Error Detection and Correction in Computer Networks
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Questions and Answers

What happens when two bits in one data unit are damaged and two bits in the same positions in another data unit are also damaged?

  • The VRC checker detects an error.
  • Both LRC and VRC checkers detect an error.
  • The LRC checker detects an error.
  • The LRC checker does not detect an error. (correct)

    • What type of parity is used for the LRC?

  • Odd parity
  • Both odd and even parity
  • Even parity (correct)
  • No parity

    • What is the binary representation of the LRC for the message 'THE CAT'?

  • 00101111 (correct)
  • 11101010
  • 10101110
  • 11010011

    • What is the purpose of the LRC checker?

    <p>To detect errors in transmission</p> Signup and view all the answers

    What is the approximate percentage of transmission errors detected by CRC?

    <p>99.999%</p> Signup and view all the answers

    What is the purpose of the CRC?

    <p>To detect transmission errors</p> Signup and view all the answers

    If data is sent at a rate of 1Mbps, how many bits can a noise of 1/100 sec affect?

    <p>10,000 bits</p> Signup and view all the answers

    What is the primary purpose of error detection in data communications?

    <p>To decide whether the received data is correct or not</p> Signup and view all the answers

    What type of redundancy checking is also referred to as character parity?

    <p>Vertical redundancy checking (VRC)</p> Signup and view all the answers

    What is the main advantage of using longitudinal redundancy checking (LRC)?

    <p>It increases the likelihood of detecting burst errors</p> Signup and view all the answers

    What happens to the LRC bits in the transmitter?

    <p>They are computed and appended to the end of the message</p> Signup and view all the answers

    What type of error can VRC detect, but only if the total number of errors is odd?

    <p>Burst error</p> Signup and view all the answers

    What is the purpose of the checksum in error detection?

    <p>To detect errors involving an odd number of bits</p> Signup and view all the answers

    What is the process of creating a checksum?

    <p>Adding sections together using one's complement and then complementing the sum</p> Signup and view all the answers

    What is the primary requirement for reliable communication in networks?

    <p>Data transmission with complete accuracy</p> Signup and view all the answers

    What is the function of the generator polynomial in CRC?

    <p>To create a checksum for error detection</p> Signup and view all the answers

    Which type of error is most likely to occur in serial data transmission?

    <p>Burst error</p> Signup and view all the answers

    What operation is performed on the data and generator polynomial in CRC?

    <p>Modulo-2 division</p> Signup and view all the answers

    What is the purpose of the CRC code?

    <p>To detect errors in transmission</p> Signup and view all the answers

    What is the duration of a bit in serial data transmission at a rate of 1Mbps?

    <p>1 μs</p> Signup and view all the answers

    Which layer of the OSI model is responsible for error detection and correction?

    <p>Data link layer and transport layer</p> Signup and view all the answers

    What type of errors can the checksum detect?

    <p>Most errors involving an even number of bits and all errors involving an odd number of bits</p> Signup and view all the answers

    What is the term for two or more bits in the data unit changing from 1 to 0 or from 0 to 1?

    <p>Burst error</p> Signup and view all the answers

    Why is single-bit error less likely to occur in serial data transmission?

    <p>The noise must have a very short duration</p> Signup and view all the answers

    What is the primary advantage of using forward error correction over retransmission?

    <p>It is faster and more efficient.</p> Signup and view all the answers

    What is the condition that must be satisfied by the value of r in error correction?

    <p>2r ≥ m+r+1</p> Signup and view all the answers

    How does the receiver correct a single-bit error using an error-correcting code?

    <p>It reverses the value of the altered bit.</p> Signup and view all the answers

    What is the purpose of redundancy bits in error correction?

    <p>To enable error detection and correction.</p> Signup and view all the answers

    What type of error can be corrected using a Hamming code?

    <p>Single-bit error.</p> Signup and view all the answers

    What is the benefit of using a Hamming code for error detection?

    <p>It can detect and correct single-bit errors.</p> Signup and view all the answers

    Study Notes

    Error Detection and Correction

    • Data can be corrupted during transmission, and errors must be detected and corrected for reliable communication.
    • Error detection and correction are implemented at the data link layer or the transport layer of the OSI model.

    Types of Errors

    • Single-bit error: a rare type of error that occurs in serial data transmission, requiring a noise duration of very short duration.
    • Multiple-bit error: also known as burst error, occurs when two or more bits in the data unit change from 1 to 0 or from 0 to 1.

    Error Detection Methods

    • Redundancy: adding extra bits for detecting errors at the destination.
    • Vertical Redundancy Checking (VRC): a simple error-detection scheme that uses parity to detect single-bit errors and burst errors with an odd number of errors.
    • Longitudinal Redundancy Checking (LRC): uses parity to determine if a transmission error has occurred within a message, increasing the likelihood of detecting burst errors.
    • Cyclic Redundancy Checking (CRC): a convolutional coding scheme that detects approximately 99.999% of all transmission errors.

    CRC Calculation

    • CRC generates an n-bit sequence, frame check sequence (FCS), such that the resulting frame is exactly divisible by some predetermined number.
    • The receiver divides the incoming frame by the same number, and if there is no remainder, assumes no error.

    Checksum

    • The checksum is calculated by dividing the unit into k sections, each of n bits, and adding them together using one's complement.
    • The sum is complemented and becomes the checksum, which is sent with the data.
    • At the receiver, the unit is divided into k sections, and the sum is complemented; if the result is zero, the data are accepted.

    Error Correction

    • Error correction can be handled by retransmitting the entire data unit or using an error-correcting code.
    • Single-bit error correction can be done by reversing the value of the altered bit, requiring the receiver to know which bit is in error.
    • Hamming Code: an error-correcting code that uses redundancy bits to detect and correct single-bit errors.

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    Description

    This quiz covers the basics of error detection and correction in computer networks, including types of errors and correction methods.

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