The relationship between mobility and wireless is analogous for both telephony and data communications. Wireless telephony technology should be familiar to most readers and, as such, is used for description purposes in this chapter. Wireless telephony is already ubiquitouscordless phones in the home and cell phones on the gobut how do they work and why are they different? The difference between cordless phones and mobile phones is the network infra-structure and wireless technology. Cordless phones are purchased as two parts: the handset and the base station. A handset can only talk to one base station without reprogramming. This is done to prevent our neighbors from making phone calls on our phone line. Cordless phones are often very-short-range devices; they are typically only usable within a few hundred feet of the base station. Increased range requires increased power. This is how early mobile phone systems operated: A single base station would serve an entire city. However, just as with radio and television, a limited number of channels are available to make calls. To make a business out of mobile phone service, operators needed a way to make tens of thousands of calls at a time, not just tens of calls.
Cellular telephone systems provide more efficient use of the radio spectrum. Cellular systems work by using many base stations with lower-power radios that each serve a small area. As shown in Figure 1-1, many base stations within a city can use the same channels for different calls. Building a network that allows users to maintain a call while roaming in and out of the coverage area of multiple base stations is much more complex than building a cordless phone. The need to change access links while maintaing a call is similar to the challenges of moving between LAN, MAN, and WAN data networks. Whether the topic is a cellular phone system or a mobile data network, the solution must address several challenges, which are discussed in the next section.
Figure 1-1. Broadcast Radio Technology Allows Only One User per Channel in a City, Whereas Cellular Radio Technology Allows Channels To Be Reused