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# lec11 - UCI University of California Irvine EECS 161...

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EECS 161 - Introduction to Computer Networking Scott Jordan LAN performance UCI University of California, Irvine Shared Ethernet -- Performance: We want to find the performance of shared Ethernet (the hub version). We consider a simplified model, without adaptive backoff, and we redefine a "slot time". Q. How long does it take to determine there has been a collision?

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EECS 161 - Introduction to Computer Networking Scott Jordan LAN performance UCI University of California, Irvine Shared Ethernet -- Performance: Q. How long does it take to determine there has been a collision? A. , where is maximum end-to-end propagation time. So consider a timeline marked in "slots" of length each. We want to know the proportion of slots used for a sucessful transmission. Denote the transmission time of a packet as . Efficiency is given by: where A = average number of slots wasted due to collisions & idle. A = ? 2 ρρ 2 ρ τ η CSMA/CD useful time overall time ---------------------------- τ τ A 2 ρ () + ------------------------ ==
EECS 161 - Introduction to Computer Networking Scott Jordan LAN performance UCI University of California, Irvine Shared Ethernet -- Performance: A = average number of slots wasted due to collisions & idle. A = ? Model: If a collision occurs, you flip a coin. If H, then you retransmit in the next slot. If T, then wait 1 slot & flip again. (This is not quite the same as the Binary Exponential Backoff used by Ethernet, but it’s close.) start stop fail (idle or collision) succeed (1 transmission w/o collision)

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lec11 - UCI University of California Irvine EECS 161...

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