Communication system : A communication system is the set-up used in the transmission of information from one place to another . The present day communication systems are electrical , electronic or optical in nature.
A schematic model of an electrical communication system is shown in figure .
It consists of three major parts:
Transmitter , Communication channel and Receiver.
A transmitter transmits the information after modifying it to a form suitable for transmission. This modification is achieved by a process called Modulation.
The communication channel carries the modulated wave from the transmitter to the receiver. A receiver reconstructs the original message after propagation through the channel. This is achieved by a process, called ‘ Demodulation ’, which is the reverse of modulation.
(i) Information : Information itself is that which is conveyed. The amount of information contained in a message is measured in bits. The set or total number of messges, consists of individual messages-which may be distinguished from one another.
(ii)Transmitter : For sending information, the incoming signals are converted into electrical variations. A transmitter is required to process and possibly encode the incoming information so as to make it suitable for transmission and subsequent reception.
In a transmitter, the information modulates the carrier i.e. information is impressed on a high frequency carrier wave. The system may involve amplitude modulation, frequency modulation, pulse modulation or any combination of these.
(iii)Communication channel : the communication channel carries the modulated wave from the transmitter to the receiver. In case of telephony and telegraphy, communication channel is a transmission line. In optical communication, an optical fibre is the communication channel and in radio / TV broadcasting, communication channel is the free space itself.
(iv)Channel Noise : The term ‘channel’ usually refers to the frequency range allocated to a particular service or transmission.
Noise is unwanted energy, usually of random character, present in a transmission system, due to some known or unknown causes. Noise may distort the information at any point in a communications system. Obviously, noise will have its greatest effect, when the signal is weakest.
(v) Receiver : The most important function of receiver is demodulation and sometimes decoding as well. Both these processes are the reverse of the corresponding transmitter (modulator) processes.
Receivers range from a very simple crystal receiver with head phones, to a complex radar receiver. The output of a receiver may be fed to a loud speaker, video display unit, tele typewriter, TV picture tube, pen recorder or computer.
Note that the transmitter and receiver must be in agreement with the modulation and coding methods used.
The schematics of an arrangement for transmission and reception of a message signal are shown in figure :
Basic Forms of Communications:
The various forms of communications in common use are :
(ii) Radio broadcast
(iii) Television broadcast
(v) Radar communications
(vi) Sonar communications
(vii) Electronic mail
(viii) Tele printing
(x) Mobile phones
On the basis of links between transmitter and receiver, the various forms of communication are :
(i) Cable communications
(ii) Ground wave communications
(iii) Sky wave communications
(iv) Satellite communications
(v) Optical communications
On the basis of the signal used, there are two forms of communication :
(i) Analog communications
(ii) Digital communications
Message signals are electrical signals generated from the original information to be transmitted, using an appropriate transducer. A message signal is a single valued function of time that conveys the information.
These signals are of two types :
(i) Analog signals
(ii) Digital signals
An analog signal is that in which current or voltage value varies continuously with time .
Figure represents the simplest form of sinusoidal analog voltage signal, having single frequency.
Mathematically , we represent such a signal as
F(t) = A sin (ωt)
Such signals can have all sorts of values at different instants, but these values shall remain within the range of a maximum value (Vmax) and a minimum value (Vmin).
The sound produced by a vibrating tuning fork provides such a signal. A microphone converts pressure variations in air, produced by sound waves into corresponding current/voltage variations.
Similarly, a photocell converts variations in light intensity to corresponding current/voltage variations .
A digital signal has two levels of current or voltage, represented by 0 and 1. Digital signals are discrete signals as shown in figure .
Zero (0) of a digital signal refers to open circuit and (1) of a digital signal refers to close circuit. 0 is also referred to as ‘ No ’ or ‘ space ’ and 1 is referred to as ‘ Yes ’ or mark.
Both 0 and 1 are called bits. A group of bits is called a binary word or a byte. A byte made of 2 bits can give four code combinations : 00, 01, 10, 11.
In general, number of code combinations = N = 2x, where x is the number of bits in a byte. Thus a popular 8 bit byte can give code combinations N = 28 = 256.
The output of a digital computer is an example of a digital signal.
An analog signal can be converted into a digital signal by sampling it in time, quantizing and coding it.
The digital communication is far more advantageous than the analog communication. This is because in the former, the receiver has to recognize whether a pulse is present or not in any prescribed time interval. Further, a large number of digital signals can be sent through a single channel only.