Lecture-05 - Lecture#05 Todays Lecture Objectives Questions...

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Lecture #05
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Today’s Lecture Objectives • Questions about HW#1? • Data Link Layer: functions and a description of each – The precursor to finally talking about a protocol.
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Data Link Layer Overview Function: frames across a single hop Specific Tasks: – framing – error detection (correction) – flow control – medium access control (depending)
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Framing Goal : successful transfer of bits Problem : bits can be changed, added, or deleted Solution : use framing
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Framing Summary • Framing gives us a basic technique to identify bits from non-bits and identify groups of bits. • Allows data to be divided into “chunks” for attempted delivery over a link. – Layer 2: frames – Layer 3: packets – Layer 4: segments • Techniques – timing – character counter (less robust) – starting (and/or) ending flags or characters
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Framing Flags or Characters • Flag may look like data, need to distinguish from data – Need to be clear to the sender AND receiver • Find a way to distinguish flag from data – Use “bit stuffing” – Use “character stuffing” • Examples of each…
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Error Detection/Correction detection : identify that an error occurred correction : identify and fix the error – harder to correct – can correct some, but not all – trying to correct may lead to incorrect results
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Hamming Distance distance, such that d errors are required to change one valid bit string into another Example want to send a single bit “0” or “1” send it four times!! how many errors can be detected? how many errors can be corrected?
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Hamming Distance detection: HD -1 correction: the “floor” of (HD-1)/2 for the last example: can detect 3 bit errors as an error can only correct 1 bit errors if 3 bit errors occur, correction will be wrong!
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Detection Techniques Parity odd and even Variations of Hamming Codes CRC Codes Message, M(x) Generator Polynomial, G(x)
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CRC Computation Steps Add zeros to end equal to the degree of G(x) Divide G(x) into M(x) use modulo-2 binary division Put remainder in extra bits Transmit Divide received M(x) by G(x) if no remainder, then received correctly if remainder, then error occurred
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This note was uploaded on 02/19/2012 for the course ENGR 361 taught by Professor Drexel during the Spring '12 term at Bloomsburg.

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Lecture-05 - Lecture#05 Todays Lecture Objectives Questions...

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