ft, softer, semisoft, or hard as needed. The messages associated with handoffs are simulated, and under marginal radio conditions, these messages can be lost. Calls can be lost because of poor signal quality, or finally, they can end because their callers have nothing left to say; that is, they end normally. What we can do in the simulation environment is keep detailed statistics of what happens to each call. We do not need to wait for a subscriber to tell us there was some static on the line because our simulation will detect lower performance for that period of time. We know when and where every moment of substandard call quality is occur- ring. More important, we know the particular interferers that are causing lower call quality or other impairment each time it occurs. Computer simulation is a fantastic tool for investigating system problems. Periods of high blocking, services with reduced call quality, or regions with a lot of lost calls can be simulated and diagnosed. Not only can we determine the root cause of a system problem, but we also can test solution strategies on the computer before we install equipment in the actual system. Simulations are generally not a good way to ensure overall QoS in a wireless tele- phone system. When a system problem is bad enough to be noticed, then a simulation can explore its cause, but telling the difference between good enough and not quite good enough is a hard thing to do with a random process. The finer the line between good enough and not good enough, the longer it takes to see the difference. Let's take a simple example from statistics. A coin is fair if it is equally likely to flip heads or tails. If the coin is actually fair, then it takes about 200 tosses to have a percent probability that the fraction of heads will be between 40 and 60 percent. To get 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 th