ams. The example is easier to follow with round numbers of 30 and 20 than with the actual rates of 37 and 23 cents in use as we are writing this book. 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. Business-Case Models Business-Case Models Cents Ounces Figure 46.1 Postage as a function of weight. then it does not make sense to use a linear approximation that takes multiple MSCs into account. The linear function that makes sense for a single-MSC wireless telephone system is the first step of the function in Fig. 46.3 with a higher fixed cost and lower per-cell cost. Another example is at a more detailed level of base station design. Suppose that a base station manufacturer has a design in which a channel backplane in a base station supports 32 channel cards and has a cost of B. The base station has to have one chan- nel backplane, and it requires another one at 33 channel cards and another one after that at 65 channel cards. We have a step function of channel backplane cost for a base station. If base stations vary considerably in the number of channel cards, then it makes sense to use the approximation that the channel backplane cost is a per-cell cost of B/2 and a per-channel-card cost of B/32. We can use the postage model and its linear approximation when there is a step- function cost that we want to estimate over several steps. 46.2 Cost-per-Subscriber Models We can put all this together to form a per-subscriber-cost model for a wireless telephone system. This model is generic. The different CDMA air interfaces have different capac- ities, and vendors vary in their equipment architectures. Thus the most we can hope to 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.