ly the modems are software-based. The military underwrote the cost to develop software- based radios and was willing to pay high prices for radio as well as modem functions in software. However, radios based in software are prohibitively expensive for com- mercial use. SDRs, which are generally located within base stations in cell sites, enable carri- ers to support multiple air interfaces in a single radio. In addition to defraying infra- structure costs, carriers now can share cell sites because of the added flexibility of supporting multiple spectrum bands and air interfaces in a single base station. SDRs dramatically lower the cost of supporting numerous types of air interfaces and of add- ing new protocols. Without SDRs, each type of air interface requires hardware control- lers in cell sites and a technician to manually install the hardware. Air interface flexibility is an important factor that allows customers to use their mobile handsets when they travel. The speed at which the radio software completes operations is a factor in supporting handsets in automobiles and trains. Reliable cell phone operation when customers are moving at high speeds depends on a network's Mobile and Wi-Fi Networks capability to support handovers between cell sites without dropping voice or data ses- sions. It also depends on networks that support many different air interfaces including 2G, 3G, and advanced 3G protocols such as High-Speed Packet Access plus (HSPA+) and High-Speed Downlink Packet Access (HSDPA). The advent of LTE, HSPA+, and Worldwide Interoperability for Microwave Access (WiMAX) increases the number of air interfaces that base stations, tablet computers, and handsets need to support. SDRs commonly support three air interfaces and five spectrum bands. There can be fewer air interfaces because the same air interface can be used on more than one spectrum band. In addition to more easily being compatible with numerous mobile protocols, SDRs enable carriers to add new air interfaces without changing hardware. With ra