Telecommunication, often used in its plural form or abbreviated astelecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of transmission may be divided intocommunication channels formultiplexing, allowing for a single medium to transmit several concurrentcommunication sessions. Long-distance technologies invented during the 20th and 21st centuries generally use electric power, and include thetelegraph,telephone,television, andradio.

Early telecommunication networks used metal wires as the medium for transmitting signals. These networks were used fortelegraphy and telephony for many decades. In the first decade of the 20th century, a revolution inwireless communication began with breakthroughs including those made inradio communications byGuglielmo Marconi, who won the 1909Nobel Prize in Physics. Other early pioneers in electrical and electronic telecommunications include co-inventors of the telegraphCharles Wheatstone andSamuel Morse, numerousinventors and developers of the telephone includingAntonio Meucci,Philipp Reis,Elisha Gray andAlexander Graham Bell, inventors of radioEdwin Armstrong andLee de Forest, as well as inventors of television likeVladimir K. Zworykin,John Logie Baird andPhilo Farnsworth.

Since the 1960s, the proliferation of digital technologies has meant thatvoice communications have gradually been supplemented by data. The physical limitations of metallic media prompted the development of optical fibre.^[1][2][3] TheInternet, a technology independent of any given medium, has provided global access to services for individual users and further reduced location and time limitations on communications.

At the 1932Plenipotentiary Telegraph Conference and the International Radiotelegraph Conference in Madrid, the two organizations merged to form theInternational Telecommunication Union (ITU).^[4] They definedtelecommunication as "any telegraphic or telephonic communication of signs, signals, writing, facsimiles and sounds of any kind, by wire, wireless or other systems or processes of electric signaling or visual signaling (semaphores)."

The definition was later reconfirmed, according to Article 1.3 of theITU Radio Regulations, which defined it as "Any transmission, emission or reception of signs, signals, writings, images and sounds or intelligence of any nature bywire, radio, optical, or other electromagnetic systems".

As such, slow communications technologies likepostal mail andpneumatic tubes are excluded from the telecommunication's definition.^[5][6]

Telecommunication is a compound noun of the Greek prefixtele- (τῆλε), meaningdistant,far off, orafar,^[7] and the Latin verbcommunicare, meaningto share. Its modern use is adapted from the French,^[8] because its written use was recorded in 1904 by the French engineer and novelistÉdouard Estaunié.^[9][10]Communication was first used as an English word in the late 14th century. It comes from Old French comunicacion (14c., Modern French communication), from Latin communicationem (nominative communication), noun of action from past participle stem of communicare, "to share, divide out; communicate, impart, inform; join, unite, participate in," literally, "to make common", from communis.^[11]

Manytransmission media have been used for long-distance communication throughout history, fromsmoke signals,beacons,semaphore telegraphs,signal flags, and opticalheliographs towires and empty space made to carryelectromagnetic signals.

Homing pigeons have been used throughout history by different cultures.Pigeon post hadPersian roots and was later used by the Romans to aid their military.Frontinus claimedJulius Caesar used pigeons as messengers in his conquest ofGaul.^[12] TheGreeks also conveyed the names of the victors at theOlympic Games to various cities using homing pigeons.^[13] In the early 19th century, the Dutch government used the system inJava andSumatra. And in 1849,Paul Julius Reuter started a pigeon service to fly stock prices betweenAachen andBrussels, a service that operated for a year until the gap in the telegraph link was closed.^[14]

In the Middle Ages, chains ofbeacons were commonly 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 theSpanish Armada, when a beacon chain relayed a signal fromPlymouth toLondon.^[15]

In 1792,Claude Chappe, a French engineer, built the first fixed visualtelegraphy system (orsemaphore line) betweenLille and Paris.^[16] However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres (six to nineteen miles). As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880.^[17]

Besides beacons and pigeons, long-distance communication used sounds likecoded drumbeats, the blowing ofhorns, andwhistles.

On July 25, 1837, the first commercialelectrical telegraph was demonstrated by English inventor SirWilliam Fothergill Cooke and English scientist SirCharles Wheatstone.^[18][19] Both inventors viewed their device as "an improvement to the [existing] electromagnetic telegraph" and not as a new device.^[20]

Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on September 2, 1837.His code was an important advance over Wheatstone's signaling method. The firsttransatlantic telegraph cable was successfully completed on July 27, 1866, allowing transatlantic telecommunication for the first time.^[21]

After early attempts to develop atalking telegraph byAntonio Meucci and atelefon byJohann Philipp Reis, a patent for the conventional telephone was filed byAlexander Bell in February 1876 (just a few hours beforeElisha Gray filed apatent caveat for a similar device).^[22][23] The first commercial telephone services were set up by the Bell Telephone Company in 1878 and 1879 on both sides of the Atlantic in the cities ofNew Haven and London.^[24][25]

In 1894, Italian inventorGuglielmo Marconi began developing a wireless communication using the then-newly discovered phenomenon ofradio waves, demonstrating, by 1901, that they could be transmitted across the Atlantic Ocean.^[26] This was the start ofwireless telegraphy by radio. On 17 December 1902, a transmission from theMarconi station in Glace Bay, Nova Scotia, Canada, became the world's first radio message to cross the Atlantic from North America. In 1904, a commercial service was established to transmit nightly news summaries to subscribing ships, which incorporated them into their onboard newspapers.^[27]

World War I accelerated the development of radio formilitary communications. After the war, commercial radioAM broadcasting began in the 1920s and became an important mass medium for entertainment and news.World War II again accelerated the development of radio for the wartime purposes of aircraft and land communication, radio navigation, and radar.^[28] Development of stereoFM broadcasting of radio began in the 1930s in the United States and the 1940s in the United Kingdom,^[29] displacing AM as the dominant commercial standard in the 1970s.^[30]

On March 25, 1925,John Logie Baird demonstrated the transmission of moving pictures at the London department storeSelfridges. Baird's device relied upon theNipkow disk byPaul Nipkow and thus became known as themechanical television. It formed the basis of experimental broadcasts done by theBritish Broadcasting Corporation beginning on 30 September 1929.^[31] However, for most of the 20th century, televisions depended on thecathode ray tube invented byKarl Ferdinand Braun. The first version of such a television to show promise was produced byPhilo Farnsworth and demonstrated to his family on 7 September 1927.^[32] After World War II, interrupted experiments resumed and television became an important home entertainment broadcast medium.

The type of device known as athermionic tube orthermionic valve usesthermionic emission of electrons from aheated cathode for a number of fundamental electronic functions such as signalamplification and currentrectification.

The simplest vacuum tube, thediode invented in 1904 byJohn Ambrose Fleming, contains only a heated electron-emitting cathode and an anode. Electrons can only flow in one direction through the device—from the cathode to the anode. Adding one or morecontrol grids within the tube enables the current between the cathode and anode to be controlled by the voltage on the grid or grids.^[33] These devices became a key component of electronic circuits for the first half of the 20th century and were crucial to the development of radio, television, radar,sound recording and reproduction, long-distance telephone networks, and analogue and early digitalcomputers. While some applications had used earlier technologies such as thespark gap transmitter for radio ormechanical computers for computing, it was the invention of the thermionic vacuum tube that made these technologies widespread and practical, leading to the creation ofelectronics.^[34]

In the 1940s, the invention ofsemiconductor devices made it possible to producesolid-state devices, which are smaller, cheaper, and more efficient, reliable, and durable than thermionic tubes. Starting in the mid-1960s, thermionic tubes were replaced with thetransistor. Thermionic tubes still have some applications for certain high-frequency amplifiers.

On 11 September 1940,George Stibitz transmitted problems for his Complex Number Calculator in New York using ateletype and received the computed results back atDartmouth College inNew Hampshire.^[35] This configuration of a centralized computer (mainframe) with remotedumb terminals remained popular well into the 1970s. In the 1960s,Paul Baran and, independently,Donald Davies started to investigatepacket switching, a technology that sends a message in portions to its destinationasynchronously without passing it through a centralizedmainframe. A four-nodenetwork emerged on 5 December 1969, constituting the beginnings of theARPANET, which by 1981 had grown to 213nodes.^[36] ARPANET eventually merged with other networks to form the Internet. While Internet development was a focus of theInternet Engineering Task Force (IETF) who published a series ofRequest for Comments documents, other networking advancements occurred inindustrial laboratories, such as thelocal area network (LAN) developments ofEthernet (1983),Token Ring (1984)^{[citation needed]} andStar network topology.

The effective capacity to exchange information worldwide through two-way telecommunication networks grew from 281petabytes (PB) of optimally compressed information in 1986 to 471 PB in 1993 to 2.2exabytes (EB) in 2000 to 65 EB in 2007.^[37] This is the informational equivalent of two newspaper pages per person per day in 1986, and six entire newspapers per person per day by 2007.^[38] Given this growth, telecommunications play an increasingly important role in the world economy and the global telecommunications industry was about a $4.7 trillion sector in 2012.^[39][40] The service revenue of the global telecommunications industry was estimated to be $1.5 trillion in 2010, corresponding to 2.4% of the world's gross domestic product (GDP).^[39]

Modern telecommunication is founded on a series of key concepts that experienced progressive development and refinement in a period of well over a century:

Telecommunication technologies may primarily be divided intowired and wireless methods. Overall, a basic telecommunication system consists of three main parts that are always present in some form or another:

• Atransmitter that takes information and converts it to asignal
• Atransmission medium, also called thephysical channel, that carries the signal (e.g., the"free space channel")
• Areceiver that takes the signal from the channel and converts it back into usable information for the recipient

In aradio broadcasting station, the station's largepower amplifier is the transmitter and the broadcastingantenna is the interface between the power amplifier and the free space channel. 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, theradio receiver is the destination of the radio signal, where it is converted from electricity to sound.

Telecommunication systems are occasionally"duplex" (two-way systems) with a single box of electronics working as both the transmitter and a receiver, or atransceiver (e.g., amobile phone).^[41] The transmission electronics and the receiver electronics within a transceiver are quite independent of one another. This can be explained by the fact that radio transmitters contain power amplifiers that operate with electrical powers measured in watts or kilowatts, but radio receivers deal with radio powers measured in microwatts ornanowatts. Hence, transceivers have to be carefully designed and built to isolate their high-power circuitry and their low-power circuitry from each other to avoid interference.

Telecommunication over fixed lines is calledpoint-to-point communication because it occurs between a transmitter and a receiver. Telecommunication through radio broadcasts is calledbroadcast communication because it occurs between a powerful transmitter and numerous low-power but sensitive radio receivers.^[41]

Telecommunications in which multiple transmitters and multiple receivers have been designed to cooperate and share the same physical channel are calledmultiplex systems. The sharing of physical channels using multiplexing often results in significant cost reduction. Multiplexed systems are laid out in telecommunication networks and multiplexed signals are switched at nodes through to the correct destination terminal receiver.

Communications can be encoded asanalogue ordigital signals, which may in turn be carried byanalogue ordigital communication systems. Analogue signals vary continuously with respect to the information, while digital signals encode information as a set of discrete values (e.g., a set of ones and zeroes).^[42] During propagation and reception, information contained in analogue signals is degraded by undesirablenoise. Commonly, the noise in a communication system can be expressed as adding or subtracting from the desirable signal via arandom process. This form of noise is calledadditive noise, with the understanding that the noise can be negative or positive at different instances.

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 analogue signals. However, digital systemsfail catastrophically when noise exceeds the system's ability to autocorrect. On the other hand, analogue systems fail gracefully: as noise increases, the signal becomes progressively more degraded but still usable. Also, digital transmission ofcontinuous data unavoidably addsquantization noise to the output. This can be reduced, but not eliminated, only at the expense of increasing the channel bandwidth requirement.

The termchannel has two different meanings. In one meaning, a channel is the physical medium that carries a signal between the transmitter and the receiver. Examples of this include theatmosphere for sound communications, glassoptical fibres for some kinds ofoptical communications,coaxial cables for communications by way of the voltages and electric currents in them, andfree space for communications usingvisible light,infrared waves,ultraviolet light, andradio waves. Coaxial cable types are classified by RG type orradio guide, terminology derived from World War II. The various RG designations are used to classify the specific signal transmission applications.^[43] This last channel is called thefree 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. Radio waves travel through a perfectvacuum just as easily as they travel through air, fog, clouds, or any other kind of gas.

The other meaning of the termchannel in telecommunications is seen in the phrasecommunications channel, 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 neighbourhood of 94.5 MHz (megahertz) while another radio station can simultaneously broadcast radio waves at frequencies in the neighbourhood of 96.1 MHz. Each radio station would transmit radio waves over a frequencybandwidth of about 180 kHz (kilohertz), centred 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.

In the example above, thefree space channel has been divided into communications channels according tofrequencies, and each channel is assigned a separate frequency bandwidth in which to broadcast radio waves. This system of dividing the medium into channels according to frequency is calledfrequency-division multiplexing. Another term for the same concept iswavelength-division multiplexing, which is more commonly used in optical communications when multiple transmitters share the same physical medium.

Another way of dividing a communications medium into channels is to allocate each sender a recurring segment of time (atime slot, for example, 20milliseconds 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 calledtime-division multiplexing (TDM), and is used in optical fibre communication. Some radio communication systems use TDM within an allocated FDM channel. Hence, these systems use a hybrid of TDM and FDM.

The shaping of a signal to convey information is known asmodulation. Modulation can be used to represent a digital message as an analogue waveform. This is commonly called"keying"—a term derived from the older use of Morse Code in telecommunications—and several keying techniques exist (these includephase-shift keying,frequency-shift keying, andamplitude-shift keying). TheBluetooth system, for example, uses phase-shift keying to exchange information between various devices.^[44][45] In addition, there are combinations of phase-shift keying and amplitude-shift keying which is called (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 analogue signals at higher frequencies. This is helpful because low-frequency analogue signals cannot be effectively transmitted over free space. Hence the information from a low-frequency analogue signal must be impressed into a higher-frequency signal (known as thecarrier wave) before transmission. There are several different modulation schemes available to achieve this [two of the most basic beingamplitude modulation (AM) andfrequency modulation (FM)]. An example of this process is a disc jockey's voice being impressed into a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel 96 FM).^[46] In addition, modulation has the advantage that it may use frequency division multiplexing (FDM).

A telecommunications network is a collection of transmitters, receivers, andcommunications channels that send messages to one another. Some digital communications networks contain one or morerouters that work together to transmit information to the correct user. An analogue communications network consists of one or moreswitches that establish a connection between two or more users. For both types of networks,repeaters may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combatattenuation that can render the signal indistinguishable from the noise.^[47] Another advantage of digital systems over analogue is that their output is easier to store in memory, i.e., two voltage states (high and low) are easier to store than a continuous range of states.

Telecommunication has a significant social, cultural and economic impact on modern society. In 2008, estimates placed thetelecommunication industry's revenue at US$4.7 trillion or just under three per cent of thegross world product (official exchange rate).^[39] Several following sections discuss the impact of telecommunication on society.

On themicroeconomic scale, companies have used telecommunications to help build global business empires. This is self-evident in the case of online retailerAmazon.com but, according to academic Edward Lenert, even the conventional retailerWalmart has benefited from better telecommunication infrastructure compared to its competitors.^[48] In cities throughout the world, home owners use their telephones to order and arrange a variety of home services ranging from pizza deliveries to electricians. Even relatively poor communities have been noted to use telecommunication to their advantage. InBangladesh'sNarsingdi District, isolated villagers use cellular phones to speak directly to wholesalers and arrange a better price for their goods. InCôte d'Ivoire, coffee growers share mobile phones to follow hourly variations in coffee prices and sell at the best price.^[49]

On themacroeconomic scale, Lars-Hendrik Röller andLeonard Waverman suggested a causal link between good telecommunication infrastructure and economic growth.^[50][51] Few dispute the existence of a correlation although some argue it is wrong to view the relationship as causal.^[52]

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 thedigital divide. A 2003 survey by theInternational Telecommunication Union (ITU) revealed that roughly a 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. 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.^[53] Using this measure, Sweden, Denmark andIceland received the highest ranking while the African countriesNiger,Burkina Faso andMali received the lowest.^[54]

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 (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. New promotions started appealing to consumers' emotions, stressing the importance of social conversations and staying connected to family and friends.^[55]

Since then the role that telecommunications has played in social relations has become increasingly important. In recent years,^[when?] the popularity ofsocial 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, sexual preference and relationship status. In this way, these sites can play important role in everything from organising social engagements tocourtship.^[56]

Prior to social networking sites, technologies likeshort message service (SMS) and the telephone also had a significant impact on social interactions. In 2000, market research groupIpsos 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.^[57]

In cultural terms, telecommunication has increased the public's ability to access 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. 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 2006 survey (right table) of slightly more than 3,000 Americans by the non-profit Pew Internet and American Life Project in the United States the majority specified television or radio over newspapers.

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 media that depend upon telecommunication.^[59]

Many countries have enacted legislation which conforms to the International Telecommunication Regulations established by the International Telecommunication Union (ITU), which is the "leading UN agency for information and communication technology issues".^[60] 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". According to the ITU'sRadio Regulations adopted in Atlantic City, all frequencies referenced in theInternational Frequency Registration Board, examined by the board and registered on theInternational Frequency List "shall have the right to international protection from harmful interference".^[61]

From a global perspective, there have been political debates and legislation regarding the management of telecommunication and broadcasting. Thehistory of broadcasting discusses some debates in relation to balancing conventional communication such as printing and telecommunication such as radio broadcasting.^[62] The onset ofWorld War II brought on the first explosion of international broadcasting propaganda.^[62] Countries, their governments, insurgents, terrorists, and militiamen have all used telecommunication and broadcasting techniques to promote propaganda.^[62][63] Patriotic propaganda for political movements and colonization started the mid-1930s. In 1936, the BBC broadcast propaganda to the Arab World to partly counter similar broadcasts from Italy, which also had colonial interests in North Africa.^[62] Modern political debates in telecommunication include the reclassification ofbroadband Internet service as a telecommunications service (also callednet neutrality),^[64][65] regulation ofphone spam,^[66][67] and expanding affordable broadband access.^[68]

According to data collected by Gartner^[69][70] and Ars Technica^[71] sales of main consumer's telecommunication equipment worldwide in millions of units was:

In a telephone network, the caller is connected to the person to whom they wish to talk by switches at varioustelephone exchanges. The switches form an electrical connection between the two users and the setting of these switches is determined electronically when the callerdials the number. Once the connection is made, the caller's voice is transformed to an electrical signal using a smallmicrophone in the caller'shandset. 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 smallspeaker in that person's handset.

As of 2015^[update], the landline telephones in most residential homes are analogue—that is, the speaker's voice directly determines the signal's voltage.^[72] Although short-distance calls may be handled from end-to-end as analogue signals, increasingly telephone service providers are transparently converting the signals to digital signals for transmission. 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 analogue signals that are inevitably impacted by noise).

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), Western Europe (164 m), CEMEA (Central Europe, the Middle East and Africa) (153.5 m), North America (148 m) and Latin America (102 m).^[73] In terms of new subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth.^[74] Increasingly these phones are being serviced by systems where the voice content is transmitted digitally such asGSM orW-CDMA with many markets choosing to deprecate analog systems such asAMPS.^[75]

There have also been dramatic changes in telephone communication behind the scenes. Starting with the operation ofTAT-8 in 1988, the 1990s saw the widespread adoption of systems based on optical fibres. The benefit of communicating with optical fibres 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 optical fibre cables are able to carry 25 times as many telephone calls as TAT-8.^[76] This increase in data capacity is due to several factors: First, optical fibres are physically much smaller than competing technologies. Second, they do not suffer fromcrosstalk which means several hundred of them can be easily bundled together in a single cable.^[77] Lastly, improvements in multiplexing have led to an exponential growth in the data capacity of a single fibre.^[78][79]

Assisting communication across many modern optical fibre networks is a protocol known asAsynchronous Transfer Mode (ATM). The ATM protocol allows for the side-by-sidedata 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 atraffic contract with that pathway. 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. 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.^[80] There are competitors to ATM, such asMultiprotocol Label Switching (MPLS), that perform a similar task and are expected to supplant ATM in the future.^[81][82]

In a broadcast system, the central high-poweredbroadcast tower transmits a high-frequencyelectromagnetic wave to numerous low-powered receivers. The high-frequency wave sent by the tower is modulated with a signal containing visual or audio information. The receiver is thentuned so as to pick up the high-frequency wave and ademodulator is used to retrieve the signal containing the visual or audio information. The broadcast signal can be either analogue (signal is varied continuously with respect to the information) or digital (information is encoded as a set of discrete values).^[41][83]

Thebroadcast media industry is at a critical turning point in its development, with many countries moving from analogue to digital broadcasts. This move is made possible by the production of cheaper, faster and more capableintegrated circuits. The chief advantage of digital broadcasts is that they prevent a number of complaints common to traditional analogue broadcasts. For television, this includes the elimination of problems such assnowy pictures,ghosting and other distortion. These occur because of the nature of analogue 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. In a simplified example, if a binary message 1011 was transmitted with signal amplitudes [1.0 0.0 1.0 1.0] and received with signal amplitudes [0.9 0.2 1.1 0.9] it would still decode to the binary message 1011— a perfect reproduction of what was sent. 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. Usingforward 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.^[84][85]

In digital television broadcasting, there are three competing standards that are likely to be adopted worldwide. These are theATSC,DVB andISDB standards; the adoption of these standards thus far is presented in the captioned map. All three standards useMPEG-2 for video compression. ATSC usesDolby Digital AC-3 for audio compression, ISDB usesAdvanced Audio Coding (MPEG-2 Part 7) and DVB has no standard for audio compression but typically usesMPEG-1 Part 3 Layer 2.^[86][87] 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 theDigital Audio Broadcasting standard (also known as theEureka 147 standard). The exception is the United States which has chosen to adoptHD Radio. HD Radio, unlike Eureka 147, is based upon a transmission method known asin-band on-channel transmission that allows digital information to piggyback on normal AM or FM analog transmissions.^[88]

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^[89] after twice delaying the switchover deadline. Kenya also ended analog television transmission in December 2014 after multiple delays. For analogue television, there were three standards in use for broadcasting colour TV (see a map on adoptionhere). These are known asPAL (German designed),NTSC (American designed), andSECAM (French-designed). For analogue radio, the switch to digital radio is made more difficult by the higher cost of digital receivers.^[90] The choice of modulation for analogue radio is typically between amplitude (AM) or frequency modulation (FM). To achievestereo playback, an amplitude modulated subcarrier is used forstereo FM, and quadrature amplitude modulation is used for stereo AM orC-QUAM.

The Internet is a worldwide network of computers and computer networks that communicate with each other using theInternet Protocol (IP).^[91] Any computer on the Internet has a uniqueIP address that can be used by other computers to route information to it. 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. The Internet is thus an exchange of messages between computers.^[92]

It is estimated that 51% of the information flowing through two-way telecommunications networks in the year 2000 were flowing through the Internet (most of the rest (42%) through thelandline telephone). By 2007 the Internet clearly dominated and captured 97% of all the information in telecommunication networks (most of the rest (2%) throughmobile phones).^[37] As of 2008^[update], 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%).^[93] In terms ofbroadband access, Iceland (26.7%), South Korea (25.4%) and the Netherlands (25.3%) led the world.^[94]

The Internet works in part because ofprotocols that govern how the computers and routers communicate with each other. 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. A practical example of why this is important is because it allows aweb 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. 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.^[95]

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. 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. 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 forlogical addressing. For the World Wide Web, these IP addresses are derived from the human-readable form using theDomain Name System (e.g., 72.14.207.99 is derived fromGoogle.com). At the moment, the most widely used version of the Internet Protocol is version four but a move to version six is imminent.^[96]

At the transport layer, most communication adopts either the Transmission Control Protocol (TCP) or theUser Datagram Protocol (UDP). TCP is used when it is essential every message sent is received by the other computer whereas UDP is used when it is merely desirable. 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 nor retransmitted if lost. Both TCP and UDP packets carryport numbers with them to specify what application orprocess the packet should be handled by.^[97] 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 assigningpriority.

Above the transport layer, there are certain protocols that are sometimes used and loosely fit in the session and presentation layers, most notably theSecure Sockets Layer (SSL) andTransport Layer Security (TLS) protocols. These protocols ensure that data transferred between two parties remains completely confidential.^[98] Finally, at the application layer, are many of the protocols Internet users would be familiar with such asHTTP (web browsing),POP3 (e-mail),FTP (file transfer),IRC (Internet chat),BitTorrent (file sharing) andXMPP (instant messaging).

Voice over Internet Protocol (VoIP) allows data packets to be used forsynchronous voice communications. The data packets are marked as voice-type packets and can be prioritized by the network administrators so that the real-time, synchronous conversation is less subject to contention with other types of data traffic which can be delayed (i.e., file transfer or email) or buffered in advance (i.e., audio and video) without detriment. That prioritization is fine when the network has sufficient capacity for all the VoIP calls taking place at the same time and the network is enabled for prioritization, i.e., a private corporate-style network, but the Internet is not generally managed in this way and so there can be a big difference in the quality of VoIP calls over a private network and over the public Internet.^[99]

Despite the growth of the Internet, the characteristics oflocal area networks (LANs)—computer networks that do not extend beyond a few kilometres—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. 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 does not provide assured protection from hackers, military forces, or economic powers. These threats exist if there are any methods for connecting remotely to the LAN.

Wide area networks (WANs) are private computer networks that may extend for thousands of kilometres. Once again, some of their advantages include privacy and security. Prime users of private LANs and WANs include armed forces and intelligence agencies that must keep their information 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 theOSI reference model. These includedAppleTalk,IPX, andNetBIOS with the dominant protocol set during the early 1990s being IPX due to its popularity withMS-DOS users.TCP/IP existed at this point, but it was typically only used by large government and research facilities.^[100]

As the Internet grew in popularity and its traffic was required to be routed into private networks, the TCP/IP protocols replaced existing local area network technologies. Additional technologies, such asDHCP, allowed TCP/IP-based computers to self-configure in the network. Such functions also existed in the AppleTalk/ IPX/ NetBIOS protocol sets.^[101]

Whereas Asynchronous Transfer Mode (ATM) or Multiprotocol Label Switching (MPLS) are typical data-link protocols for larger networks such as WANs; 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 asquality of service guarantees, and offermedium access control. Both of these differences allow for more economical systems.^[102]

Despite the modest popularity of Token Ring in the 1980s and 1990s, virtually all LANs now use either wired or wireless Ethernet facilities. At the physical layer, most wired Ethernet implementations usecopper twisted-pair cables (including the common10BASE-T networks). However, some early implementations used heavier coaxial cables and some recent implementations (especially high-speed ones) use optical fibres.^[103] When optic fibres are used, the distinction must be made between multimode fibres and single-mode fibres.Multimode fibres can be thought of as thicker optical fibres that are cheaper to manufacture devices for, but that suffer from less usable bandwidth and worse attenuation—implying poorer long-distance performance.^[104]

• Goggin, Gerard,Global Mobile Media (New York: Routledge, 2011), p. 176.ISBN 978-0-415-46918-0.
• OECD,Universal Service and Rate Restructuring in Telecommunications, Organisation for Economic Co-operation and Development (OECD) Publishing, 1991.ISBN 92-64-13497-2.
• Wheen, Andrew.Dot-Dash to Dot.Com: How Modern Telecommunications Evolved from the Telegraph to the Internet (Springer, 2011).

• International Teletraffic Congress
• International Telecommunication Union (ITU)
• ATIS Telecom Glossary
• Federal Communications Commission
• IEEE Communications Society
• International Telecommunication Union
• Ericsson's Understanding Telecommunications at theWayback Machine (archived 13 April 2004) (Ericsson removed the book from their site in September 2005)