The physical layer is level one in the seven level OSI model. It performs services requested by the data link layer.
This level is referring to network hardware, physical cabling or a wireless electromagnetic connection. It also deals with electrical specifications, collision control and other low-level functions.
The physical layer is the very simplest, defining only exactly what a bit is: in other words how to transmit a one or a zero. For example, you would specify at this layer things like what shapes the electrical connectors are, what frequencies to broadcast at, and what frequencies are allowed and will not blow up the network cards. In a snail-mail network, that is a network made up of people posting letters to one another, the physical layer is all about how you write and read individual letters of the alphabet.
The major functions and services performed by the physical layer are: