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 (C)</p> Signup and view all the answers

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

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

What is the purpose of the CRC?

<p>To detect transmission errors (A)</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 (C)</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 (D)</p> Signup and view all the answers

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

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

What is the primary requirement for reliable communication in networks?

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

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

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

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

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

What is the purpose of the CRC code?

<p>To detect errors in transmission (A)</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 (C)</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 (A)</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 (A)</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 (D)</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 (A)</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. (A)</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 (D)</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. (D)</p> Signup and view all the answers

What is the purpose of redundancy bits in error correction?

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

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

<p>Single-bit error. (A)</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. (B)</p> Signup and view all the answers

Flashcards

Error Detection and Correction

The process of identifying and fixing errors that occur during data transmission.

Single-bit Error

A single bit in a data unit flips from 0 to 1 or vice versa.

Multiple-bit Error (Burst Error)

When multiple bits in a data unit change, often due to a burst of interference.

Redundancy (Error Detection)

The process of adding extra bits to the data to help detect errors on the receiving end.

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Vertical Redundancy Checking (VRC)

A simple error detection scheme that checks the parity (even or odd) of bits in a data unit.

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Longitudinal Redundancy Checking (LRC)

Uses parity to check if errors occurred within a message, improving detection of burst errors compared to VRC.

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Cyclic Redundancy Checking (CRC)

A powerful error detection method that uses a mathematical algorithm to generate a checksum for the data.

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Frame Check Sequence (FCS)

A sequence of bits generated by CRC that is appended to the data and used for error detection.

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Checksum Calculation

The process of calculating a checksum for a data unit by dividing it into sections and adding them together.

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Checksum Verification

The receiver checks the checksum by performing the same calculation on the received data and comparing it to the sent checksum.

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Error Correction

The process of correcting errors that occur during transmission.

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Retransmission

A simple error correction method where the entire data unit is re-sent if an error is detected.

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Error-Correcting Codes

Codes that use redundancy to detect and correct errors directly, without retransmission.

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Single-bit Error Correction

A code designed to correct single-bit errors by reversing the value of the flipped bit.

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Hamming Code

An error-correcting code that uses redundant bits (parity bits) to detect and correct single-bit errors.

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Parity Bits

Extra bits added to data to help detect and correct errors, but not part of the original data.

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Error Control

The process of identifying and correcting errors in data transmission.

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Error Resilience

The ability of a system to detect and correct multiple errors.

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Error Detection Rate

A measure of how well a system can detect and correct errors.

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Hamming Bound

A theoretical limit on the number of errors that can be corrected using a specific error-correcting code.

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Forward Error Correction (FEC)

A type of error correction that corrects all errors up to a certain number of bits.

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Data Integrity

The process of detecting and correcting errors in transmitted data to maintain data integrity.

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Error Indication

When errors are detected but not corrected, they are typically flagged for the receiver to handle.

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Noise Duration

The amount of time a signal needs to be corrupted to introduce an error, usually shorter for single-bit errors.

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Real-time Error Detection

A method to detect errors in a real-time stream of data, like in video streaming.

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Error Budget

A measure of how many errors are allowed before a system fails, often used in reliability engineering.

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Request Retransmission

A strategy where the receiver requests a retransmission if it detects an error, instead of trying to correct it on its own.

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Error Probability

The likelihood of an error occurring during transmission, affected by factors like noise and signal strength.

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

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