This document discusses error detection and correction techniques used in data communication networks. It describes common types of transmission errors like single bit errors and burst errors. It then explains different error detection methods including parity checking, longitudinal redundancy checking (LRC), and checksum checking. It also covers error correction techniques like Go-Back-N and Selective Repeat that allow retransmission of only corrupted frames. Flow control mechanisms like stop-and-wait and sliding window protocols are also summarized.

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)