The way in which data is transmitted from one place to another is called data transmission mode. It is also called the data communication mode. It indicates the direction of flow of information. Sometimes, data transmission modes are also referred to as directional modes.
When a person is giving a lecture, information is conveyed in one direction. Similarly, during a conversation between two persons, spoken messages are exchanged in both directions. These messages may be exchanged alternatively or simultaneously.
There are three data transmission modes.
(i) Simplex mode:
In simplex mode, data is transmitted in only one direction. A terminal can only send data and cannot receive it or it can only receive data but cannot send data.
Today, this mode of data communication is not popular, because most of the modem communications require two-way exchange of data. However, this mode of communication is used in business field at certain point-of-sale terminals in which sales data is entered without a corresponding reply. The other examples of simplex communication modes are Radio and T.V transmissions.
In computer system, the keyboard, monitor and printer are examples of simplex devices. The keyboard can only be used to enter data into computer, while monitor and printer can only accept (display/print) output.
(ii) Half-Duplex Mode:
In half-duplex mode, data can be transmitted in both directions but only in one direction at a time. In this mode, data is sent and received alternatively. It is like a one-lane bridge where two-way traffic must give way in order to cross the other.
In half-duplex mode, at a time only one end transmits data while other end receives. In addition, it is possible to perform error detection and request the sender to re-transmit information.
