The physical layer transfers a stream of bits (in the form of a signal) from the sender to the receiver. The transfer is node-to-node, from one node to the next. The physical layer of the two adjacent nodes provides a logical pipe through which the bits can travel. The logical pipe under the physical layer is transmission media (cable or air). Since a transmission medium can not carry bits, we need to represent the bits by a signal, electromagnetic energy that can propagate through a medium. Although the transmission medium determines the upper limit of the data rate, the physical layer is the controller. The design of the physical layer hardware and software determine the data rate. The timing of the bit transfer is crucial in data communications. The physical layer governs the synchronization of bits by providing clocking mechanisms that control both the sender and the receiver. The physical layer by using different techniques also provides multiplexing. Although the medium itself is not actually changed, the result is several channels instead of one. Switching in data communications can be done in several layers. We have circuit switching, packet switching and message switching. Circuit switching, a method that allows two nodes to have a dedicated link, is mostly a function of the physical layer.
