Most Unix platforms offer powerful serial connections that support both modem and terminal services, such as remote login and file transfer services. A properly wired and configured serial interface is a basic requirement that is common to both types of services.
The standard interface used to perform external modems/terminal connections is RS232C/D. It defines the interface between DTE and DCE. In practical terms, and for the purposes of this section, this means that it defines the physical interface between a computer that is the Data Terminal Equipment (the DTE) and the modem that is the Data Communication Equipment (the DCE). It also defines the connection between two DTEs.
The RS232C/D standard interface ...