f a par- ticular layout and deciding among several plans, we consider each of these separate components and calculate the total cost of each plan. If we are calculating the cost of the system at a high level, however, then we may add up all the components of an entire cell and call that the base station cost. We can say that each cell has three sectors, the average sector has four of the five carriers, and the per-cell cost of the MSC is 1/200 of the total MSC cost. As we shall see in Sec. 46.2, this level of aggregation gives us cost estimates per subscriber for financial planning, but it does not give us the level of detail needed to compare two system plans. Equipment has variable costs as well. Most equipment has time component of cost, a monthly cost of operation and maintenance. Of course, each piece of equipment also has a life cycle, time period after which we expect to replace it. A $3000 piece of equip- ment with a 6-year life has a $500 per year cost associated with it for its periodic replacement. And some equipment has a cost of usage. An antenna does not cost any more money because we are using it for more wireless telephone calls, but an amplifier can use more electricity and run up a power bill. While few components of a base station have usage-sensitive life expectancy, user terminals can wear out faster with increased use. Even if the electronics last forever, the buttons on the keypad wear out with use. 46.1.3 Proportional-cost model Another way we can simplify cost calculations is to distribute costs of shared equip- ment proportionally. For example, if all the cells have three sectors, then we can take the per-cell costs of a base station controller, tower, antennas, power, and building and count one-third of that as a per-sector cost. This is a convenient way to combine costs for a simpler calculation. Let us take this example one step further. If the average sector has four CDMA car- riers, then each carrier on a sector inherits one-quarter of the per-sector cost. If all the carriers on one