How to compute the CRC?
Shortcut method
5: DataLink Layer
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CRC Example
R = remainder[
Compute the CRC value for the character ¡°1011 1001¡± considering: – the algorithm uses a standard polynomial method with 16 bits
– the generator is 0x8005
Given: D=1011 1001
G=1 1000 0000 0000 0101, r = 16
Note that the Generator G always starts with 1.
5: DataLink Layer
CRC Example
R=remainder[ D.2r] G
Compute the CRC value for the character ¡°1011 1001¡± considering: – the algorithm uses a standard polynomial method with 16 bits
– the generator is 0x8005
Given: D=1011 1001
G=1 1000 0000 0000 0101, r = 16
Append r zero bits on the given data. (equivalently, shift D by r bits to the left)
5: DataLink Layer
CRC Example
R=remainder[ D.2r] G
Compute the CRC value for the character ¡°1011 1001¡± considering: – the algorithm uses a standard polynomial method with 16 bits
– the generator is 0x8005
Given: D=1011 1001
G=1 1000 0000 0000 0101, r = 16
Align the left-most bit of the generator with the first nonzero bit of data, then calculate the XOR value.
This is a short-cut technique! Remember, we are only interested in the remainder.
5: DataLink Layer
CRC Example
R=remainder[ D.2r] G
Compute the CRC value for the character ¡°1011 1001¡± considering: – the algorithm uses a standard polynomial method with 16 bits
– the generator is 0x8005
Given: D=1011 1001
G=1 1000 0000 0000 0101, r = 16
Repeat until you have used all bits in the data.
CRC, in hex form
5: DataLink Layer
For sending: 1011 1001 1000 0011 1001 0101
CRC Example
R=remainder[ D.2r] G
Compute the CRC value for the character ¡°1011 1001¡± considering: – the algorithm uses a standard polynomial method with 16 bits
– the generator is 0x8005
Given: D=1011 1001
G=1 1000 0000 0000 0101, r = 16
At the receiver side:
Remainder is zero, therefore no error is detected!
5: DataLink Layer
Exercise: CRC
R=remainder[ D.2r] G
Compute the CRC value for the character ¡°1000 1001¡± considering: – the algorithm uses a standard polynomial method with 16 bits
– the generator is 0x8005
Given: D=1000 1001
G=1 1000 0000 0000 0101, r = 16
5: DataLink Layer
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