Direction of transmission flows are Simplex, Half-Duplex & Full-Duplex

Besides signal type (analog or digital) and manner of data transmission  (synchronous or asynchronous), data communications technology must also consider the direction of data traffic flow supported by communications links such as modems. 
simplex transmission

In simplex transmission, data can travel in only one direction at all times. For example, in some museum rooms, environmental devices send information about temperature, humidity, and other conditions to a computer that monitors and adjusts office environmental settings automatically. However, the computer does not send information back to the devices. The simplex mode is used occasionally in some local area networks, which we will discuss later. 
In half-duplex transmission, data can travel in two directions, but in only one direction at one time. This mode of transmission is similar to using a CB (citizens’ band) radio. When you press the transmit button you can talk, but you cannot receive. After you release the transmit button, you can receive, but you cannot transmit. Transmission of data in this mode over on distances can greatly increase the time it takes to communicate data. This delay is due to three factors: a) the time needed for device to change from receive to transmit mode, b) the time required for device A to transmit to device B a request for confirmation that all is ready for transmission, and c) the time required for device A to receive the confirmation that device B is ready to receive. The half-duplex transmission mode is frequently used for linking microcomputers via telephone lines.
In full-duplex transmission, data is sent in both directions simultaneously, similar to two trains passing in opposite directions on side-by-side tracks. This transmission mode eliminates the problem of transmission delay but it is more expensive than the other two modes because it requires special equipment. Full-duplex transmission is used primarily for mainframe communications. 

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