Parity bit and CRC question (1 Viewer)

[BlueGas]

The answer is D, but why?

 

[Katsumi]

This is going off very old knowledge but iirc

• A parity bit is an extra bit added to a group that makes the number of 0's or 1's odd or even.
• A CRC does something to do with the order of bits

A problem with the parity bit is that it may not detect changes in the order of bits. For example, if the order of bits change, but the number of 0's or 1's is still even/odd then that change will not be detected

Original: 010010101 - Number of 1's is Even
Will be detected: 010110101 - Number of 1's is Odd
Won't be detected: 010101001 - The order is different, the file will still be corrupted, but it won't be detected as the number of 1's is Even

Pretty much everything but Parity can detect an issue in the order of bits. It's only parity that has that problem.

 

[BlueGas]

This is going off very old knowledge but iirc

• A parity bit is an extra bit added to a group that makes the number of 0's or 1's odd or even.
• A CRC does something to do with the order of bits

A problem with the parity bit is that it may not detect changes in the order of bits. For example, if the order of bits change, but the number of 0's or 1's is still even/odd then that change will not be detected

Original: 010010101 - Number of 1's is Even
Will be detected: 010110101 - Number of 1's is Odd
Won't be detected: 010101001 - The order is different, the file will still be corrupted, but it won't be detected as the number of 1's is Even

Pretty much everything but Parity can detect an issue in the order of bits. It's only parity that has that problem.

So why can't it be A?

 

[Katsumi]

This will have something to do with how a CRC works. Go back and study it to see if you can find out.

 

[hindsight2020]

Edit:nvm didn't realise this post was a year old. Hope this helps someone anyway.

Hey, I'm in year 8 so haven't done the course, but know a bit about networking, I think I know why it can't be A.

It can't be a because if an even number of bits change, it still checks out.

Eg: say the message we're sending is 0101010 with even parity, the parity would be 1 to make it even.

Say we mess up an even number of bits (we'll do 2 for example), 0101111. The parity bit would still be a 1 because 5 is still an odd number of 1s. Therefore the error would be unnoticed.

If we switch an odd number of bits (we'll do 2), let's see what happens

0000011

5 0s and 2 1s. If you got a parity bit of 1 here, you'd know something went wrong, because 2+1=3 which is not even. 5 is also not even.

Let me know if you need further explanation.

Cheers!

Sorry for typos, on mobile and Tapatalk won't work so had to type using the website.