The communication system is a collection of individual telecommunications networks, transmission systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interoperation to form an integrated whole.
Communication pervades all stages of life of all living creatures. Irrespective of its nature, every communication system has three essential elements transmitter, medium/channel and receiver.
There are two basic modes of communication: point-to-point and broadcast. In 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. In contrast, in the broadcast mode, there are a large number of receivers corresponding to a single transmitter. Radio and television are examples of broadcast mode of communication.
Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. It is typically measured in hertz, and depending on context, may specifically refer to passband bandwidth or baseband bandwidth.
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. 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.
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.
Coaxial cable offers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz. 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.
Once the energy of the electromagnetic wave is reemitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then reemitted as an electromagnetic wave.
Ground wave propagation is a type of radio propagation which is also known as a surface wave. These waves propagate over the earth's surface in low and medium frequencies. These are mainly used for transmission between the surface of the earth and the ionosphere. These are made up of the number of constituent waves.
In space wave propagation radio wave moves along line of sight or reflected by the troposphere of reflected by the surface of earth and is received by the receiving antenna. In sky wave propagation radio waves are reflected by the ionosphere and then those are received by the receiving antenna.
Space wave propagation is defined for the radio waves that occur within the 20km of the atmosphere ie; troposphere, comprising of a direct and reflected waves. These waves are also known as tropospheric propagation as they can travel directly from the earth's surface to the troposphere surface of the earth.
Modulation allows us to send a signal over a bandpass frequency range. If every signal gets its own frequency range, then we can transmit multiple signals simultaneously over a single channel, all using different frequency ranges. Another reason to modulate a signal is to allow the use of a smaller antenna.
Ideally, the aerial needs to be the same length as the wavelength for best possible reception. However, at many frequencies, the wavelength is extremely long, making it impractical to make aerials that are the full wavelength. FM radio for example, would need aerials of a little over three meters long.
Effective radiated power (ERP), synonymous with equivalent radiated power, is an IEEE standardized definition of directional radio frequency (RF) power, such as that emitted by a radio transmitter. It is equal to the input power to the antenna multiplied by the gain of the antenna.
Mixing up of signals from different transmitters results in signal interference and can cause a big problem in communication systems. A signal wave gets generated that may be deviated from the original waveform and hence is of no use.
In amplitude modulation the amplitude of the carrier is varied in accordance with the information signal. Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio carrier wave. In amplitude modulation, the amplitude (signal strength) of the carrier wave is varied in proportion to that of the message signal, such as an audio signal.
Modulating signal is added to the carrier signal and the resultant signal is passed through the square law device. The unrequired frequency components are rejected from this signal with the help of a band-pass filter and the resultant is used as the modulated wave.
The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore to be followed by an amplifier and a detector. In addition, to facilitate further processing, the carrier frequency is usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the detection. The detected signal may not be strong enough to be made use of and hence is required to be amplified.
It is a system with billions of users worldwide. It permits communication and sharing of all types of information between any two or more computers connected through a large and complex network. It was started in 1960’s and opened for public use in 1990’s. With the passage of time it has witnessed tremendous growth and it is still expanding its reach. Its applications include
(i) E mail – It permits exchange of text/graphic material using email software. We can write a letter and send it to the recipient through ISP’s (Internet Service Providers) who work like the dispatching and receiving post offices.
(ii) File transfer – A FTP (File Transfer Programmes) allows transfer of files/software from one computer to another connected to the Internet.
(iii) World Wide Web (WWW) – Computers that store specific information for sharing with others provide websites either directly or through web service providers. Government departments, companies, NGO’s (Non-Government Organisations) and individuals can post information about their activities for restricted or free use on their websites.
(iv) E-commerce – Use of the Internet to promote business using electronic means such as using credit cards is called E-commerce. Customers view images and receive all the information about various products or services of companies through their websites.
(v) Chat – Real time conversation among people with common interests through typed messages is called chat. Everyone belonging to the chat group gets the message instantaneously and can respond rapidly.
It scans the contents of a document (as an image, not text) to create electronic signals. These signals are then sent to the destination (another FAX machine) in an orderly manner using telephone lines. At the destination, the signals are reconverted into a replica of the original document. Note that FAX provides image of a static document unlike the image provided by television of objects that might be dynamic.
The concept of mobile telephony was developed first in 1970’s and it was fully implemented in the following decade. The central concept of this system is to divide the service area into a suitable number of cells centred on an office called MTSO (Mobile Telephone Switching Office). Each cell contains a low-power transmitter called a base station and caters to a large number of mobile receivers (popularly called cell phones). Each cell could have a service area of a few square kilometers or even less depending upon the number of customers.
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