ving call waiting or a three-way call does not increase the amount of speech handling required. Thus we have a simple Erlang-B traffic engineering problem: There is a stochastic demand for speech handling, some number of erlangs, and our job is to make sure that there are enough speech handlers to provide an adequately low blocking rate for the speech coding service. The blocking percentages required, however, are typically much lower than the 1 to 10 percent we engineer on the air interface. We can take advantage of the larger numbers of voice calls for an entire MSC community and engineer the speech handling for very low blocking rates on the order of 0.1 or even 0.01 percent, as shown in Tables 23.5 and 23.6. The blocking of speech handlers is virtually independent of air-interface blocking, SO the two blocking rates are almost exactly added. 5 Because of the large numbers speech handlers for the entire MSC voice-call community, we should be able to maintain busy- hour occupancy of around 90 percent even at low blocking levels of around 0.01 percent. Thus it seems that there is not much economy to be gained and significant performance to be lost by engineering fewer speech coders and raising their blocking rates. 24.6 Conclusion Now that we have completed our general look at MSC switching capacity, we will turn our attention to the capacity required for mobile-specific activities, as defined by the ANSI-41 standard. So long as both air-interface and speech coding have low blocking rates, adding the two block- ing rates is a reasonable approximation of the combined blocking rate. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Switching Capacity Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright© 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use 