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Sunday, March 27, 2011

Electrical engineering

Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronicsand electromagnetism(SONY Vaio VGN-CR11Z/R battery). The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical power supply. It now covers a range of subtopics including power(SONY Vaio VGN-CR13/B battery), electronics, control systems, signal processing and telecommunications.

Electrical engineering may include electronic engineering. Where a distinction is made, usually outside of the United States, electrical engineering is considered to deal with the problems associated with large-scale electrical systems such as power transmission and motor control(SONY Vaio VGN-CR13/L battery), whereas electronic engineering deals with the study of small-scale electronic systems including computers and integrated circuits. Alternatively, electrical engineers are usually concerned with using electricity to transmitenergy(SONY Vaio VGN-CR13/P battery), while electronic engineers are concerned with using electricity to process information. More recently, the distinction has become blurred by the growth of power electronics(SONY Vaio VGN-CR13/R battery).

History

Electricity has been a subject of scientific interest since at least the early 17th century. The first electrical engineer was probably William Gilbertwho designed the versorium: a device that detected the presence of statically charged objects(SONY Vaio VGN-CR13/W battery). He was also the first to draw a clear distinction between magnetism and static electricity and is credited with establishing the term electricity. In 1775 Alessandro Volta's scientific experimentations devised the electrophorus(SONY Vaio VGN-CR13G battery), a device that produced a static electric charge, and by 1800 Volta developed the voltaic pile, a forerunner of the electric battery.

However, it was not until the 19th century that research into the subject started to intensify. Notable developments in this century include the work of Georg Ohm(SONY Vaio VGN-CR13G/B battery), who in 1827 quantified the relationship between the electric current and potential difference in a conductor, Michael Faraday, the discoverer of electromagnetic induction in 1831, and James Clerk Maxwell, who in 1873 published a unified theory of electricity and magnetism in his treatise Electricity and Magnetism(SONY Vaio VGN-CR13G/L battery).

During these years, the study of electricity was largely considered to be a subfield of physics. It was not until the late 19th century that universitiesstarted to offer degrees in electrical engineering(SONY Vaio VGN-CR13G/W battery). The Darmstadt University of Technology founded the first chair and the first faculty of electrical engineering worldwide in 1882. In the same year, under Professor Charles Cross, the Massachusetts Institute of Technologybegan offering the first option of Electrical Engineering within a physics department(SONY Vaio VGN-CR13G/P battery). In 1883 Darmstadt University of Technology and Cornell University introduced the world's first courses of study in electrical engineering, and in 1885 the University College London founded the first chair of electrical engineering in the United Kingdom(SONY Vaio VGN-CR13G/R battery). The University of Missouri subsequently established the first department of electrical engineering in the United States in 1886.

During this period, the work concerning electrical engineering increased dramatically. In 1882(SONY Vaio VGN-CR13T/L battery), Edison switched on the world's first large-scale electrical supply network that provided 110 volts direct current to fifty-nine customers in lower Manhattan. In 1884 Sir Charles Parsonsinvented thesteam turbine which today generates about 80 percent of the electric power in the world using a variety of heat sources(SONY Vaio VGN-CR13T/P battery). In 1887, Nikola Teslafiled a number of patents related to a competing form of power distribution known as alternating current. In the following years a bitter rivalry between Tesla and Edison, known as the "War of Currents", took place over the preferred method of distribution(SONY Vaio VGN-CR13T/R battery). AC eventually replaced DC for generation and power distribution, enormously extending the range and improving the safety and efficiency of power distribution.

The efforts of the two did much to further electrical engineering—Tesla's work on induction motors and polyphase systems influenced the field for years to come(SONY Vaio VGN-CR13T/W battery), while Edison's work on telegraphy and his development of the stock ticker proved lucrative for his company, which ultimately became General Electric. However, by the end of the 19th century, other key figures in the progress of electrical engineering were beginning to emerge(SONY Vaio VGN-CR15/B battery).

Modern developments

During the development of radio, many scientists and inventors contributed to radio technology and electronics. In his classic UHF experiments of 1888, Heinrich Hertz transmitted (via a spark-gap transmitter) and detected radio waves using electrical equipment(SONY Vaio VGN-CR190 battery). In 1895, Nikola Tesla was able to detect signals from the transmissions of his New York lab at West Point (a distance of 80.4 km / 49.95 miles). In 1897, Karl Ferdinand Braun introduced the cathode ray tube as part of an oscilloscope(SONY Vaio VGN-CR190E/L battery), a crucial enabling technology forelectronic television. John Fleminginvented the first radio tube, the diode, in 1904. Two years later, Robert von Lieben and Lee De Forest independently developed the amplifier tube, called the triode. In 1895, Guglielmo Marconi furthered the art of hertzian wireless methods(SONY Vaio VGN-CR190E/P battery). Early on, he sent wireless signals over a distance of one and a half miles. In December 1901, he sent wireless waves that were not affected by the curvature of the Earth. Marconi later transmitted the wireless signals across the Atlantic between Poldhu, Cornwall, and St. John's, Newfoundland, a distance of 2,100 miles (3,400 km) (SONY Vaio VGN-CR190E/R battery). In 1920 Albert Hull developed the magnetron which would eventually lead to the development of the microwave oven in 1946 byPercy Spencer. In 1934 the British military began to make strides toward radar (which also uses the magnetron) under the direction of Dr Wimperis(SONY Vaio VGN-CR190E/W battery), culminating in the operation of the first radar station atBawdsey in August 1936.

In 1941 Konrad Zuse presented the Z3, the world's first fully functional and programmable computer(SONY Vaio VGN-CR21/B battery). In 1946 the ENIAC (Electronic Numerical Integrator and Computer) of John Presper Eckert and John Mauchlyfollowed, beginning the computing era. The arithmetic performance of these machines allowed engineers to develop completely new technologies and achieve new objectives(SONY Vaio VGN-CR21E/L battery), including the Apollo missions and theNASA moon landing.

The invention of the transistor in 1947 by William B. Shockley, John Bardeen and Walter Brattain opened the door for more compact devices and led to the development of the integrated circuit in 1958 by Jack Kilbyand independently in 1959 by Robert Noyce(SONY Vaio VGN-CR21E/P battery). Starting in 1968, Ted Hoffand a team at Intel invented the first commercial microprocessor, which presaged the personal computer. The Intel 4004 was a 4-bit processor released in 1971, but in 1973 the Intel 8080, an 8-bit processor, made the first personal computer, the Altair 8800, possible(SONY Vaio VGN-CR21E/W battery).

Education

Electrical engineers typically possess an academic degree with a major in electrical engineering. The length of study for such a degree is usually four or five years and the completed degree may be designated as aBachelor of Engineering, Bachelor of Science(SONY Vaio VGN-CR21S/L battery), Bachelor of Technologyor Bachelor of Applied Science depending upon the university. The degree generally includes units covering physics, mathematics, computer science, project management and specific topics in electrical engineering(SONY Vaio VGN-CR21S/P battery). Initially such topics cover most, if not all, of the sub-disciplines of electrical engineering. Students then choose to specialize in one or more sub-disciplines towards the end of the degree.

Some electrical engineers also choose to pursue a postgraduate degree such as a Master of Engineering/Master of Science (M.Eng./M.Sc.) (SONY Vaio VGN-CR21S/W battery), a Master of Engineering Management, a Doctor of Philosophy (Ph.D.) in Engineering, an Engineering Doctorate (Eng.D.), or an Engineer's degree. The Master and Engineer's degree may consist of either research, coursework or a mixture of the two(SONY Vaio VGN-CR21Z/N battery). The Doctor of Philosophy and Engineering Doctorate degrees consist of a significant research component and are often viewed as the entry point to academia. In the United Kingdom and various other European countries, the Master of Engineering is often considered an undergraduate degree of slightly longer duration than the Bachelor of Engineering(SONY Vaio VGN-CR21Z/R battery).

Practicing engineers

In most countries, a Bachelor's degree in engineering represents the first step towards professional certification and the degree program itself is certified by a professional body. After completing a certified degree program the engineer must satisfy a range of requirements (including work experience requirements) before being certified(SONY Vaio VGN-CR220E/R battery). Once certified the engineer is designated the title of Professional Engineer (in the United States, Canada and South Africa ), Chartered Engineer (in India, the United Kingdom, Ireland and Zimbabwe), Chartered Professional Engineer (in Australia and New Zealand) or European Engineer (in much of theEuropean Union) (SONY Vaio VGN-CR23/B battery).

The advantages of certification vary depending upon location. For example, in the United States and Canada "only a licensed engineer may seal engineering work for public and private clients"(SONY Vaio VGN-CR23/P battery). This requirement is enforced by state and provincial legislation such as Quebec's Engineers Act. In other countries, no such legislation exists. Practically all certifying bodies maintain a code of ethics that they expect all members to abide by or risk expulsion(SONY Vaio VGN-CR23/R battery). In this way these organizations play an important role in maintaining ethical standards for the profession. Even in jurisdictions where certification has little or no legal bearing on work, engineers are subject to contract law(SONY Vaio VGN-CR23/L battery). In cases where an engineer's work fails he or she may be subject to the tort of negligence and, in extreme cases, the charge of criminal negligence. An engineer's work must also comply with numerous other rules and regulations such as building codes and legislation pertaining to environmental law(SONY Vaio VGN-CR23/N battery).

Professional bodies of note for electrical engineers include the Institute of Electrical and Electronics Engineers (IEEE) and the Institution of Engineering and Technology (IET). The IEEE claims to produce 30% of the world's literature in electrical engineering(SONY Vaio VGN-CR23/W battery), has over 360,000 members worldwide and holds over 3,000 conferences annually. The IET publishes 21 journals, has a worldwide membership of over 150,000, and claims to be the largest professional engineering society in Europe(Dell Latitude E5400 battery). Obsolescence of technical skills is a serious concern for electrical engineers. Membership and participation in technical societies, regular reviews of periodicals in the field and a habit of continued learning are therefore essential to maintaining proficiency(Dell Latitude E6400 battery).

In Australia, Canada and the United States electrical engineers make up around 0.25% of the labor force (see note). Outside of Europe and North America, engineering graduates per-capita, and hence probably electrical engineering graduates also, are most numerous in Taiwan, Japan, and South Korea(Dell Latitude E6500 battery).

Tools and work

From the Global Positioning System to electric power generation, electrical engineers have contributed to the development of a wide range of technologies. They design, develop, test and supervise the deployment of electrical systems and electronic devices(SONY VGP-BPS10 battery). For example, they may work on the design of telecommunication systems, the operation of electric power stations, the lighting and wiring of buildings, the design ofhousehold appliances or the electrical control of industrial machinery(SONY VGP-BPL10 battery).

Fundamental to the discipline are the sciences of physics and mathematics as these help to obtain both a qualitative and quantitative description of how such systems will work. Today most engineering work involves the use of computers and it is commonplace to use computer-aided designprograms when designing electrical systems(SONY VGP-BPS22A battery). Nevertheless, the ability to sketch ideas is still invaluable for quickly communicating with others.

Although most electrical engineers will understand basic circuit theory (that is the interactions of elements such as resistors, capacitors, diodes, transistors and inductorsin a circuit) (SONY VGP-BPS22/A battery), the theories employed by engineers generally depend upon the work they do. For example, quantum mechanics and solid state physics might be relevant to an engineer working on VLSI (the design of integrated circuits), but are largely irrelevant to engineers working with macroscopic electrical systems(SONY VGP-BPS18 battery). Even circuit theorymay not be relevant to a person designing telecommunication systems that use off-the-shelf components. Perhaps the most important technical skills for electrical engineers are reflected in university programs, which emphasize strong numerical skills(SONY VGP-BPS22 battery), computer literacy and the ability to understand the technical language and concepts that relate to electrical engineering.

For many engineers, technical work accounts for only a fraction of the work they do. A lot of time may also be spent on tasks such as discussing proposals with clients(SONY VGP-BPS14B Battery), preparing budgets and determining project schedules. Many senior engineers manage a team of technicians or other engineers and for this reason project managementskills are important. Most engineering projects involve some form of documentation and strong written communication skills are therefore very important(SONY VGP-BPS14/S Battery).

The workplaces of electrical engineers are just as varied as the types of work they do. Electrical engineers may be found in the pristine lab environment of a fabrication plant, the offices of a consulting firm or on site at a mine(SONY VGP-BPS14/B Battery). During their working life, electrical engineers may find themselves supervising a wide range of individuals including scientists, electricians, computer programmers and other engineers.

Sub-disciplines

Electrical engineering has many sub-disciplines(SONY VGP-BPS14 Battery), the most popular of which are listed below. Although there are electrical engineers who focus exclusively on one of these sub-disciplines, many deal with a combination of them. Sometimes certain fields, such as electronic engineering and computer engineering, are considered separate disciplines in their own right(SONY VGP-BPS12 battery).

Power

Power engineering deals with the generation, transmission and distribution of electricity as well as the design of a range of related devices. These include transformers,electric generators, electric motors, high voltage engineering and power electronics(SONY VGP-BPL12 battery). In many regions of the world, governments maintain an electrical network called apower grid that connects a variety of generators together with users of their energy. Users purchase electrical energy from the grid, avoiding the costly exercise of having to generate their own(SONY VGP-BPL11 battery). Power engineers may work on the design and maintenance of the power grid as well as the power systems that connect to it. Such systems are calledon-grid power systems and may supply the grid with additional power, draw power from the grid or do both(SONY VGP-BPS11 battery). Power engineers may also work on systems that do not connect to the grid, called off-grid power systems, which in some cases are preferable to on-grid systems. The future includes Satellite controlled power systems, with feedback in real time to prevent power surges and prevent blackouts(Dell Studio 1457 battery).

Control

Control engineering focuses on the modeling of a diverse range of dynamic systems and the design of controllers that will cause these systems to behave in the desired manner. To implement such controllers electrical engineers may use electrical circuits(Dell Inspiron 1764 battery), digital signal processors, microcontrollers and PLCs (Programmable Logic Controllers). Control engineering has a wide range of applications from the flight and propulsion systems of commercial airliners to the cruise control present in many modern automobiles. It also plays an important role in industrial automation(Dell Inspiron 1564 battery).

Control engineers often utilize feedback when designing control systems. For example, in an automobile with cruise control the vehicle's speed is continuously monitored and fed back to the system which adjusts the motor's power output accordingly(Dell Inspiron 1464 battery). Where there is regular feedback, control theory can be used to determine how the system responds to such feedback.

Electronics

Electronic engineering involves the design and testing of electronic circuits that use the properties of components such as resistors, capacitors, inductors, diodes andtransistors to achieve a particular functionality(Dell Inspiron 1320N battery). The tuned circuit, which allows the user of a radioto filter out all but a single station, is just one example of such a circuit. Another example (of a pneumatic signal conditioner) is shown in the adjacent photograph(Dell Studio 1450 battery).

Prior to the second world war, the subject was commonly known as radio engineering and basically was restricted to aspects of communications and radar, commercial radio and early television. (Dell Inspiron 1320 battery) Later, in post war years, as consumer devices began to be developed, the field grew to include modern television, audio systems, computers andmicroprocessors. In the mid-to-late 1950s, the term radio engineering gradually gave way to the name electronic engineering(Dell Latitude E6500 battery).

Before the invention of the integrated circuit in 1959, electronic circuits were constructed from discrete components that could be manipulated by humans. These discrete circuits consumed much space and power and were limited in speed, although they are still common in some applications(Dell Latitude E6400 battery). By contrast, integrated circuits packed a large number—often millions—of tiny electrical components, mainly transistors, into a small chip around the size of a coin. This allowed for the powerful computers and other electronic devices we see today(Dell Studio 1737 battery).

Microelectronics

Microelectronics engineering deals with the design and microfabrication of very small electronic circuit components for use in an integrated circuit or sometimes for use on their own as a general electronic component(Dell Studio 1735 battery). The most common microelectronic components are semiconductortransistors, although all main electronic components (resistors, capacitors, inductors) can be created at a microscopic level. Nanoelectronics is the further scaling of devices down to nanometer levels(Dell Vostro 1710 battery).

Microelectronic components are created by chemically fabricating wafers of semiconductors such as silicon (at higher frequencies, compound semiconductors like gallium arsenide and indium phosphide) to obtain the desired transport of electronic charge and control of current(Dell Inspiron 1501 battery). The field of microelectronics involves a significant amount of chemistry and material science and requires the electronic engineer working in the field to have a very good working knowledge of the effects of quantum mechanics(Dell Inspiron 6400 battery).

Signal processing

Signal processing deals with the analysis and manipulation of signals. Signals can be either analog, in which case the signal varies continuously according to the information, or digital, in which case the signal varies according to a series of discrete values representing the information(Dell Inspiron 6000 battery). For analog signals, signal processing may involve the amplification and filtering of audio signals for audio equipment or the modulationand demodulation of signals for telecommunications. For digital signals, signal processing may involve the compression, error detectionand error correction of digitally sampled signals(Dell INSPIRON 1525 Battery).

Signal Processing is a very mathematically oriented and intensive area forming the core of digital signal processing and it is rapidly expanding with new applications in every field of electrical engineering such as communications(Dell N3010 battery), control, radar, TV/Audio/Video engineering, power electronics and bio-medical engineering as many already existing analog systems are replaced with their digital counterparts.

Although in the classical era, analog signal processing only provided a mathematical description of a system to be designed(Dell RM791 battery), which is actually implemented by the analog hardware engineers, Digital Signal Processing both provides a mathematical description of the systems to be designed and also actually implements them (either by software programming or by hardware embedding) (Dell Latitude D620 Battery) without much dependency on hardware issues, which exponentiates the importance and success of DSP engineering.

The deep and strong relations between signals and the information they carry makes signal processing equivalent of information processing. Which is the reason why the field finds so many diversified applications(Dell Inspiron E1505 Battery). DSP processor ICs are found in every type of modern electronic systems and products including, SDTV | HDTV sets, radios and mobile communication devices, Hi-Fi audio equipments, Dolby noise reductionalgorithms(Dell INSPIRON 1420 Battery, GSM mobile phones, mp3 multimedia players, camcorders and digital cameras, automobile control systems, noise cancellingheadphones, digital spectrum analyzers, intelligent missile guidance, radar, GPS based cruise control systems and all kinds of image processing, video processing, audio processing and speech processing systems(HP PAVILION DV3000 Battery).

Telecommunications

Telecommunications engineering focuses on the transmission of information across a channel such as a coax cable, optical fiber or free space. Transmissions across free space require information to be encoded in a carrier wave in order to shift the information to a carrier frequency suitable for transmission, this is known as modulation( HP PAVILION DV3 Battery). Popular analog modulation techniques include amplitude modulation and frequency modulation. The choice of modulation affects the cost and performance of a system and these two factors must be balanced carefully by the engineer(HP PAVILION DV2000 Battery).

Once the transmission characteristics of a system are determined, telecommunication engineers design the transmitters and receivers needed for such systems. These two are sometimes combined to form a two-way communication device known as a transceiver(HP PAVILION DV2 Battery). A key consideration in the design of transmitters is their power consumption as this is closely related to their signal strength. If the signal strength of a transmitter is insufficient the signal's information will be corrupted by noise(Sony VAIO PCG-5K1L battery).

Instrumentation

Instrumentation engineering deals with the design of devices to measure physical quantities such as pressure, flow and temperature. The design of such instrumentation requires a good understanding of physics that often extends beyond electromagnetic theory(Sony VAIO PCG-6W2L battery). For example, radar gunsuse the Doppler effect to measure the speed of oncoming vehicles. Similarly, thermocouples use the Peltier-Seebeck effect to measure the temperature difference between two points(Sony VAIO PCG-7112L battery).

Often instrumentation is not used by itself, but instead as the sensors of larger electrical systems. For example, a thermocouple might be used to help ensure a furnace's temperature remains constant. For this reason, instrumentation engineering is often viewed as the counterpart of control engineering(Sony VAIO PCG-8Z1L battery).

Computers

Computer engineering deals with the design of computers and computer systems. This may involve the design of new hardware, the design of PDAs or the use of computers to control an industrial plant(Sony VAIO PCG-8Z2L battery). Computer engineers may also work on a system's software. However, the design of complex software systems is often the domain of software engineering, which is usually considered a separate discipline(Sony VAIO PCG-8Y2L battery). Desktop computersrepresent a tiny fraction of the devices a computer engineer might work on, as computer-like architectures are now found in a range of devices including video game consoles and DVD players(Sony VAIO PCG-8Y1L battery).

Related disciplines

Mechatronics is an engineering discipline which deals with the convergence of electrical and mechanical systems. Such combined systems are known aselectromechanical systems and have widespread adoption(Sony VAIO PCG-7Z2L battery). Examples include automated manufacturing systems, heating, ventilation and air-conditioning systems and various subsystems of aircraft and automobiles.

The term mechatronics is typically used to refer to macroscopic systems but futurists have predicted the emergence of very small electromechanical devices(Sony VAIO PCG-7Z1L battery). Already such small devices, known as Microelectromechanical systems (MEMS), are used in automobiles to tell airbags when to deploy, in digital projectors to create sharper images and in inkjet printers to create nozzles for high definition printing(Sony VAIO PCG-7133L battery). In the future it is hoped the devices will help build tiny implantable medical devices and improveoptical communication.

Biomedical engineering is another related discipline, concerned with the design of medical equipment(Sony VAIO PCG-7113L battery). This includes fixed equipment such as ventilators, MRI scannersand electrocardiograph monitors as well as mobile equipment such as cochlear implants, artificial pacemakersand artificial hearts(Sony VAIO PCG-6W3L battery).

Saturday, March 26, 2011

Audio compression (data)

Audio compression is a form of data compression designed to reduce the transmission bandwidth requirement of digital audio streams and the storage size of audio files(SONY VGN NR11Z/S battery). Audio compression algorithms are implemented in computer software as audio codecs. Generic data compression algorithms perform poorly with audio data, seldom reducing data size much below 87% from the original, and are not designed for use in real time applications(SONY VGN NR11M/S battery). Consequently, specifically optimized audio lossless and lossy algorithms have been created. Lossy algorithms provide greater compression rates and are used in mainstream consumer audio devices(SONY VGN NR11S/S battery).

In both lossy and lossless compression, information redundancy is reduced, using methods such as coding, pattern recognition and linear prediction to reduce the amount of information used to represent the uncompressed data(SONY VGP-BPS13B/Q Battery).

The trade-off between slightly reduced audio quality and transmission or storage size is outweighed by the latter for most practical audio applications in which users may not perceive the loss in playback rendition quality(SONY VGP-BPS13Q Battery). For example, one Compact Disc holds approximately one hour of uncompressed high fidelity music, less than 2 hours of music compressed losslessly, or 7 hours of music compressed in the MP3 format at medium bit rates(SONY Vaio VGN-FW21M Battery).

Lossless audio compression

Lossless audio compression produces a representation of digital data that can be expanded to an exact digital duplicate of the original audio stream. This is in contrast to the irreversible changes upon playback from lossy compression techniques such as Vorbis and MP3(SONY Vaio VGN-FW21L Battery). Compression ratios are similar to those for generic lossless data compression (around 50–60% of original size), and substantially less than for lossy compression, which typically yield 5–20% of original size.

Applications

The primary application areas of lossless encoding are(SONY Vaio VGN-FW21J Battery):

Thursday, March 24, 2011

Telecommunication

Telecommunication is the transmission of information, over significant distances, for the purpose of communication. In earlier times, telecommunications involved the use of visual signals(Sony VGP-BPS13B/B battery), such as beacons, smoke, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded drumbeats, lung-blown horns, or sent by loud whistles, for example. In the modern age of electricity and electronics(Sony VGP-BPS13A/B battery), telecommunications now also includes the use of electrical devices such astelegraphs, telephones, and teletypes, the use of radio and microwave communications, as well as fiber optics and their associated electronics, plus the use of the orbiting satellites and the Internet(Sony VGP-BPS13/S battery).

The first breakthrough into modern electrical telecommunications came with the push to fully develop the telegraph starting in the 1830s. The use of these electrical means of communications exploded into use on all of the continents of the world during the 19th century(Sony VGP-BPS13/B battery), and these also connected the continents via cables on the floors of the ocean. The use of the first three popular systems of electrical telecommunications, the telegraph, telephone and teletype, all required the use of conducting metal wires(Sony PCGA-BP2EA battery).

A revolution in wireless telecommunications began in the first decade of the 20th century, with Guglielmo Marconi winning the Nobel Prize in Physics in 1909 for his pioneering developments in wireless radio communications(Sony VGP-BPS13AS battery). Other highly notable pioneering inventors and developers in the field of electrical and electronic telecommunications include Charles Wheatstone and Samuel Morse (telegraph), Alexander Graham Bell(telephone), Nikola Tesla, Edwin Armstrong, and Lee de Forest(radio), as well as John Logie Baird and Philo Farnsworth (television) (Sony VGP-BPS13S battery).

Telecommunications play an important role in the world economy and the worldwide telecommunication industry's revenue was estimated to be $3.85 trillion in 2008. The service revenue of the global telecommunications industry was estimated to be $1.7 trillion in 2008, and is expected to touch $2.7 trillion by 2013(Sony VGP-BPS13B/S battery).

History

Ancient systems

Greek hydraulic semaphore systems were used as early as the 4th century BCE. The hydraulic semaphores, which worked with water filled vessels and visual signals, functioned as optical telegraphs(Sony VGP-BPS13A/S battery). However, they could only utilize a very limited range of pre-determined messages, and as with all such optical telegraphs could only be deployed during good visibility conditions.

During the Middle Ages, chains of beacons were commonly used on hilltops as a means of relaying a signal(Sony VGN-FW31J battery). Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada(Sony VGP-BPS21 battery), when a beacon chain relayed a signal fromPlymouth to London that signaled the arrival of the Spanish warships.

Systems since the Middle Ages

In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system (or semaphore line) between Lille and Paris(Sony VGP-BPS21B battery). However semaphore systems suffered from the need for skilled operators and the expensive towers at intervals of ten to thirty kilometers (six to twenty miles). As a result of competition from the electrical telegraph, Europe's last commercial semaphore line in Sweden was abandoned in 1880(Sony VGP-BPS21/S battery).

The telegraph and telephone

The first commercial electrical telegraph was constructed by Sir Charles Wheatstone and Sir William Fothergill Cooke, and its use began on April 9, 1839. Both Wheatstone and Cooke viewed their device as "an improvement to the [already-existing, so-called] electromagnetic telegraph" not as a new device(Sony VGP-BPS21A/B battery).

The businessman Samuel F.B. Morse and the physicist Joseph Henry of the United States developed their own, simpler version of the electrical telegraph, independently. Morse successfully demonstrated this system on September 2, 1837(Sony VAIO PCG-5G2L battery). Morse's most important technical contribution to this telegraph was the rather simple and highly efficient Morse Code, which was an important advance over Wheatstone's complicated and significantly more expensive telegraph system(Sony VAIO PCG-5G3L battery). The communications efficiency of the Morse Code anticipated that of the Huffman code in digital communications by over 100 years, but Morse and his associate Alfred Vail developed the code purelyempirically, unlike Huffman, who gave a detailed theoretical explanation of how his method worked(Sony VAIO PCG-5J1L battery).

The first permanent transatlantic telegraph cable was successfully completed on 27 July 1866, allowing transatlantic electrical communication for the first time. An earlier transatlantic cable had operated for a few months in 1859(Sony VAIO PCG-5J2L battery), and among other things, it carried messages of greeting back and forth between President James Buchanan of the United States and Queen Victoria of the United Kingdom(Sony VAIO PCG-5K2L battery).

However, that transatlantic cable failed soon, and the project to lay a replacement line was delayed for five years by the American Civil War. Also, these transatlantic cables would have been completely incapable of carrying telephone calls even had the telephone already been invented(Sony VAIO PCG-5L1L battery). The first transatlantic telephone cable (which incorporated hundreds of electronic amplifiers) was not operational until 1956.

The conventional telephone now in use worldwide was first patented by Alexander Graham Bell in March 1876. That first patent by Bell was the master patent of the telephone, from which all other patents for electric telephone devices and features flowed(Sony VAIO PCG-6S2L battery). Credit for the invention of the electric telephone has been frequently disputed, and new controversies over the issue have arisen from time-to-time. As with other greatinventions such as radio(Sony VAIO PCG-6S3L battery), television, the light bulb, and the digital computer, there were several inventors who did pioneering experimental work on voice transmission over a wire, and then they improved on each other's ideas. However, the key innovators were Alexander Graham Bell and Gardiner Greene Hubbard(Sony VAIO PCG-6V1L battery), who created the first telephone company, the Bell Telephone Company in the United States, which later evolved into American Telephone & Telegraph (AT&T).

The first commercial telephone services were set up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven, Connecticut, and London, England(Sony VAIO PCG-6W1L battery).

Radio and television

In 1832, James Lindsay gave a classroom demonstration of wireless telegraphy via conductive water to his students. By 1854, he was able to demonstrate a transmission across the Firth of Tay from Dundee, Scotland, to Woodhaven, a distance of about two miles (3 km) (Sony VAIO PCG-7111L battery), again using water as the transmission medium. In December 1901, Guglielmo Marconi established wireless communication between St. John's, Newfoundland and Poldhu, Cornwall (England), earning him the Nobel Prize in Physics for 1909, one which he shared with Karl Braun(Sony VAIO PCG-6W3L battery). However small-scale radio communication had already been demonstrated in 1893 by Nikola Tesla in a presentation before the National Electric Light Association.

On March 25, 1925, John Logie Baird of Scotland was able to demonstrate the transmission of moving pictures at the Selfridge's department storein London, England(Sony VAIO PCG-7113L battery). Baird's system relied upon the fast-rotatingNipkow disk, and thus it became known as the mechanical television. It formed the basis of experimental broadcasts done by the British Broadcasting Corporation beginning September 30, 1929(Sony VAIO PCG-7133L battery). However, for most of the 20th century, television systems were designed around the cathode ray tube, invented by Karl Braun. The first version of such an electronic television to show promise was produced by Philo Farnsworthof the United States, and it was demonstrated to his family in Idaho on September 7, 1927(Sony VAIO PCG-7Z1L battery).

Computer networks and the Internet

On 11 September 1940, George Stibitz was able to transmit problems using teletype to his Complex Number Calculator in New York and receive the computed results back at Dartmouth College in New Hampshire(Sony VAIO PCG-7Z2L battery). This configuration of a centralized computer or mainframe computerwith remote "dumb terminals" remained popular throughout the 1950s and into the 60's. However, it was not until the 1960s that researchers started to investigate packet switching — a technology that allows chunks of data to be sent between different computers without first passing through a centralized mainframe(Sony VAIO PCG-8Y1L battery). A four-node network emerged on December 5, 1969. This network soon became the ARPANET, which by 1981 would consist of 213 nodes.

ARPANET's development centred around the Request for Comment process and on 7 April 1969, RFC 1 was published(Sony VAIO PCG-8Y2L battery). This process is important because ARPANET would eventually merge with other networks to form the Internet, and many of the communication protocols that the Internet relies upon today were specified through the Request for Comment process(Sony VAIO PCG-8Z2L battery). In September 1981, RFC 791 introduced the Internet Protocol version 4 (IPv4) and RFC 793 introduced the Transmission Control Protocol (TCP) — thus creating the TCP/IP protocol that much of the Internet relies upon today(Sony VAIO PCG-8Z1L battery).

However, not all important developments were made through the Request for Comment process. Two popular link protocols for local area networks (LANs) also appeared in the 1970s(Sony VAIO PCG-7112L battery). A patent for the token ringprotocol was filed by Olof Soderblom on October 29, 1974, and a paper on the Ethernet protocol was published by Robert Metcalfe and David Boggs in the July 1976 issue of Communications of the ACM(Sony VAIO PCG-6W2L battery). The Ethernet protocol had been inspired by the ALOHAnet protocol which had been developed by electrical engineering researchers at the University of Hawaii.

Key concepts

A number of key concepts reoccur throughout the literature on modern telecommunication systems. Some of these concepts are discussed below(Sony VAIO PCG-5K1L battery).

Basic elements

A basic telecommunication system consists of three primary units that are always present in some form:

A transmitter that takes information and converts it to a signal.

A transmission medium, also called the "physical channel" that carries the signal. An example of this is the "free space channel"(HP PAVILION DV2 Battery).

A receiver that takes the signal from the channel and converts it back into usable information.

For example, in a radio broadcasting station the station's large power amplifier is the transmitter; and the broadcasting antenna is the interface between the power amplifier and the "free space channel"(HP PAVILION DV2000 Battery). The free space channel is the transmission medium; and the receiver's antenna is the interface between the free space channel and the receiver. Next, the radio receiver is the destination of the radio signal, and this is where it is converted from electricity to sound for people to listen to( HP PAVILION DV3 Battery).

Sometimes, telecommunication systems are "duplex" (two-way systems) with a single box of electronics working as both a transmitter and a receiver, or a transceiver. For example, a cellular telephone is a transceiver(HP PAVILION DV3000 Battery). The transmission electronics and the receiver electronics in a transceiver are actually quite independent of each other. This can be readily explained by the fact that radio transmitters contain power amplifiers that operate with electrical powers measured in the watts or kilowatts(SONY VGP-BPS11 battery), but radio receivers deal with radio powers that are measured in the microwatts or nanowatts. Hence, transceivers have to be carefully designed and built to isolate their high-power circuitry and their low-power circuitry from each other(SONY VGP-BPL11 battery).

Telecommunication over telephone lines is called point-to-point communication because it is between one transmitter and one receiver. Telecommunication through radio broadcasts is called broadcast communication because it is between one powerful transmitter and numerous low-power but sensitive radio receivers(SONY VGP-BPL12 battery).

Telecommunications in which multiple transmitters and multiple receivers have been designed to cooperate and to share the same physical channel are called multiplex systems(SONY VGP-BPS12 battery).

Analog versus digital communications

Communications signals can be either by analog signals or digital signals. There are analog communication systems and digital communicationsystems. For an analog signal, the signal is varied continuously with respect to the information(SONY VGP-BPS14 Battery). In a digital signal, the information is encoded as a set of discrete values (for example, a set of ones and zeros). During the propagation and reception, the information contained in analog signals will inevitably be degraded by undesirable physical noise. (The output of a transmitter is noise-free for all practical purposes.) Commonly(SONY VGP-BPS14/B Battery), the noise in a communication system can be expressed as adding or subtracting from the desirable signal in a completely random way. This form of noise is called "additive noise", with the understanding that the noise can be negative or positive at different instants of time(SONY VGP-BPS14/S Battery). Noise that is not additive noise is a much more difficult situation to describe or analyze, and these other kinds of noise will be omitted here.

On the other hand, unless the additive noise disturbance exceeds a certain threshold, the information contained in digital signals will remain intact. Their resistance to noise represents a key advantage of digital signals over analog signals(SONY VGP-BPS14B Battery).

Telecommunication networks

A communications network is a collection of transmitters, receivers, and communications channels that send messages to one another. Some digital communications networks contain one or more routers that work together to transmit information to the correct user(SONY VGP-BPS22 battery). An analog communications network consists of one or more switches that establish a connection between two or more users. For both types of network,repeaters may be necessary to amplify or recreate the signal when it is being transmitted over long distances(SONY VGP-BPS18 battery). This is to combat attenuation that can render the signal indistinguishable from the noise.

Communication channels

The term "channel" has two different meanings. In one meaning, a channel is the physical medium that carries a signal between the transmitter and the receiver(SONY VGP-BPS22/A battery). Examples of this include the atmosphere for sound communications, glass optical fibers for some kinds of optical communications, coaxial cables for communications by way of the voltages and electric currents in them, and free space for communications usingvisible light, infrared waves(SONY VGP-BPS22A battery), ultraviolet light, and radio waves. This last channel is called the "free space channel". The sending of radio waves from one place to another has nothing to do with the presence or absence of an atmosphere between the two(SONY VGP-BPL10 battery). Radio waves travel through a perfect vacuum just as easily as they travel through air, fog, clouds, or any other kind of gas besides air.

The other meaning of the term "channel" in telecommunications is seen in the phrase communications channel(SONY VGP-BPS10 battery), which is a subdivision of a transmission medium so that it can be used to send multiple streams of information simultaneously. For example, one radio station can broadcast radio waves into free space at frequencies in the neighborhood of 94.5 MHz (Dell Latitude E6500 battery) (megahertz) while another radio station can simultaneously broadcast radio waves at frequencies in the neighborhood of 96.1 MHz. Each radio station would transmit radio waves over a frequency bandwidth of about 180 kHz (kilohertz) (Dell Latitude E6400 battery), centered at frequencies such as the above, which are called the "carrier frequencies". Each station in this example is separated from its adjacent stations by 200 kHz, and the difference between 200 kHz and 180 kHz (20 kHz) is an engineering allowance for the imperfections in the communication system(Dell Latitude E5400 battery).

In the example above, the "free space channel" has been divided into communications channels according to frequencies, and each channel is assigned a separate frequency bandwidth in which to broadcast radio waves(SONY Vaio VGN-CR23/W battery). This system of dividing the medium into channels according to frequency is called "frequency-division multiplexing" (FDM).

Another way of dividing a communications medium into channels is to allocate each sender a recurring segment of time (SONY Vaio VGN-CR23/N battery) (a "time slot", for example, 20 milliseconds out of each second), and to allow each sender to send messages only within its own time slot. This method of dividing the medium into communication channels is called "time-division multiplexing" (TDM) (SONY Vaio VGN-CR23/L battery), and is used in optical fiber communication. Some radio communication systems use TDM within an allocated FDM channel. Hence, these systems use a hybrid of TDM and FDM.

Modulation

The shaping of a signal to convey information is known as modulation(SONY Vaio VGN-CR23/R battery). Modulation can be used to represent a digital message as an analog waveform. This is commonly called "keying" - a term derived from the older use of Morse Code in telecommunications - and several keying techniques exist (these include phase-shift keying(SONY Vaio VGN-CR23/P battery), frequency-shift keying, and amplitude-shift keying). The "Bluetooth" system, for example, uses phase-shift keying to exchange information between various devices. In addition, there are combinations of phase-shift keying and amplitude-shift keying which is called(SONY Vaio VGN-CR23/B battery) (in the jargon of the field) "quadrature amplitude modulation" (QAM) that are used in high-capacity digital radio communication systems.

Modulation can also be used to transmit the information of low-frequency analog signals at higher frequencies(SONY Vaio VGN-CR220E/R battery). This is helpful because low-frequency analog signals cannot be effectively transmitted over free space. Hence the information from a low-frequency analog signal must be impressed into a higher-frequency signal (known as the "carrier wave") before transmission(SONY Vaio VGN-CR21Z/R battery). There are several different modulation schemes available to achieve this [two of the most basic being amplitude modulation (AM) and frequency modulation (FM)]. An example of this process is a disc jockey's voice being impressed into a 96 MHz carrier wave using frequency modulation(SONY Vaio VGN-CR21Z/N battery) (the voice would then be received on a radio as the channel "96 FM").In addition, modulation has the advantage of being about to use frequency division multiplexing (FDM).

Society and telecommunication

Telecommunication has a significant social(SONY Vaio VGN-CR21S/W battery), cultural. and economic impact on modern society. In 2008, estimates placed the telecommunication industry's revenue at $3.85 trillion (USD) or just under 3.0 percent of the gross world product (official exchange rate) (SONY Vaio VGN-CR21S/P battery). Several following sections discuss the impact of telecommunication on society.

Economic impact

Microeconomics

On the microeconomic scale, companies have used telecommunications to help build global business empires. This is self-evident in the case of online retailer Amazon.com but, according to academic Edward Lenert, even the conventional retailer Wal-Mart has benefited from better telecommunication infrastructure compared to its competitors(SONY Vaio VGN-CR21S/L battery). In cities throughout the world, home owners use their telephones to organize many home services ranging from pizza deliveries to electricians. Even relatively-poor communities have been noted to use telecommunication to their advantage(SONY Vaio VGN-CR21E/W battery). In Bangladesh's Narshingdi district, isolated villagers use cellular phones to speak directly to wholesalers and arrange a better price for their goods. In Côte d'Ivoire, coffee growers share mobile phones to follow hourly variations in coffee prices and sell at the best price(SONY Vaio VGN-CR21E/P battery).

Macroeconomics

On the macroeconomic scale, Lars-Hendrik Röller and Leonard Waverman suggested a causal link between good telecommunication infrastructure and economic growth.[32] Few dispute the existence of a correlation although some argue it is wrong to view the relationship as causal(SONY Vaio VGN-CR21E/L battery).

Because of the economic benefits of good telecommunication infrastructure, there is increasing worry about the inequitable access to telecommunication services amongst various countries of the world—this is known as the digital divide. A 2003 survey by the International Telecommunication Union (ITU) (SONY Vaio VGN-CR21/B battery) revealed that roughly one-third of countries have fewer than one mobile subscription for every 20 people and one-third of countries have fewer than one land-line telephone subscription for every 20 people(SONY Vaio VGN-CR190E/W battery). In terms of Internet access, roughly half of all countries have fewer than one out of 20 people with Internet access. From this information, as well as educational data, the ITU was able to compile an index that measures the overall ability of citizens to access and use information and communication technologies(SONY Vaio VGN-CR190E/R battery). Using this measure, Sweden, Denmark and Iceland received the highest ranking while the African countries Nigeria, Burkina Faso and Mali received the lowest.

Social impact

Telecommunication has played a significant role in social relationships. Nevertheless devices like the telephone system were originally advertised with an emphasis on the practical dimensions of the device(SONY Vaio VGN-CR190E/P battery) (such as the ability to conduct business or order home services) as opposed to the social dimensions. It was not until the late 1920s and 1930s that the social dimensions of the device became a prominent theme in telephone advertisements(SONY Vaio VGN-CR190E/L battery). New promotions started appealing to consumers' emotions, stressing the importance of social conversations and staying connected to family and friends.

Since then the role that telecommunications has played in social relations has become increasingly important(SONY Vaio VGN-CR190 battery). In recent years, the popularity of social networking sites has increased dramatically. These sites allow users to communicate with each other as well as post photographs, events and profiles for others to see. The profiles can list a person's age, interests(SONY Vaio VGN-CR15/B battery), sexual preference and relationship status. In this way, these sites can play important role in everything from organising social engagements to courtship.

Prior to social networking sites, technologies like short message service(SMS) and the telephone also had a significant impact on social interactions(SONY Vaio VGN-CR13T/W battery). In 2000, market research group Ipsos MORI reported that 81% of 15 to 24 year-old SMS users in the United Kingdom had used the service to coordinate social arrangements and 42% to flirt(SONY Vaio VGN-CR13T/R battery).

Other impacts

In cultural terms, telecommunication has increased the public's ability to access to music and film. With television, people can watch films they have not seen before in their own home without having to travel to the video store or cinema(SONY Vaio VGN-CR13T/P battery). With radio and the Internet, people can listen to music they have not heard before without having to travel to the music store.

Telecommunication has also transformed the way people receive their news. A survey by the non-profit Pew Internet and American Life Project found that when just over 3,000 people living in the United States were asked where they got their news "yesterday",(SONY Vaio VGN-CR13T/L battery) more people said television or radio than newspapers. The results are summarised in the following table (the percentages add up to more than 100% because people were able to specify more than one source) (SONY Vaio VGN-CR13G/R battery).

Telecommunication has had an equally significant impact on advertising. TNS Media Intelligence reported that in 2007, 58% of advertising expenditure in the United States was spent on mediums that depend upon telecommunication. The results are summarised in the following table(SONY Vaio VGN-CR13G/P battery).

Telecommunication and government

Many countries have enacted legislation which conform to the International Telecommunication Regulations establish by the International Telecommunication Union (ITU), which is the "leading United Nationsagency for information and communication technology issues(SONY Vaio VGN-CR13G/W battery)." In 1947, at the Atlantic City Conference, the ITU decided to "afford international protection to all frequencies registered in a new international frequency list and used in conformity with the Radio Regulation(SONY Vaio VGN-CR13G/L battery)." According to the ITU's Radio Regulations adopted in Atlantic City, all frequencies referenced in the International Frequency Registration Board, examined by the board and registered on the International Frequency List "shall have the right to international protection from harmful interference(SONY Vaio VGN-CR13G/B battery)."

From a global perspective, there have been political debates and legislation regarding the management of telecommunication and broadcasting. The history of broadcasting discusses some of debates in relation to balancing conventional communication such as printing and telecommunication such as radio broadcasting(SONY Vaio VGN-CR13G battery). The onset of World War II brought on the first explosion of international broadcasting propaganda. Countries, their governments, insurgents, terrorists, and militiamen have all used telecommunication and broadcasting techniques to promote propaganda(SONY Vaio VGN-CR13/W battery). Patriotic propaganda for political movements and colonization started the mid 1930s. In 1936, the BBC did broadcast propaganda to the Arab World to partly counter similar broadcasts from Italy, which also had colonial interests in North Africa(SONY Vaio VGN-CR13/R battery).

Modern insurgents, such as those in the latest Iraq war, often use intimidating telephone calls, SMSs and the distribution of sophisticated videos of an attack on coalition troops within hours of the operation(SONY Vaio VGN-CR13/P battery). "The Sunni insurgents even have their own television station, Al-Zawraa, which while banned by the Iraqi government, still broadcasts from Erbil, Iraqi Kurdistan, even as coalition pressure has forced it to switch satellite hosts several times(SONY Vaio VGN-CR13/L battery)."

Modern telecommunication

Telephone

In an analog telephone network, the caller is connected to the person he wants to talk to by switches at various telephone exchanges. The switches form an electrical connection between the two users and the setting of these switches is determined electronically when the caller dialsthe number(SONY Vaio VGN-CR13/B battery). Once the connection is made, the caller's voice is transformed to an electrical signal using a small microphone in the caller's handset. This electrical signal is then sent through the network to the user at the other end where it is transformed back into sound by a small speaker in that person's handset(SONY Vaio VGN-CR11Z/R battery). There is a separate electrical connection that works in reverse, allowing the users to converse.

The fixed-line telephones in most residential homes are analog — that is, the speaker's voice directly determines the signal's voltage. Although short-distance calls may be handled from end-to-end as analog signals(SONY Vaio VGN-CR11S/W battery), increasingly telephone service providers are transparently converting the signals to digital for transmission before converting them back to analog for reception. The advantage of this is that digitized voice data can travel side-by-side with data from the Internet and can be perfectly reproduced in long distance communication (as opposed to analog signals that are inevitably impacted by noise) (SONY Vaio VGN-CR11S/P battery).

Mobile phones have had a significant impact on telephone networks. Mobile phone subscriptions now outnumber fixed-line subscriptions in many markets. Sales of mobile phones in 2005 totalled 816.6 million with that figure being almost equally shared amongst the markets of Asia/Pacific (204 m) (SONY Vaio VGN-CR11S/L battery), Western Europe (164 m), CEMEA (Central Europe, the Middle East and Africa) (153.5 m), North America (148 m) and Latin America (102 m). In terms of new subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth(SONY VAIO VGN-NR11Z battery). Increasingly these phones are being serviced by systems where the voice content is transmitted digitally such as GSM or W-CDMA with many markets choosing to depreciate analog systems such as AMPS(SONY VAIO VGN-NR11S battery).

There have also been dramatic changes in telephone communication behind the scenes. Starting with the operation of TAT-8 in 1988, the 1990s saw the widespread adoption of systems based on optic fibres(SONY VAIO VGN-NR110E battery). The benefit of communicating with optic fibers is that they offer a drastic increase in data capacity. TAT-8 itself was able to carry 10 times as many telephone calls as the last copper cable laid at that time and today's optic fibre cables are able to carry 25 times as many telephone calls as TAT-8. This increase in data capacity is due to several factors(SONY VAIO VGN-NR110E/T battery): First, optic fibres are physically much smaller than competing technologies. Second, they do not suffer from crosstalk which means several hundred of them can be easily bundled together in a single cable(SONY VAIO VGN-NR110E/S battery). Lastly, improvements in multiplexing have led to an exponential growth in the data capacity of a single fibre.

Assisting communication across many modern optic fibre networks is a protocol known as Asynchronous Transfer Mode (ATM) (SONY VAIO VGN-NR110E/W battery). The ATM protocol allows for the side-by-side data transmission mentioned in the second paragraph. It is suitable for public telephone networks because it establishes a pathway for data through the network and associates a traffic contract with that pathway(SONY VGN-CR11SR battery). The traffic contract is essentially an agreement between the client and the network about how the network is to handle the data; if the network cannot meet the conditions of the traffic contract it does not accept the connection(SONY VGN-CR11Z battery). This is important because telephone calls can negotiate a contract so as to guarantee themselves a constant bit rate, something that will ensure a caller's voice is not delayed in parts or cut-off completely(SONY VGN-CR11S battery). There are competitors to ATM, such as Multiprotocol Label Switching (MPLS), that perform a similar task and are expected to supplant ATM in the future.

Radio and television

In a broadcast system, the central high-powered broadcast tower transmits a high-frequency electromagnetic wave to numerous low-powered receivers(SONY VGN-CR11M battery). The high-frequency wave sent by the tower is modulated with a signal containing visual or audio information. The receiver is then tuned so as to pick up the high-frequency wave and a demodulator is used to retrieve the signal containing the visual or audio information(SONY VGN-CR11E battery). The broadcast signal can be either analog (signal is varied continuously with respect to the information) or digital (information is encoded as a set of discrete values) (SONY VGN-CR21E battery).

The broadcast media industry is at a critical turning point in its development, with many countries moving from analog to digital broadcasts. This move is made possible by the production of cheaper, faster and more capable integrated circuits(SONY VGN-CR21S battery). The chief advantage of digital broadcasts is that they prevent a number of complaints common to traditional analog broadcasts. For television, this includes the elimination of problems such as snowy pictures(SONY VGN-CR21Z battery), ghostingand other distortion. These occur because of the nature of analog transmission, which means that perturbations due to noise will be evident in the final output. Digital transmission overcomes this problem because digital signals are reduced to discrete values upon reception and hence small perturbations do not affect the final output(SONY VGN-CR21SR battery). In a simplified example, if a binary message 1011 was transmitted with signal amplitudes and received with signal amplitudes it would still decode to the binary message 1011 — a perfect reproduction of what was sent(SONY VGN-CR31SR battery). From this example, a problem with digital transmissions can also be seen in that if the noise is great enough it can significantly alter the decoded message. Using forward error correction a receiver can correct a handful of bit errors in the resulting message but too much noise will lead to incomprehensible output and hence a breakdown of the transmission(SONY VGN-CR31S battery).

In digital television broadcasting, there are three competing standards that are likely to be adopted worldwide. These are the ATSC, DVBand ISDB standards; the adoption of these standards thus far is presented in the captioned map(SONY VGN-CR31E battery). All three standards use MPEG-2 for video compression. ATSC uses Dolby Digital AC-3 for audio compression, ISDB uses Advanced Audio Coding (MPEG-2 Part 7) and DVB has no standard for audio compression but typically uses MPEG-1 Part 3 Layer 2(SONY VGN-CR31Z battery). The choice of modulation also varies between the schemes. In digital audio broadcasting, standards are much more unified with practically all countries choosing to adopt the Digital Audio Broadcasting standard (SONY VGN-CR41Z battery) (also known as the Eureka 147 standard). The exception being the United States which has chosen to adopt HD Radio. HD Radio, unlike Eureka 147, is based upon a transmission method known as in-band on-channel transmission that allows digital information to "piggyback" on normal AM or FM analog transmissions(SONY VGN-CR41S battery).

However, despite the pending switch to digital, analog television remains being transmitted in most countries. An exception is the United States that ended analog television transmission (by all but the very low-power TV stations) on 12 June 2009[62] after twice delaying the switchover deadline(SONY VGN-CR41E battery). For analog television, there are three standards in use for broadcasting color TV (see a map on adoption here). These are known as PAL(British designed), NTSC (North American designed), and SECAM (French designed) (SONY VGN-CR41SR battery). (It is important to understand that these are the ways from sending color TV, and they do not have anything to do with the standards for black & white TV, which also vary from country to country.) For analog radio, the switch to digital radio is made more difficult by the fact that analog receivers are sold at a small fraction of the price of digital receivers(SONY VGN-CR42ZR battery). The choice of modulation for analog radio is typically between amplitude modulation (AM) or frequency modulation (FM). To achieve stereo playback, an amplitude modulated subcarrier is used for stereo FM(SONY VGN-CR42Z battery).

The Internet

The Internet is a worldwide network of computers and computer networks that can communicate with each other using the Internet Protocol. Any computer on the Internet has a unique IP address that can be used by other computers to route information to it(SONY VGN-CR42S battery). Hence, any computer on the Internet can send a message to any other computer using its IP address. These messages carry with them the originating computer's IP address allowing for two-way communication(SONY VGN-CR42E battery). The Internet is thus an exchange of messages between computers.

As of 2008, an estimated 21.9% of the world population has access to the Internet with the highest access rates (measured as a percentage of the population) in North America (73.6%), Oceania/Australia (59.5%) and Europe (48.1%)(SONY Vaio VGN-FW31J Battery). In terms of broadband access, Iceland (26.7%), South Korea (25.4%) and the Netherlands (25.3%) led the world.

The Internet works in part because of protocols that govern how the computers and routers communicate with each other(SONY Vaio VGN-FW32J Battery). The nature of computer network communication lends itself to a layered approach where individual protocols in the protocol stack run more-or-less independently of other protocols. This allows lower-level protocols to be customized for the network situation while not changing the way higher-level protocols operate(SONY Vaio VGN-FW17W Battery). A practical example of why this is important is because it allows an Internet browser to run the same code regardless of whether the computer it is running on is connected to the Internet through an Ethernet orWi-Fi connection(SONY Vaio VGN-FW31E Battery). Protocols are often talked about in terms of their place in the OSI reference model (pictured on the right), which emerged in 1983 as the first step in an unsuccessful attempt to build a universally adopted networking protocol suite(SONY Vaio VGN-FW139E Battery).

For the Internet, the physical medium and data link protocol can vary several times as packets traverse the globe. This is because the Internet places no constraints on what physical medium or data link protocol is used(SONY Vaio VGN-FW139E/H Battery). This leads to the adoption of media and protocols that best suit the local network situation. In practice, most intercontinental communication will use the Asynchronous Transfer Mode (ATM) protocol (or a modern equivalent) on top of optic fibre(SONY Vaio VGN-FW465J Battery). This is because for most intercontinental communication the Internet shares the same infrastructure as the public switched telephone network.

At the network layer, things become standardized with the Internet Protocol (IP) being adopted for logical addressing(SONY Vaio VGN-FW31M Battery). For the World Wide Web, these "IP addresses" are derived from the human readable form using the Domain Name System (e.g. 72.14.207.99 is derived from www.google.com). At the moment, the most widely used version of the Internet Protocol is version four but a move to version six is imminent(SONY VAIO VGN-FZ21E Battery).

At the transport layer, most communication adopts either the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). TCP is used when it is essential every message sent is received by the other computer where as UDP is used when it is merely desirable(SONY VAIO VGN-FZ21Z Battery). With TCP, packets are retransmitted if they are lost and placed in order before they are presented to higher layers. With UDP, packets are not ordered or retransmitted if lost. Both TCP and UDP packets carry port numbers with them to specify what application or process the packet should be handled by(SONY VAIO VGN-FZ21J Battery). Because certain application-level protocols usecertain ports, network administrators can manipulate traffic to suit particular requirements. Examples are to restrict Internet access by blocking the traffic destined for a particular port or to affect the performance of certain applications by assigning priority(SONY Vaio VGN-FW11 Battery).

Above the transport layer, there are certain protocols that are sometimes used and loosely fit in the session and presentation layers, most notably the Secure Sockets Layer (SSL) and Transport Layer Security(TLS) protocols(SONY Vaio VGN-FW11M Battery). These protocols ensure that the data transferred between two parties remains completely confidential and one or the other is in use when a padlock appears in the address bar of your web browser. Finally, at the application layer, are many of the protocols Internet users would be familiar with such as HTTP (web browsing) (SONY Vaio VGN-FW11S Battery), POP3 (e-mail), FTP (file transfer), IRC (Internet chat), BitTorrent (file sharing) and OSCAR (instant messaging).

Local Area Networks and Wide Area Networks

Despite the growth of the Internet, the characteristics of local area networks ("LANs" - computer networks that do not extend beyond a few kilometers in size) remain distinct. This is because networks on this scale do not require all the features associated with larger networks and are often more cost-effective and efficient without them(SONY Vaio VGN-FW21E Battery). When they are not connected with the Internet, they also have the advantages of privacy and security. However, purposefully lacking a direct connection to the Internet will not provide 100% protection of the LAN from hackers, military forces, or economic powers. These threats exist if there are any methods for connecting remotely to the LAN(SONY Vaio VGN-FW21J Battery).

There are also independent wide area networks ("WANs" - private computer networks that can and do extend for thousands of kilometers.) Once again, some of their advantages include their privacy, security, and complete ignoring of any potential hackers - who cannot "touch" them(SONY Vaio VGN-FW21L Battery). Of course, prime users of private LANs and WANs include armed forces and intelligence agencies that must keep their information completely secure and secret.

In the mid-1980s, several sets of communication protocols emerged to fill the gaps between the data-link layer and the application layer of the OSI reference model. These included Appletalk, IPX, and NetBIOS with the dominant protocol set during the early 1990s being IPX due to its popularity with MS-DOS users. (SONY Vaio VGN-FW21M Battery) TCP/IP existed at this point, but it was typically only used by large government and research facilities.

As the Internet grew in popularity and a larger percentage of traffic became Internet-related, LANs and WANs gradually moved towards the TCP/IP protocols, and today networks mostly dedicated to TCP/IP traffic are common(SONY VGP-BPS13Q Battery). The move to TCP/IP was helped by technologies such as DHCP that allowed TCP/IP clients to discover their own network address — a function that came standard with the AppleTalk/ IPX/ NetBIOS protocol sets(SONY VGP-BPS13B/Q Battery).

It is at the data-link layer, though, that most modern LANs diverge from the Internet. Whereas Asynchronous Transfer Mode (ATM) or Multiprotocol Label Switching (MPLS) are typical data-link protocols for larger networks such as WANs(SONY VGN NR11S/S battery); Ethernet and Token Ring are typical data-link protocols for LANs. These protocols differ from the former protocols in that they are simpler (e.g. they omit features such as Quality of Service guarantees) and offer collision prevention. Both of these differences allow for more economical systems(SONY VGN NR11M/S battery). Despite the modest popularity of IBM token ring in the 1980s and 90's, virtually all LANs now use either wired or wireless Ethernets. At the physical layer, most wired Ethernet implementations use copper twisted-pair cables (including the common 10BASE-T networks) (SONY VGN NR11Z/S battery). However, some early implementations used heavier coaxial cables and some recent implementations (especially high-speed ones) use optical fibers. When optic fibers are used, the distinction must be made between multimode fibers and single-mode fiberes(SONY VGN NR11Z/T battery). Multimode fibers can be thought of as thicker optical fibers that are cheaper to manufacture devices for but that suffers from less usable bandwidth and worse attenuation - implying poorer long-distance performance(SONY VGP-BPS13A/Q Battery).