2.2 and 2.3 Methods of error detection and Encryption

Why do we need to check for errors during data transmission?

As errors can occur during data transmission due to interference which can cause data loss, data gain and data change.

Parity Check

The numbers of 1s are counted in each byte

The parity bit is added to each byte before transmission to make the sum of the bits in each byte odd or even

After the transmission if it turns changes from even or odd to odd or even an error is detected

Otherwise no error is detected

Checksum

Value calculated using the other elements in the data stream

Value transmitted at the end of a block of data

Value recalculated after transmission

Values compared after transmission to check for error

Echo check

Data sent is transmitted back

Both sets of data compared

If they are identical there is no error

If not then error is detected

ARQ

Error checking protocol
Timer is started when sending device transmits a data packet to receiver
Receiving device checks packet data for errors
If error detected then request sent to resend data and negative acknowledgement is used
Resend request is repeated until data is sent correctly or it reaches its limit
Sends acknowledgement that data is received using positive acknowledgement
If acknowledgement not received in time timeout occurs data is resent until acknowledgement received or maximum number of attempts reached

Check digit

Digit is calculated from the data

It is appended to the data

Digit is recalculated when data is entered

Digits compared to check for errors

Examples of where check digit is used

International standard book numbers

Bar codes

Positive ARQ only

Error checking protocol
Timer is started when sending device transmits a data packet to receiver
Receiving device checks packet data for errors
If there are no error positive acknowledgement is sent

If error detected then no acknowledgement sent
If acknowledgement not received in time timeout occurs data is resent until acknowledgement received or maximum number of attempts reached

Parity block check

Parity byte is added to the end of the data so if the data was arranged in a grid the number on ones in each column and row would be odd/even,

This allows the exact location of the error to be pinpointed as the location in which the error has occurred’s row and column will be detected

Need and purpose of encryption

Protects from unautorised access by making it meaningless

Makes it unable to be deciphered even if intercepted

Encryption

Jumbling up or scrambling data so that the message makes no sense

Plain text

data that is not encrypted

Cypher text

Data that has been encrypted

How is data encrypted symmetrically

Encryption key is used

Encryption algorithm is used

Encryption key/ algorithm is applied to plain text to convert it so cypher text

Same key is used to encrypt and decrypt data

Key

Binary string of certain length that when applied to an encryption algorithm can encrypt plain text and decrypt cipher text

Asymmetric encryption

Public and private key

Public key is known to everyone and used to encrypt messages

Private key is known only to receiver and decrypts messages

Sender sends data encrypted by the public key

Receiver receives data decrypted by the private key only known to them