lecture13 - Alan Mislove amislove at ccs.neu.edu...

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Unformatted text preview: Alan Mislove amislove at ccs.neu.edu Northeastern University 1 CS4700/CS5700 Fundamentals of Computer Networks Lecture 13: Reliability Slides used with permissions from Edward W. Knightly, T. S. Eugene Ng, Ion Stoica, Hui Zhang Alan Mislove amislove at ccs.neu.edu Northeastern University 2 Overview Goal: transmit correct information Problem: bits can get corrupted Electrical interference, thermal noise Problem: packets can be lost Solution Detect errors Recover from errors Correct errors Retransmission Alan Mislove amislove at ccs.neu.edu Northeastern University 3 Outline Revisit error detection Reliable Transmission Alan Mislove amislove at ccs.neu.edu Northeastern University 4 Nave approach Send a message twice Compare two copies at the receiver If different, some errors exist How many bits of error can you detect? What is the overhead? Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Error Detection Problem: detect bit errors in packets (frames) Solution: add extra bits to each packet Goals: Reduce overhead, i.e., reduce the number of redundancy bits Increase the number and the type of bit error patterns that can be detected Examples: Two-dimensional parity Checksum Cyclic Redundancy Check (CRC) Hamming Codes Alan Mislove amislove at ccs.neu.edu Northeastern University 6 Parity Even parity Add a parity bit to 7 bits of data to make an even number of 1s How many bits of error can be detected by a parity bit? Whats the overhead? 0110100 1011010 Alan Mislove amislove at ccs.neu.edu Northeastern University 6 Parity Even parity Add a parity bit to 7 bits of data to make an even number of 1s How many bits of error can be detected by a parity bit? Whats the overhead? 0110100 1011010 1 Alan Mislove amislove at ccs.neu.edu Northeastern University 6 Parity Even parity Add a parity bit to 7 bits of data to make an even number of 1s How many bits of error can be detected by a parity bit? Whats the overhead? 0110100 1011010 1 Alan Mislove amislove at ccs.neu.edu Northeastern University 7 Two-dimensional Parity Add one extra bit to a 7-bit code such that the number of 1s in the resulting 8 bits is even (for even parity, and odd for odd parity) Add a parity byte for the packet Example: five 7-bit character packet, even parity 0110100 1011010 0010110 1110101 1001011 Alan Mislove amislove at ccs.neu.edu Northeastern University 7 Two-dimensional Parity Add one extra bit to a 7-bit code such that the number of 1s in the resulting 8 bits is even (for even parity, and odd for odd parity) Add a parity byte for the packet Example: five 7-bit character packet, even parity 0110100 1011010 0010110 1110101 1001011 1 Alan Mislove amislove at ccs.neu.edu Northeastern University 7 Two-dimensional Parity Add one extra bit to a 7-bit code such that the number of 1s in...
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lecture13 - Alan Mislove amislove at ccs.neu.edu...

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