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Efficiency is the long-run fraction of successful slots when there’s many nodes, each with many frames to send. How to calculate the efficiency of Aloha, Slotted Aloha, and CSMA/CD? Slotted Aloha efficiency Suppose N nodes with many frames to send, each transmits in slot with probability p prob that node 1 has success in a slot = p(1-p)N-1 prob that any node has a success = Np(1-p)N-1 For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1 For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37 At best: channel used for useful transmissions 37% of time! Pure Aloha efficiency P(success by given node) = P(node transmits) . P(no other node transmits in [p0-1,p0]. P(no other node transmits in [p0-1,p0] = p . (1-p)N-1 . (1-p)N-1 = p . (1-p)2(N-1) … choosing optimum p and then letting n -> infty ... = 1/(2e) = .18 Even worse!