6 Chapter 3.2 Error Error Detection and Error Control

3.2 Errors, Error Detection, and Error Control

Learning Objectives

  • Identify the different types of noise commonly found in computer networks.

  • Specify error-prevention techniques and apply them to different types of noise.

  • Compare error-detection techniques, considering efficiency and efficacy.

  • Perform simple parity and longitudinal parity calculations and enumerate their strengths and weaknesses.

  • Cite the advantages of arithmetic checksum.

  • Cite the advantages of cyclic redundancy checksum and which types of errors it can detect.

  • Differentiate error control methods and describe circumstances for their use.

  • Follow examples of Hamming self-correcting code.

Introduction to Errors

  • Noise is pervasive in communications.

  • High levels of noise can corrupt or destroy signals, requiring systems to check for errors.

  • Systems may discard erroneous data, or attempt to correct it if the error is detected.

Types of Noise

1. White Noise

  • Known as thermal or Gaussian noise.

  • Persistent but can be reduced.

  • High intensity can disrupt signals.

2. Impulse Noise

  • Characterized by random spikes that disrupt bits of information.

  • Difficult to distinguish from the original signal in analog.

  • More destructive to adjacent bits at higher transmission rates.

3. Crosstalk

  • Unwanted signal coupling between different paths (e.g., overhearing a conversation).

  • Can be diminished using appropriate shielding measures.

4. Echo

  • Reflective feedback of a signal in transmission mediums (particularly coaxial cables).

  • Can significantly interfere with the original signal.

  • Methods exist for reducing echo.

5. Jitter

  • Caused by small timing irregularities in digital signal transmission.

  • Affects the smooth flow of digital data, necessitating system slowdowns.

  • Can be mitigated to some extent.

6. Delay Distortion

  • Results from frequency-dependent signal propagation speeds in mediums.

  • Can be minimized with corrective measures.

7. Attenuation

  • Continuous signal strength loss as it travels through a medium.

Error Prevention Techniques

  • Apply multiple strategies to avert errors:

    • Proper shielding of cables to minimize interference.

    • Employ equalization on telephone lines.

    • Upgrade old media and equipment to digital components.

    • Optimize the use of digital repeaters and analog amplifiers.

    • Adhere to the stated operational capacities of communication media.

Error Detection Techniques

  • Despite prevention methods, errors can still occur:

    1. Parity Checking:

      • Simple Parity: Maintains an even or odd number of bits.

      • Longitudinal Parity: Adds additional parity checks for data blocks, increasing redundancy.

    2. Arithmetic Checksum:

      • Converts data to numerical form, sums values, and includes the sum with the transmitted data.

      • Receiver performs a similar summation and comparison.

    3. Cyclic Redundancy Check (CRC):

      • Involves polynomial arithmetic to divide data by a generating polynomial; remainder indicates errors.

      • High efficacy in detecting various error types based on the polynomial used.

Error Control Mechanisms

  • Upon detecting an error, actions include:

    • Do Nothing: Discarding packets error-free, like lower-layer protocols.

    • Return an Error Message: Sends feedback to the transmitter using:

      • Stop-and-Wait Protocol: Acknowledges receipt after each frame.

      • Sliding Window Protocol: Allows several frames to be sent before awaiting acknowledgment.

    • Correct Errors: Utilizes forward error correction (e.g., Hamming code), which includes redundant bits for self-correction.

Examples of Error Control

Hamming Codes

  • Utilize redundancy for error correction, with additional parity check bits for various segments of the data stream.

  • The receiver can detect and correct errors without transmitter interference based on parity checks from the redundancy provided.

Summary

  • Noise inherently exists in computer networks, leading to potential errors in data transmission.

  • Various error detection and correction mechanisms help ensure data integrity, with cyclic redundancy checksum (CRC) being highly effective.

  • Understanding protocol differences like stop-and-wait vs. sliding window is critical for efficient data transmission and error handling.