Cable television is a system of delivering television programming to consumers via radio frequency signals transmitted through coaxial cables, or in more recent systems, light pulses through fiber-optic cables. This contrasts with broadcast television, in which the television signal is transmitted over the air by radio waves and received by a television antenna attached to the television. FM radio programming, high-speed Internet, telephone services, similar non-television services may be provided through these cables. Analog television was standard in the 20th century, but since the 2000s, cable systems have been upgraded to digital cable operation. A "cable channel" is a television network available via cable television; when available through satellite television, including direct broadcast satellite providers such as DirecTV, Dish Network and Sky, as well as via IPTV providers such as Verizon FIOS and AT&T U-verse is referred to as a "satellite channel". Alternative terms include "non-broadcast channel" or "programming service", the latter being used in legal contexts.
Examples of cable/satellite channels/cable networks available in many countries are HBO, Cinemax, MTV, Cartoon Network, AXN, E!, FX, Discovery Channel, Canal+, Fox Sports, Disney Channel, Nickelodeon, CNN International, ESPN. The abbreviation CATV is used for cable television, it stood for Community Access Television or Community Antenna Television, from cable television's origins in 1948. In areas where over-the-air TV reception was limited by distance from transmitters or mountainous terrain, large "community antennas" were constructed, cable was run from them to individual homes; the origins of cable broadcasting for radio are older as radio programming was distributed by cable in some European cities as far back as 1924. To receive cable television at a given location, cable distribution lines must be available on the local utility poles or underground utility lines. Coaxial cable brings the signal to the customer's building through a service drop, an overhead or underground cable. If the subscriber's building does not have a cable service drop, the cable company will install one.
The standard cable used in the U. S. is RG-6, which has a 75 ohm impedance, connects with a type F connector. The cable company's portion of the wiring ends at a distribution box on the building exterior, built-in cable wiring in the walls distributes the signal to jacks in different rooms to which televisions are connected. Multiple cables to different rooms are split off the incoming cable with a small device called a splitter. There are two standards for cable television. All cable companies in the United States have switched to or are in the course of switching to digital cable television since it was first introduced in the late 1990s. Most cable companies require a set-top box or a slot on one's TV set for conditional access module cards to view their cable channels on newer televisions with digital cable QAM tuners, because most digital cable channels are now encrypted, or "scrambled", to reduce cable service theft. A cable from the jack in the wall is attached to the input of the box, an output cable from the box is attached to the television the RF-IN or composite input on older TVs.
Since the set-top box only decodes the single channel, being watched, each television in the house requires a separate box. Some unencrypted channels traditional over-the-air broadcast networks, can be displayed without a receiver box; the cable company will provide set top boxes based on the level of service a customer purchases, from basic set top boxes with a standard definition picture connected through the standard coaxial connection on the TV, to high-definition wireless DVR receivers connected via HDMI or component. Older analog television sets are "cable ready" and can receive the old analog cable without a set-top box. To receive digital cable channels on an analog television set unencrypted ones, requires a different type of box, a digital television adapter supplied by the cable company. A new distribution method that takes advantage of the low cost high quality DVB distribution to residential areas, uses TV gateways to convert the DVB-C, DVB-C2 stream to IP for distribution of TV over IP network in the home.
In the most common system, multiple television channels are distributed to subscriber residences through a coaxial cable, which comes from a trunkline supported on utility poles originating at the cable company's local distribution facility, called the "headend". Many channels can be transmitted through one coaxial cable by a technique called frequency division multiplexing. At the headend, each television channel is translated to a different frequency. By giving each channel a different frequency "slot" on the cable, the separate television signals do not interfere with each other. At an outdoor cable box on the subscriber's residence the company's service drop cable is connected to cables distributing the signal to different rooms in the building. At each television, the subscriber's television or a set-top box provided by the cable company translates the desired channel back to its original frequency, it is displayed onscreen. Due to widespread cable theft in earlier analog systems, the signals are encrypted on m
Shortwave radio is radio transmission using shortwave radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band, extends from 1.7–30 MHz. Radio waves in the shortwave band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the ionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon; this is called skywave or "skip" propagation. Thus shortwave radio can be used for long distance communication, in contrast to radio waves of higher frequency which travel in straight lines and are limited by the visual horizon, about 64 km. Shortwave radio is used for broadcasting of voice and music to shortwave listeners over large areas, it is used for military over-the-horizon radar, diplomatic communication, two-way international communication by amateur radio enthusiasts for hobby and emergency purposes, as well as for long distance aviation and marine communications.
The widest popular definition of the shortwave frequency interval is the ITU Region 1 definition, is the span 1.6–30 MHz, just above the medium wave band, which ends at 1.6 MHz. There are other definitions of the shortwave frequency interval: 1.71 to 30 MHz in ITU Region 2 1.8 to 30 MHz 2.3 to 30 MHz 2.3 to 26.1 MHz In Germany and Austria the ITU Region 1 shortwave radio frequency interval can be subdivided in: de:Grenzwelle: 1.605–3.8 MHz In Germany these shortwave radio frequency intervals have been seen used: the above other definitions The name "shortwave" originated during the early days of radio in the early 20th century, when the radio spectrum was considered divided into long wave, medium wave and short wave bands based on the wavelength of the radio waves. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used for radio communications; the broadcast medium wave band now extends above the 200 m/1,500 kHz limit, the amateur radio 1.8 MHz – 2.0 MHz band is the lowest-frequency band considered to be'shortwave'.
Early long distance radio telegraphy used long waves, below 300 kilohertz. The drawbacks to this system included a limited spectrum available for long distance communication, the expensive transmitters and gigantic antennas that were required, it was difficult to beam the radio wave directionally with long wave, resulting in a major loss of power over long distances. Prior to the 1920s, the shortwave frequencies above 1.5 MHz were regarded as useless for long distance communication and were designated in many countries for amateur use. Guglielmo Marconi, pioneer of radio, commissioned his assistant Charles Samuel Franklin to carry out a large scale study into the transmission characteristics of short wavelength waves and to determine their suitability for long distance transmissions. Franklin rigged up a large antenna at Poldhu Wireless Station, running on 25 kW of power. In June and July 1923, wireless transmissions were completed during nights on 97 meters from Poldhu to Marconi's yacht Elettra in the Cape Verde Islands.
In September 1924, Marconi transmitted daytime and nighttime on 32 meters from Poldhu to his yacht in Beirut. Franklin went on to refine the directional transmission, by inventing the curtain array aerial system. In July 1924, Marconi entered into contracts with the British General Post Office to install high speed shortwave telegraphy circuits from London to Australia, South Africa and Canada as the main element of the Imperial Wireless Chain; the UK-to-Canada shortwave "Beam Wireless Service" went into commercial operation on 25 October 1926. Beam Wireless Services from the UK to Australia, South Africa and India went into service in 1927. Shortwave communications began to grow in the 1920s, similar to the internet in the late 20th century. By 1928, more than half of long distance communications had moved from transoceanic cables and longwave wireless services to shortwave and the overall volume of transoceanic shortwave communications had vastly increased. Shortwave stations had cost and efficiency advantages over massive longwave wireless installations, however some commercial longwave communications stations remained in use until the 1960s.
Long distance radio circuits reduced the load on the existing transoceanic telegraph cables and hence the need for new cables, although the cables maintained their advantages of high security and a much more reliable and better quality signal than shortwave. The cable companies began to lose large sums of money in 1927, a serious financial crisis threatened the viability of cable companies that were vital to strategic British interests; the British government convened the Imperial Wireless and Cable Conference in 1928 "to examine the situation that had arisen as a result of the competition of Beam Wireless with the Cable Services". It recommended and received Government approval for all overseas cable and wireless resources of the Empire to be merged into one system controlled by a newly formed company in 1929, Imperial and International Communications Ltd; the name of the company was changed to Cable and Wireless Ltd. in 1934. Long-distance cables had a
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
The Marconi Company was a British telecommunications and engineering company that did business under that name from 1963 to 1987. It was derived from earlier variations in the name and incorporation, spanning a period from its inception in 1897 until 2006, during which time it underwent numerous changes and acquisitions; the company was founded by the Italian inventor Guglielmo Marconi and began as the Wireless Telegraph & Signal Company. The company was a pioneer of wireless long distance communication and mass media broadcasting becoming one of the UK's most successful manufacturing companies. In 1999, its defence manufacturing division, Marconi Electronic Systems, merged with British Aerospace to form BAE Systems. In 2006, extreme financial difficulties led to the collapse of the remaining company, with the bulk of the business acquired by the Swedish telecommunications company, Ericsson. 1897–1900: The Wireless Telegraph & Signal Company 1900–1963: Marconi's Wireless Telegraph Company 1963–1987: Marconi Company Ltd 1987–1998: GEC-Marconi Ltd 1998–1999: Marconi Electronic Systems Ltd 1999–2003: Marconi plc 2003–2006: Marconi Corporation plc Marconi's "Wireless Telegraph and Signal Company" was formed on 20 July 1897 after the granting of a British patent for wireless in March of that year.
The company opened the world's first radio factory on Hall Street in Chelmsford in 1898 and was responsible for some of the most important advances in radio and television. These include: In 1900 the company's name was changed to "Marconi's Wireless Telegraph Company" and Marconi's Wireless Telegraph Training College was set up in 1901; the company and factory was moved to New Street Works in 1912, to allow for production expansion in light of the RMS Titanic disaster. Along with private entrepreneurs, Marconi company formed in 1924 the Unione Radiofonica Italiana, granted by Mussolini's regime a monopoly of radio broadcasts in 1924. After the war, URI became the RAI. In 1939, the Marconi Research Laboratories at Great Baddow were founded and in 1941 there was a buyout of Marconi-Ekco Instruments to form Marconi Instruments. English Electric acquired the Marconi Company in 1946. In 1948 the company was reorganised into four divisions: These had expanded to 13 manufacturing divisions by 1965 when a further reorganisation took place.
The divisions were placed into three groups: At this time the Marconi Company had facilities at New Street Chelmsford, Basildon and Writtle as well as in Wembley and Hackbridge. It owned Marconi Instruments, Sanders Electronics, Eddystone Radio and Marconi Italiana. In 1967 Marconi took over Company to form Eddystone Radio. In 1903 Marconi founded the Marconi's Wireless Telegraph Company of Canada, renamed as the Canadian Marconi Company in 1925; the radio business of the Canadian Marconi Company is known as Ultra Electronics TCS since 2002 and its avionic activities as CMC Electronics, owned by Esterline since 2007. In 1967 or 1968 English Electric was subject to a takeover bid by the Plessey Company but chose instead to accept an offer from GEC. Under UK government pressure, the computer section of GEC, English Electric Leo Marconi, merged with International Computers and Tabulators to form International Computers Limited; the computer interests of Elliott Automation which specialised in real-time computing were amalgamated with those of Marconi's Automation Division to form Marconi-Elliott Computers renamed as GEC Computers.
In 1968 Marconi Space and Defence Systems and Marconi Underwater Systems were formed. The Marconi Company continued as the primary defence subsidiary of GEC-Marconi. Marconi was renamed GEC-Marconi in 1987. During the period 1968–1999 GEC-Marconi/MES underwent significant expansion. Acquisitions which were folded into the company and partnerships established include: Other acquisitions include: Divisions of Plessey in 1989. Plessey Avionics Plessey Naval Systems Plessey Cryptography Plessey Electronic Systems Sippican Leigh InstrumentsIn a major reorganisation of the company, GEC-Marconi was renamed Marconi Electronic Systems in 1996 and was separated from other non-defence assets. In 1999 GEC underwent a major transformation. Marconi Electronic Systems which included its wireless assets was demerged and sold to British Aerospace which formed BAE Systems. GEC, realigning itself as a telecommunications company following the MES sale, retained the Marconi brand and renamed itself Marconi plc. BAE were granted limited rights to continue its use in existing partnerships, however by 2005 no BAE businesses use the Marconi name.
Major spending and the dot-com collapse led to a major restructuring of that group, in a debt-for-equity swap shareholders were given 0.5% of the new company, Marconi Corporation plc. In 1999 Reltec and Fore Systems were acquired at the height of the "dot-com" boom. With its subsequent collapse the Marconi Corporation got into financial difficulties. In October 2005 the Swedish firm Ericsson offered to buy most of the assets; the transaction was completed on 23 January 2006 effective as of 1 January 2006. The Marconi name will still be used as a brand within Ericsson. At the time of the acquisition Ericsson announced that they would be rebranding Marconi assets Ericsson and retaining Marconi only as the name of the Italian research facility; however the company has since labelled its OMS line and its Long Haul Digital Radio system Marconi. The rest of the Marconi company was renamed as Telent. Aerospace industry
A computer network is a digital telecommunications network which allows nodes to share resources. In computer networks, computing devices exchange data with each other using connections between nodes; these data links are established over cable media such as wires or optic cables, or wireless media such as Wi-Fi. Network computer devices that originate and terminate the data are called network nodes. Nodes are identified by network addresses, can include hosts such as personal computers and servers, as well as networking hardware such as routers and switches. Two such devices can be said to be networked together when one device is able to exchange information with the other device, whether or not they have a direct connection to each other. In most cases, application-specific communications protocols are layered over other more general communications protocols; this formidable collection of information technology requires skilled network management to keep it all running reliably. Computer networks support an enormous number of applications and services such as access to the World Wide Web, digital video, digital audio, shared use of application and storage servers and fax machines, use of email and instant messaging applications as well as many others.
Computer networks differ in the transmission medium used to carry their signals, communications protocols to organize network traffic, the network's size, traffic control mechanism and organizational intent. The best-known computer network is the Internet; the chronology of significant computer-network developments includes: In the late 1950s, early networks of computers included the U. S. military radar system Semi-Automatic Ground Environment. In 1959, Anatolii Ivanovich Kitov proposed to the Central Committee of the Communist Party of the Soviet Union a detailed plan for the re-organisation of the control of the Soviet armed forces and of the Soviet economy on the basis of a network of computing centres, the OGAS. In 1960, the commercial airline reservation system semi-automatic business research environment went online with two connected mainframes. In 1963, J. C. R. Licklider sent a memorandum to office colleagues discussing the concept of the "Intergalactic Computer Network", a computer network intended to allow general communications among computer users.
In 1964, researchers at Dartmouth College developed the Dartmouth Time Sharing System for distributed users of large computer systems. The same year, at Massachusetts Institute of Technology, a research group supported by General Electric and Bell Labs used a computer to route and manage telephone connections. Throughout the 1960s, Paul Baran and Donald Davies independently developed the concept of packet switching to transfer information between computers over a network. Davies pioneered the implementation of the concept with the NPL network, a local area network at the National Physical Laboratory using a line speed of 768 kbit/s. In 1965, Western Electric introduced the first used telephone switch that implemented true computer control. In 1966, Thomas Marill and Lawrence G. Roberts published a paper on an experimental wide area network for computer time sharing. In 1969, the first four nodes of the ARPANET were connected using 50 kbit/s circuits between the University of California at Los Angeles, the Stanford Research Institute, the University of California at Santa Barbara, the University of Utah.
Leonard Kleinrock carried out theoretical work to model the performance of packet-switched networks, which underpinned the development of the ARPANET. His theoretical work on hierarchical routing in the late 1970s with student Farouk Kamoun remains critical to the operation of the Internet today. In 1972, commercial services using X.25 were deployed, used as an underlying infrastructure for expanding TCP/IP networks. In 1973, the French CYCLADES network was the first to make the hosts responsible for the reliable delivery of data, rather than this being a centralized service of the network itself. In 1973, Robert Metcalfe wrote a formal memo at Xerox PARC describing Ethernet, a networking system, based on the Aloha network, developed in the 1960s by Norman Abramson and colleagues at the University of Hawaii. In July 1976, Robert Metcalfe and David Boggs published their paper "Ethernet: Distributed Packet Switching for Local Computer Networks" and collaborated on several patents received in 1977 and 1978.
In 1979, Robert Metcalfe pursued making Ethernet an open standard. In 1976, John Murphy of Datapoint Corporation created ARCNET, a token-passing network first used to share storage devices. In 1995, the transmission speed capacity for Ethernet increased from 10 Mbit/s to 100 Mbit/s. By 1998, Ethernet supported transmission speeds of a Gigabit. Subsequently, higher speeds of up to 400 Gbit/s were added; the ability of Ethernet to scale is a contributing factor to its continued use. Computer networking may be considered a branch of electrical engineering, electronics engineering, telecommunications, computer science, information technology or computer engineering, since it relies upon the theoretical and practical application of the related disciplines. A computer network facilitates interpersonal communications allowing users to communicate efficiently and via various means: email, instant messaging, online chat, video telephone calls, video conferencing. A network allows sharing of computing resources.
Users may access and use resources provided by devices on the network, such as printing a document on a shared network printer or use of a shared storage device. A network allows sharing of files, and
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final detection is done. Conversion to an intermediate frequency is useful for several reasons; when several stages of filters are used, they can all be set to a fixed frequency, which makes them easier to build and to tune. Lower frequency transistors have higher gains so fewer stages are required. It's easier to make selective filters at lower fixed frequencies. There may be several such stages of intermediate frequency in a superheterodyne receiver. Intermediate frequencies are used for three general reasons.
At high frequencies, signal processing circuitry performs poorly. Active devices such as transistors cannot deliver much amplification. Ordinary circuits using capacitors and inductors must be replaced with cumbersome high frequency techniques such as striplines and waveguides. So a high frequency signal is converted to a lower IF for more convenient processing. For example, in satellite dishes, the microwave downlink signal received by the dish is converted to a much lower IF at the dish, to allow a inexpensive coaxial cable to carry the signal to the receiver inside the building. Bringing the signal in at the original microwave frequency would require an expensive waveguide. A second reason, in receivers that can be tuned to different frequencies, is to convert the various different frequencies of the stations to a common frequency for processing, it is difficult to build multistage amplifiers and detectors that can have all stages track in tuning different frequencies, but it is comparatively easy to build tunable oscillators.
Superheterodyne receivers tune in different frequencies by adjusting the frequency of the local oscillator on the input stage, all processing after, done at the same fixed frequency, the IF. Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the frequency was changed, as was necessary in the early tuned radio frequency receivers. A more important advantage is; the bandwidth of a filter is proportional to its center frequency. In receivers like the TRF in which the filtering is done at the incoming RF frequency, as the receiver is tuned to higher frequencies its bandwidth increases; the main reason for using an intermediate frequency is to improve frequency selectivity. In communication circuits, a common task is to separate out or extract signals or components of a signal that are close together in frequency; this is called filtering. Some examples are, picking up a radio station among several that are close in frequency, or extracting the chrominance subcarrier from a TV signal.
With all known filtering techniques the filter's bandwidth increases proportionately with the frequency. So a narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing the filtering at that frequency. FM and television broadcasting with their narrow channel widths, as well as more modern telecommunications services such as cell phones and cable television, would be impossible without using frequency conversion; the most used intermediate frequencies for broadcast receivers are around 455 kHz for AM receivers and 10.7 MHz for FM receivers. In special purpose receivers other frequencies can be used. A dual-conversion receiver may have two intermediate frequencies, a higher one to improve image rejection and a second, lower one, for desired selectivity. A first intermediate frequency may be higher than the input signal, so that all undesired responses can be filtered out by a fixed-tuned RF stage. In a digital receiver, the analog to digital converter operates at low sampling rates, so input RF must be mixed down to IF to be processed.
Intermediate frequency tends to be lower frequency range compared to the transmitted RF frequency. However, the choices for the IF are most dependent on the available components such as mixer, filters and others that can operate at lower frequency. There are other factors involved in deciding the IF frequency, because lower IF is susceptible to noise and higher IF can cause clock jitters. Modern satellite television receivers use several intermediate frequencies; the 500 television channels of a typical system are transmitted from the satellite to subscribers in the Ku microwave band, in two subbands of 10.7 - 11.7 and 11.7 - 12.75 GHz. The downlink signal is received by a satellite dish. In the box at the focus of the dish, called a low-noise block downconverter, each block of frequencies is converted to the IF range of 950 - 2150 MHz by two fixed frequency local oscillators at 9.75 and 10.6 GHz. One of the two blocks is selected by a control signal from the set top box inside, which switches on one of the local oscillators.
This IF is carried into the building to the television receiver on a coaxial cable. At the cable company's set top box, the signal is converted to a lower IF of 480 MHz for filtering, by a variable frequency oscillator; this is sent through a 30 MHz bandpass filter, which selects the signal from one of the transponders on the satellite, which carries several channels. Further
History of the Internet
The history of the Internet begins with the development of electronic computers in the 1950s. Initial concepts of wide area networking originated in several computer science laboratories in the United States, United Kingdom, France; the U. S. Department of Defense awarded contracts as early as the 1960s, including for the development of the ARPANET project, directed by Robert Taylor and managed by Lawrence Roberts; the first message was sent over the ARPANET in 1969 from computer science Professor Leonard Kleinrock's laboratory at University of California, Los Angeles to the second network node at Stanford Research Institute. Packet switching networks such as the NPL network, ARPANET, Merit Network, CYCLADES, Telenet, were developed in the late 1960s and early 1970s using a variety of communications protocols. Donald Davies first demonstrated packet switching in 1967 at the National Physics Laboratory in the UK, which became a testbed for UK research for two decades; the ARPANET project led to the development of protocols for internetworking, in which multiple separate networks could be joined into a network of networks.
The Internet protocol suite was developed by Robert E. Kahn and Vint Cerf in the 1970s and became the standard networking protocol on the ARPANET, incorporating concepts from the French CYCLADES project directed by Louis Pouzin. In the early 1980s the NSF funded the establishment for national supercomputing centers at several universities, provided interconnectivity in 1986 with the NSFNET project, which created network access to the supercomputer sites in the United States from research and education organizations. Commercial Internet service providers began to emerge in the late 1980s; the ARPANET was decommissioned in 1990. Limited private connections to parts of the Internet by commercial entities emerged in several American cities by late 1989 and 1990, the NSFNET was decommissioned in 1995, removing the last restrictions on the use of the Internet to carry commercial traffic. In the 1980s, research at CERN in Switzerland by British computer scientist Tim Berners-Lee resulted in the World Wide Web, linking hypertext documents into an information system, accessible from any node on the network.
Since the mid-1990s, the Internet has had a revolutionary impact on culture and technology, including the rise of near-instant communication by electronic mail, instant messaging, voice over Internet Protocol telephone calls, two-way interactive video calls, the World Wide Web with its discussion forums, social networking, online shopping sites. The research and education community continues to develop and use advanced networks such as JANET in the United Kingdom and Internet2 in the United States. Increasing amounts of data are transmitted at higher and higher speeds over fiber optic networks operating at 1 Gbit/s, 10 Gbit/s, or more; the Internet's takeover of the global communication landscape was instant in historical terms: it only communicated 1% of the information flowing through two-way telecommunications networks in the year 1993 51% by 2000, more than 97% of the telecommunicated information by 2007. Today the Internet continues to grow, driven by greater amounts of online information, commerce and social networking.
However, the future of the global internet may be shaped by regional differences in the world. The concept of data communication – transmitting data between two different places through an electromagnetic medium such as radio or an electric wire – pre-dates the introduction of the first computers; such communication systems were limited to point to point communication between two end devices. Semaphore lines, telegraph systems and telex machines can be considered early precursors of this kind of communication; the Telegraph in the late 19th century was the first digital communication system. Fundamental theoretical work in data transmission and information theory was developed by Claude Shannon, Harry Nyquist, Ralph Hartley in the early 20th century. Early computers had remote terminals; as the technology evolved, new systems were devised to allow communication over longer distances or with higher speed that were necessary for the mainframe computer model. These technologies made it possible to exchange data between remote computers.
However, the point-to-point communication model was limited, as it did not allow for direct communication between any two arbitrary systems. The technology was considered unsafe for strategic and military use because there were no alternative paths for the communication in case of an enemy attack. With limited exceptions, the earliest computers were connected directly to terminals used by individual users in the same building or site; such networks became known as local area networks. Networking beyond this scope, known as wide area networks, emerged during the 1950s and became established during the 1960s. J. C. R. Licklider, Vice President at Bolt Beranek and Newman, Inc. proposed a global network in his January 1960 paper Man-Computer Symbiosis: A network of such centers, connected to one another by wide-band communication lines the functions of present-day libraries together with anticipated advances in information storage and retrieval and symbiotic functions suggested earlier in this paper In August 1962, Licklider and Welden Clark published the paper "On-Line Man-Computer Communication", one of the first descriptions of a networked future.
In October 1962, Licklider was hired by Jack Ruina as director of the newly established Information Processing Techniques Office w