lec-34-35 - 11-13 Nov 2008 BITS C481 (Computer...

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Unformatted text preview: 11-13 Nov 2008 BITS C481 (Computer Networks)1Whats NextData Link LayerIntroduction and servicesError detection and correction Multiple access protocolsLink-layer AddressingEthernetLink-layer switches (Hub, Bridge and Switch)PPP and HDLC Link virtualization: ATM, MPLS11-13 Nov 2008 BITS C481 (Computer Networks)2Multiple Access protocolsSingle shared broadcast channel Two or more simultaneous transmissions by nodes: interference Collisionif node receives two or more signals at the same timeMultiple access protocolDistributed algorithm that determines how nodes share channel, i.e., determine when node can transmitCommunication about channel sharing must use channel itself! No out-of-band channel for coordination11-13 Nov 2008 BITS C481 (Computer Networks)3Multiple Access Protocols11-13 Nov 2008 BITS C481 (Computer Networks)4Random Access ProtocolsWhen node has packet to sendTransmit at full channel data rate R.No a prioricoordination among nodesTwo or more transmitting nodes collision,Random access MAC protocolspecifies: How to detect collisionsHow to recover from collisions (e.g., via delayed retransmissions)Examples of random access MAC protocols:Slotted ALOHAALOHACSMA, CSMA/CD, CSMA/CA11-13 Nov 2008 BITS C481 (Computer Networks)5Pure (unslotted) ALOHAUnslotted Aloha: simpler, no synchronizationWhen frame first arrivesTransmit immediately Collision probability increases:Frame sent at tcollides with other frames sent in [t-1,t+1]11-13 Nov 2008 BITS C481 (Computer Networks)6Vulnerable Time for Pure ALOHA11-13 Nov 2008 BITS C481 (Computer Networks)7Procedure11-13 Nov 2008...
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lec-34-35 - 11-13 Nov 2008 BITS C481 (Computer...

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