lecture6 - CS4700/CS5700 Fundamentals of Computer Networks...

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Alan Mislove amislove at ccs.neu.edu Northeastern University 1 CS4700/CS5700 Fundamentals of Computer Networks Lecture 6: Datalink layer problems Slides used with permissions from Edward W. Knightly, T. S. Eugene Ng, Ion Stoica, Hui Zhang
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Alan Mislove amislove at ccs.neu.edu Northeastern University 2 Bit Stream Encoding • Specify how bits are represented in the analog signal – This service is provided by the physical layer • Challenges: – Efficiency: ideally, bit rate is maximized – Robust: avoid de-synchronization between sender and receiver when there is a large sequence of 1’s or 0’s
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Alan Mislove amislove at ccs.neu.edu Northeastern University 3 Assumptions • We use two discrete signals, high and low, to encode 1 and 0 • The transmission is synchronous, i.e., there is a clock used to sample the signal • If the amplitude and duration of the signals is large enough, the receiver can do a reasonable job of looking at the distorted signal and estimating what was sent.
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Alan Mislove amislove at ccs.neu.edu Northeastern University 4 Non-Return to Zero (NRZ) 1 high signal; 0 low signal 0 0 1 0 1 0 1 1 0 Clock
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Alan Mislove amislove at ccs.neu.edu Northeastern University 4 Non-Return to Zero (NRZ) 1 high signal; 0 low signal 0 0 1 0 1 0 1 1 0 NRZ (non-return to zero) Clock
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Alan Mislove amislove at ccs.neu.edu Northeastern University 4 Non-Return to Zero (NRZ) 1 high signal; 0 low signal 0 0 1 0 1 0 1 1 0 NRZ (non-return to zero) Clock Disadvantages: when there is a long sequence of 1’s or 0’s – Sensitive to clock skew, i.e., difficult to do clock recovery
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Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Non-Return to Zero Inverted (NRZI) • 1 make transition; 0 stay at the same level • Solve previous problems for long sequences of 1’s, but not for 0’s 0 0 1 0 1 0 1 1 0 Clock
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Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Non-Return to Zero Inverted (NRZI) • 1 make transition; 0 stay at the same level • Solve previous problems for long sequences of 1’s, but not for 0’s 0 0 1 0 1 0 1 1 0 Clock NRZI (non-return to zero inverted)
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Alan Mislove amislove at ccs.neu.edu Northeastern University 6 Manchester 0 0 1 0 1 0 1 1 0 Clock
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Alan Mislove amislove at ccs.neu.edu Northeastern University 6 Manchester 0 0 1 0 1 0 1 1 0 Clock Manchester
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amislove at ccs.neu.edu Northeastern University
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This note was uploaded on 03/04/2011 for the course CS 4700 taught by Professor Mislove during the Spring '11 term at Northeastern.

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lecture6 - CS4700/CS5700 Fundamentals of Computer Networks...

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