CRC calculations ...

johnifanx98johnifanx98 Member Posts: 329
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

Comments

  • Patel128Patel128 Member Posts: 339
    Isn't CCNA4U a **** site? Sounds like one. I probably suggest that you not use that as study material
    Studying For:
    B.S. in Computer Science at University of Memphis
    Network+
    Currently Reading:
    CompTIA Network+ Study Guide - Lammle
  • mapletunemapletune Member Posts: 316
    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
    Studying: vmware, CompTIA Linux+, Storage+ or EMCISA
    Future: CCNP, CCIE
  • YFZbluYFZblu Member Posts: 1,462 ■■■■■■■■□□
    Yep - According to CertGuard, ccna4u.net is indeed a braindump.
  • johnifanx98johnifanx98 Member Posts: 329
    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?
  • mapletunemapletune Member Posts: 316
    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
    Studying: vmware, CompTIA Linux+, Storage+ or EMCISA
    Future: CCNP, CCIE
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