Lets' start by understanding what Data communication processors are,
A data communication processor is an I/O processor that distributes and collects data from numerous remote terminals connected through telephone and other communication lines to the computer. It is a specialized I/O processor designed to communicate with data communication networks.
Such a communication network consists of variety of devices such as printers, display devices, digital sensors etc. serving many users at once. The data communication processor communicates with each terminal through a single pair of wire. It also communicates with CPU and memory in the same manner as any I/O processor does.
In a Data Communication Network, the remote terminals are connected to the data communication processor through telephone lines or other wires. Such telephone lines are specially designed for voice communication and computers use them to communicate in digital signals, therefore some conversion is required. These conversions are called modem (modulator-demodulator).
A modem converts digital signal into audio tones to be transmitted over telephone lines and also converts audio tones into digital signal for machine use.
Data can be transmitted between 2 points by three different modes:
A simplex line carries information in one direction only. In this mode receiver cannot communicate with the sender to indicate the occurrence of errors that means only sender can send data but receiver cannot. For example: Radio and Television Broadcasting.
In half duplex mode, system is capable of transmitting data in both directions but data can be transmitted in one direction only at a time. A pair of wires is needed for this mode. For example: Walkie - Talkie.
In this mode data can be send and received in both directions simultaneously. In this four wire link is used. For example: Video Calling, Audio calling etc.
The communication lines, modems and other devices used in any transmission are collectively called a Data Link. The orderly transmission of data in a data link can be accomplished by a protocol.
A Protocol is a set of rules that are followed by interconnecting devices to ensure that all data is passed correctly without any error.
There are two types of protocols:
It is based on the binary code of character set. The code is mostly used in ASCII. It includes upper case and lower case letters, numerals and variety of special symbols. The characters that control the transmission is called communication control characters.
It does not use characters in its control field and is independent of any code. It allows the transmission of serial bit stream of any length without the implication of character boundaries.