5.5 Information Coding Systems

How many bits for Unicode

Either 16 or 32

How many bits for standard ASCII

7

Disadvantage of using 4 bytes to store data

Takes more storage space

4 causes of transmission errors

Electrical interference, power surges, synchronisation issues, wear and tear on the cable or connection

Can data be corrected if there is an error

No, a request to resend the data is invoked

2 types of parity bits

Odd or even

Method for calculating the parity bit

The bits are XOR’d with each other

How is parity used for error checking

The parity bit is set at the transmitting end and parity is checked at the receiving end

If the wrong number of bits are ‘on’, an error has occurred

The receiving computer notifies the transmitting end and the data is resent

Limitation of parity bits

If two bits are flipped the parity would be the same despite an error

Majority voting

Each bit of a message is sent three times and if a bit value is flipped erroneously the recipient computer uses the majority rule

Majority rule

Assume that the two bits that have not changed are correct

2 disadvantages of majority voting

Sending messages 3 times takes a lot of storage, if there are 2 errors they would be the majority

Check digit

An additional digit at the end of a string of other numbers designed to check for mistakes in input or transmission

How are check digits calculated

Using a specific algorithm

Checksum

A total sum of all bytes in a transmission of data is calculated using an algorithm. The receiving computer recalculates the checksum based on the data it received and compares it

Character code

A unique number used to represent a character