2.2 Error Checking

Question 1

why do we need error checking

Answer 1

with the complexity of computers and vast amount of 1s and 0s being transmitted, there is always room for error, data corruptions or loss

Question 2

how can errors occur in data transmission

Answer 2

electrical interference such as electrical fields or waves can cause data to be corrupted, ie 1s become 0s or lost
packet switching can be incorrect and packets can be lost enroute or corrupted or gained

Question 3

what are the reasons for transmission errors

Answer 3

, imperfections in the wire, or electricl fields or intereference with the wire, can be stopped with insulating outer material

Question 4

what are the common methods of error checking

Answer 4

parity checks
parity block checks
checksums
echo checks
check digits
ARQ

Question 5

what is a parity check

Answer 5

a error detection method that stores the most significant bit (MSB = the biggest bit) as a parity bit to show the amount of 1s in a byte as odd or even
example 10001001 using odd parity
the 1 at the start, MSB, means that the amount of 1s are odd, there are 3 ones in that byte so there is no error according to the parity check

Question 6

pros and cons of parity check

Answer 6

simple errors can be found however the exact corrupted bit cannot be identified as the computer recognises if its odd or even, so any bit can be changed

Question 7

how can you advance parity checks to detect the exact bit and correct it

Answer 7

parity block checks

Question 8

what are parity block checks

Answer 8

when a whole block of bytes has been sent, parity checks can be used horizontally an vertically to identify the exact bit corrupted and fix the error without asking for a retransmission

Question 9

what is checksum

Answer 9

an error detection method where a set of bytes is sent along with an extra byte which is a checksum used to evaluate the data at the reciever

Question 10

how does checksum works

Answer 10

an extra checksum byte is added, this checksum is calculated using a pre agreed algorithm and send along with the data, the reciever recalculated the checksum using the same preagreed algorith, generating another checksum, if both sender and reciever checksums match then the data is error free, otherwise not

Question 11

what is an echo checkk

Answer 11

when the sender sends the data and reciever resends what it has recieved to the sender in order for it make sure what the sender has recieved is the same than what it has sent, if it does not match then the data is resent

Question 12

what is a con of echo check (2)

Answer 12

you cannot tell wether the data has been corrupted while sending, or corrupted while being recieved
it also creates unessesary network traffic because 2-3 transmissions are send back and forth for one set of data

Question 13

what are check digits

Answer 13

the last digit in a code calculated with an algorithm used the predescesing digits in order to check for errors

Question 14

what are check digits used for

Answer 14

in order to detect errors during data entry normally caused by mistyping or scanning error ig barcode

Question 15

what are check digits used IN

Answer 15

ISBN NUMERS
bank card numbers
VIN car numbers

Question 16

how to calculate check digits using modulus 11

Answer 16

• from left to right, give the numbers a weighting of 10-2
• multiply the numbers with their weighting
• add all the sums up
• divide by 11 and the remainder will become the check digit

Question 17

how to check a check digit

Answer 17

• from left to right, give the numbers their weighting INCLUDING the check digit
• multiply the numbers with their weight, INCLUDING the check digit
• add all the sums up
• divide by 11 and if the remainder is 0, the check digit is correct

Question 18

what are some commen errors in data entry

Answer 18

incorrect digits entered

transposition errors, so 493 becomes 439

phonetic errors like 13 instead of 30

omitted extra digits, like 4567 instead of 456

Question 19

what is ARQ

Answer 19

Automatic Repeat reQuest or Query
an error detection method

Question 20

how does ARQ check errors

Answer 20

the sender sends a data packet along with an error detection code, usually a Cylic redundancy code or CRC, the reciever the checsk for errors, if there are no errors then it sends an acklowedgement signal either acknowledging it was error free or not, if it was error free then the sender does not resend the data, otherwise it does. the sender has a time limit of recieving an acknowledgement signal and if it doesnt not recieve anything before a timelimit, then it will keep resending the data unless there is a cap on the amount of times it will resend it

Question 21

what is enryption

Answer 21

encoding a message so it can only be read by the sender and recipient

Question 22

what is symmetric encryption

Answer 22

when one key is used to encrypt and decrypt data being sent

Question 23

what are the drawbacks of symmetric encryption

Answer 23

very prone to data interception and can be encrypted with a key copy

Question 24

what is assymetric encryption

Answer 24

when 4 keys are used to encrypt and decrypt data, a public key and private key the recievers public and recievers private both sender and reciever exchange public keys and if a message is encrypted using person A private key, it can be decrypted by Person B with Person As public key, the public key is public and can be used by anyone however the private key is private and never shared

Question 25

how does assymetric encryption authentic user messages

Answer 25

By using a private key to encrypt data to be sent, you might think why would you do this because it can be decrypted using a public key, however if it CAN be decrypted using a public key it shows that the message has came from the private key, authenticating the message, but for the encryption to not be intercepted the private key ALONG with the public key is used togethor, a combined encryption key, so the private to public authenticates message, and the public to private encrypts it