## Tuesday, November 20, 2012

### Data and Signals

* Data must be transformed into electromagnetic signals prior to transmission across a network.
* Data and signals can be either analog or digital.
* A signal is periodic if it consists of a continuously repeating pattern.
* Each sine wave can be characterized by its amplitude, frequency, and phase.
* Frequency and period are inverses of each other.
* A time-domain graph plots amplitude as a function of time.
* A frequency-domain graph plots each sine wave’s peak amplitude against its frequency.
* By using Fourier analysis, any composite signal can be represented as a combination of simple sine waves.

* The spectrum of a signal consists of the sine waves that make up the signal.
* The bandwidth of a signal is the range of frequencies the signal occupies. Bandwidth is determined by finding the difference between the highest and lowest frequency components.
* Bit rate (number of bits per second) and bit interval (duration of 1 bit) are terms used to describe digital signals.
* A digital signal is a composite signal with an infinite bandwidth.
* Bit rate and bandwidth are proportional to each other.
* The Nyquist formula determines the theoretical data rate for a noiseless channel.
* The Shannon capacity determines the theoretical maximum data rate for a noisy channel.
* Attenuation, distortion, and noise can impair a signal.
* Attenuation is the loss of a signal’s energy due to the resistance of the medium.
* The decibel measures the relative strength of two signals or a signal at two different points.
* 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.
* We can evaluate transmission media by throughput, propagation speed, and propagation time.
* The wavelength of a frequency is defined as the propagation speed divided by the frequency.

Reference:
Data Communication and Networking
Fourth Edition

Behrouza A. Forouzan