Data must be transformed to electromagnetic signals to be transmitted.
Data can be analog or digital. Analog data are continuous and take continuous values. Digital data have discrete states and take discrete values.
Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital ,signals can have only a limited number of values.
In data communications, we commonly use periodic analog signals and nonperiodic digital signals.
Frequency and period are the inverse of each other.
Frequency is the rate of change with respect to time.
Phase describes the position of the waveform relative to time O.
A complete sine wave in the time domain can be represented by one single spike in the frequency domain.
A single-frequency sine wave is not useful in data communications; we need to send a composite signal, a signal made of many simple sine waves.
According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases.
The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal.
A digital signal is a composite analog signal with an infinite bandwidth.
Baseband transmission of a digital signal that preserves the shape of the digital signal is possible only if we have a low-pass channel with an infinite or very wide bandwidth.
If the available channel is a bandpass channel, we cannot send a digital signal directly to the channel; we need to convert the digital signal to an analog signal before transmission.
For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate. For a noisy channel, we need to use the Shannon capacity to find the maximum bit rate.
Attenuation, distortion, and noise can impair a signal.
Attenuation is the loss of a signal’s energy due to the resistance of the medium.
Distortion is the alteration of a signal due to the differing propagation speeds of each of the frequencies that make up a signal.
Noise is the external energy that corrupts a signal.
The bandwidth-delay product defines the number of bits that can fill the link.