nes in POTS and local area networks (LANs), discussed a little later in this chapter. 2.3.2 Terrestrial Microwave Another way in which information can be sent from one point to another is by use of (1) a modulator, which turns the incoming information signal into a high-frequency electrical signal on a wire; and (2) an antenna, which turns the high-frequency signal into an electromagnetic wave sent through the atmosphere. At the receiver side, you use (1) a receiver antenna, which picks up the incoming electromagnetic wave and turns it back into the high-frequency electrical signal, and (2) a demodulator, which returns the high-frequency electrical signal back to the original information signals. Some examples of communication systems which send information in this way are radio stations, wireless communication systems (later in this chapter), and terrestrial microwave, which I'll explain right now SO you can get a better understanding of this idea. A terrestrial microwave transmitter is shown in Figure 2.14. I'll explain its work- ings here in three points. 1. In Figure 2.14 you see the incoming information signal. In this example, the incoming information signal contains voice signals; specifically, it contains two DS-3 signals combined on a single wire using TDM methods. 2. The incoming information signal enters a modulator, and the modulator maps the incoming signal into a high-frequency electrical signal. For example, the modulator may map the incoming signal SO that it is now centered around 3 GHz, 11 GHz, or 23 GHz. 3. The antenna takes the incoming electrical signal and maps it into an electro- magnetic wave of frequency 3 GHz, 11 GHz, or 23 GHz (for example). These frequencies correspond to microwave frequencies on the EM (electromagnetic) spectrum, SO the system is called a microwave system. Telecommunication Networks Between the transmitter and receiver we place some devices called repeaters, shown in Figure 2.15. Repeaters are placed every 26 miles (40 kilometers). The re- peater may do one of two thi