Unit – I
Communication is the process of exchange of information between two entities. It involves sending, receiving and processing of information.
The communication system is a device or setup used in the transmission of the information from one place to another. The nature of communication may be electrical, electronic or optical.
Elements of Communications System:
The Basic Elements of Communications System is shown in the following block diagram.
1. In a communications system, the transmitter is located at one place, the receiver is located at some other place (far or near) separate from the transmitter and the channel is the
physical medium that connects them.
2. Depending upon the type of communications system, a channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless.
3. The purpose of the transmitter is to convert the message signal produced by the source of information into a form suitable for transmission through the channel. If the output
of the information source is a non-electrical signal like a voice signal, a transducer converts it to electrical form before giving it as an input to the transmitter.
4. When a transmitted signal propagates along the channel it may get distorted due to channel imperfection. Moreover, noise adds to the transmitted signal and the receiver receives a
corrupted version of the transmitted signal.
5. The receiver has the task of operating on the received signal. It reconstructs a recognizable form of the original message signal for delivering it to the user of information.
Types of Communication System:
1. There are two basic modes of communication: point-to-point and broadcast.
2. In the point-to-point communication mode, communication takes place over a link between a single transmitter and a receiver.
Telephony is an example of such a mode of communication.
3. In the broadcast mode, there are a large number of receivers corresponding to a single transmitter. Radio and television are examples of the broadcast mode of communication.
The terminology of Communication System:
1. Any device that converts one form of energy into another can be termed as a transducer.
2. In electronic communication systems, we usually come across devices that have either their inputs or outputs in the electrical form.
3. An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature,
etc) into corresponding variations in the electrical signal at its output.
1. Information converted in electrical form and suitable for transmission is called a signal.
2. Signals can be either analog or digital.
3. Analog signals are continuous variations of voltage or current. They are essentially single-valued functions of time. The sine wave is a
fundamental analog signal. All other analog signals can be fully understood in terms of their sine wave components.Sound and picture signals in TV are analog in nature.
4. Digital signals are those which can take only discrete stepwise values. Binary system that is extensively used in digital electronics employs just two levels of a signal. ‘0’ corresponds to a
low level and ‘1’ corresponds to a high level of voltage/ current. There are several coding schemes used for digital communication. They employ suitable combinations of number
systems such as the binary coded decimal (BCD)*. American Standard Code for Information Interchange (ASCII)** is a universally popular digital code to represent
numbers, letters, and certain characters.
1. Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system.
2. The source generating the noise may be located inside or outside the system.
A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception
A receiver extracts the desired message signals from the received signals at the channel output.
The loss of strength of a signal while propagating through a medium is known as attenuation.
1. It is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier (reference Chapter 14).
2. Amplification is necessary to compensate for the attenuation of the signal in communication systems.
3. The energy needed for additional signal strength is obtained from a DC power source. Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength.
It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.
Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal.
1. The original low-frequency message/ information signal cannot be transmitted to long distances. Therefore, at the transmitter, information contained in the low-frequency message signal is superimposed on a high-frequency wave, which acts as a carrier of the information. This process is known as modulation.
2. There are several types of modulation, abbreviated as AM, FM, and PM.
The process of retrieval of information from the carrier wave at the receiver is termed demodulation. This is the reverse process of modulation.
1. A repeater is a combination of a receiver and a transmitter. A repeater picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency.
2. Repeaters are used to extend the range of a communications system. A communication satellite is essentially a repeater station in space.
These are the Elements of Communications System.
Bandwidth of a Signal:
1. In a communication system, the message signal can be voice, music, picture or computer data.
Each of these signals has different ranges of frequencies.
2. The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for the communication process.
a) For speech signals, frequency range 300 Hz to 3100 Hz is considered adequate. Therefore speech signal requires a bandwidth of 2800 Hz (3100 Hz – 300 Hz) for commercial telephonic communication.
b) To transmit music, an approximate bandwidth of 20 kHz is required because of the high frequencies produced by the musical instruments. The audible range of frequencies extends from 20 Hz to 20 kHz.
c) Video signals for transmission of pictures require about 4.2 MHz of bandwidth. A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission.
Bandwidth of Transmission Medium:
1. Similar to message signals, different types of transmission media offer different bandwidths.
2. The commonly used transmission media are wire, free space, and fiber optic cable.
3. Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz.
4. Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz. This range of frequencies is further subdivided and allocated for various services.
5. Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz.
6. Spectrum allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.