The Physical Layer
The physical layer controls how the raw bit stream of 0s and 1s is transmitted onto the physical media – e.g. the copper cable.
Data is transmitted onto the physical media by encoding the binary digits that represent data into signals.
Signaling
The physical layer must generate the electrical, optical, or wireless signals that represent the “1” and “0” on the media. The method of representing the bits is called the signaling method. The physical layer standards must define what type of signal represents a 1 and what type of signal represents a 0. This can be as simple as a change in the level of an electrical signal or optical pulse. For example, a long pulse might represent a 1, whereas a short pulse represents a 0.
This is similar to how Morse code is used for communication. Morse code is another signaling method that uses a series of on-off tones, lights, or clicks to send text over telephone wires or between ships at sea.
Play the following sound file to hear the Morse code alphabet.
It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:
What signal state represents a binary 1
How the receiving station knows when a "bit-time" starts
How the receiving station delimits a frame