 capable of sending information at rates of 1014 bits/s. Because of the incredible promise of fiber-optic cable, these cables are being placed everywhere. They have even been placed on the ocean floor between North America and Europe, competing with and in many cases replacing satellite links. Fiber- optic links are being used to connect switching centers in POTS. They are limiting the use of coaxial cable and twisted-pair cable to areas where they have already been put in place. Telecommunication Networks 2.4 Data Communication Networks We've seen POTS, and we've studied the different mediums through which a communi- cation system can send its information. Let's take some time here to explore some of the different ways and networks (other than POTS) which are in place to help people com- municate with one another. We'll start with a discussion of data communication. So far, in all the examples we've considered, we've discussed voice communication between different locations. But now we'll talk about the transmission of data, which here we'll define to be computer communication (that is, communication between computers). One of the most common ways in which people's computers communicate is through the use of a modem and POTS (plain old telephone system), shown in Figure 2.19. Here we see a computer ready to send data on the far left. The data it wants to communicate is sent to a device called a modem, which is short for modulator/ demodular. The modem next to the computer on the left (sending computer) acts like a modulator, which means that it turns the digital signal (data) into a waveform ready for transmission over the telephone-specifically, it turns the data into a waveform that looks like a speech signal as far as the telephone is concerned (but it sure doesn't sound like one). Then, the modem is connected to the twisted-pair cable that leaves the house. The data signal enters into the five-layer telephone system POTS, and travels through this system of wires and switching centers until it arrives at the 