Physical Layer

The bottom layer of the OSI model is the physical layer. This is where the bits hit the wire. Depending on the transmission technology, the frames will be encoded to match. Each transmission technology (Ethernet, Token Ring, FDDI) has it's own standard for how the data should be transmitted. The protocols at the above layer know what kind of technology the network is transmitting on and tells the physical layer what voltage and signaling scheme should be used. The network interface card serves as a bridge between the data link layer and the physical layer. It's the NIC's responsibility to physically encode the frames.