developing nations, WLL may be an ancillary service to cellular with better quality. Once the cellular networks are growing, multiunit residences and businesses could add WLL terminal units. Sec- ond, WLL has some specialized uses in the first world as well. WLL is an excellent way to provide high-quality phone service to temporary locations and in emergencies. In the construction industry, projects and costs can be managed much more effectively if there is reliable telephone service to construction sites. It can be cost-effective to bring either satellite or WLL phone service into a construction site that is in a remote area and not served by landline service yet. Also, if disaster damages or overloads the local PSTN, WLL can assist. After the collapse of the World Trade Center, WLL pay phones pro- vided phone service in lower Manhattan for several months, as shown in Fig. 3.3. 3.2.2 Wireless data terminals Wireless data terminals include simple devices such as pagers, as well as full-powered laptop computers and PDAs with wireless Internet access built in. Although these devices are the first that spring to mind, wireless terminals are extremely diverse in function and in design. In this section we will discuss the units that have a human Figure 3.3 Wireless pay phone near World Trade Center, New York City. (Yoav Levy.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The User Terminal The User Terminal interface, and in the next section we will discuss fully automated units with no human interface. Wireless data terminals can be divided into two major categories. Devices such as pagers receive and send data over the Short Message Service (SMS), which transmits packets of data up to 160 characters long. The SMS protocol, more fully covered in Chap. 19, uses free space on the signaling channel of cellular networks to transmit short packets o