The Internet is the global system of interconnected computer networks that use the Internet protocol suite to link devices worldwide. It is a network of networks that consists of private, academic and government networks of local to global scope, linked by a broad array of electronic and optical networking technologies; the Internet carries a vast range of information resources and services, such as the inter-linked hypertext documents and applications of the World Wide Web, electronic mail and file sharing. Some publications no longer capitalize "internet"; the origins of the Internet date back to research commissioned by the federal government of the United States in the 1960s to build robust, fault-tolerant communication with computer networks. The primary precursor network, the ARPANET served as a backbone for interconnection of regional academic and military networks in the 1980s; the funding of the National Science Foundation Network as a new backbone in the 1980s, as well as private funding for other commercial extensions, led to worldwide participation in the development of new networking technologies, the merger of many networks.
The linking of commercial networks and enterprises by the early 1990s marked the beginning of the transition to the modern Internet, generated a sustained exponential growth as generations of institutional and mobile computers were connected to the network. Although the Internet was used by academia since the 1980s, commercialization incorporated its services and technologies into every aspect of modern life. Most traditional communication media, including telephony, television, paper mail and newspapers are reshaped, redefined, or bypassed by the Internet, giving birth to new services such as email, Internet telephony, Internet television, online music, digital newspapers, video streaming websites. Newspaper and other print publishing are adapting to website technology, or are reshaped into blogging, web feeds and online news aggregators; the Internet has enabled and accelerated new forms of personal interactions through instant messaging, Internet forums, social networking. Online shopping has grown exponentially both for major retailers and small businesses and entrepreneurs, as it enables firms to extend their "brick and mortar" presence to serve a larger market or sell goods and services online.
Business-to-business and financial services on the Internet affect supply chains across entire industries. The Internet has no single centralized governance in either technological implementation or policies for access and usage; the overreaching definitions of the two principal name spaces in the Internet, the Internet Protocol address space and the Domain Name System, are directed by a maintainer organization, the Internet Corporation for Assigned Names and Numbers. The technical underpinning and standardization of the core protocols is an activity of the Internet Engineering Task Force, a non-profit organization of loosely affiliated international participants that anyone may associate with by contributing technical expertise. In November 2006, the Internet was included on USA Today's list of New Seven Wonders; when the term Internet is used to refer to the specific global system of interconnected Internet Protocol networks, the word is a proper noun that should be written with an initial capital letter.
In common use and the media, it is erroneously not capitalized, viz. the internet. Some guides specify that the word should be capitalized when used as a noun, but not capitalized when used as an adjective; the Internet is often referred to as the Net, as a short form of network. As early as 1849, the word internetted was used uncapitalized as an adjective, meaning interconnected or interwoven; the designers of early computer networks used internet both as a noun and as a verb in shorthand form of internetwork or internetworking, meaning interconnecting computer networks. The terms Internet and World Wide Web are used interchangeably in everyday speech. However, the World Wide Web or the Web is only one of a large number of Internet services; the Web is a collection of interconnected documents and other web resources, linked by hyperlinks and URLs. As another point of comparison, Hypertext Transfer Protocol, or HTTP, is the language used on the Web for information transfer, yet it is just one of many languages or protocols that can be used for communication on the Internet.
The term Interweb is a portmanteau of Internet and World Wide Web used sarcastically to parody a technically unsavvy user. Research into packet switching, one of the fundamental Internet technologies, started in the early 1960s in the work of Paul Baran and Donald Davies. Packet-switched networks such as the NPL network, ARPANET, the Merit Network, CYCLADES, Telenet were developed in the late 1960s and early 1970s; the ARPANET project led to the development of protocols for internetworking, by which multiple separate networks could be joined into a network of networks. ARPANET development began with two network nodes which were interconnected between the Network Measurement Center at the University of California, Los Angeles Henry Samueli School of Engineering and Applied Science directed by Leonard Kleinrock, the NLS system at SRI International by Douglas Engelbart in Menlo Park, California, on 29 October 1969; the third site was the Culler-Fried Interactive Mathematics Center at the University of California, Santa Barbara, followed by the University of
In communication systems, signal processing, electrical engineering, a signal is a function that "conveys information about the behavior or attributes of some phenomenon". In its most common usage, in electronics and telecommunication, this is a time varying voltage, current or electromagnetic wave used to carry information. A signal may be defined as an "observable change in a quantifiable entity". In the physical world, any quantity exhibiting variation in time or variation in space is a signal that might provide information on the status of a physical system, or convey a message between observers, among other possibilities; the IEEE Transactions on Signal Processing states that the term "signal" includes audio, speech, communication, sonar, radar and musical signals. In a effort of redefining a signal, anything, only a function of space, such as an image, is excluded from the category of signals, it is stated that a signal may or may not contain any information. In nature, signals can take the form of any action by one organism able to be perceived by other organisms, ranging from the release of chemicals by plants to alert nearby plants of the same type of a predator, to sounds or motions made by animals to alert other animals of the presence of danger or of food.
Signaling occurs in organisms all the way down to the cellular level, with cell signaling. Signaling theory, in evolutionary biology, proposes that a substantial driver for evolution is the ability for animals to communicate with each other by developing ways of signaling. In human engineering, signals are provided by a sensor, the original form of a signal is converted to another form of energy using a transducer. For example, a microphone converts an acoustic signal to a voltage waveform, a speaker does the reverse; the formal study of the information content of signals is the field of information theory. The information in a signal is accompanied by noise; the term noise means an undesirable random disturbance, but is extended to include unwanted signals conflicting with the desired signal. The prevention of noise is covered in part under the heading of signal integrity; the separation of desired signals from a background is the field of signal recovery, one branch of, estimation theory, a probabilistic approach to suppressing random disturbances.
Engineering disciplines such as electrical engineering have led the way in the design and implementation of systems involving transmission and manipulation of information. In the latter half of the 20th century, electrical engineering itself separated into several disciplines, specialising in the design and analysis of systems that manipulate physical signals. Definitions specific to sub-fields are common. For example, in information theory, a signal is a codified message, that is, the sequence of states in a communication channel that encodes a message. In the context of signal processing, signals are analog and digital representations of analog physical quantities. In terms of their spatial distributions, signals may be categorized as point source signals and distributed source signals. In a communication system, a transmitter encodes a message to create a signal, carried to a receiver by the communications channel. For example, the words "Mary had a little lamb" might be the message spoken into a telephone.
The telephone transmitter converts the sounds into an electrical signal. The signal is transmitted to the receiving telephone by wires. In telephone networks, for example common-channel signaling, refers to phone number and other digital control information rather than the actual voice signal. Signals can be categorized in various ways; the most common distinction is between discrete and continuous spaces that the functions are defined over, for example discrete and continuous time domains. Discrete-time signals are referred to as time series in other fields. Continuous-time signals are referred to as continuous signals. A second important distinction is between continuous-valued. In digital signal processing, a digital signal may be defined as a sequence of discrete values associated with an underlying continuous-valued physical process. In digital electronics, digital signals are the continuous-time waveform signals in a digital system, representing a bit-stream. Another important property of a signal is its information content.
Two main types of signals encountered in practice are digital. The figure shows a digital signal that results from approximating an analog signal by its values at particular time instants. Digital signals are quantized. An analog signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e. analogous to another time varying signal. For example, in an analog audio signal, the instantaneous voltage of the signal varies continuously with the pressure of the sound waves, it differs from a digital signal, in which the continuous quantity is a representation of a sequence of discrete values which can only take on one of a finite number of values. The term analog signal refers to electrical signals. An analog signal uses some property of the medium to convey the signal's information. For ex
Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. 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, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio
A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder. Communications satellites are used for television, radio and military applications. There are 2,134 communications satellites in Earth’s orbit, used by both private and government organizations. Many are in geostationary orbit 22,200 miles above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it; the high frequency radio waves used for telecommunications links travel by line of sight and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between separated geographical points. Communications satellites use a wide range of microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use.
This allocation of bands minimizes the risk of signal interference. The concept of the geostationary communications satellite was first proposed by Arthur C. Clarke, along with Vahid K. Sanadi building on work by Konstantin Tsiolkovsky. In October 1945 Clarke published an article titled "Extraterrestrial Relays" in the British magazine Wireless World; the article described the fundamentals behind the deployment of artificial satellites in geostationary orbits for the purpose of relaying radio signals. Thus, Arthur C. Clarke is quoted as being the inventor of the communications satellite and the term'Clarke Belt' employed as a description of the orbit. Decades a project named Communication Moon Relay was a telecommunication project carried out by the United States Navy, its objective was to develop a secure and reliable method of wireless communication by using the Moon as a passive reflector and a natural communications satellite. The first artificial Earth satellite was Sputnik 1. Put into orbit by the Soviet Union on October 4, 1957, it was equipped with an on-board radio-transmitter that worked on two frequencies: 20.005 and 40.002 MHz.
Sputnik 1 was launched as a major step in the exploration of rocket development. However, it was not placed in orbit for the purpose of sending data from one point on earth to another; the first satellite to relay communications was an intended lunar probe. Though the spacecraft only made it about halfway to the moon, it flew high enough to carry out the proof of concept relay of telemetry across the world, first from Cape Canaveral to Manchester, England; the first satellite purpose-built to relay communications was NASA's Project SCORE in 1958, which used a tape recorder to store and forward voice messages. It was used to send a Christmas greeting to the world from U. S. President Dwight D. Eisenhower. Courier 1B, built by Philco, launched in 1960, was the world's first active repeater satellite; the first artificial satellite used to further advances in global communications was a balloon named Echo 1. Echo 1 was the world's first artificial communications satellite capable of relaying signals to other points on Earth.
It soared 1,600 kilometres above the planet after its Aug. 12, 1960 launch, yet relied on humanity's oldest flight technology — ballooning. Launched by NASA, Echo 1 was a 30-metre aluminized PET film balloon that served as a passive reflector for radio communications; the world's first inflatable satellite — or "satelloon", as they were informally known — helped lay the foundation of today's satellite communications. The idea behind a communications satellite is simple: Send data up into space and beam it back down to another spot on the globe. Echo 1 accomplished this by serving as an enormous mirror, 10 stories tall, that could be used to reflect communications signals. There are two major classes of communications satellites and active. Passive satellites only reflect the signal coming from the source, toward the direction of the receiver. With passive satellites, the reflected signal is not amplified at the satellite, only a small amount of the transmitted energy reaches the receiver. Since the satellite is so far above Earth, the radio signal is attenuated due to free-space path loss, so the signal received on Earth is very weak.
Active satellites, on the other hand, amplify the received signal before retransmitting it to the receiver on the ground. Passive satellites are little used now. Telstar was the second direct relay communications satellite. Belonging to AT&T as part of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British General Post Office, the French National PTT to develop satellite communications, it was launched by NASA from Cape Canaveral on July 10, 1962, in the first sponsored space launch. Relay 1 was launched on December 13, 1962, it became the first satellite to transmit across the Pacific Ocean on November 22, 1963. An immediate antecedent of the geostationary satellites was the Hughes Aircraft Company's Syncom 2, launched on July 26, 1963. Syncom 2 was the first communications satellite in a geosynchronous orbit, it revolved around the earth once per day at constant speed, but because it still had north-south motion, special equipment was needed to track it. Its successor, Syncom 3 was the first geostationary communications satellite.
Syncom 3 obt
A transmission is a machine in a power transmission system, which provides controlled application of the power. The term transmission refers to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device. In British English, the term transmission refers to the whole drivetrain, including clutch, prop shaft and final drive shafts. In American English, the term refers more to the gearbox alone, detailed usage differs; the most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a high rotational speed, inappropriate for starting and slower travel; the transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are used on pedal bicycles, fixed machines, where different rotational speeds and torques are adapted. A transmission has multiple gear ratios with the ability to switch between them as speed varies.
This switching may be done automatically. Directional control may be provided. Single-ratio transmissions exist, which change the speed and torque of motor output. In motor vehicles, the transmission is connected to the engine crankshaft via a flywheel or clutch or fluid coupling because internal combustion engines cannot run below a particular speed; the output of the transmission is transmitted via the driveshaft to one or more differentials, which drives the wheels. While a differential may provide gear reduction, its primary purpose is to permit the wheels at either end of an axle to rotate at different speeds as it changes the direction of rotation. Conventional gear/belt transmissions are not the only mechanism for speed/torque adaptation. Alternative mechanisms include power transformation. Hybrid configurations exist. Automatic transmissions use a valve body to shift gears using fluid pressures in response to speed and throttle input. Early transmissions included the right-angle drives and other gearing in windmills, horse-powered devices, steam engines, in support of pumping and hoisting.
Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft. This means that the output shaft of a gearbox rotates at a slower rate than the input shaft, this reduction in speed produces a mechanical advantage, increasing torque. A gearbox can be set up to do the opposite and provide an increase in shaft speed with a reduction of torque; some of the simplest gearboxes change the physical rotational direction of power transmission. Many typical automobile transmissions include the ability to select one of several gear ratios. In this case, most of the gear ratios are used to slow down the output speed of the engine and increase torque. However, the highest gears may be "overdrive" types. Gearboxes have found use in a wide variety of different—often stationary—applications, such as wind turbines. Transmissions are used in agricultural, construction and automotive equipment. In addition to ordinary transmission equipped with gears, such equipment makes extensive use of the hydrostatic drive and electrical adjustable-speed drives.
The simplest transmissions called gearboxes to reflect their simplicity, provide gear reduction, sometimes in conjunction with a right-angle change in direction of the shaft. These are used on PTO-powered agricultural equipment, since the axial PTO shaft is at odds with the usual need for the driven shaft, either vertical, or horizontally extending from one side of the implement to another. More complex equipment, such as silage choppers and snowblowers, have drives with outputs in more than one direction; the gearbox in a wind turbine converts the slow, high-torque rotation of the turbine into much faster rotation of the electrical generator. These are more complicated than the PTO gearboxes in farm equipment, they weigh several tons and contain three stages to achieve an overall gear ratio from 40:1 to over 100:1, depending on the size of the turbine. The first stage of the gearbox is a planetary gear, for compactness, to distribute the enormous torque of the turbine over more teeth of the low-speed shaft.
Durability of these gearboxes has been a serious problem for a long time. Regardless of where they are used, these simple transmissions all share an important feature: the gear ratio cannot be changed during use, it is fixed at the time. For transmission types that overcome this issue, see Continuously variable transmission known as CVT. Many applications require the availability of multiple gear ratios; this is to ease the starting and stopping of a mechanical system, though another important need is that of maintaining good fuel efficiency. The need for a transmission in an automobile is a consequence of the characteristics of the internal combustion engine. Eng
Submarine communications cable
A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean and sea. The first submarine communications cables laid beginning in the 1850s carried telegraphy traffic, establishing the first instant telecommunications links between continents, such as the first transatlantic telegraph cable which became operational on 16 August 1858. Subsequent generations of cables carried telephone traffic data communications traffic. Modern cables use optical fiber technology to carry digital data, which includes telephone and private data traffic. Modern cables are about 1 inch in diameter and weigh around 2.5 tons per mile for the deep-sea sections which comprise the majority of the run, although larger and heavier cables are used for shallow-water sections near shore. Submarine cables first connected all the world's continents when Java was connected to Darwin, Northern Territory, Australia in 1871 in anticipation of the completion of the Australian Overland Telegraph Line in 1872 connecting to Adelaide, South Australia and thence to the rest of Australia.
After William Cooke and Charles Wheatstone had introduced their working telegraph in 1839, the idea of a submarine line across the Atlantic Ocean began to be thought of as a possible triumph of the future. Samuel Morse proclaimed his faith in it as early as 1840, in 1842, he submerged a wire, insulated with tarred hemp and India rubber, in the water of New York Harbor, telegraphed through it; the following autumn, Wheatstone performed a similar experiment in Swansea Bay. A good insulator to cover the wire and prevent the electric current from leaking into the water was necessary for the success of a long submarine line. India rubber had been tried by Moritz von Jacobi, the Prussian electrical engineer, as far back as the early 19th century. Another insulating gum which could be melted by heat and applied to wire made its appearance in 1842. Gutta-percha, the adhesive juice of the Palaquium gutta tree, was introduced to Europe by William Montgomerie, a Scottish surgeon in the service of the British East India Company.
Twenty years earlier, Montgomerie had seen whips made of gutta-percha in Singapore, he believed that it would be useful in the fabrication of surgical apparatus. Michael Faraday and Wheatstone soon discovered the merits of gutta-percha as an insulator, in 1845, the latter suggested that it should be employed to cover the wire, proposed to be laid from Dover to Calais, it was tried on a wire laid across the Rhine between Cologne. In 1849, C. V. Walker, electrician to the South Eastern Railway, submerged a two-mile wire coated with gutta-percha off the coast from Folkestone, tested successfully. Having earlier obtained a concession from the French government, in August 1850 John Watkins Brett's English Channel Submarine Telegraph Company laid the first line across the English Channel, using the converted tug Goliath, it was a copper wire coated with gutta-percha, without any other protection, was not successful. The experiment served to secure renewal of the concession, in September 1851, a protected core, or true, cable was laid by the reconstituted Submarine Telegraph Company from a government hulk, the Blazer, towed across the Channel.
In 1853 further successful cables were laid, linking Great Britain with Ireland and the Netherlands, crossing The Belts in Denmark. The British & Irish Magnetic Telegraph Company completed the first successful Irish link on May 23 between Portpatrick and Donaghadee using the collier William Hutt; the same ship was used for the link from Dover to Ostend in Belgium, by the Submarine Telegraph Company. Meanwhile, the Electric & International Telegraph Company completed two cables across the North Sea, from Orford Ness to Scheveningen, The Netherlands; these cables were laid by the Monarch, a paddle steamer which became the first vessel with permanent cable-laying equipment. In 1858 the steamship Elba was used to lay a telegraph cable from Jersey to Guernsey, on to Alderney and to Weymouth, the cable being completed in September of that year. Problems soon developed with eleven breaks occurring by 1860 due to storms and sand movements and wear on rocks. A report to the Institution of Civil Engineers in 1860 set out the problems to assist in future cable laying operations.
The first attempt at laying a transatlantic telegraph cable was promoted by Cyrus West Field, who persuaded British industrialists to fund and lay one in 1858. However, the technology of the day was not capable of supporting the project. Subsequent attempts in 1865 and 1866 with the world's largest steamship, the SS Great Eastern, used a more advanced technology and produced the first successful transatlantic cable. Great Eastern went on to lay the first cable reaching to India from Aden, Yemen, in 1870. From the 1850s until 1911, British submarine cable systems dominated the most important market, the North Atlantic Ocean; the British had demand side advantages. In terms of supply, Britain had entrepreneurs willing to put forth enormous amounts of capital necessary to build and maintain these cables. In terms of demand, Britain's vast colonial empire led to business for the cable companies from news agencies and shipping companies, the British government. Many of Britain's colonies had significant populations of European settlers, making news about them of interest to the general public in the home country.
British officials believed that depending on telegraph lines that passed through non-British territory posed a security