Lectures15_16

Lectures15_16 - Lectures 15 & 16 Local Area...

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Unformatted text preview: Lectures 15 & 16 Local Area Networks Eytan Modiano Eytan Modiano Slide 1 Carrier Sense Multiple Access (CSMA) In certain situations nodes can hear each other by listening to the channel - Carrier Sensing CSMA: Polite version of Aloha Nodes listen to the channel before they start transmission Channel idle => Transmit Channel busy => Wait (join backlog) When do backlogged nodes transmit? When channel becomes idle backlogged nodes attempt transmission with probability q r = 1 Persistent protocol, q r = 1 Non-persistent protocol, q r < 1 Eytan Modiano Slide 2 CSMA Let = the maximum propagation delay on the channel When a node starts/stops transmitting, it will take this long for all nodes to detect channel busy/idle For initial understanding, view the system as slotted with "mini- slots" of duration equal to the maximum propagation delay Normalize the mini-slot duration to = /D tp and packet duration = 1 > minislots packet <----------- <----------------> 1 Actual systems are not slotted, but this hypothetical system simplifies the analysis and understanding of CSMA Eytan Modiano Slide 3 Rules for slotted CSMA When a new packet arrives If current mini-slot is idle, start transmitting in the next mini-slot If current mini-slot is busy, node joins backlog If a collision occurs, nodes involved in collision become backlogged Backlogged nodes attempt transmission after an idle mini-slot with probability q r < 1 (non-persistent) Transmission attempts only follow an idle mini-slot Eachbusy-period (success or collision) is followed by an idle slot before a new transmission can begin Time can be divided into epochs: A successful packet followed by an idle mini-slot (duration = +1) A collision followed by an idle mini-slot (duration = +1) An idle minislot (duration = ) Eytan Modiano Slide 4 Analysis of CSMA Let the state of the system be the number of backlogged nodes Let the state transition times be the end of idle slots Let T(n) = average amount of time between state transitions when the system is in state n T(n) = + (1 - e- (1-q r ) n ) When qr is small (1-q r ) n ~ e-q r n => T(n) = + (1 - e- nq r ) At the beginning of each epoch, each backlogged node transmits with probability q r New arrivals during the previous idle slot are also transmitted With backlog n, the number of packets that attempt transmission at the beginning of an epoch is approximately Poisson with rate g(n) = + nq r Eytan Modiano Slide 5 Analysis of CSMA The probability of success (per epoch) is P s = g(n) e-g(n) The expected duration of an epoch is approximately T(n) ~ + (1 - e-g(n) ) Thus the success rate per unit time is g ( n ) e g ( n ) < departurerate = + 1 e g ( n ) Eytan Modiano Slide 6 Maximum Throughput for CSMA The optimal value of g(n) can again be obtained: 1 g ( n ) 2...
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Lectures15_16 - Lectures 15 & 16 Local Area...

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