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Detecting and correcting transmission errors

  • 3 things to know
  • A2 Unit 1

Overview

Three causes of errors in transmitted data, parity bit and how echo checking works.

What you need to know

Answer each one before you reveal it.

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Key term

Explain how a two-dimensional parity check can locate a single incorrect bit.

Key term

How is a checksum calculated and used?

Key term

Name three causes of errors in transmitted data.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Name three causes of errors in transmitted data.

    Model answer

    Electrical interference from nearby equipment, attenuation that weakens the signal until bits are misread, and crosstalk between adjacent cables all cause errors in transmitted data.

  2. 2

    What is a parity bit?

    Model answer

    A parity bit is one extra bit added to a group of data bits and set so that the total number of ones in the group meets an agreed rule, either always even or always odd.

  3. 3

    The seven data bits 1011001 are to be sent using even parity. State the parity bit and explain your answer.

    Model answer

    The parity bit is 0, because the data already contains four ones and four is an even count, so no extra one is needed.

  4. 4

    Which errors can a single parity bit not detect?

    Model answer

    A single parity bit cannot detect any error that flips an even number of bits, because two changes leave the count of ones with the same parity as before.

  5. 5

    Why can a single parity bit not correct an error?

    Model answer

    A single parity bit shows only that the count of ones no longer matches the rule, and gives no information about which of the bits in the group was altered.

  6. 6

    Explain how a two-dimensional parity check can locate a single incorrect bit.

    Model answer

    A two-dimensional parity check adds a parity bit to every row of a block and a further parity byte across the columns, so a single flipped bit fails exactly one row check and one column check, and the intersection of that row and column identifies the bit to invert.

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