 MHz-894 MHz). The problem with that was that many people wanted to use the wireless communica- tion systems but with the limited frequencies, the systems could only support a few users. Then, an engi- neer had an idea he called the cellular Cell B concept, and wireless telecommunica- tions was changed forever. The cellular concept idea is shown in Figure 2.22. 1. You can see that the entire area is divided up into small, funny- shaped regions. Each region is called a cell (as in cells in the body). Each cell is typically between 1 mile to 12 miles long. Figure 2.22 Cellular concept Telecommunication Networks 2. Each cell has its own base station. Mobile phones in a cell communicate through the base station in its cell. 3. The transmit power of a mobile phone in a cell is kept low. Any transmissions in the cell A that travel in the direction of cell B are effectively zero by the time they get to cell B. 4. Cell A and B use the exact same frequencies. They can do this because they are separated in space by a distance that makes cell A's transmissions unnotice- able in cell B (and vice-versa). In this way, phone companies were able to support a large number of wireless users. When I teach this in my class, there are two common questions. I'll ask these questions here and then provide the answers. Q: What happens if you are in cell A and want to call someone in cell B? A: If you are making a call while in cell A, and the user you want to talk to is in cell B, then your call goes to the base station in cell A. It is sent from there to the MTSO (Mobile Telephone Switching Center); from there the call is sent to the base station in cell B, which sends it down to the intended listener in cell B. Q: What happens if, when driving in your car, you head out of one cell and into another? A: If you move out of one cell and into another, then the wireless communication system switches your call from communication with the base station in the old cell to communication with the base station in the new cell. 2.6 Local Area Networ