Devices attached to an Ethernet hub must operate in ___ duplex mode.

Devices attached to an Ethernet hub must operate in half duplex mode. This is because Ethernet hubs operate at the physical layer of the OSI model and use a simple broadcasting method to transmit data. In half duplex mode, a device can either send or receive data at any one time, but not both simultaneously. When one device is sending data, the others must wait until the transmission is complete before they can send their own data.

Hubs do not have the intelligence to manage traffic or to direct data packets; they merely act as repeaters, reflecting incoming signals to all connected devices. This lack of management means that collisions can occur when multiple devices attempt to send data at the same time, further necessitating the half duplex operation to prevent data loss.

In contrast, full duplex mode allows devices to send and receive data simultaneously, which is supported by switches and more advanced networking devices capable of managing data flows. However, since a hub lacks this capability, it does not support full duplex operations. The terms variable or adaptive duplex are not standardized in this context, as they do not relate directly to how hubs function.