- Within the OSI Reference Model for the implementation of communications protocols, the first, or lowest, of the seven layers. This layer deals with the hardware aspects of sending and receiving data in the network. Layer 1 provides the specifications for the transmission medium, such as fiber-optic cable or coax, for the adapter cards, connectors and their pinouts, and so on. At the sending end this layer accepts the bit stream from layer 2, places it in the physical medium, and at the receiving end extracts the bit stream and returns it to layer 2. Also called physical layer.