Link Layer Contention EE 122 Intro to Communication Networks Fall 2007 WF 4 5 30 in Cory 277 Vern Paxson TAs Lisa Fowler Daniel Killebrew Jorge Ortiz http inst eecs berkeley edu ee122 Materials with thanks to Jennifer Rexford Ion Stoica and colleagues at Princeton and UC Berkeley 1 Our Story So Far Single shared broadcast channel Avoid having multiple nodes speaking at once Otherwise collisions lead to garbled data Multiple access mechanism Distributed algorithm for sharing the channel Algorithm determines which node can transmit Classes of techniques Channel partitioning divide channel into pieces o TDMA and FDMA time division frequency division Taking turns scheme for trading off who gets to transmit Random access allow collisions and then recover o Optimizes for the common case of only one sender 2 Taking Turns MAC protocols Polling Token passing Master node invites Control token passed from one slave nodes to node to next sequentially transmit in turn Node must have token to send data poll master data slaves Concerns Token overhead Latency Single point of failure token Concerns Polling overhead Latency Single point of failure master 3 Random Access Protocols When node has packet to send Transmit at full channel data rate No a priori coordination among nodes Two or more transmitting nodes collision Data lost Random access MAC protocol specifies How to detect collisions How to recover from collisions Examples ALOHA and Slotted ALOHA CSMA CSMA CD CSMA CA 4 Slotted ALOHA Assumptions All frames same size Time divided into equal slots time to transmit a frame Operation Nodes are synchronized When node obtains fresh frame transmits in next slot Nodes begin to transmit frames only at start of slots No collision node can send new frame in next slot No carrier sense If two or more nodes transmit all nodes detect collision Collision node retransmits frame in each subsequent slot with probability p until success 5 Slotted ALOHA Pros Cons Single active node can continuously transmit at full rate of channel Collisions wasting slots Highly decentralized only slots in nodes need to be in sync Nodes may be able to detect collision in less than time to transmit packet Simple Clock synchronization Idle slots 6 Slotted Aloha efficiency Efficiency long run fraction of successful slots many nodes all with many frames to send Suppose N nodes with many frames to send each transmits in slot with probability p Maximum efficiency find p that maximizes Np 1 p N 1 Probability that given node has success in a slot At best under heavy load channel wasted 63 of the time p 1 p N 1 Probability that any node has a success Np 1 p N 1 For many nodes take limit of Np 1 p N 1 as N goes to infinity gives Maximum efficiency 1 e 37 Can also show that without slots efficiency drops to 1 2e 18 7
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