The physical layer must generate the electrical, optical, or wireless signals that represent the binary numbers of the encoded frame. The method of representing the binary bits is called the signaling method.
The physical layer standards must define what type of signal represents a "1" and a "0" and how they will be transmitted. This can be as simple as a change in the level of an electrical signal or optical pulse or a more complex signaling method. The receiving node must convert the signals back into bits. The bits are then examined for the start of frame and end of frame bit patterns to determine that a complete frame has been received. The physical layer then delivers all the bits of a frame to the data link layer.
As shown in Figure 1, signals can be transmitted in one of two ways:
- Asynchronous: Data signals are transmitted without an associated clock signal. The time spacing between data characters or blocks may be of arbitrary duration, meaning the spacing is not standardized; therefore, frames require start and stop indicator flags.
- Synchronous: Data signals are sent along with a clock signal which occurs at evenly spaced time. This is referred to as the bit time.
There are many ways to transmit signals. A common method to send data is using modulation techniques. Modulation is the process by which the characteristic of one wave (the signal) modifies another wave (the carrier). The following modulation techniques have been widely used in transmitting data on a medium:
- Frequency modulation (FM): A method of transmission in which the carrier frequency varies in accordance with the signal.
- Amplitude modulation (AM): A transmission technique in which the amplitude of the carrier varies in accordance with the signal.
- Pulse-coded modulation (PCM): A technique in which an analog signal, such as a voice, is converted into a digital signal by sampling the signal’s amplitude and expressing the different amplitudes as a binary number. The sampling rate must be at least twice the highest frequency in the signal.
The nature of the actual signals representing the bits on the media will depend on the signaling method in use. Some methods may use one attribute of signal to represent a single 0 and use another attribute of signal to represent a single 1.
Figure 2 illustrates the how AM and FM techniques are used to send a signal.
The video in Figure 3 is an example of amplitude modulation displayed on an oscilloscope.