Data, Signals, Transmission and Channel
Data is the entity that is to be exchanged between different computers. Signals are nothing but electrical or electromagnetic representation of data. Data transmission is nothing but communication of data by the transmission or propagation of electromagnetic signals through either wired/wireless media (channel). Thus the basic building blocks of data communication are data, signals, data transmission and communication channels.
- Electrons when propagated either in a guided media or free space creates electromagnetic waves. They are so called because they have both an electical as well as a magnetic field associated with them.
- Signals are the actual electromagnetic voltages/waves transmitted on a physical wired/wireless medium.
- Just like data, electromagnetic signals can be either analog (continuous & can take infinite values) or digital (discrete and take only a finite set of values)
- Also, electromagnetic signals can be expressed either as a function of time (time domain) or frequency (frequency/spectral domain), as shown in the diagrams given below
Analog Electromagnetic Signals
The above diagram shows the time and frequency domain representation of an analog electromagnetic signal (that of a typical human voice). As seen in the diagram, an analog signal typically consists of a range of continuous voltage values in both the time and frequency domains. In this example, in the frequency domain, the analog signal is made up of a weighted combination of frequency components with frequency values ranging from 300Hz to 3200Hz. Thus the signal bandwidth or the range of frequencies contained in this signal is equal to 2900 Hz (3200 – 300). Also, this particular signal, does not have frequency components between 0 to 300 Hz and also does not have any frequency component above 3200 Hz.
The above diagram shows the time and frequency domain representation of a digital electromagnetic signal. As seen in the diagram, in the time domain, the digital signal takes only two voltage values and it alternates between them repeatedly. In the frequency domain too, the digital signal is made up of a discrete set of frequency components, with different proportions. Typically, if it is a periodic digital signal, then the frequency components would only be made up of a fundamental frequency and its harmonics (integer multiples).
The frequency perspective of an electromagnetic signal plays a crucial role in the communication world. Wonder how different wireless transmissions like radio channels, satellite TV channels, mobile communication signals, Wireless LAN signals all coexist together in the free space medium without interference!!. The answer lies in the different bandwidth (or range of frequencies) of each of these signals . In order to receive a particular radio/TV channel, the receiver just extracts the signal corresponding to a particular frequency (or range). Thus it is extremelly important to understand the frequency/spectrum component of an electromagnetic signal.
- Though the electromagnetic signals span a wide range of frequencies, only the signals in the radio, microwave and optical region are used for data communication purposes. Radio and micro waves are used in wireless LAN, satellite, twisted pair copper and coaxial cable medias. Optical waves are used in high speed broadband fibre optical communication.
- Since X-rays, UV and gamma rays are dangerous to human beings, they are not used for communication purposes, even though they are high frequency signals capable of carrying huge amounts of information.
- Even optical signals are a type of electromagnetic signals. The only difference is that the optical signals fall in the range of visible light (that can be seen by a human eye).
- In general, higher the frequency component of a signal, more the information carrying capacity of that signal. This is because frequency of a signal indicates the rate at which the signal changes its values (or number of cycles per second for a sinusoidal signal) and so more the rate of changes of signal values per second, more the number of bits that can be transmitted, assuming that each instance of signal change represents a bit of information.
Types of Signal Transmissions used in Data Communication
In Computer communication, though data can either be analog or digital, all data are first converted into digital form before transmission.
Also, computer communication can use either analog signal transmission or digital signal transmission or both. It depends on the capability of the underlying media. For example, all wireless media typically support only analog signal transmission. Some wired media like Ethernet and Fibre support digital signal transmission.
- For transmission purposes, digital data are converted into analog or digital signals using different line encoding/modulation techniques and then transmitted through wired/wireless media.