The document discusses various communication devices and interfaces used for computer networking. It describes how modems modulate digital signals to analog for transmission over telephone lines and demodulate the analog signals back to digital. The RS-232C interface standard defines electrical signaling and connections between Data Terminal Equipment (DTE) like computers and Data Communication Equipment (DCE) like modems. Multiplexers combine multiple signals for transmission over high-speed lines and separate them at the receiving end.

1. Communication Devices