f user data. Cell phones, pagers, and other devices can use SMS. For example, when any of us chooses the option to leave a callback number, we are using SMS, but not when we leave a voice message. As a result, the person we are calling or paging can see the callback number without dialing in to pick up messages. This works because the mes- sage is transmitted over the control channel and not over a voice channel. SMS traffic is currently about 1,000,000,000 (an American billion) messages per day worldwide. However, this could increase significantly. Public institutions, such as ele- mentary and secondary school systems, are starting to use SMS broadcasts to an- nounce school closings for bad weather. As people come to expect to be notified of im- portant events wherever they are, the cellular network will supplement and in some cases perhaps replace broadcast radio and television as media of public notification and communications. Once a service becomes common and inexpensive enough to be used by public institutions, it is likely to expand rapidly as many sectors of society find uses for it that were never predicted by the planners and developers. The second category of wireless data terminals includes those devices which send large quantities of data over the cellular network traffic channels. Among these are PDAs and laptops for Internet access, as well as for access to proprietary or secure cor- porate and government networks using dedicated lines, encryption, or virtual private networks (VPNs). The demand for higher-speed data access is a major driver behind the 3G/UMTS standards initiative, as discussed in Part 9. As more people send data over wireless for more reasons, and as new technologies develop, what can we expect to see? One possible growth area is in the use of the cell phone itself, especially if Bluetooth technology takes off. Bluetooth is a wireless net- work technology designed to allow digital devices to talk to one another up to a radius of 10 m (30 ft) using very inexpensive digital transceiv