Error is a condition when the output information does not match with the input information. During transmission, digital signals suffer from noise that can introduce errors in the binary bits travelling from one system to other. That means a 0 bit may change to 1 or a 1 bit may change to 0.
Error-Detecting
codes
Whenever a
message is transmitted, it may get scrambled by noise or data may get
corrupted. To avoid this, we use error-detecting codes which are additional
data added to a given digital message to help us detect if an error occurred
during transmission of the message. A simple example of error-detecting code is
parity check.
Error-Correcting
codes
Along with
error-detecting code, we can also pass some data to figure out the original
message from the corrupt message that we received. This type of code is called
an error-correcting code. Error-correcting codes also deploy the same strategy
as error-detecting codes but additionally, such codes also detect the exact
location of the corrupt bit.
In
error-correcting codes, parity check has a simple way to detect errors along
with a sophisticated mechanism to determine the corrupt bit location. Once the
corrupt bit is located, its value is reverted (from 0 to 1 or 1 to 0) to get
the original message.
How to Detect
and Correct Errors?
To detect and
correct the errors, additional bits are added to the data bits at the time of
transmission.
·
The
additional bits are called parity bits. They allow detection or correction of
the errors.
·
The
data bits along with the parity bits form a code word.
Parity Checking
of Error Detection
It is the
simplest technique for detecting and correcting errors. The MSB of an 8-bits
word is used as the parity bit and the remaining 7 bits are used as data or message
bits. The parity of 8-bits transmitted word can be either even parity or odd
parity.
Even parity --
Even parity means the number of 1's in the given word including the parity bit
should be even (2,4,6,....).
Odd parity -- Odd
parity means the number of 1's in the given word including the parity bit
should be odd (1,3,5,....).
Use of
Parity Bit
The parity bit
can be set to 0 and 1 depending on the type of the parity required.
·
For
even parity, this bit is set to 1 or 0 such that the no. of "1 bits"
in the entire word is even. Shown in fig. (a).
· For odd parity, this bit is set to 1 or 0 such that the no. of "1 bits" in the entire word is odd. Shown in fig. (b).
How Does Error
Detection Take Place?
Parity checking
at the receiver can detect the presence of an error if the parity of the
receiver signal is different from the expected parity. That means, if it is
known that the parity of the transmitted signal is always going to be
"even" and if the received signal has an odd parity, then the receiver
can conclude that the received signal is not correct. If an error is detected,
then the receiver will ignore the received byte and request for retransmission
of the same byte to the transmitter.
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