Communication system | electronic tutorial - electronics and communication9

Communication system:

The word communication means exchange or sharing of ideas or thoughts with one another. The physical medium between transmitter and receiver is called communication channel. Channel is divided into two categories namely wired channel and wireless channel.

Signal is a physical variable which is a function of time. If the signal is a continuous time dependent variable, signal is Analog and if the signal is discrete time dependent then it is called Digital or Discrete signal.

The communication linked with analog signal is called Analog Communication and the signal, linked with digital signal is called Digital Communication. The essential components in a basic communication system are shown in the following block diagram.

Elements of a communication system

a). Source:

The message or information is generated in the information source, the information may be voice or related to it

b). Input Transducer

The input transducer converts the non-electrical signal to electrical, which is suitable for transmission. The electrical signal is called baseband or message signal.

c). Transmitter Unit

The information cannot be transmitted in its raw format through the communication channel. Transmitter unit is used to convert raw information into the format that is understandable by the communication channel. Different methods like modulation and coding may be used depending on the requirement. Mobile phones and AM radio transmitters are some of the most familiar transmitters. Modulation is employed to superimpose a low frequency message signal on to a high frequency carrier to protect the signal from getting dispersed.

d). Communication channel

Communication channel is defined as the medium through which the signal is send from transmitter to receiver. When signal is propagated through the channel, it gets affected by noise. And also channel attenuation degrades the signal strength, so signal power decreases with distance. In radio communication systems, air is the medium and in satellite communication systems, both air and vacuum are the medium.

e). Receiver

Signals sent through the communication channel reaches the receiver, where it is decoded or demodulated to extract the message. Since channel attenuation degrades the signal power, amplifier is used at the receiver to compensate for the transmission losses. Both selectivity and sensitivity of a receiver should be high only then the message signal can be extracted from the received signal.

f).Output transducer

Output transducer converts electrical signal to the original message.

Noise:

Noises are unwanted signal which interfere the original signal. A noise is always random in nature. The art of communication engineering is to suppress the noise through advanced digital signal processing technique. The function of information source is to generate signal. Input transducer converts the physical signal into electrical signal. The function of transmitter is to transmit the electrical signal into space through which the signal is being transmitted. The function of receiver is to receive the electrical signal. The transducer, connected at the receiving end converts electrical signal again into physical signal which is being collected at the receiver.in electronics and communication systems, noise is a random fluctuation or variation of an electromagnetic analog signal such as a voltage or a current.

Communication Protocol

Communication protocol is defined as the set of rules that the devices agree for communication. When messages are exchanged between two systems, parameters like rate of transmission, synchronous or asynchronous, half-duplex or full-duplex mode should be agreed to before transmission. Detecting and correction of transmission errors and encoding and decoding data will also come under communication protocol. It is implemented in both hardware and software.

Elements in communication protocol

(a). Syntax

Syntax in communication protocol defines the structure of data, that is, how each data is arranged in the transmitted data. For example, transmitted data may contain 12 bits in which first four bits represent sender’s address, next four bits shows the receiver’s address and the final four bits are for data. Syntax defines this structure only after which the receiver can understand and be able to fetch the corresponding data.

(b). Semantic

Transmitted data contains digital representation of any real time variable. Semantic defines the meaning of a particular data and how it is to be interpreted. For example, in the receiver’s address part, if data is coded as 0010 it means that data has to be moved to third module.

(c). Timing

‘When to send’ and ‘at what rate’ comes under timing protocol. If the sender produces data at one rate and receiver process at a lower rate, then there is a possibility of data loss causing incorrect interpretation or system failure.

In this method, signal is directly transmitted from the transmitter to the receiver. Most common example of baseband communication is direct conversation between two people. In this method, the signal amplitude gets distorted with the distance. If the second person is standing 200 meters away from the first person, he cannot hear what the other person is saying.

Describe why modulation is necessary for communication?

The necessity of modulation serves the following purposes.

i) Frequency shifting

basically in modulation, the frequency of modulating signal become shifted to higher frequency region. so it can be said that frequency shifting is one kind of frequency shifting is one kind of frequency translation.

ii) Frequency multiplexing

Multiplexing means many into one in modulation carrier signal is added to the modulating signal to make a common envelop upon the modulating signal. Hence frequency multiplexing is another purpose of modulation.

iii) Antenna size:

For efficient transmission the transmitting antennas should have length at least equal to a quarter of the wavelength of the signal to be transmitted. For an electromagnetic wave of frequency 15 kHz, the wavelength λ is 20 km and one-quarter of this will be equal to 5 km. Obviously, a vertical antenna of this size is impracticable. On the other hand, for a frequency of 1 MHz, this height is reduced to 75 m.

Also, the power radiated by an antenna of length l is proportional to (l/λ). This shows that for the same antenna length, power radiated is large for shorter wavelength. Thus, our signal which is of low frequency must be translated to the high frequency spectrum of the electromagnetic wave. This is achieved by the process of modulation.