Classification of Electronic Communication System
The communication systems may be classified into various cataeories as shown in fig. 1.
Fig 1: Classification of electronic communication system
It shows that the electronic communication system may be basically categorised into three groups based on :
- Whether the system is unidirectional or bidirectional
- Whether it uses as analog or digital signal
- Whether the system uses baseband transmission or uses some kind of modulation
Classification Based on Direction of Communication
Based on whether the system communicates only in one direction or otherwise, the communication systems are classified as under :
- Simplex System
- Half duplex System
- Full duplex System
Fig. 2 shows this classification .
In these systems, the information is communicated in only one direction .
For example, the radio or TV broadcasting system can only transmit, they can not receive .
Another example of simplex communication is the information transmitted by the telemetry system of a satellite to earth . The telemetry system transmits information about the physical status of the satellite such as its position or temperature .
Half duplex System
These systems are bidirectional, i.e. they can transmit as well as receive but not simultaneously .
At a time, these systems can either transmit or receive, for example, a transreceiver or walky talky set .
The direction of communication alternates . The radio communications such as those in military, fire fighting, citizen band (CB) and amateur radio are half duplex system .
Full duplex System
These are truly bidirectional systems as they allow the communication to take place in both the directions simultaneously .
These systems can transmit as well as receive simultaneously . For example, the telephone systems.
However, the bulk of electronic communications is two -way.
The best example of full duplex communication system is telephone system .
Classification Based On The Nature of Information Signal
Fig.3 shows another way of classifying the electronic communication system .
They are classified into two categories namely :
- Analog communication system
- Digital communication system
The modulation systems or techniques in which one of the characteristics of the carrier is varied in proportion with the instantaneous value of modulating signal is called as analog modulation system .
If the carrier is sinusoidal, then its amplitude, frequency or phase is changed in accordance with the modulating signal to obtain AM, FM or PM respectively . These are continuous wave modulation systems.
Analog modulation can be pulsed modulation as well . Here the carrier is in the form of rectangular pulse . The amplitude, width or position of the carrier pulses is varied in accordance with the modulating signal to obtain the PAM, PWM or PPM outputs .
Examples of analog modulation
Following are the examples of analog modulation systems :
- Amplitude modulation (AM)
- Frequency modulation (FM)
- Phase modulation (PM)
- Pulse Amplitude Modulation (PAM)
- Pulse Width Modulation (PWM)
- Pulse Position Modulation (PPM)
Advantages of analog communication
- Transmitters and receivers are simple
- Low bandwidth requirement
- FDM (Frequency division multiplexing) can be used
Drawbacks of analog communication
- Noise affects the signal quality
- It is not possible to separate noise and signal
- Repeaters can not be used between transmitter and receiver
- Coding is not possible
- It is not suitable for the transmission of secret information
- Radio broadcasting (AM and FM)
- TV broadcasting
The modulation system or technique in which the transmitted signal is in the form of digital pulses of constant amplitude, constant frequency and phase is called as digital modulation system .
Pulse code modulation (PCM) and delta modulation (DM) are the examples of digital modulation .
In PCM and DM , a train of digital pulses is transmitted by the transmitter . All the pulses are of constant amplitude, width and position . The information is contained in the combination of the transmitted pulses .
Advantages of Digital Communication
- Due to the digital nature of the transmitted signal, the interference of additive noise does not introduce many errors . Hence, digital communication has a better noise immunity .
- Due to the channel coding techniques used in digital communication, it is possible to detect and correct the errors introduced during the data transmission .
- Repeaters can be used between transmitter and receiver to regenerate the digital signal . This improves the noise immunity further .
- Due to the digital nature of the signal, it is possible to use the advanced data processing techniques such as digital signal processing, image processing, data compression etc .
- TDM (Time Division Multiplexing ) technique can be used to transmit many voice channels over a single common transmission channel .
- Digital communication is useful in military applications where only a few permitted receivers can receive the transmitted signal .
- Digital communication is becoming simpler and cheaper as compared to the analog communication due to the nvention of high speed computers and integrated circuits (ICs) .
Drawbacks of Digital Communication
- The bit rates of digital systems are high . Therefore, they require a larger channel bandwidth as compared to analog system .
- Digital modulation needs synchronization in case of synchronous modulation .
Applications of Digital Communications
- Long distance communication between earth and space ships .
- Satellite communication
- Military communication
- Telephone systems
- Data and computer communications
Classification Based on the Technique of Transmission
Based on the technique used for the signal transmission, we can categorise the electronic communication system as under :
- Baseband transmission system
- Communication systems using modulation
In baseband transmission systems, the baseband signals (original information signals) are directly transmitted .
Example of these type of systems are telephone networks where the sound signal converted into the electrical signal is placed directly on the telephone lines for transmission .
Another example of baseband transmission is computer data transmission over the coaxial cables in the computer networks .
Thus, the baseband transmission is the transmission of the original information signal as it is .
Limitation of Baseband Transmission
The baseband transmission can not be used with certain mediums e.g., it can not be used gor the radio transmission where the medium is free space . This is because the voice signal can not travel long distance in air . It gets suppressed after a short distance . Therefore, for the radio communication of baseband signals, a technique called modulation is used .
In the modulation process, two signals are used namely the modulating signal and the carrier .
The modulating signal is nothing but the baseband signal or information signal while the carrier is a high frequency sinusoidal signal .
In the modulation process, some parameter of the carrier wave (such as amplitude, frequency or phase ) is varied in accordance with the modulating signal . This modulated signal is then transmitted by the transmitter .
The receiver demodultes the received modulated signal and gets the original information signal back .
Thus, demodulation is exactly opposite to modulation .
In the process of modulation the carrier wave actually acts as carrier which carries the information signal from the transmitter to receiver .
Frequency Translation in Modulation Process
The baseband signal or modulating signal is a low frequency signal . For example, the audio signal is present in the frequency range from 20 Hz TO 20 kHz . But due to modulation, the same signal now gets translated to a higher frequency range .