Now, in wider sense we may understand that a transmitter or sender may be a terminal (computer) responsible for with communication and application software controls the terminal and processes data. There may be more than one terminal connected to the sender.
Communications software is generally embedded in the computer operating system. Alternatively, it can take the form of a systems task under the control of the computer’s operating system; the role-of communications software is to assist the operating System in managing local and remote terminal access to host resources, to manage security and to perform certain check point activities.
The following Figure explains more clearly the components of data communication system with devices as DTE, DCE and medium. DTE, DCE and medium as depicted in Figure have been replaced by computer with communication software, MODEM (Modulator and demodulator) and telephone line, respectively.