al that comes out of the DTE is compatible with the requirements of the transmission channel. So, for instance, with an analog voice-grade line, the DCE would be responsible for translating the digital data coming from the PC into an analog form that could be transmitted over that voice-grade line. A variety of different conversions (e.g., digital-to-analog conversion, conversion in voltage levels) might need to take place in a network, depending on the network service. The DCE contains the signal coding that makes these conversions possible. For example, a DCE might have to determine what voltage level to assign to a one bit and what level to assign to a zero bit. There are rules about how many of one type of bit can be sent in a row, and if too many of them are sent in sequence, the network can lose synchronization, at which point transmission errors might be introduced. The DCE applies such rules and performs the needed signal conversions. DCEs all perform essentially the Chapter 5 Data Communications Basics same generic function, but the names differ depending on the type of network ser- vice to which they're attached. Examples of DCEs include channel service units (CSUs), data service units (DSUs), network termination units, PBX data terminal interfaces, and modems. Another part of a data network is the physical interface, which defines how many pins are in the connector, how many wires are in the cable, and what signal is carried over which of the pins and over which of the wires, to ensure that the infor- mation is viewed compatibly. In Figure 5.1, the lines that join the DTE and DCE rep- resent the physical interface. There are many different forms of physical interfaces. Modems and Modulation No discussion of data communications is complete without a discussion of modula- tion. As mentioned in Chapter 1, "Telecommunications Technology Fundamentals," the term modem is a contraction of the terms modulate and demodulate, and these terms refer to the fact that a modem alters a carrier signal based on whether