Chap2 - DATA LINK LAYER Sunghyun Choi Adopted from Prof....

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ATA LINK LAYER DATA LINK LAYER Sunghyun Choi Adopted from Prof. Saewoong ahk’s material Bahk s material
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Data Link Layer: Introduction • Efficient communication between two adjacent machines • Providing a well defined service interface to the network layer • Framing • Error control (providing error-free channel) l t l • Flow control edium access control (MAC) for Medium access control (MAC) for broadcast channel
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Services for the Network Layer nacknowledged connectionless service • Unacknowledged connectionless service – No connection establishment – No attempt to recover a frame ppropriate for lo error te links – Appropriate for low error-rate links • Acknowledged connectionless service – No connection establishment – Each frame sent is individually acknowledged If ACK has not arrived within a specified time Cs ov e d w s p e c e d e interval, it can be sent again
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seful for unreliable channels – Useful for unreliable channels • Acknowledged connection-oriented service – Connection establishment h f t h t i i b d – Each frame over the connection is numbered and acknowledged – Frame order conservation
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Error detection and correction xtra bits must be appended to a frame to • Extra bits must be appended to a frame to detect errors (redundancy) • Single Parity Check – simplest – a single bit , “parity check” appended – This parity check bit is 1 if the number of 1’s in e bit string is odd and 0 otherwise the bit string is odd, and 0 otherwise – single error detection (odd number of errors can be detected)
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orizontal and vertical parity checks • Horizontal and vertical parity checks – 2-dimensional parity check (row and column) – If an even number of errors are confined to a ngle row each of them can be detected by single row, each of them can be detected by column parity checks • Multi-parity checks et K be the length of the data string Let K be the length of the data string – Let L be the number of parity checks
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ach string of length K is mapped into a frame – Each string of length K is mapped into a frame of length K+L – Minimum distance of a code : the smallest number of bit positions in which two codewords differ he smallest # of errors that can convert one code The smallest # of errors that can convert one code word into another o detect d errors need a distance d+1 code • To detect d errors, need a distance d+1 code • To correct d errors, need a distance 2d+1 code
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ffectiveness of parity check codes • Effectiveness of parity check codes • Measures of effectiveness – Min. distance – Burst detecting capability – Probability that a completely random string is accepted as error free
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ength of a burst of errors: # of bits from the first Length of a burst of errors: # of bits from the first error to the last, inclusive urst detecting capability: the largest integer B s t • Burst detecting capability: the largest integer B s.t.
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Chap2 - DATA LINK LAYER Sunghyun Choi Adopted from Prof....

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