays
- 29. Selective reject Tradeoff made for Selective Repeat: ◦ More complex than Go-Back-N due to buffering at receiver and multiple timers at sender ◦ More efficient use of link bandwidth as only lost frames are resent (with low error rates)