Contents
Physics Topics are also essential for space exploration, allowing scientists to study phenomena such as gravitational waves and cosmic rays.
Name Two Widely Used Transmission Media? What is Transmission Band?
Introduction
The word ‘communication’ has a widespread meaning in our daily life, as expanded by the modern technology. It means transmission of information or ideas by means of words and gestures, books and periodicals, radio-television, video, mobile, telephone and internet. The importance of communication sys-tem as exemplified by the above can hardly be exaggerated.
One-way communication: Here the transmitter only communicates message or information to the receiver, but the receiver, in exchange, cannot data communicate anything. [Fig.]
Example: Books, music, radio, TV, one-way intercom are examples of this kind.
Both-way or full-duplex or interactive communication: In this type of communication, the receiver receives information from the sender, and at the same time it communicates counter-message or information to the sender [Fig.]. In this way-there goes on a mutual message-transfer between the sender and the receiver.
Example: Exchange of dialogues (between people) in a crowd, telephone or Internet system.
Half-duplex communication: In between these two categories, there is a system where there is a scope of exchanging messages both ways, but which never happens at the same time.
Example: SMS ore-mail, through which only after the completion of sending message from one end, the counter-message can be communicated from the other end. There is no scope of simultaneous message transfer between the sender and the receiver.
In this chapter, we will concern ourselves with distant communication. For communication, within our audible and visible range, no technology is necessary. On the other hand, by applying electromagnetic waves through various technological processes, the distant communication has made an incredible development in the last 20/30 years. A list of development of communication science, is given in Fig.
Elements of Communication System
Message signal or data signal: The data or message to be transmitted, is converted into a similar electrical wave at the transmitter end. As for example, an audio wave is converted into a similar electrical wave by a microphone. The next step is to transmit this electrical wave through transmitting antenna, as a similar electromagnetic wave (i.e., as message signal or data signal) to distant places. In reality, however, the direct transmission of this data signal is practically impossible. Hence, the help of electromagnetic carrier wave is taken.
Carrier wave and transmission band: Carrier waves are used for carrying the data signal effectively from transmitter end to receiver end. The frequency of carrier wave is much higher than the frequency of the data signal i.e., its wavelength is small.
Thus the antennas of transmitters and receivers can function properly. Morever, the distortion created by superposition of different data signals at the receiver end is eliminated.
Electromagnetic wave of any frequency-chosen from a wide range of frequencies-can be used as a carrier wave. But first, the main objective of transmission has to be determined. Then the frequency of the carrier wave is chosen accordingly. For example, for radio-transmission up to 200 km, a medium wave (with frequency ≈ 3 × 105Hz-3 × 106Hz) would be adequate. For a particular transmission, the range of frequency used is called transmission band.
Communication medium or transmission medium:
To reach the receiver end, the carrier wave essentially requires a medium. A medium is selected in such a way that the wave pattern of data signal is received in an undistorted condition. Generally the media used are—
- atmosphere for wireless communication;
- coaxial cable and optical fibre for wired communication.
Communication channel: The transmission medium and the transmission band together form a communication channel. ‘A short wave is being transmitted through the atmosphere’- this statement indicates a particular communication channel with frequencies between 3 × 106Hz to 30 × 106Hz. Different zones of atmosphere are chosen for transmission of different frequencies. The technological means to be adopted for this are also predetermined. Hence, in communication science, the concept of communication channel is of paramount importance.
Terminologies used in communication system
Modulation: As the frequency of the original message or data signal is very low, it cannot travel a very long distance on its own. So, for transmitting it through a long distance, it is super imposed on a high frequency carrier wave. The process of super imposing data signal on a carrier wave of specific amplitude and frequency is called modulation.
Some important modulations are-
- Amplitude Modulation (AM),
- Frequency Modulation (FM) and
- Phase Modulation (PM).
Demodulation: Modulated wave being transmitted from transmitter end is collected at receiver end. Once again the data signal is separated from the wave. This is called demodulation. Arrangements are made to convert the separated data signal again into the original message or data. As for example, to convert demodulated audio signal into sound, loudspeaker is used.
Noise: The disturbance or distortion of the original wave form of the transmitted signal is called noise. In practice, however, when a signal is transmitted from the transmitting station to the receiving station, or from some intermediate receiving station to the next receiving station in steps, some noise gets involved with the original signal at every step. As a result, the received data cannot be exactly similar to the transmitted data.
As for example, we can easily differentiate the original voice of the speaker, from the sound emitted from the loudspeaker. Generally the noise is classified into two categories—
i) Controllable noise: Defects in the equipment or in their components give rise to this type of ‘noise: which can be largely eliminated by using improved instruments. Hence, this defect is most often pot treated as noise at all. Obviously, by using high quality loudspeaker, the degree of noise can be reduced substantially.
ii) Random noise: These noises are not at all within our control. Hence these type of noises can never be eliminated.
Block diagram of a communication system:
In Fig., the main components of a standard communication system is shown in the form of a block diagram: