CRC calculations ...

johnifanx98

How many unique CRC calculations will take place as traffic routes from the PC to the laptop?

The answer is 8?!


http://answers.ccna4u.net/wp-content/uploads/2011/02/0712-300x94.jpg

Patel128

Isn't CCNA4U a **** site? Sounds like one. I probably suggest that you not use that as study material

mapletune

Think about it. At what Layer of the OSI model does error detection with CRC checksums work at?

That's right, Layer 2. The L2 frame header contains a 32 bit CRC frame check sequence.

So, the host computer has to encapsulate that L2 frame in order to send it through the L1 wire, correct? That's 1 calculation.

Now, the first Router gets it, it checks for errors, 2nd calculation.

However, because it decapsulated the frame to read L3 IP packet header, it must encap it again, now, with a new MAC address source/destination, everything, including a new 32 bit CRC frame check sequence. That's the 3rd calculation.

Repeat for every link and you get 2 calculations per link. one to calculate CRC and one to check CRC.

Cheers,
Mike

YFZblu

Yep - According to CertGuard, ccna4u.net is indeed a braindump.

johnifanx98

mapletune wrote: »

Think about it. At what Layer of the OSI model does error detection with CRC checksums work at?

That's right, Layer 2. The L2 frame header contains a 32 bit CRC frame check sequence.

So, the host computer has to encapsulate that L2 frame in order to send it through the L1 wire, correct? That's 1 calculation.

Now, the first Router gets it, it checks for errors, 2nd calculation.

However, because it decapsulated the frame to read L3 IP packet header, it must encap it again, now, with a new MAC address source/destination, everything, including a new 32 bit CRC frame check sequence. That's the 3rd calculation.

Repeat for every link and you get 2 calculations per link. one to calculate CRC and one to check CRC.

Cheers,
Mike

Thanks for clarification. I've reviewed the book, and realized only error detection in layer 2 is called CRC. I've mixed up header checksum with CRC. But is there any obvious difference between checksum and CRC?

mapletune

A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage.

-Wikipedia

anyway, CRC is one of the algorithms used to generate a checksum value