# L27 - Efficiency of CSMA = End-to-end propagation time TF =...

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Efficiency of CSMA ô = End-to-end propagation time F T = Average frame duration F â = ô / T Beta is the key parameter in determining efficiency. With CSMA/CD, For values of â < 1, approximate limiting efficiency is For â << 1, efficiency is almost perfect. For â near 1, efficiency is no better than unslotted ALOHA.

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Timing with a center repeater station: Each sees the synchronized timing and the echo of its own signal as well as others. This allows the establishment of synchronized slots - as in Slotted ALOHA, reservation minislots, reserved slots.
The collision resolution algorithm Assume time is slotted and all users observe (before next slot) if slot is unused, has a success, or has a collision ( number of colliders unknown). If collision occurs, only colliders will attempt until all are known successful. Then others may send. Goal: have an algorithm that resolves a collision in as few slots as possible, while also allowing all users to know when the collision has been resolved. The basic idea is called a splitting algorithm .

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Illustration of the splitting algorithm Three colliders whose selected numbers are: A= .3, B=.4, C=.6. Let .5 divide the subsets, so A and B will transmit in the next slot and collide again, as shown: Efficiency and stability of ALOHA or CSMA/CD are improved with this algorithm.
Wireless LAN protocols Not always true that all stations can hear each other. Suppose A and C both send to B. B and D see a collision, But A, C, E and F don’t see any collision.

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