Back-of-the-Envelope Computation of Throughput

Abstract — This paper presents a simple method for computing throughputs of links in a CSMA network. We call our method back-of-the-envelop (BoE) computation, because for many network configurations, very accurate results can be obtained by simple hand computation. BoE beats prior methods in terms of both speed and accuracy. To explain BoE, we construct a theory based on the model of an “ideal CSMA network ” (ICN). We find that link throughputs are insensitive to the distributions of the backoff countdown time and transmission time in ICN given the ratio of their mean c. The BoE computation method emerges from ICN in the limit 0→c. The insensitivity result explains why BoE works so well for IEEE 802.11 networks, in which the backoff countdown process is one that has memory and the transmission time can be arbitrarily distributed. Furthermore, c does not have to be very small for BoE to be highly accurate. BoE allows us to make shortcuts in performance evaluation, bypassing complicated stochastic analysis. An immediate application of BoE is for quick identification of starved links in the network so that remedies can be devised to solve the problem. Index Terms—CSMA, 802.11, Wi-Fi, multiple access. I