In which duplex mode can an interface not send and receive data at the same time?

In half duplex mode, an interface is capable of sending and receiving data, but not simultaneously. This means that while one device is sending data, the other must wait until the transmission is complete before it can respond. This mode is akin to a walkie-talkie system, where one party must finish speaking before the other can reply. This limitation can result in delays, particularly in environments where quick back-and-forth communication is essential.

In contrast, full duplex mode allows for simultaneous sending and receiving of data, which enhances performance and reduces the potential for collision, thereby enabling smoother communication. Automatic and simultaneous are not standard duplex modes and can cause confusion as they don't apply in the same context as half and full duplex operations.