The Physical Layer

   A Stream of Bits

Physical Layer processes are concerned only with transmitting and receiving physical signals that represent data bits. They do this without any knowledge of the meaning or structure of the data itself, just as a conveyor belt has no understanding of the packages it carries.

A Physical Layer process is only aware of the transmission medium itself, and is not aware of any communicating device that may be sending or receiving transmissions over that medium. Thus, when a Layer 1 process receives a signal, it is only aware that it is coming from a cable or radio channel. If the process is sending a signal, it is only aware that it is placing the signal onto the wire, or modulating a radio wave. It is not aware that any other device at the other end of that physical connection is either the source or destination of a signal.

Physical Layer processes use different transmission protocols, depending on the nature of the physical communication link (copper, fiber, or radio). Physical transmission protocols are concerned with issues such as:

How bits (1s and 0s) are represented. For example, a bit may be a flash of light over a fiber optic cable, high voltage on a copper cable, or change of frequency or amplitude on a radio channel.

How to tell when each bit, or a whole transmission, starts and ends. Synchronous protocols use coordinated clocks to detect the start and end of a message. Asynchronous protocols use special signals to mark each message.

Whether bits can flow in one direction only or in both directions simultaneously.