Error Control

During the transmission there can be various Error at receiver end and there may be various reason like:

  • When many packet are being send to the router ,some time what happend is,the entire buffer of router is filled and procesor is not able to process the packet so fast and in this case packet is lost.
  • You will be able to receive the packet but inside the packet either one bit or more bits will get corrupted.

               Packet loss is handle by some protocol now we have to do something when a bit  is lost.

Error handling: Error can be handle by these many ways and these are:

1)CRC(Cyclic Redundancy Check):

  • A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. Blocks of data entering these systems get a short check value attached, based on the remainder of a polynomial division of their contents.
  • There is a CRC Generator which is well known to both Sender and Receiver.
  • If data with the CRC ,which means if there is no error in data then the CRC you get wil be 000 at that time receiver will be sure that there is no error in the data.
  • CRC can detect the error but not correct the error.

CRC Example

Given CRC as a sequence of bits they give it to as a polynomial.

X4 +X3 +X+ 1 ,data at sender side is given as 11100101.

sol:

At Sender Side

A Receiver Side 

                     

             ---------------------------

11011) 111001010100

                  ⊕

               11011

        -----------------------------

              0011110

                        ⊕

                   11011

-----------------------------------------     

                   0010110

                         ⊕

                        11011

-----------------------------------------

                          011011

                                  ⊕

                           11011

-----------------------------------------------

                          0000000

that means in this code there is no error

 

2)Checksum:

 

  • In checksum error detection scheme, the data is divided into k segments each of m bits.
  • In the sender’s end the segments are added using 1’s complement arithmetic to get the sum. The sum is complemented to get the checksum.
  • The checksum segment is sent along with the data segments.
  • At the receiver’s end, all received segments are added using 1’s complement arithmetic to get the sum. The sum is complemented.
  • If the result is zero, the received data is accepted; otherwise discarded.

In TCP,UDP as well as IP we have 16 bit checksum.In case of data linked layer we use 32 bit CRC.

 

 

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