Data and Signals
One of the major functions of the physical layer is to move data in the form of electromagnetic signals across a transmission medium. Whether you are collecting numerical
statistics from another computer, sending animated pictures from a design workstation,
or causing a bell to ring at a distant control center, you are working with the transmission of data across network connections.
Generally, the data usable to a person or application are not in a form that can be
transmitted over a network. For example, a photograph must first be changed to a form
that transmission media can accept. Transmission media work by conducting energy
along a physical path.
ANALOG AND DIGITAL
Both data and the signals that represent them can be either analog or digital in form.
Analog and Digital Data
Data can be analog or digital. The term analog data refers to information that is continuous; digital data refers to information that has discrete states. For example, an analog
clock that has hour, minute, and second hands gives information in a continuous form;
the movements of the hands are continuous. On the other hand, a digital clock that
reports the hours and the minutes will change suddenly from 8:05 to 8:06.
Analog data, such as the sounds made by a human voice, take on continuous values.
When someone speaks, an analog wave is created in the air. This can be captured by a
microphone and converted to an analog signal or sampled and converted to a digital
signal.
Digital data take on discrete values. For example, data are stored in computer
memory in the form of Os and 1s. They can be converted to a digital signal or modulated into an analog signal for transmission across a medium.
Analog and Digital Signals
Like the data they represent, signals can be either analog or digital. An analog signal
has infinitely many levels of intensity over a period of time. As the wave moves from
value A to value B, it passes through and includes an infinite number of values along its
path. A digital signal, on the other hand, can have only a limited number of defined
values. Although each value can be any number, it is often as simple as 1 and O.
The simplest way to show signals is by plotting them on a pair of perpendicular
axes. The vertical axis represents the value or strength of a signal. The horizontal axis
represents time. Figure 3.1 illustrates an analog signal and a digital signal. The curve
representing the analog signal passes through an infinite number of points. The vertical
lines of the digital signal, however, demonstrate the sudden jump that the signal makes
from value to value.
