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To communicate information the sender and the receiver must be able to send and receive signals through the medium with a minimum of noise or be able to filter out the noise.

Communication is the process of giving or exchanging of information, signals, or messages. In electronic communication systems signals can be either digital or analog. It involves using equipment and communications media to transport signals from one location to another.

Both digital and analog signals are used in Digital Communication. Analog signals are used in many telephone communications. Digital signals are used with digital computers sending communication. Examples of digital data are text, integers, and Morse code.

Digital signals are made up of separate units, usually represented by a series of 1's and 0's. Analog signals, on the other hand, vary continuously; an example of an analog signal is a sound wave.

An analog signal in electronic communication uses an electromagnetic/ photonic wave. This wave generally corresponds to the wave pattern of the message. Sound for example is transmitted using this signal by making the electromagnetic wave analogous to the sound waves. Shorter wave lengths represent higher frequencies and greater amplitudes represent louder sounds.

A communications system is responsible for the transmission of information

from the sender to the recipient. At its simplest, the system contains (see Figure):

• a transmission channel that is the physical link between the communicating parties.
• a modulator that takes the source signal and transforms it so that it is physically suitable for the transmission channel.
• a transmitter that actually introduces the modulated signal into the channel, usually amplifying the signal as it does so.
• a receiver that detects the transmitted signal on the channel and usually amplifies it (as it will have been attenuated by its journey through the channel).
• a demodulator that receives the original source signal from the received signal and passes it to the sink.

Communication systems are distinguished by the type of signal presented to the modulator. An analogue system is designed for transmitting analogue signals.

There is considerable scope for confusion if the use of the words analogue and digital are not qualified. An analogue communication system can transmit digital signals, so long as they are made to conform to the analogue expectations of the system. This is the function of the modem (modulator/demodulator) used to connect computers to the PSTN. On the other hand, digital systems cannot transmit analogue signals, because their greater speed relies on the digital form of the message. If the message source is analogue, like speech, then it must first be converted to a digital message by an Analog to Digital Converter (ADC).

The one disadvantage of digital communication is the increased expense of transmitters and receivers. This is particularly true of of real-time communication of analogue signals. Complex and expensive circuitry is needed to perform real-time ADC and DAC. Radio and television broadcast are analogue, because the terminal cost must be kept low. However, digital manipulation of television images is now common prior to transmission, where equipment expense is less relevant. This situation is changing. Audio and video frequency digital players (compact disc (CD), digital audio tape (DAT), digital compact cassette (DCC -- LaserDisc) have been commercially available for some time. The price to be paid for these gains is the greatly increased complexity of the encoding and decoding procedures. In particular, timing control and the correct identification of the digital structure are crucial. These problems do not exist with analogue communication.

In communications, modulation is the process by which some characteristic of a carrier wave is made to vary in accordance with an information-bearing signal wave (the modulating wave).

The original, unmodulated wave may be of any kind, such as sound or, most often, , including optical waves. The carrier wave can be a direct current, an alternating current, or a pulse chain. In modulation, it is processed in such a way that its amplitude, frequency, or some other property varies.

When used to describe the modulation, the words analogue and digital refer to the message. Analogue modulation describes the modulation of an analogue signal onto the analogue carrier. Radio, for example, uses analogue modulation. Digital modulation describes the modulation of a digital signal onto the analogue carrier. Thus analogue systems may use analogue or digital modulation; digital systems use digital modulation.

The use of digital signals and modulation has great advantages over analogue

systems. These are:

Amplitude modulation (AM) is the modulation method used in the AM broadcast band. In this system the intensity, or amplitude, of the carrier wave varies in accordance with the modulating signal wave.

When the carrier wave is thus modulated, a fraction of the power is converted to extending above and below the carrier frequency by an amount equal to the highest modulating frequency. If the modulated carrier is rectified and the carrier frequency filtered out, the modulating signal can be recovered. This form of modulation is not a very efficient way to send information; the power required is relatively large because the carrier, which contains no information, is sent along with the information.

In a variant of amplitude modulation, called single sideband modulation (SSB), the modulated signal contains only one sideband and no carrier. The information can be demodulated only if the carrier is used as a reference. This is normally accomplished by generating a wave in the receiver at the carrier frequency. SSB modulation is used for long-distance telephony (such as in the amateur radio bands) and telegraphy over land and submarine cables.

In frequency modulation (FM), the frequency of the carrier wave is varied in such a way that the change in frequency at any instant is proportional to another signal that varies with time.

Its principal application is also in radio, where it offers increased immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth. The FM band has become the choice of music listeners because of its low-noise, wide-bandwidth qualities; it is also used for the audio portion of a television broadcast.

Phase modulation, like frequency modulation, is a form of angle modulation (so called because the angle of the sinewave carrier is changed by the modulating wave). The two methods are very similar in the sense that any attempt to shift the frequency or phase is accomplished by a change in the other.

Pulse modulation involves modulating a carrier that is a train of regularly recurrent pulses. The modulation might vary the amplitude (PAM or pulse amplitude modulation), the duration (PDM or pulse duration modulation), or the presence of the pulses (PCM or pulse code modulation). PCM can be used to send digital data; audio signals on a use pulse code modulation. Developed in 1939 by the English inventor Alec H. Reeves, pulse code modulation is the most important form of pulse modulation because it can be used to transmit information over long distances with hardly any interference or distortion; for this reason it has become increasingly important in the transmission of data in the space program and between computers. Although PCM transmits digital instead of analog signals, the modulating wave is continuous. Digital modulation begins with a digital modulating signal. The two most common digital modulating techniques are phase-shift keying (PSK) and frequency-shift keying (FSK).

Demodulation is the process by which the original signal is recovered from the wave produced by modulation.

Data is information in the form of text and numbers. Computers are machines that can handle data digitally. Thus, a computer is most often the  Data Terminal Equipment. A computer in conjunction with a user generates data as well as receives it for further processing.

Data communication involves using equipment and communications media to transport signals from one location to another. Data communication includes the content and context of the messages along with the people using the messages.

The digital bits in data communication are the signals that flow between the two computers. It can be illustrated in the following picture.

Signals can not be seen in the picture, but are assumed to be moving across the communication network and through the modems which connect the two computers. Also not illustrated are the human beings who would be using the two computers.

Data communication involves certain well accepted codes for transmission and retreival of data, one such code is the ASCII code depicted in the table below, which is a code to convert alphabetical letters of English language into digital form.

Thus a 8 bit digital message 1000001 represents the letter A.

Modem is a device that converts between analog and digital signals. They are often used to enable computers to communicate with each other across telephone lines.

As we know already, digital signals are made up of separate units, usually represented by a series of 1's and 0's. and analog signals vary continuously.. A modem converts the digital signals of the sending computer to analog signals that can be transmitted through telephone lines. When the signal reaches its destination, another modem reconstructs the original digital signal, which is processed by the receiving computer.

If both modems can transmit data to each other simultaneously, the modems are operating in full duplex mode; if only one modem can transmit at a time, the modems are operating in half duplex mode.

To convert a digital signal to an analog one, the modem generates a carrier wave and modulates it according to the digital signal. The kind of modulation used depends on the application and the speed of operation for which the modem is designed. For example, many high-speed modems use a combination of amplitude modulation, where the amplitude of the carrier wave is changed to encode the digital information, and phase modulation, where the phase of the carrier wave is changed to encode the digital information. The process of receiving the analog signal and converting it back to a digital signal is called demodulation. The word "modem" is a contraction of its two basic functions: modulation and demodulation.

A fax (fascimile) machine is a device that converts an image ( a photograph or a written document) into electrical signals which are transmitted through telephone communication network. At the receivers end the fax machine converts the electrical signal received into a printed image.

Space communication is communication through free space. It is characterized by the absence of any medium, like wires or air, for the transmission channel. The most familiar form of space communication are radio and television. It is a system of communication employing electromagnetic waves propagated through space. Because of their varying characteristics, radio waves of different lengths are employed for different purposes.

Because electromagnetic waves in a uniform atmosphere travel in straight lines and because the earth's surface is approximately spherical, long-distance radio communication is made possible by the reflection of radio waves from the ionosphere. Radio waves shorter than about 10 m in wavelength—designated as very high, ultrahigh, and superhigh frequencies (VHF, UHF, and SHF)—are usually not reflected by the ionosphere; thus, in normal practice, such very short waves are received only within line-of-sight distances. Wavelengths shorter than a few centimeters are absorbed by water droplets or clouds; those shorter than 1.5 cm (0.6 in) may be absorbed selectively by the water vapor present in a clear atmosphere.

The propagation of electromagnetic waves through the atmosphere is strongly influenced by the atmosphere. From the point of view of wave propagation, there are two layers. The troposphere is the lowest layer of the atmosphere. It extends (typically) from the surface to a height of 50 Km. It contains all the Earth's weather, all the liquid water, most of the water vapour, most of the gaseous atmosphere, and most of the pollution. The ionosphere extends from the top of the troposphere into outer space. The ionosphere plays a crucial historical role in radio communication. It consists of oxygen molecules that are ionised by the action of the sun. During the day, the quantity of ions rises. At night, the ions recombine to form uncharged oxygen molecules. The ionisation of the atmosphere converts the ionosphere into a plasma: an electrically neutral gas of positive and negative charges.

A plasma has a wave-speed that is a strong function of frequency. The consequence of this is that, to a low frequency wave, the ionosphere behaves as a mirror. Waves are simply reflected. This permits a mode of propagation in which the wave bounces forwards and backwards between the ionosphere and the Earth (see Figure). It was this mode of propagation that permitted Marconi, against the experts' advice, to achieve cross-Atlantic radio communication, and, in the process, discover the ionosphere. The Earth itself also acts as a mirror for electromagnetic waves. As the frequency increases to around 50MHz, the ionospheric effect reduces, and at higher frequencies becomes invisible.

The ionosphere permits communication around the globe over great distances: these are the frequencies of the radio ham. However, the ionosphere is not dependable, signal strengths can vary considerably -- effect is maximum in early evening.

Space waves propagate in straigt lines only. In order to communicate through space waves between two locations not in line of sight, objects are necessary to divert them in space towards the direction of the receiver. Satellites are such objects. Satellites carry signals between stations where cable links or microwave towers cannot be built. Thus, communications satellites serve as relay stations, receiving radio signal messages from one location and transmitting them to another For example, satellites relay television signals across oceans.

An artificial satellite is a manufactured object that continuously orbits the earth or some other body in space. Most artificial satellites orbit the earth. They are used to study the universe, forecast the weather, transfer telephone calls over the oceans, to assist in the navigation of ships and aircraft, monitor crops and other resources, and observe movements of military equipment on the ground.

A communications satellite can relay several television programmes or many thousands of telephone calls at once. Communications satellites are usually put in a high altitude, geosynchronous orbit over a ground station. A ground station has a large dish aerial for transmitting and receiving radio signals. Countries and commercial organizations such as television broadcasters and telephone companies use these satellites continuously.

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Remote sensing is a technique used to gather information about an object from a remote distance.without actually touching it. We practise remote sensing with our eyes, ears, and even our skin. These sensors obtain information about the size, colour, location, and temperature of objects.

Television is also a form of remote sensing. A TV camera acts as a sensor when it picks up an image and transmits it to a studio. The image is then relayed by cable, broadcasting station, or satellite into viewers' homes. Sensors similar to TV cameras are flown in aircraft and satellites. They relay images of the earth to stations on the ground. Cloud maps used on TV weather forecasts are created from images relayed by satellites orbiting about 35,900 kilometres above the earth.

Some sensors detect invisible forms of energy, especially infrared rays (heat rays) that the earth sends out. A computer converts the data into TV pictures or photographs. The colours created by computer are called false colours because they do not correspond to the colours we normally see. Radar is a sensor that uses radio waves to make images of moving or fixed objects. Sonar uses sound waves to map the ocean floor and locate underwater objects.

Remote sensing is useful for obtaining information about the earth. Images from satellites are used for estimating crop yields and searching for mineral and petroleum deposits. Remote sensing also helps scientists understand how human activity affects the environment. For example, sensors monitor the health of forests threatened by pollution, map the destruction of tropical rain forests, and measure the warming of the earth's atmosphere known as the greenhouse effect.

We can even learn about past environments. Imaging radar has mapped stream channels under the Sahara in southern Egypt, showing that this desert once had a wetter climate.

A transmission line is a pair a conducting wires held apart by an insulator or dielectric. They come in a variety of construction geometries. The simplest and least expensive form is two-wire (ribbon) cable. Twisted pair cable consists of two wires sheathed in an insulator and twisted together. Shielded pair cable contains two wires surrounded and separated by a solid dielectric. The dielectric is contained within a copper braid, that shields the conductors from external noise sources. The entire construction is housed in a flexible, waterproof cover.

The use of this kind of cable is limited by two factors: attenuation and cross-talk. There are three principle sources of attenuation. Resistance (or impedance) losses are simply the loss resulting from the resistance of the wires. This loss is minimised by the choice of a metal with low resistivity. Copper is chosen for this reason. (Gold is even better, and is in fact used on satellites to reduce losses.) Dielectric losses are caused by the heating effects when a varying electric field passes through a dielectric (insulator). Radiation losses occur because the cable acts as an antenna. All these losses increase with frequency.

When a transmission line can act as an antenna, it can also act as a receiver. Lines prone to radiation loss are also susceptable to pick-up, or cross-talk. The first two types of transmissio lines, described above, are particularly prone to this fault. The shielded pair is designed to reduce this pick-up.

All these lines have strong attenuation at frequencies above 1MHz. They are generally used for for low bit-rate communication.

A two-wire line is standard for the connection of individual telephone receivers. Twisted pair(s) is the normal method of connection for computer terminals and short high bit-rate connections. A twisted pair can support rates of several Mb/s over short distances (metres), but over long distances (kilometres) will be completely unsuitable at these data-rates.

However, many telephone calls, especially long-distance calls, travel over coaxial cables. When used for telephone conversations, coaxials work in pairs. One coaxial carries signals in one direction, while the other handles signals from the other direction. A pair of coaxials may handle 13,200 telephone conversations at a time.

Cable is an insulated bundle of metal wires or threadlike fibres that carry electric current. Cables are widely used to distribute electric power and to transmit communications signals. They are also used to connect parts of computers and other electronic devices.

Simple cables are made up of a single pair of insulated wires twisted together. Multiconductor cables, such as telephone lines, contain hundreds--or even thousands--of conductors bound together. In many cases, multiconductor cables are enclosed in a heavy sheath made up of several layers of aluminium or plastic. Some thick cables also contain steel wire to provide strength.

Communications cables are laid underground and along the ocean floor, or they are mounted on poles. Burying cable protects it from harsh weather and keeps the land uncluttered. As a result, few new communications cables are strung above ground today. Submarine cables serve as a communications link between continents. These cables need tough outer coverings so that they can withstand strong ocean currents.

Most communications cables consist of conductors (metal wires that carry electric current) and insulation. Metals commonly used as conductors include copper and aluminium. Insulation holds the electricity in the conductors, protects the conductors, and metal shielding reduces noise interference. Interference occurs when a conductor picks up stray electricity from other conductors or from the air. Insulation is made from such non conducting materials as plastic and paper pulp.

There are two main kinds of cables that can carry especially large quantities of messages at once. They are (1) coaxial cables and (2) fibre optic cables.

Coaxial cable is the most familiar cable -- it is the cable used to connect your television ariel. It is often used for long distances, or data-rates in excess of several Mega bits per sec. Coaxial cables are made up of special conductors called coaxials. A coaxial is a copper tube with a copper wire running through its centre. Insulation holds the wire in place. The tube and the wire have the same axis (centre) and are therefore called coaxial. A typical coaxial has about the same diameter as a pencil, and a typical coaxial cable contains 22 coaxials.

A coaxial's copper tube shields the electric signals it carries from outside interference and helps prevent the signals from escaping. In addition, special amplifiers called repeaters are installed at various points along many coaxial cable systems to strengthen the signals. These amplifiers help prevent cable loss. Cable loss is the gradual weakening of signals as they travel along a cable. Repeaters consist of such electronic devices as transistors. The cable is finally surrounded in a water-proof, flexible sheath. The supreme advantage of this method of construction is its resistance to radiation losses. The outer conductor acts to shield out any external fields, whist preventing any internal fields escaping.

Cable television systems use coaxial cables to transmit TV programmes. A single cable television system can carry as many as 60 television signals at once. As a result, cable TV systems can transmit regular programmes as well as a variety of special features.

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Most cables are designed to carry more than one message at a time. For example, a process called carrier transmission enables two pairs of wires to transmit as many as 96 telephone conversations simultaneously. Each conversation is carried by an electric current travelling at a different frequency (rate of vibration) so that the individual signals do not interfere with one another. Electronic machinery at each end of the cable returns the telephone conversations to their normal frequencies.

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For many years it has been appreciated that the use of optical (light) waves as a carrier wave provides an enormous potential bandwidth. Optical carriers are in the region of 10¹³ Hz to 10¹⁶ Hz, i.e. three to six orders of magnitude higher than microwave frequencies. However, the atmosphere is a poor transmission medium for light waves. Optical communication became a widespread option only with the development of low-loss dielectric waveguide that is optical fibres.

An Optical fibre is a waveguide that carries messages in the form of light. The fibre (in its simplest form) consists of a core of glass of one refractive index, and a cladding of a slightly lower refractive index (Figure). Typical fibre dimensions are 100mm to 1500 mm diameter. Fibre optic cables consist of a bundle of glass optical fibres, which look like transparent threads. Coded light signals travel through the core of the fibres. A thin layer of glass called cladding surrounds the core and helps prevent the light from escaping.

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In simple terms, the action of a optical fibre waveguide can be partially understood by considering the rays down the fibre. A light-wave entering the fibre is either refracted into the cladding, and attenuated, or is totally internally reflected at the core/cladding boundary. In this manner it travels along the length of the fibre.

The largest fibre-optic cables can carry hundreds of thousands of telephone conversations or hundreds of television channels. Many communications companies have begun using fibre-optic cables instead of coaxial cables. No electrical interference occurs in fibre-optic cables, and there is less cable loss than there is in coaxial cables. Fibre-optic cables measure only 1 to 13 millimetres in diameter and thus take up much less space than coaxial cables.

In addition to the potential bandwidth, optical fibre communication offers a number of benefits:

Two fibre optic cables are used at a time, one cable for sending messages in each direction. Fibre optic cables are not affected by magnetic fields from motor, transformers, other wires or cables near by as are copper wires. Fibre optic cables do not carry electricity; therefore they can safely be used to transfer information in areas were the danger of electrical sparks are a problem such as in gas plants, mines, explosives plants, etc. Also it is harder to detect the presents of cable or to steal a signal from a fibre optic cable so security of the transmitted is improved.

The main negative of fibre optics communication systems is their cost. The glass fibre cables are getting cheaper, but the senders, receivers, and the detector/repeaters are still very expensive. Technology has been improving. The distance that can be obtained without having to install a detector/repeater has increased greatly.

Special lasers are used to transmit the coded light signals through fibre-optic cables. The lasers flash on and off at extremely high speeds. Electronic machinery at the receiving end of fibre-optic cables decodes the light signals.

When the carrier is a light wave, signal encoding is physically done either through direct modulation or external modulation of the light source. For example, varying the current of a laser diode (and therefore its light output) is a form of direct modulation.

Within this modulation method exist a variety of signal-encoding schemes that can be classified as either analog or digital. Analog transmitters change the amplitude (intensity), phase, or frequency of a light wave in a smooth, continuous fashion, while digital transmitters shift these same attributes between distinct states, like the dots and dashes of Morse code.

The compact, solid-state structure of LEDs and diode lasers, as well as their compatibility with direct intensity modulation, has made them overwhelmingly successful for fiberoptic communications (particularly diode lasers). And the invention of stable, tunable, single- frequency diode lasers such as distributed-feedback (DFB) lasers and distributed-Bragg- reflector (DBR) cavities has stimulated the growth of coherent fiberoptic communications systems for external modulation of phase or frequency.

Coherent communications systems actually require at least two single-frequency lasers, one at the transmitter and one at the receiver. With this arrangement, modulated light from the transmitting laser can be heterodyned (or homodyned) with the brighter light from the receiver's laser, which is called a local oscillator. The result is a hundredfold improvement in receiver sensitivity over simpler systems that detect the light signal directly (direct detection).

Modulation schemes for coherent communications systems include amplitude shift keying, frequency shift keying, phase shift keying (PSK), and differential phase shift keying. Homodyne PSK offers the highest sensitivity of any coherent detection system.

Another big advantage of coherent optical communications systems is that they allow a narrower channel spacing for wavelength-division multiplexing (WDM). Multiplexing refers to any of several techniques used to pack more information on a single fiber by simultaneously transmitting several signals over the same fiber. To avoid senseless gibberish at the receiving end, each signal is uniquely tagged in a way that the receiver can recognize. WDM accomplishes this by delivering each signal on a slightly different laser frequency that is then optically filtered by the receiver.