Communications media can be classified in terms of whether or not they can send messages in two directions. In the vernacular of communications, transmission mode is said to be simplex, half-duplex, or full-duplex.
1. In simplex transmission, data can be transmitted only in a single, prespecified direction. An example from everyday life is a doorbell- the signal can go only from the button to the chime.
Although simplex lines are cheap, they are uncommon for computer-based telecommunications applications, which generally involve two-way communication. Even devices that are designed primarily to receive information, such as printers, communicate an acknowledgment back to the sender device.
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2. In half-duplex transmission, messages can be carried in either direction, but only one way at a time.
The press-to-talk radio phones used in police cars employ this mode of transmission; only one person can talk at a time- Often the line between a terminal and its host CPU is half-duplex.
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If the computer is transmitting to the terminal, the operator cannot send new messages until the computer if finished.
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3. Full-duplex transmission is like traffic on a busy two-way street: The flow moves in two directions at the same time. Full-duplexing is ideal for hardware units that need to pass large amounts of data between themselves, as in computer-to-computer communication.
Full-duplex channels generally are not needed for terminal-to-host links, because usually depends on the results sent back from the computer.