What is the lowest layer of the OSI model called?

The lowest layer of the OSI model is the Physical layer, which is responsible for the transmission and reception of raw bit streams over a physical medium. This layer defines the hardware elements involved in the network, such as cables, switches, and other network devices, as well as electrical, optical, or radio signal specifications. The Physical layer ensures that the data sent from a source device can be received correctly by a destination device by taking care of aspects like voltage levels, timing of voltage changes, physical data rates, and the layout of pins in connection interfaces.

In contrast, the Application layer, which is at the top of the model, focuses on providing network services directly to end-user applications; the Transport layer is responsible for end-to-end communication, including error recovery and flow control; and the Data link layer deals with node-to-node data transfer, framing, and addressing. Each of these layers builds upon the capabilities provided by the layers below, making the Physical layer the foundational layer that underpins the entire OSI model.