Frequency division multiplexing (FDM) is a technique of multiplexing, where the users are allowed the total available bandwidth on time sharing basis. Here the time domain is divided into several recurrent slots of fixed length, and each signal is allotted a time slot on a round-robin basis.Concept and ProcessIn TDM, the data flow of each input stream is divided into units. One unit may be 1 bit, 1 byte, or a block of few bytes. Each input unit is allotted an input time slot. One input unit corresponds to one output unit and is allotted an output time slot. During ... Read More

A communication satellite is an artificial satellite that acts as a large repeater in the sky. It receives signals from the source transmitter, amplifies using transponders, and relays them to the receiver. Thus, it creates a communication channel between locations of the earth that would not have been able to communicate due to long distance or obstruction by earth’s curvature.Communication satellites may be owned by government or private organizations. Presently, there are more than 2000 communication satellites in the sky. Some of its uses are −InternetMilitary operationsTelevisionTelephoneRadioCommunication satellites are broadly categorized into three types depending upon the orbit in which ... Read More

Electromagnetic waves within the frequency range of 400 THz – 790 THz are detected by the human eye. Light transmission is line of sight propagation and is blocked by obstacles. Laser Signals Optical signalling can be obtained by laser signals. For example, the LANs in two buildings can be connected by installing laser signalling system on the rooftops. Laser rays are unidirectional. So both the transmitter and the receiver need perfectly aligned photo-emitter and photo-detector. Visual Light Communication A more used variant of light transmission is visual light communication (VLC). VLC refers to data communication using visible ... Read More

Infrared waves are those between the frequencies 300GHz and 400THz in the electromagnetic spectrum. Their wavelengths are shorter than microwaves but longer than visible light. Infrared propagation is line of sight. They cannot penetrate walls and sun’s infrared rays interfere with these rays. So cannot be used for long – range communication. As their usage is confined within closed space, they do not need any government permissions for their applications. Applications of Infrared Waves in Communications Remote controls for television, stereos and other home appliances. Wireless LANs Wireless modem, keyboard, mouse, printer etc Fire detectors Night vision ... Read More

In the electromagnetic spectrum, waves within the frequencies 1GHz to 300GHz are called microwaves.Features of MicrowavesMicrowaves travel in straight lines, and so the transmitter and receiver stations should be accurately aligned to each other.Microwave propagation is line – of – sight propagation. So, towers hoisting the stations should be placed so that the curvature of the earth or any other obstacle does not interfere with the communication.Since it is unidirectional, it allows multiple receivers in a row to receive the signals without interference.Microwaves do not pass through buildings. So, indoor receivers cannot be used effectively.Microwaves are often refracted by the ... Read More

In the electromagnetic spectrum, all omnidirectional waves in the frequencies 3KHz to 1GHz are called radio waves. They are widely used for communications since they are easy to generate, can travel long distances and can penetrate buildings. Radio waves have omnidirectional antennas, i.e. antennas that can send signals in all directions. The properties of radios waves vary according to their frequencies. However, radio waves at all frequencies are prone to interference from electrical equipments like motors etc. Low and Medium Frequency Radio Waves Low and medium frequency radio waves can pass through obstacles and have ground propagation. However, the ... Read More

The electromagnetic spectrum is the entire range of electromagnetic radiation according to the wavelength or frequencies. It has a range of frequencies from 1Hz to Hz. The waves in order of increasing frequencies are radio waves, microwaves, infrared rays, visible light, UV rays, X-rays and gamma rays.Among these range, the frequencies between Hz to Hz are used for communication. The following diagram shows the electromagnetic spectrum. The first figure gives the full range of frequency spectrum while the second figure elaborates that part of the spectrum which can be used for data communication − The ... Read More

In passband transmission, the amplitude, phase or frequency of the carrier signal is regulated to transmit the bits. The incoming data stream is modulated onto a carrier and then transmitted over a band-pass channel. The types of passband transmission are illustrated as − Amplitude Shift Keying (ASK) In ASK, the amplitude of the signal is varied to represent the signal levels, while frequency and phase remains constant. In order to represent 0 and 1, two different amplitudes are used. Frequency Shift Keying (FSK) In FSK, the frequency of the signal is modulated to represent the signal levels, while ... Read More

In baseband transmission, the data bits are directly converted into signals. Generally a higher voltage level represents the bit 1, while a lower voltage level represents bit 0.The different encoding schemes are shown in the diagram. Among these, the first three are come in the category of polar encoding. In polar signaling, one logical state is represented by only one voltage state. In bipolar schemes, two voltage levels may be used to represent a logical state.NRZ (Non – Return to Zero)NRZ is an unipolar coding scheme. Here, a high voltage represents 1, while a low voltage represents 0. Non-return to ... Read More

Fiber optic communications and satellite communications are complementary to each other. Their properties are very different from one another and consequently their usage varies.Comparison between satellite communication and optical fiber communication can be done based upon the following areas −TerrainSatellite communication is best suited for rough terrains, poorly connected areas and places where it is difficult to lay wires.On the other hand, fibers are suited for urban areas with good infrastructures, where it is convenient to lay communication lines.BandwidthFiber optic promises extremely higher bandwidth with negligible electromagnetic interference. Satellites have lesser bandwidth and are prone to interferences.Data Rate and DelayThe ... Read More