ng coverage inside buildings where it is relatively inexpen- sive to add access points. This trend has been made possible by the advent of handsets with chips that sup- port both Wi-Fi and mobile air interfaces. These interfaces are now tightly integrated and have the capability to automatically hand traffic off between cell sites and Wi-Fi networks. They additionally support both voice and data. However, using voice on Wi-Fi networks drains batteries more quickly than on mobile networks. The downside of operating large numbers of small cell sites and hotspots is main- tenance. Large carriers can have 50,000 of these sites to manage. Many mobile opera- tors outsource management of their picocell sites, small cellular base stations, and hotspots to other firms. Picocells are another option for coverage inside buildings. For more information about picocells, see the section "The Microcell Technology of Pico- cells and Femtocells," earlier in this chapter. Using Wi-Fi Hot Zones to Cover Outdoor Areas Another way that carriers are starting to use Wi-Fi is for offloading data traffic from cellular networks in outside areas as well as within buildings. They do this by deploying Mobile and Wi-Fi Networks multiple access points in congested areas such as Times Square in New York City. These Wi-Fi areas are referred to as Wi-Fi hot zones. The hot zone in Times Square was built by AT&T, which is building hot zones in other areas, as well. Carriers such as PCCW Ltd in Hong Kong, and China Telecom are also constructing Wi-Fi hot zones. A hot zone consists of multiple access points that have the capability to hand traffic off to other access points and to cellular networks as people move out of the Wi-Fi coverage area. The downside of this strategy is that older phones do not automatically switch between Wi-Fi and cellular networks. In addition, Wi-Fi service drains handset bat- teries more quickly than cellular service. Wi-Fi hot zone strategies are attractive to carriers whose amount of spectrum cannot keep up with heavy da