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Cisco CCNA Certification: Error Detection vs. Error Recovery |
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Passing the CCNA, Intro, and ICND exam is all about knowing
and noticing the details. (Which makes perfect sense, since
becoming a master networking administrator or engineer is
also about noticing the details!) One such detail knows the
difference between error detection and error recovery. While
the terms are sometimes used interchangeably, they are not
the same thing.
Error detection is just that - error detection only. Two common
error detection methods are found at the Data Link layer of
the OSI model, the FCS (Frame Check Sequence) and CRC (Cyclical
Redundancy Check). A mathematical equation is run against
the data in the frame, and the result is sent along with the
data. The receiver runs the equation again, but this time.
If the result is the same, the frame is considered valid;
if the result is different, the frame is considered corrupt
and is discarded.
Note that the FCS and CRC do nothing in regards to retransmission.
They are strictly error detection schemes.
For an example of error recovery, we look to the Transport
layer, where TCP runs. TCP performs reliable delivery, and
the reason we call it "reliable" is that TCP uses
sequence numbers to detect missing segments. If the sender
determines from the sequence numbers that the remote host
did not receive transmitted segments, the sender will retransmit
the missing segments.
The key to keeping the terms straight in your head is to
remember that while both error detection and error recovery
both detect problems, only error recovery does anything about
it. It's also worth reading an exam question twice when you
see either term!
About the Author:
Article by Chris Bryant.
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