Data Communications Networking Devices - 4th Ed.pdf

832 _______________________________ SIZING DATA COMMUNICATIONS NETWORK DEVICESFigure A.6Channel requirementsencountering a busy signal while 10 channels would provide a 0.0318 probability.To obtain a 98% probability of access statistically would require 11 channels. Sincethere are only 10 terminals, logic would override statistics and 10 channels or onechannel per terminal would suf®ce. It should be noted that the statistical approachis based upon a level of traf®c which can be generated from an in®nite number ofterminals. Thus, one must also use logic and recognize the limits of the statisticalapproach when sizing equipment. Since a 0.0681 probability of encountering a busysignal is associated with nine channels and a 0.1334 probability with eight channels,nine channels would be required to obtain a 90% probability of access.In Figure A.6 the sizing required for average peak daily traf®c is listed for both 90and 98% probability of obtaining access. Note that the difference betweensupporting the average and peak traf®c loads is four channels for both the 90 and98% probability of access scenarios, even though peak traf®c is 2 1 2times averagetraf®c.The last process in the sizing procedure is to determine the number of channelsand associated equipment to install. Whether to support the average or peak loadwill depend upon the critical nature of the application, funds availability, how oftenpeak daily traf®c can be expected, and perhaps organizational politics. If peak traf®conly occurs once per month, we could normally size equipment for the average dailytraf®c expected. If peak traf®c was expected to occur twice each day, we wouldnormally size equipment based upon peak traf®c. Traf®c between these extremesmay require the ®nal step in the sizing procedure to one of human judgement,incorporating knowledge of economics, and the application into the decisionprocess.