Telecommunication is the transmission of information of any kind such as signals, messages, images and sounds by wire, radio system, optical fiber or other electromagnetic systems. Telecommunication occurs between communication participants when there is a exchange of information that includes the use of technology. It is transmitted through a transmission media, such as over physical media, for eg, over optical fiber cable or electrical cable, or via electromagnetic radiation through space such as radio or light.

Technologies for long distance Communication in 20th- and 21st-century usually involve electrical and electromagnetic technologies, such as telegraph, telephone, and teleprinter, networks, radio, communications satellites, microwave transmission and fiber optics,

Guglielmo Marconi the great inventor & nobel laureate with his pioneering developments in radio communications, revolution in wireless communication began in the first decade of the 20th century

i) Basic element of Telecommunication

Telecommunication technologies may primarily be divided into methods, Wired and wireless. A basic telecommunication system consists of three main parts & that are:

• A transmitter takes information in any form and converts it to a signal. As the name depicts Transmitter transmits the information using any device such as antenna (specially for wireless communication) or any other electronic devices.

• A transmission medium carries the signal that is also called the physical channel . An example of this is the “free space channel, fiber optics or electrical cable”.

• A receiver that takes the signal from the transmission medium (channel) and converts it back into usable information for the recipient.

ii) Analog and Digital Communication

Analog communication is transferring the data from transmitter and receiver with the help of analog signal. Using Analog signal Any type of data such as voice, sound etc. can be transferred. Firstly, the data needs to be converted into electrical form. As voice, sound is non-electric in nature, it can be converted into electric form with the help of transducer. Then this signal is passed through the communication channel.

The crucial difference between Analog and Digital Communication is that in Analog communication analog signals are used for transmission and reception of data while in digital communication digital signals are used for transmitting and receiving data. Analog signals are the continuous time-varying signal while digital signals are those which consist of discrete values.

Digital communication provides various advantages such as it is immune to noise and distortion as it possesses greater signal to noise ratio. The uses of repeaters in case of digital communication improve the SNR (Signal to Noise Ratio) further, however in Analog communication with the use of repeaters there is more chance of distortion of signal. Moreover, digital communication having one more crucial advantage that it requires less power in comparison to analog communication.

Nowadays analog communication almost becomes obsolete & only digital communication is in use.

iii) Communication Channel

A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used to carry or convey an information signal, for example a digital bit stream, from one or several senders (or transmitters) to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second.

For sending data from one location to another requires some form of pathway or medium. These pathways or mediums, called communication channels, use two types of media: cable (twisted-pair wire, co-axial cable, and optical fibre cable) and broadcast (microwave, satellite, radio, and infrared). Wire line media use physical wires of cables to transmit data and information. Twisted-pair wire and coaxial cables are made of copper, and optical fibre cable is made of glass.

Different modes in Communication channel

Basically there are three modes of communication channels in telecommunications and computer networking & they are Simplex, half duplex and full duplex

1) Simplex

A simplex communication channel only sends information in one direction. For example, a radio station usually sends information or signals to the audience but never receives information or signals from them, thus a radio station is a simplex channel. It is also common to use simplex channel in fiber optic communication. In Fiber optic communication optical fiber is used in pair. One pair is used for transmitting signals and the other is for receiving signals. The bad part of simplex mode is that its one way communication but good part is that its entire bandwidth can be used during the transmission.

2) Half duplex

In half duplex mode, data can be transmitted in both directions on a single carrier except not at the same time. At a certain point, it is actually a simplex channel whose transmission direction can be switched. Walkie-talkie is a typical half duplex device. It has a “push-to-talk” button which can be used to turn on the transmitter but turn off the receiver. Therefore, once you push the button, you cannot hear the person you are talking to but your partner can hear you. Biggest disadvantage is that efficiency of the system reduces & advantage of half-duplex is that the single track is cheaper than the double tracks.

3) Full duplex

A full duplex communication channel is able to transmit data in both directions on a signal carrier at the same time. It is constructed as a pair of simplex links that allows bidirectional simultaneous transmission. Take an example of a voice call using telephone, people at both ends of a call can speak and be heard by each other at the same time because there are two communication paths between them. Thus the full duplex mode can greatly increase the efficiency of communication system.

iv) Modulation Technique

The process of changing the characteristics of the wave to be transmitted by superimposing the message signal on the high frequency signal is called modulation. In this process video, voice and other data signals are modified using high frequency signals – also known as carrier wave. Usually high frequency sine wave is used as a carrier wave signal.

These modulation techniques are classified into two major types: analog and digital or pulse modulation. Prior to discussing further about the different types of modulation techniques, let us understand why it is important.

Why modulation is so important in communication?

1. The message signal frequency is raised to a range using modulation so that it is more useful for transmission.
2. The signals from various sources are transmitted through a common channel simultaneously by using multiplexers. If these signals are transmitted simultaneously with certain bandwidth, they cause interference. To overcome this, speech signals are modulated to various carrier frequencies in order for the receiver to tune them to desired bandwidth of his own choice within the range of transmission.
3. Another reason is antenna size. the antenna size is inversely proportional to the frequency of the radiated signal. Antenna size is not practicable if the signal is 10 KHz. Therefore, raising frequency by modulating the signal, will certainly reduce the size of the antenna.
4. Modulation is important to transfer the signals over large distances, since it is not possible to send low-frequency signals for longer distances due to very high attenuation.
5. Modulation is also important to allocate more channels for users and to increase noise immunity.

Different Types of Modulation

The two types of modulation: analog and digital modulation techniques & in both the techniques, the baseband information is converted to Radio Frequency signals, but in analog modulation these RF communication signals are continuous range of values, whereas in digital modulation these are discrete in values.

Analog Modulation

In this modulation, a continuously varying sine wave is used as a carrier wave that modulates the message signal or data signal. The amplitude, frequency and phase of the Sinusoidal wave’s modified according to the message or information signal. So the types of analog modulation are:

• Amplitude modulation (AM)
• Frequency modulation (FM)
• Phase modulation (PM)

Analog modulation (AM, FM and PM) is more sensitive to noise. If noise enters into a system, it persists and gets carried till the end receiver & also increases with the use of repeaters. Therefore, this drawback can be overcome by the digital modulation technique.

Digital Modulation

For a better quality and efficient communication, digital modulation technique is taken over analog communication. The main advantages of the digital modulation are permissible power, available bandwidth and high noise immunity. In digital modulation, a message signal is converted from analog to digital message, and then modulated using a carrier wave.

The carrier wave is keyed or switched on and off to create pulses such that the signal is modulated. Similar to the analog communication, here the parameters like amplitude, frequency and phase variation of the carrier wave decides the type of digital modulation.

Digital modulation is of several types depending on the type of signal and application used such as Amplitude Shift Keying, Frequency Shift Keying, Phase Shift Keying, Differential Phase Shift Keying, Quadrature Phase Shift Keying, Quadrature amplitude modulation, Minimum Shift Keying, Gaussian Minimum Shift Keying, Orthogonal Frequency Division Multiplexing, etc.