A half duplex transmission is a transmission that can be sent in both directions (sending and receiving), but not at the same time. For instance, on a local area network that uses half duplex one workstation can send the data on the network, and then that workstation can immediately receive data from the direction that the data was sent from.
Duplex systems are used in many different communications networks. They allow communication in one of two ways. The first is a communication or a two way street between two connecting parties. The second is a reverse path connection. This reverse path connection can be used for the monitoring of and adjustment of equipment in the field.
With half duplex transmissions one party will start to send data to a second party and the second party must wait for that data to complete the transmission before they are able to respond and send data back to the first party. The reason for this is because both parties receive and send data on the same frequency.
These half duplex systems are typically used to conserve bandwidth. Since only one direction can use the system at a time there is not a high volume of bandwidth usage. Half duplex systems are usually controlled by the hardware.
This allows no waste of the channel for switching. For an example a station we will call A will be allotted a time of 1 second to send data to station B. Then station be will be allotted the same amount of time to reply to station A. This allows for full control over the system and less instances of data collision within the system.