Error-free Transmission (34)[detail]
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Packetsmall blocks of data send and received individually Packets benefitsallow efficient error detection/recovery and fair sharing of comm links Framesencapsulates frames for transmission on on physical layer frame contentsheader, payload, trailer unack connless serviceethernet ack connless servicewifi ack conn-oriented serviceATM sender data link layerbreak bit stream into discrete frames, compute checksum, send w/ frame receiver data link layerrecomputes checksum, if does not match an error occurrerrrd byte stuffingadd special byte to beginning and end of each frame bit stuffingsimilar to byte stuffing but no requirement for 8 bits, helps physical layer maintain serialisation transmission error typesinterference causing received incorrectly, and interference causing data to be received when none sent two basic strats for errorserror correcting codes and error detecting codes error detectioncheck if error occurs and if so, request frame be resubmitted, no need for error correcting data parityeven or odd number of 1s in message checksumsender calculates complement of sum and sends with message, receiver checks if total is zero Cyclic redundancy checkA polynomial code, sender and receiver agree on G(x) Error correctionRedundancy to sent information, most useful on noisy channels where retrans likely to error hamming distancegiven 2 valid codewords, count number of corresponding bits that differ hamming error detecting hrequires code with hamming distance h+1 hamming error correcting hrequires code with hamming distance 2h+1 stop and waitsender waits for ack before sending more sliding windowsender and receiver maintain set of seq numbers for frames they can send/receive broadcast channelsshared medium, used by LANs, reduces cost but adds coord overhead and high bandwidth req. LAN topologiesstar, ring, bus medium access controlsublayer that determines who goes next on multiaccess channel static channel allocationsplit channel by multiplexing, works well with small constant users dynamical channel allocation assumptionsindependent traffic, single channel, observable collisions, continuous or slotted time, carrier sense or no carrier sense ALOHAUsers transmit whenever they have data, central computer broadcasts each frame it receives, if frame not heard wait a random time and retransmit Slotted ALOHAALOHA synced with clock to improve performance CSMA/CDCarrier Sense Multple Access w/ Collision Detection CSMAdetect when stations are transmitting and wait for idle, wait random time, or use slots Bridgeconnects multiple LANs VLANvirtual LANs based on organisational structure rather than geography