The data communication between devices takes place using one or more of the following data flow methods
In simplex mode, the data flows only in one direction. The one device is sender and other device is always a receiver. For example printer is always a receiver, data flows from computer to printer in one direction. Keyboard is another example of simplex mode. TV and radio broadcast are also forms of simplex transmission mode of data.
One advantage of simplex mode is sender can use the entire bandwidth of the channel
In half duplex systems both terminals can communicate with each other but not at the same time. If computer A is sending at a time, then computer B is only receiving and if computer B is sending data then computer A is only receiving. Most popular example is walky-talky where only one terminal is able to send information at one time.
As only one terminal is sending at a time so full capacity of the channel can be utilized.
In full duplex, both terminals can share data simultaneously. Here no need to wait when channel will be free. Both terminals are sending data to each other at the same time. For example two people can talk to each other at the same time on a mobile phone call. You can upload and download data in parallel while using internet.
As both terminals are sharing data simultaneously so channel is shared. But other possibility is you can also use separate channels for both sides. Most recent technologies use separate upload and download channels.