Data Communications Networking Devices - 4th Ed.pdf

A.4 APPLYING THE EQUIPMENT SIZING PROCESS ___________________________________ 831Figure A.5Terminal traf®c surveyNormally, busy hour traf®c can be estimated fairly accurately from historical dataor from computer billing and accounting tape data. Suppose that either sourceshows a busy hour traf®c equal to twice the average daily traf®c based upon an 8-hour normal operational shift. Then the traf®c would be 8/8) x 2 or 2 erlangs whilethe busy hour peak traf®c would be 20/8) x 2 or 5 erlangs.The next process in the sizing procedure is to determine the appropriate sizingformula to apply to the problem. If we assume that users encountering a busy signalwill tend to redial the telephone numbers associated with the multiplexer, thePoisson formula will be applicable. From Table A.8 the 2.0 erlang traf®c columnshows a 0.0165 probability 1.65%) of all channels busy for a device containing sixchannels, 0.0527 for ®ve channels and 0.1428 for four channels, thus, to obtain a98% probability of access based upon the daily average traf®c would require sixchannels while a 90% probability of access would require ®ve channels.If we want to size the equipment based upon the daily peak traf®c load, howwould sizing differ? We would now use the 5 erlang traf®c column of Table A.6from the table, 11 channels would provide a 0.0137 probability 1.37%) ofTable A.8 Poisson distribution program result extract C. Probability that all channels arebusy when call attemptedTraf®c in erlangs grade of service)Channel 0.500 1.000 1.500 2.000 2.5001 0.393 469 0.632 120 0.776 870 0.864 665 0.917 9152 0.090 204 0.264 241 0.442 174 0.593 994 0.712 7023 0.014 387 0.080 301 0.191 152 0.323 323 0.456 1864 0.001 751 0.018 988 0.065 642 0.142 875 0.242 4225 0.000 172 0.003 659 0.018 575 0.052 652 0.108 8206 0.000 014 0.000 594 0.004 455 0.016 562 0.042 0197 0.000 001 0.000 083 0.000 925 0.004 532 0.014 1858 0.000 000 0.000 010 0.000 169 0.001 095 0.004 2459 0.000 000 0.000 000 0.000 027 0.000 236 0.001 13810 0.000 000 0.000 000 0.000 003 0.000 045 0.000 27511 0.000 000 0.000 000 0.000 000 0.000 007 0.000 05912 0.000 000 0.000 000 0.000 000 0.000 000 0.000 010