The physical layer (layer 1) serialises the bytes (i.e. converts it to a series of bits/bauds) and sends it across a communications circuit (i.e. cable) on a link towards the destination (or an intermediate) system. There are a number of types of cable (sometimes called "media") that may be used.
The physical layer defines the representation of ecah bit as a voltage, current, phase, or frequency. Various encoding schemes are used:
At the receiver, the remote system reassembles the series of bits to form a frame and forwards the frame for processing by the link layer. A clock (timing signal) is needed to identify the boundaries between the bits (in practice it is preferable to identify the centre of the bit - since this usually indicates the point of maximum signal power). There are two systems used to providing timing: