Information Coding Systems

4.5.5 (14 questions). Public Notes: AQA's A Level Computer Science

Describe the ASCII system.

Each character from English A-Z, a-z, 0-9 and other symbols each have a 7 bit binary character code and are in sequence.

Describe the Unicode system.

Each character from many alphabet systems, all punctuation, special symbols and emojis have a 8-48 bit bit binary character code and are in sequence.

ASCII was introduced in 1963. Explain why Unicode was later introduced in 1991.

More bits per character code, wider range of different characters.

Explain even parity bit error checking.

  1. A single bit is added to the end of a transmission
    • This is to be 1, if after adding the bit, the total number of 1s is even
    • This is to be 0, if after adding the bit, the total number of 1s is odd
  2. During the parity check if there is an odd number of 1s, an error in transmission has detected.

Explain odd parity but checking.

  1. A single bit is added to the end of a transmission
    • This is to be 1, if after adding the bit, the total number of 1s is odd
    • This is to be 0, if after adding the bit, the total number of 1s is even
  2. During the parity check if there is an odd number of 1s, an error in transmission has detected.

Describe the major issue with parity bit error checking.

  1. If an even number of bits are changed during transmission, the number of 1s will remain even or odd
  2. The error is not therefore detected in the corrupt data

Explain majority voting error checking.

  1. Data is transmitted three times.
  2. Each bit is compared with two other bits in the same position.
  3. The bit which was most frequently received is assumed to be correct.

State the advantage of majority voting error checking over parity bits, checksums and check-digits.

Majority voting requires no retransmission of data if an error has been detected, because it can correct for errors

State the disadvantage of majority voting error checking over parity bits, checksums and check-digits.

A larger volume of data has to be transmitted and a larger number of transmission must be made. This will significantly increase the time taken to transmit data.

Explain checksum error checking.

  1. The data to be sent is processed using an algorithm, and appended to the end of original data in binary.
  2. The recipient uses the same algorithm to recalculate a checksum, and compares for a match with the checksum received.
  3. If the two results don't match, then error has been detected and a request to resend is sent.

Explain check-digit error checking

  1. The data to be sent is processed using an algorithm, and a single bit is appended to the end of original data in binary.
  2. The recipient uses the same algorithm to recalculate a check-digit, and compares for a match with the check-digit received.
  3. If the two check-digits don't match, then error has been detected and a request to resend is sent.

State the disadvantage of check-digit error checking to error checking.

Smaller range of algorithms can be used to calculate a check-digit, so a smaller range of errors can be detected using check-digit over checksum.

State the disadvantage of checksum error checking to check-digit error checking.

Algorithm for calculating checksums can be more complex and less efficient than the algorithm for calculating check-digits.

Compare the efficiency of parity bit, majority voting, checksum and check-digit error checking

Parity bit > Check-digit > Checksum > Majority vote