ERRORERROR DETECTION


When data is being transmitted from one machine to another machine.

It may be possible that data become corrupted on its way.

Some of the bits may be altered, damaged or last during transmission.

Such a condition is known as error.

The error may occur because of noise online, attenuation and delay this dissertation for reliable communication, it is important that errors are detected and corrected.

There are two main types of error in transmission:

§  Single Bit Error

§  Burst Error


Single Bit Error

§  It means only 1 bit of data unit changed from 1 to 0 or 0 to 1.

§  Single Bit error can happen in parallel transmission where all the data are transmitted using separate wires.

Burst Error:

§  It means two or more bits in data unit are changed from 1 to 0 and 0 to 1.

§  In Burst Error, it is not necessary that only consecutive bits are changed.

§  The length of burst error is measured from 1st changed bit to last changed bit.

§  Burst Error is most likely occur in serial transmission.

§  The noise occurring for a longer duration affects multiple bits.

§  The number of bit affected depends upon on data rate and duration of noise,

§  For example if data rate is one kilo bit per second and noises is 1/100 second can affect 10 bits.

Error Detecting Techniques:

§  Error Detection needs inspecting the data so as to check whether or not data is changed during the transmission.

§  An error detection system is needed at the receiver end that should check the incoming data and report the number and type of errors that has occur.

Types of Error Detecting Techniques:

§  Vertical Redundancy Check(VRC)

§  Cyclic Redundancy Check(CRC)

Vertical Redundancy Check:

§  This method is also known as parity check.

§  In this method, a redundant bit called parity bit is added to each data unit.

§  This method may include even parity or odd parity.

§  Even parity means that total number of ones in data.

§  Odd parity means that total number of ones in data including parity bit is odd.

§  For example: if source wants to transmit a data unit 110111 using even parity to the destination, the source will pass this unit to even parity generator.

§  The parity generator will count the number of ones and generates the parity bit accordingly.

§  Now this data along with parity bit is then transmitted across the network.

§  At the destination this data is passed to parity checker.

§  The parity checker counts the number of ones in data.

§  If the number of one’s is odd, then the receiver rejects that data unit.

§  This method can detect all single bit error.

§  It can also detect burst errors, but only in those cases where number of bit changed is odd.

§  It is not able to detect burst error, if number of bits changed is even.

§  This is the major disadvantages of this method.

Cyclic Redundancy Check:

§  CRC is more powerful than VRC in detecting errors.

§  This method is not based on binary addition.

§  It is based on binary division method.

§  At the sender side, data unit to be transmitted is divided by a predetermined binary number in order to obtain the remainder.

§  The CRC has one bit less than the divisor.

§  It means that if CRC is of ‘n’ bits, divisor is of n+1 bit.

§  The sender appends this CRC to the end of the data unit such that the resulting data unit becomes exactly divisible by predetermined divisor that remainder becomes zero.

§  At the destination this incoming data unit that is data + CRC is divided by the same number.

§  If the remainder after division is zero, then there is no error in data unit and receiver accepts.

§  If remainder after division is not zero, it indicates that the data unit has been damaged and therefore it is rejected.

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