Duplex communication refers to the direction of data transmission between two devices. Half-duplex communications limit the exchange of data to one direction at a time while full-duplex allows the sending and receiving of data at the same time.
In this type of communication where one side can talk at a time. When one side has complete transmitting its data, the other side can reply. Only one node can talk at a time. If both try to talk at the same time, a collision will take place on the network. So both device can transmit and receive on the media but cannot do so at the same time. The half-duplex mode is used in legacy bus topologies and with Ethernet hubs. WLANs also operate in half-duplex. Half-Duplex is used with contention-based access methods. this method of communication is not very efficient and requires more time to send/receive larger amounts of data. Older networks work in half-duplex mode, due to the constraints of the network medium (coax cable) and hardware equipment (hubs). Figure 3.22 illustrate the half-duplex communication.
Full-duplex is the type of communication where Both devices can transmit and receive on the media at the same time. The data link layer assumes that the media is available for transmission for both nodes at any time. Of this type, there is no danger of a collision and therefore the transfer of data is completed much faster. Ethernet switches operate in full-duplex mode by default but can operate in half-duplex if connecting to a device such as an Ethernet hub. Figure 3.23 shows full-duplex communication.