s, and radio signal valleys. In making these decisions, it is also important to consider the future consequences of cell placement decisions. A few off-grid cells early in the service-providing game may not be a problem for the first year, but then it may become more difficult to grow the system to meet demand. The system plan should consider the short-term installation issues and the longer-term issues of growth. A system plan is also a financial plan and should take into account financial details such as revenue streams and cost tradeoff decisions. 32.5.1 Sharing existing towers The best starting place for cheap base stations is towers that are already there. Build- ing towers is expensive and takes a long time, and getting zoning permission for tow- ers can be even more expensive and more time-consuming. It is much quicker, easier, and cheaper to add our equipment to an existing tower (or a building that already serves as a base for radio services) than it is to get zoning permission and then build a new tower. If the tower has indoor room for base station equipment and electrical power readily available, this substantially increases the cost savings over building a new facility. Good tower locations with adequate power should be on the base station list. About half the cost of a base station is in towers and buildings, SO being able to use existing phys- ical structures may allow us to as much as double the number of cells the initial sys- tem capital can pay for. Another consideration for using existing buildings and towers is finding those loca- tions with access to low-cost telephone transport, perhaps on a high-capacity transport backbone. We should make sure that there is capacity available for backhaul on the backbone network of the public switched telephone network (PSTN). Backhaul trans- port is an ongoing expense for a wireless service provider, and having base stations where cheap transport is available is an important opportunity. Another consideration for base station location is access to mainten