twork. 26.1.5 Other backhaul issues The expenses of transport in the telephone network are more complex than a simple cost function. Some links offer the option of a microwave link in which the wireless ser- vice provider has the option of paying once for equipment rather than leasing facilities with a monthly tariff. The economics of owning versus leasing are beyond the scope of the network flow model itself; rather, these tradeoffs are part of the inputs to the opti- mization and must be weighed carefully in using backhaul optimization models. Nothing in this discussion presumes that the costs depend on distance. The tele- phone network has high-capacity, low-cost transport backbones, so the cost of transport is almost entirely dependent on the distance from the base station to the backbone net- work. This means that the minimum-cost backhaul solution may look very strange on a map. The important thing is that the backhaul solution takes maximum advantage of the available transport options. The traffic engineering of the backhaul links is typically based on load during the busy period. If the busy period is the same for all the base stations, then the network flow optimization gives a good solution for minimum-cost backhaul. If there are signif- icant time-of-day variations, there may be further economies to be realized by traffic engineering the combined links in the fan-out based on their traffic loads throughout the day. Our final backhaul issue has little to do with cost. If some of the transport links are not sufficiently reliable, then we may want to engineer the backhaul network to have some al- ternate routing. A backhaul solution with higher reliability would no longer be a tree graph, no longer be a fan-out, no longer be absolutely minimum cost. However, it might be the optimal solution for providing the best level of service to wireless subscribers. 26.2 Cellular System Components In addition to transport and switches, our cellular network has a number of other com- ponents. In planning and increasin