The physical layer is the lowest level of the OSI model and it is responsible for the actual physical connection between the network devices. It transmits information in the form of bits (0s and 1s). The bits are coded as physical states of the transmission medium. For wired cables, these are electrical signals, for optical fibers – light pulses, and for wireless connections – electromagnetic waves.
This layer includes the physical equipment involved in the data transfer, such as the cables and switches and is not concerned with the meaning of the bits. Along with the line configuration and type of signal, it also defines the topology of the network (how network devices are arranged) and the data transmission modes such as simplex mode, half-duplex mode and full-duplex mode. In simplex transmission mode, the sender can only send the data, and the receiver can only receive the data. In half-duplex mode, the sender and receiver can both send and receive the information, but only one is allowed to send at any given time. In full-duplex transmission mode, the communication between sender and receiver can occur simultaneously and they can both transmit and receive at the same time.
The physical layer mainly defines standards for devices that are used to move the data across the network. Few examples of the standards used in this layer are 10BaseT, 10Base100, CSU/DSU, DTE and DCE.