There's one more concept that every network planner should understand: the difference between data terminal equipment (DTE) and data communications equipment or data circuit-terminating equipment (DCE). If you're clear on these two types of network devices, you will avoid a lot of headaches caused by communication failures.
Data can move through a wire in only one direction. When a data link sends and receives signals at the same time, it must use separate wires to send data from the DTE to the DCE, and from the DCE to the DTE. Therefore, a network device uses separate inputs and outputs on the same multipin connector. The specific pin assignment is different in different connection types, but the inputs and outputs are always different pins or sockets.
The problem arises because every output must connect to an input. As Figure 2-5 shows, if you connect an output to another output, the two signals will collide; if you connect an input to an input, there's never any signal.
Therefore, when you connect two pieces of equipment, the outputs at each end must go to inputs at the other end. If Pin 2 on one device is an output, Pin 2 on the other device must be an input. Most standard data cables connect each connector pin to the same numbered pin at the other end, so connecting two devices through a cable is exactly the same as plugging one device directly into another.
That's why there are two categories of data devices. Data terminal equipment includes remote terminals, computers, some printers, and other network endpoints. Data communications equipment includes modems, hubs, switches, and other control devices. When you connect a terminal to a control device, the output pins on the DTE device connect to the input pins on the DCE device.
The problem arises when you want to connect two computers without a control device in between. Direct computer-to-computer communication requires a special cable because you can't connect a DTE device directly to another DTE device. When you connect two DTE devices with serial data ports, you connect the output on one computer to the output on the other computer, and the input to the input, so neither computer will actually receive any data. Therefore, you must flip the connections, so each output connects to an input. A cable or adapter that connects output pins to input pins is called a null modem. Figure 2-6 shows a typical null modem adapter.
Note
The "data moves in only one direction" rule does not apply to data moving through coaxial cable, which can handle inbound and outbound signals modulated at different frequencies through the same cable.