n overflow medium, SO there is no degradation of service to the subscriber when the dedicated facilities are blocked. These dedicated facilities have virtually unlimited overflow capability in the PSTN. Therefore, the decision is a cost is- sue rather than a capacity issue. 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. Capacity Calculations for Cellular Networks Capacity Calculations for Cellular Networks Let us look at the distribution of usage for the dedicated connection we are consider- ing. For any level of transport capacity, we are interested in the fraction of time, min- utes per day, where the demand exceeds that capacity. Figure 26.6 shows a graph of the function we are describing. 8 There is some traffic nearly 18 hours per day (1050 min- utes) and enough to fill six DS-1 links 6 hours per day (360 minutes). This means that one-quarter of the time the traffic is six DS-1 links or more. If the daily cost of a dedi- cated DS-1 link is the same as 360 minutes of PSTN usage, then the breakeven point for this connection is 360 minutes per day. Once we know the breakeven usage level, we can look at the usage graph and determine the number of DS-1 or E-1 links that minimize our transport costs for this connection, six DS-1 links in this example. Estimating the usage function is the key to making a good decision about dedicated facilities. The simplest model is a normal bell-shaped statistical distribution based on total daily usage. We could take the total minutes per day as the mean and estimate the standard deviation o based on the customer mix. Then we can compute the proba- bility of exceeding capacity x from Eq. (26.3): (26.3) SThose who have worked in the field of revenue management (which used to be called yield man- agement) will recognize this graph. It is the same as the expected marginal seat revenue (EMSR) graph the airline