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 Naïve 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 1’s • How many bits of error can be detected by a parity bit? • What’s 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 1’s • How many bits of error can be detected by a parity bit? • What’s 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 1’s • How many bits of error can be detected by a parity bit? • What’s 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 1’s 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 1’s 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 1’s in...
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lecture13 - Alan Mislove amislove at ccs.neu.edu...

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