error checking Flashcards
What are the ways errors occur during data transmission?
interference, problems during packet switching, skewing of data
What are the error checking methods?
parity checking, automatic repeat request (ARQ), checksum, echo checking, check digits
Facts about parity checking?
a byte of data is allocated a PARITY BIT before transmission takes place
systems that use even parity have an even of of 1-bits and systems that use odd parity have a odd number of 1-bits
before data is transferred an agreement is made between sender and receiver on which type of parity to use. it is a example of protocol.
how does parity checking work?
if a byte was transmitted from A to B and even parity was used a error would be flagged if a byte now had a odd number of 1-bits
the error is detected by the computer recalculating the parity of the byte sent
what is the problem with parity checking?
if there are two errors and they cancel each other out so they’d go unnoticed (ex. 2 bits may have an error or have swapped places)
parity blocks and parity byte?
Parity blocks: a block of data is sent and the number of 1-bits are totaled horizontally and vertically. the method identifies where the error is.
Parity byte: it is consists entirely of the parity bits produced by the vertical parity check and indicates the end of the block of data
parity example:
if 9 bytes were sent using even parity but byte 8 (row 8) had three 1-bits and bit 5 (column 5) had five 1-bits than the bit with an error can be found at the intersection.
bytes are rows and bits are columns
facts about automatic repeat request (ARQ)?
it uses an ACKKNOWLEDGEMNT and TIMEOUT. if the acknowledgment isn’t sent back to the sender before timeout than the message is automatically reset
What is acknowledgment and timeout in ARQ?
ACKKNOWLEDGEMNT is a message sent by the receiver indicating that the data has been received correctly
TIMEOUT is the time allowed before an acknowledgment must be received
how does ARQ work?
ARQ uses positive and negative acknowledgments and the reviewing device relieves a error decision code as part of the data transmission. (usually a cyclic redundancy check) to detect whether the data contains any errors.
what happens when an error is detected in ARQ?
a time out is used by the sending device and if no acknowledgment is received during the time out and it automatically resends the data until a positive acknowledgment is received.
Where is ARQ often used?
it is often used by mobile phone networks to guarantee data integrity
What is checksum and how does it work?
When a block of data is about to be transmitted the checksum for the bytes is calculated and then transmitted with the block of data. Then the checksum is recalculated and compared to the original. If they are the same there was no error and if they aren’t a request is sent for the data to be retransmitted.
How to calculate the checksum?
Look at the ppt :)
What is echo check and how does it work?
when data is sent to another device the data is sent back to the sender. the sender will compare the two sets of data to check for errors.
