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Telecommunication
Telecommunication is the transmission of signals over a distance for the purpose of communication. Today this process almost always involves the sending of electromagnetic waves by electronic transmitters but in earlier years it may have involved the use of smoke signals, drums or semaphores. Today, telecommunication is widespread and devices that assist the process such as the television, radio and telephone are common in many parts of the world. There is also a vast array of networks that connect these devices, including computer networks, public telephone networks, radio networks and television networks. Computer communication across the Internet, such as e-mail and internet faxing, is just one of many examples of telecommunication.
The word telecommunication was adapted from the French word télécommunication. It is a compound of the Greek prefix tele- (τηλε-), meaning 'far off', and communication, meaning 'exchange of information
Technical foundationsThe basic elements of a telecommunication system are:
For example, consider a radio broadcast. In this case, the broadcast tower is the transmitter, the radio is the receiver and the transmission medium is free space. Often telecommunication systems are two-way and devices act as both a tranmitter and receiver or transceiver. For example, a mobile phone is a transceiver. Telecommunication over a phone line is called point-to-point communication because it is between one transmitter and one receiver, telecommunication through radio broadcasts is called broadcast communication because it is between one powerful transmitter and numerous receivers.[2]
Signals can either be analogue or digital. In an analogue signal, the signal is varied continuously with respect to the information. In a digital signal, the information is encoded as a set of discrete values (e.g. 1's and 0's).[3]
A collection of transmitters, receivers or transceivers that communicate with each other is known as a network. Digital networks may consist of one or more routers that route data to the correct user. An analogue network may consist of one or more switches that establish a connection between two or more users. For both types of network, a repeater may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combat noise which can corrupt the information carried by a signal.
A channel is a division in a tranmission medium so that it can be used to send multiple independent streams of data. For example, a radio station may broadcast at 96 MHz while another radio station may broadcast at 94.5 MHz. In this case the medium has been divided by frequency and each channel received a separate frequency to broadcast on. Alternatively one could allocate each channel a segment of time over which to broadcast.
The shaping of a signal to convey information is known as modulation. Modulation is a key concept in telecommunications and is frequently used to impose the information of one signal on another. Modulation is used to represent a digital message as an analogue waveform. This is known as keying and several keying techniques exist — these include phase-shift keying, amplitude-shift keying and minimum-shift keying. Bluetooth, for example, uses phase-shift keying for exchanges between devices (see note).
However, more relevant to earlier discussion, modulation is also used to boost the frequency of analogue signals. This is because a raw signal is often not suitable for transmission over free space due to its low frequencies. Hence its information must be superimposed on a higher frequency signal (known as a carrier wave) before transmission. There are several different modulation schemes available to achieve this — some of the most basic being amplitude modulation and frequency modulation. An example of this process is a DJ's voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel “96 FM”).
HistoryEarly telecommunicationsEarly forms of telecommunication include smoke signals and drums. Drums were used by natives in Africa, New Guinea and tropical America whereas smoke signals were used by natives in America and China. Contrary to what one might think, these systems were often used to do more than merely announce the presence of a camp.
In 1792, a French engineer, Claude Chappe built the first visual telegraphy (or semaphore) system between Lille and Paris. This was followed by a line from Strasbourg to Paris. In 1794, a Swedish engineer, Abraham Edelcrantz built a quite different system from Stockholm to Drottningholm. As opposed to Chappe's system which involved pulleys rotating beams of wood, Edelcrantz's system relied only upon shutters and was therefore faster.[4] However semphore as a communication system suffered from the need for skilled operators and expensive towers often at intervals of only ten to thirty kilometres (six to nineteen miles). As a result, the last commercial line was abandoned in 1880.
Telegraph and telephoneThe first commercial electrical telegraph was constructed by Sir Charles Wheatstone and Sir William Fothergill Cooke. It used the deflection of needles to represent messages and started operating over thirteen miles (twenty-one kilometres) of the Great Western Railway on 9 April 1839. Both Wheatstone and Cooke viewed their device as "an improvement to the [existing] electromagnetic telegraph" not as a new device.
On the other side of the Atlantic Ocean, Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837. Soon after he was joined by Alfred Vail who developed the register — a telegraph terminal that integrated a logging device for recording messages to paper tape. This was demonstrated successfully over three miles (five kilometres) on 6 January 1838 and eventually over forty miles (64 kilometres) between Washington, DC and Baltimore on 24 May 1844. The patented invention proved lucrative and by 1851 telegraph lines in the United States spanned over 20,000 miles (32,000 kilometres).[5]
The first transatlantic telegraph cable was successfully completed on 27 July 1866, allowing transatlantic telegraph communications for the first time. Earlier transatlantic cables installed in 1857 and 1858 only operated for a few days or weeks before they failed.[6]
The conventional telephone was invented by Alexander Bell in 1876. Although in 1849 Antonio Meucci invented a device that allowed the electrical tranmission of voice over a line. Meucci's device depended upon the electrophonic effect and was of little practical value because it required users to place the receiver in their mouth to “hear” what was being said.[7]
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. Bell held patents needed for such services in both countries. The technology grew quickly from this point, with inter-city lines being built and exchanges in every major city of the United States by the mid-1880's.[8][9] Despite this, transatlantic communication remained impossible for customers until January 7, 1927 when a connection was esablished using radio. However no cable connection existed until TAT-1 was inaugerated on September 25, 1956 providing 36 telephone circuits. [10]
Radio and televisionIn 1832, James Lindsay gave a classroom demonstration of wireless telegraphy to his students. By 1854 he was able to demonstrate a transmission across the Firth of Tay from Dundee to Woodhaven, a distance of two miles, using water as the transmission medium.[11]
Addressing the Franklin Institute in 1893, Nikola Tesla described and demonstrated in detail the principles of wireless telegraphy. The apparatus that he used contained all the elements that were incorporated into radio systems before the development of the vacuum tube. However it was not until 1900, that Reginald Fessenden was able to wirelessly transmit a human voice. In December 1901, Guglielmo Marconi established wireless communication between Britain and the United States earning him the Nobel Prize in physics in 1909 (which he shared with Karl Braun).[12]
On March 25, 1925, John Logie Baird was able to demonstrate the tranmission of moving pictures at the London department store Selfridges. However his device did not adaquately display halftones and thus only presented a silhouette of the recorded image. This problem was rectified in October of that year leading to a public demonstration of the improved device on 26 January 1926 again at Selfridges. Baird's device relied upon the Nipkow disk and thus became known as the mechanical television. It formed the basis of experimental broadcasts done by the British Broadcasting Corporation beginning September 30, 1929.[13]
However for most of the twentieth century televisions depended upon the cathode ray tube invented by Karl Braun. The first version of such a television to show promise was produced by Philo Farnsworth and demonstrated to his family on September 7, 1927. Farnsworth's device would compete with the work of Vladimir Zworykin who also produced a television picture in 1929 on a cathode ray tube. Zworykin's camera, which later would be known as the Iconoscope, had the backing of the influential Radio Corporation of America (RCA) however eventually court action regarding "the electron image" between Farnsworth and RCA would resolve in Farnsworth's favour.[14]
Computer networks and the InternetOn September 11, 1940 George Stibitz was able to transmit problems using teletype to his Complex Number Calculator in New York and receive the computed results back at Dartmouth College in New Hampshire.[15] This configuration of a centralized computer or mainframe with remote dumb terminals remained popular throughout the 1950s. However it was not until the 1960s that researchers started to investigate packet switching — a technology that would allow chunks of data to be sent to different computers without first passing through a centralized mainframe. A four-node network emerged on December 5, 1969 between the University of California, Los Angeles, the Stanford Research Institute, the University of Utah and the University of California, Santa Barbara. This network would become ARPANET, which by 1981 would consist of 213 nodes.[16] In June 1973, the first non-US node was added to the network belonging to Norway's NORSAR project. This was shortly followed by a node in London.[17]
ARPANET's development centred around the Request for Comment process and on April 7, 1969, RFC 1 was published. This process is important because ARPANET would eventually merge with other networks to form the Internet and many of the protocols the Internet relies upon today were specified through this process. In September 1981, RFC 791 introduced the Internet Protocol v4 (IPv4) and RFC 793 introduced the Transmission Control Protocol (TCP) — thus creating the TCP/IP protocol that much of the Internet relies upon today. A more relaxed transport protocol that, unlike TCP, did not guarantee the orderly delivery of packets called the User Datagram Protocol (UDP) was submitted on 28 August 1980 as RFC 768. An e-mail protocol, SMTP, was introduced in August 1982 by RFC 821 and HTTP/1.0 a protocol that would make the hyperlinked Internet possible was introduced on May 1996 by RFC 1945.
However not all important developments were made through the Request for Comment process. Two popular link protocols for local area networks (LANs) also appeared in the 1970s. A patent for the token ring protocol was filed by Olof Soderblom on October 29, 1974.[18] And a paper on the ethernet protocol was published by Robert Metcalfe and David Boggs in the July 1976 issue of Communications of the ACM.[19] From the late 1980s until the late 1990s both protocols would have success in the LAN market with the token ring protocol fiercely promoted by IBM. But ultimately the majority of LANs would settle on the ethernet protocol by the start of the twenty-first century.
Modern operationTelephone
Optic fibres are revolutionizing long-distance communication
Today, the fixed-line telephone systems in most residential homes remain analogue and, although short-distance calls may be handled from end-to-end as analogue signals, increasingly telephone service providers are transparently converting signals to digital before, if necessary, converting them back to analogue for reception. Mobile phones have had a dramatic impact on telephone service providers. 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 (204m), Western Europe (164m), CEMEA (Central Europe, the Middle East and Africa) (153.5m), North America (148m) and Latin America (102m).[20] In terms of new subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth compared to the next largest market, Asia, which boasted 34.3% growth.[21] Increasingly these phones are being serviced by digital systems such as GSM or W-CDMA with many markets chosing to depreciate analogue systems such as AMPS.[22] However there have been equally drastic changes in telephone communication behind the scenes. Starting with the operation of TAT-8 in 1988, the 1990s saw the widespread adoption of systems based around optic fibres. The benefit of communicating with optic 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 optic fibre cables are able to carry 25 times as many telephone calls as TAT-8.[23] This rapid increase in data capacity is due to several factors. First, optic fibres are physically much smaller than competing technologies. Second, they do not suffer from crosstalk which means several hundred of them can be easily bundled together in a single cable.[24] Lastly, improvements in multiplexing have lead to an exponential growth in the data capacity of a single fibre. This is due to technologies such as dense wavelength-division multiplexing, which at its most basic level is building multiple channels based upon frequency division as discussed in the Technical foundations section.[25] However despite the advances of technologies such as dense wavelength-division multiplexing, technologies based around building multiple channels based upon time division such as synchronous optical networking and synchronous digital hierarchy remain dominant.[26]
Assisting communication across these networks is a protocol known as Asynchronous Transfer Mode (ATM). As a technology, ATM arose in the 1980s and was envisioned to be part of the Broadband Integrated Services Digital Network. The network ultimately failed but the technology gave birth to the ATM Forum which in 1992 published its first standard.[27] Today, despite competitors such as Multiprotocol Label Switching, ATM remains the protocol of choice for most major long-distance optical networks. The importance of the ATM protocol was chiefly in its notion of establishing pathways for data through the network and associating a traffic contract with these pathways. The traffic contract was essentially an agreement between the client and the network about how the network was to handle the data. This was important because telephone calls could negotiate a contract so as to guarantee themselves a constant bit rate, something that was essential to ensure the call could take place without a caller's voice being delayed in parts or cut-off completely.[28]
Radio and television
Digital television standards and their adoption worldwide.
The broadcast media industry is also at a critical turning point in its development, with many countries starting to move from analogue to digital broadcasts. The chief advantage of digital broadcasts is that they prevent a number of complaints with traditional analogue broadcasts. For television, this includes the elimination of problems such as snowy 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 binary data 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. Using forward error correction a receiver can correct a handful of bit errors in the resulting message but too much noise will lead to incomprehensible output and hence a breakdown of the tranmission.[29] In digital television broadcasting, there are three competing standards that are likely to be adopted worldwide. These are the ATSC, DVB and ISDB standards and the adoption of these standards thus far is presented in the captioned map. All three standards use MPEG-2 for video compression. ATSC uses Dolby Digital AC-3 for audio compression, ISDB uses Advanced Audio Coding (MPEG-2 Part 7) and DVB has no standard for audio compression but typically uses MPEG-1 Part 3 Layer 2.[30][31] The choice of modulation also varies between the schemes. Both DVB and ISDB use orthogonal frequency-division multiplexing (OFDM) for terrestrial broadcasts (as opposed to satellite or cable broadcasts) where as ATSC uses vestigial sideband modulation (VSB). OFDM should offer better resistance to multipath interference and the Doppler effect (which would impact reception using moving receivers).[32] However controversial tests conducted by the United States' National Association of Broadcasters have shown that there is little difference between the two for stationary receivers.[33]
In digital audio broadcasting, standards are much more unified with practically all countries (including Canada) choosing to adopt the Digital Audio Broadcasting standard (also known as the Eureka 147 standard). The exception being the United States which has chosen to adopt the HD Radio standard. HD Radio, unlike Eureka 147, is based upon a transmission method known as in-band on-channel transmission — this allows digital information to "piggyback" on normal AM or FM analogue transmissions. Hence avoiding the bandwidth allocation issues of Eureka 147 and therefore being strongly advocated National Association of Broadcasters who felt there was a lack of new spectrum to allocate for the Eureka 147 standard.[34] In the United States the Federal Communications Commission has chosen to leave licensing of the standard in the hands of a commercial corporation called iBiquity.[35] An open in-band on-channel standard exists in the form of Digital Radio Mondiale (DRM) however adoption of this standard is mostly limited to a handful of shortwave broadcasts. Despite the different names all standards rely upon OFDM for modulation. In terms of audio compression, DRM typically uses Advanced Audio Coding (MPEG-4 Part 3), DAB like DVB can use a variety of codecs but typically uses MPEG-1 Part 3 Layer 2 and HD Radio uses High-Definition Coding.
However, despite the pending switch to digital, analogue receivers still remain widespread. Analogue television is still transmitted in practically all countries. The United States of America had hoped to end analogue broadcasts by December 31st, 2006 however this was recently pushed back to February 17th, 2009.[36] For analogue, there are three standards in use (see a map on adoption here). These are known as PAL, NTSC and SECAM. The basics of PAL and NTSC are very similar; a quadrature amplitude modulated subcarrier carrying the chrominance information is added to the luminance video signal to form a composite video baseband signal (CVBS). The SECAM system, on the other hand, uses a frequency modulation scheme on its colour subcarrier. The name "Phase Alternating Line" describes the way that the phase of part of the colour information on the video signal is reversed with each line, which automatically corrects phase errors in the transmission of the signal by cancelling them out. For analogue radio, the switch to digital is made more difficult by the fact that analog receivers cost a fraction of the cost of digital receivers. For example while you can get a good analog receiver for under $20 USD[37] a digital receiver will set you back at least $75 USD.[38] The choice of modulation for analogue radio is typically between amplitude modulation (AM) or frequency modulation (FM). To achieve stereo playback, an amplitude modulated subcarrier is used for stereo FM and quadrature amplitude modulation is used for stereo AM or C-QUAM (see each of the linked articles for more details).
The InternetToday an estimated 15.7% of the world population has access to the Internet with the highest concentration in North America (68.6%), Ocenia/Australia (52.6%) and Europe (36.1%).[39] In terms of broaband access, countries such as Iceland (26.7 per 100), South Korea (25.4 per 100) and the Netherlands (25.3 per 100) lead the world.[40]The International Telecommunication Union uses this information to compile a Digital Access Index that measures the overall ability of citizens to access and use information and communication technologies. Using this measure, countries such as Sweden, Denmark and Iceland receive the highest ranking while African countries such as Niger, Burkina Faso and Mali receive the lowest.[41] ReferencesNotes
Note I — Bluetooth 2.0 uses PSK for its enhanced data rate (EDR). Specifically π/4-shifted DQPSK at 2 Mbps and 8DPSK at 3 Mbps.[42] Citations
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Wise Word
Eliminate the impossible and whatever remains, however improbable, must be the truth.
Sherlock Holmes
