link_error_mac - Link Layer Error Detection/Correction and...

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5: DataLink Layer 5-1 Link Layer – Error Detection/Correction and MAC
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5: DataLink Layer 5-2 Link Layer: Introduction Some terminology: hosts and routers are nodes communication channels that connect adjacent nodes along communication path are links wired links wireless links LANs layer-2 packet is a frame , encapsulates datagram “link” data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
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5: DataLink Layer 5-3 Adaptors Communicating link layer implemented in “adaptor” (aka NIC) Ethernet card, PCMCI card, 802.11 card sending side: encapsulates datagram in a frame adds error checking bits, rdt, flow control, etc. receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to rcving node adapter is semi- autonomous link & physical layers sending node frame rcving node datagram frame adapter adapter link layer protocol
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5: DataLink Layer 5-4 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction
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5: DataLink Layer 5-5 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity : Detect and correct single bit errors 0 0
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5: DataLink Layer 5-6 Internet checksum Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? More later …. Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only )
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5: DataLink Layer 5-7 Checksumming: Cyclic Redundancy Check view data bits, D , as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R , such that <D,R> exactly divisible by G (modulo 2) receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!
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