Data Transmission I

Introduction
Part Two of this subject is deal with the transfer of data between two devices that are directly connected; that is, the two devices are linked by a single transmission path rather than a network. Even this simple context introduces numerous technical and design issues. We need to understand something about the process of transmitting signals across a communications link. Both analog and digital transmission techniques are used. In both cases, the signal can be described as consisting of a spectrum of components across a range of electromagnetic frequencies.

Data, Signals and Transmission
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. There are three contexts in communication system: data, signaling, and transmission. Briefly, these contexts define as:
Data is entities that convey meaning, or information. Signals are electric or electromagnetic representations of data.
Signaling is the physical propagation of the signal along a suitable medium.
Transmission is the communication of data by the propagation and processing of signals.

Analog and Digital Data Transmission
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 and take continuous values; 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. Digital data refers to information that has discrete states and take discrete states values; for example, a digital clock that reports the hours and the minutes will change suddenly from 8:05 to 8:06.
? Analog and Digital Signals
Like the data they represent, signals can be either analog or digital. An analog signal is a continuously varying electromagnetic wave that may be propagated over a variety of media (i.e., has infinitely many levels of intensity over a period of time or can has infinite number of values in a range). 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, is a sequence of voltage pulses that may be transmitted over a wire medium (i.e., 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 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.