telecommunication

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Copy of the original phone of Alexander Graham Bell at the Musée des Arts et Métiers in Paris
Telecommunication is the transmission of signals over a distance for the purpose of communication. In modern times, this process typically involves the sending of electromagnetic waves by electronic transmitters, but in earlier times telecommunication may have involved the use of smoke signals, drums or semaphore or heliograph. 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 are also many networks that connect these devices, including computer networks, public telephone networks, radio networks and television networks. Computer communication across the Internet is one of many examples of telecommunication.

Telecommunication systems are generally designed by telecommunication engineers. Early inventors in the field include Alexander Graham Bell, Guglielmo Marconi and John Logie Baird. Telecommunication is an important part of the world economy with the telecommunication industry's revenue being placed at just under 3 percent of the gross world product.

Key concepts

Etymology
The word telecommunication was adapted from the French word télécommunication. It is a compound of the Greek prefix tele- (τηλε-), meaning 'far off', and the Latin communicare, meaning 'to share'.[1] The French word télécommunication was coined in 1904 by French engineer and novelist Édouard Estaunié.[2]

Basic elements

Each telecommunication system consists of three basic elements: Consider a radio broadcast for example. 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 a single device acts as both a transmitter and receiver, or transceiver. For example, a mobile phone is a transceiver.[3]

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 (or point-to-multipoint) communication because it is between one powerful transmitter and numerous receivers.<ref name="stallings-intro" />

Analogue or digital

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 (for example, ones and zeros). During transmission, the information contained in analogue signals will be degraded by noise. Conversely, unless the noise exceeds a certain threshold, the information contained in digital signals will remain intact. This represents a key advantage of digital signals over analogue signals.[4]

Networks

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, repeaters may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combat attenuation that can render the signal indistinguishable from noise.[5]

Channels

A channel is a division in a transmission medium so that it can be used to send multiple streams of information. 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 recurring segment of time over which to broadcast — this is known as time-division multiplexing and is sometimes used in digital communication.<ref name="glossary" />

Modulation

The shaping of a signal to convey information is known as modulation. Modulation can be 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, frequency-shift keying and amplitude-shift keying). Bluetooth, for example, uses phase-shift keying to exchange information between devices.[6] [7]

Modulation can also be used to transmit the information of 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 superimposed on a higher-frequency signal (known as a carrier wave) before transmission. There are several different modulation schemes available to achieve this (two of the most basic being amplitude modulation and frequency modulation). An example of this process in action 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”).[8]

Society and telecommunication

Telecommunication is an important part of modern society. In 2006, estimates placed the telecommunication industry's revenue at $1.2 trillion or just under 4% of the gross world product.[9]

On the microeconomic scale, companies have used telecommunication to help build global empires. This is self-evident in the case of online retailer Amazon.com but, according to academic Edward Lenert, even the conventional retailer Wal-Mart has benefited from better telecommunication infrastructure compared to its competitors.[10] In cities throughout the world, home owners use their telephones to organize many home services ranging from pizza deliveries to electricians. Even relatively poor communities have been noted to use telecommunication to their advantage. In Bangladesh's Narshingdi district, isolated villagers use cell phones to speak directly to wholesalers and arrange a better price for their goods. In Cote d'Ivoire, coffee growers share mobile phones to follow hourly variations in coffee prices and sell at the best price.[11] On the macroeconomic scale, Lars-Hendrik Röller and Leonard Waverman suggested a causal link between good telecommunication infrastructure and economic growth.[12] Few dispute the existence of a correlation although some argue it is wrong to view the relationship as causal.[13]

Due to the economic benefits of good telecommunication infrastructure, there is increasing worry about the digital divide. This is because the world's population does not have equal access to telecommunication systems. A 2003 survey by the International Telecommunication Union (ITU) revealed that roughly one-third of countries have less than 1 mobile subscription for every 20 people and one-third of countries have less than 1 fixed line subscription for every 20 people. In terms of Internet access, roughly half of all countries have less than 1 in 20 people with Internet access. From this information, as well as educational data, the ITU was able 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 received the highest ranking while African countries such as Niger, Burkina Faso and Mali received the lowest.[14]

History

For more details on this topic, see History of telecommunication.

Early telecommunications

Enlarge picture
A replica of one of Chappe's semaphore towers.


Early forms of telecommunication include smoke signals and drums. Drums were used by natives in Africa, New Guinea and South America whereas smoke signals were used by natives in North 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.[15] [16]

In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy (or semaphore) system between Lille and Paris.[17] 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.[18]

Telegraph and telephone

The first commercial electrical telegraph was constructed by Sir Charles Wheatstone and Sir William Fothergill Cooke and opened on 9 April 1839. Both Wheatstone and Cooke viewed their device as "an improvement to the [existing] electromagnetic telegraph" not as a new device.[19]

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 successfully completed on 27 July 1866, allowing transatlantic telecommunication for the first time.[20]

The conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876.[21] 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.[22] 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.[23] [24]

Radio and television

In 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, Scotland to Woodhaven, a distance of two miles, using water as the transmission medium.[25] In December 1901, Guglielmo Marconi established wireless communication between St. John's, Newfoundland (Canada) and Poldhu, Cornwall (England), earning him the Nobel Prize in physics in 1909 (which he shared with Karl Braun).[26] However small-scale radio communication had already been demonstrated in 1893 by Nikola Tesla in a presentation to the National Electric Light Association.[27]

On March 25, 1925, John Logie Baird was able to demonstrate the transmission of moving pictures at the London department store 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.[28] 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. [29]

Computer networks and the Internet

On 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.[30] 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; this network would become ARPANET, which by 1981 would consist of 213 nodes.[31]

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 the Request for Comment 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.

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 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.[32] [33]

Modern operation

Telephone

In a conventional telephone system, the caller is connected to the person he wants to talk to by switches at various telephone exchanges. The switches form an electrical connection between the two users and the setting of these switches is determined electronically when the caller dials the number. Once the connection is made, the caller's voice is transformed to an electrical signal using a small microphone in the caller's handset. This electrical signal is then sent through the network to the user at the other end where it transformed back into sound by a small speaker in that person's handset. There is a separate electrical connection that works in reverse, allowing the users to converse.[34] [35]

The fixed-line telephones in most residential homes are analogue — that is, the speaker's voice directly determines the signal's voltage. Although short-distance calls may be handled from end-to-end as analogue signals, usually telephone service providers transparently convert the signals to digital for switching and transmission before converting them back to analogue for reception. The advantage of this is that digitized voice data can travel side-by-side with data from the Internet and can be perfectly reproduced in long distance communication (as opposed to 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).[36] In terms of new subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth.[37] Increasingly these phones are being serviced by systems where the voice content is transmitted digitally such as GSM or W-CDMA with many markets choosing to depreciate analogue systems such as AMPS.[38]

There have also been dramatic changes in telephone communication behind the scenes. Starting with the operation of TAT-8 in 1988, the 1990s saw the widespread adoption of systems based on optic fibres. 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.[39] This drastic 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.[40] Lastly, improvements in multiplexing have lead to an exponential growth in the data capacity of a single fibre.[41] [42]

Assisting communication across these networks is a protocol known as Asynchronous Transfer Mode (ATM). The ATM protocol allows for the side-by-side data transmission mentioned in the second paragraph. It is suitable for public telephone networks because it establishes a pathway for data through the network and associates a traffic contract with that pathway. 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.[43] There are competitors to ATM, such as Multiprotocol Label Switching (MPLS), that perform a similar task and are expected to supplant ATM in the future.[44]

Radio and television

Enlarge picture
Digital television standards and their adoption worldwide.


In a broadcast system, a central high-powered broadcast tower transmits a high-frequency electromagnetic 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 antenna of the receiver is then tuned so as to pick up the high-frequency wave and a demodulator is used to retrieve the signal containing the visual or audio information. 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).[45] [46]

The broadcast 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 capable integrated circuits. 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 transmission.[47] [48]

In digital television broadcasting, there are three competing standards that are likely to be adopted worldwide. These are the ATSC, DVB and ISDB standards; 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.[49] [50] The choice of modulation also varies between the schemes. In digital audio broadcasting, standards are much more unified with practically all countries choosing to adopt the Digital Audio Broadcasting standard (also known as the Eureka 147 standard). The exception being the United States which has chosen to adopt HD Radio. HD Radio, unlike Eureka 147, is based upon a transmission method known as in-band on-channel transmission that allows digital information to "piggyback" on normal AM or FM analogue transmissions.[51]

However, despite the pending switch to digital, analogue receivers still remain widespread. Analogue television is still transmitted in practically all countries. The United States had hoped to end analogue broadcasts on December 31, 2006; however, this was recently pushed back to February 17, 2009.[52] For analogue television, there are three standards in use (see a map on adoption here). These are known as PAL, NTSC and SECAM. For analogue radio, the switch to digital is made more difficult by the fact that analogue receivers are a fraction of the cost of digital receivers.[53] [54] 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.

The Internet



The Internet is a worldwide network of computers and computer networks that can communicate with each other using the Internet Protocol.[55] Any computer on the Internet has a unique IP 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. In this way, the Internet can be seen as an exchange of messages between computers.[56]

An estimated 16.9% of the world population has access to the Internet with the highest access rates (measured as a percentage of the population) in North America (69.7%), Oceania/Australia (53.5%) and Europe (38.9%).[57] In terms of broadband access, countries such as England (89%), Iceland (26.7%), South Korea (25.4%) and the Netherlands (25.3%) lead the world.[58]

The Internet works in part because of protocols 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 an Internet browser to run the same code regardless of whether the computer it is running on is connected to the Internet through an Ethernet or Wi-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.[59]

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 for logical addressing. For the world wide web, these “IP addresses” are derived from the human readable form using the Domain Name System (e.g. 72.14.207.99 is derived from www.google.com). At the moment, the most widely used version of the Internet Protocol is version four but a move to version six is imminent.[60]

At the transport layer, most communication adopts either the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). TCP is used when it is essential every message sent is received by the other computer where as UDP is used when it is merely desirable. With TCP, packets are retransmitted if they are lost and placed in order before they are presented to higher layers. With UDP, packets are not ordered or retransmitted if lost. Both TCP and UDP packets carry port numbers with them to specify what application or process the packet should be handled by.[61] Because certain application-level protocols use certain ports, network administrators can restrict Internet access by blocking the traffic destined for a particular port.

Above the transport layer, there are certain protocols that are sometimes used and loosely fit in the session and presentation layers, most notably the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols. These protocols ensure that the data transferred between two parties remains completely confidential and one or the other is in use when a padlock appears at the bottom of your web browser.[62] Finally, at the application layer, are many of the protocols Internet users would be familiar with such as HTTP (web browsing), POP3 (e-mail), FTP (file transfer), IRC (Internet chat), BitTorrent (file sharing) and OSCAR (instant messaging).

Local area networks

Enlarge picture
A local area network.


Despite the growth of the Internet, the characteristics of local area networks (computer networks that run at most 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.

In the mid-1980s, several protocol suites emerged to fill the gap between the data link and applications layer of the OSI reference model. These were Appletalk, IPX and NetBIOS with the dominant protocol suite during the early 1990s being IPX due to its popularity with MS-DOS users. TCP/IP existed at this point but was typically only used by large government and research facilities.[63] As the Internet grew in popularity and a larger percentage of traffic became Internet-related, local area networks gradually moved towards TCP/IP and today networks mostly dedicated to TCP/IP traffic are common. The move to TCP/IP was helped by technologies such as DHCP that allowed TCP/IP clients to discover their own network address — a functionality that came standard with the AppleTalk/IPX/NetBIOS protocol suites.[64]

It is at the data link layer though that most modern local area networks diverge from the Internet. Whereas Asynchronous Transfer Mode (ATM) or Multiprotocol Label Switching (MPLS) are typical data link protocols for larger networks, Ethernet and Token Ring are typical data link protocols for local area networks. These protocols differ from the former protocols in that they are simpler (e.g. they omit features such as Quality of Service guarantees) and offer collision prevention. Both of these differences allow for more economic set-ups.[65]

Despite the modest popularity of Token Ring in the 80's and 90's, virtually all local area networks now use wired or wireless Ethernet. At the physical layer, most wired Ethernet implementations use copper twisted-pair cables (including the common 10BASE-T networks). However, some early implementations used coaxial cables and some recent implementations (especially high-speed ones) use optic fibres. Optic fibres are also likely to feature prominently in the forthcoming 10-gigabit Ethernet implementations.[66] Where optic fibre is used, the distinction must be made between multi-mode fibre and single-mode fibre. Multi-mode fibre can be thought of as thicker optical fibre that is cheaper to manufacture but that suffers from less usable bandwidth and greater attenuation (i.e. poor long-distance performance).[67]

See also

Main list: List of basic telecommunication topics

References

1. ^ Telecommunication, tele- and communication, New Oxford American Dictionary (2nd edition), 2005.
2. ^ Jean-Marie Dilhac, From tele-communicare to Telecommunications, 2004.
3. ^ Haykin, Simon (2001). Communication Systems, 4th edition, John Wiley & Sons, pp 1-3. ISBN 0-471-17869-1. 
4. ^ Ambardar, Ashok (1999). Analog and Digital Signal Processing, 2nd edition, Brooks/Cole Publishing Company, pp 1-2. ISBN 0-534-95409-X. 
5. ^ ATIS Telecom Glossary 2000, ATIS Committee T1A1 Performance and Signal Processing (approved by the American National Standards Institute), 28 February, 2001.
6. ^ Haykin, pp 344-403.
7. ^ Bluetooth Specification Version 2.0 + EDR (p 27), Bluetooth, 2004.
8. ^ Haykin, pp 88-126.
9. ^ Telecom Industry Revenue to Reach $1.2 Trillion in 2006, VoIP Magazine, 2005.
10. ^ Lenert, Edward (Dec 1998). "A Communication Theory Perspective on Telecommunications Policy". Journal of Communication 48 (4): 3-23. 
11. ^ Mireille Samaan (April 2003). "The Effect of Income Inequality on Mobile Phone Penetration" (PDF). Boston University Honors thesis. Retrieved on 2007-06-08.
12. ^ Röller, Lars-Hendrik (2001). "Telecommunications Infrastructure and Economic Development: A Simultaneous Approach". American Economic Review 91 (4): 909-923. ISSN 0002-8282. 
13. ^ Riaz, Ali (1997). "The role of telecommunications in economic growth: proposal for an alternative framework of analysis". Media, Culture & Society 19 (4): 557-583. ISSN 0163-4437. 
14. ^ World Telecommunication Development Report 2003, International Telecommunication Union, 2003.
15. ^ Native American Smoke Signals, William Tomkins, 2005.
16. ^ Talking Drums, Instrument Encyclopedia, Cultural Heritage for Community Outreach, 1996.
17. ^ Les Télégraphes Chappe, Cédrick Chatenet, l'Ecole Centrale de Lyon, 2003.
18. ^ CCIT/ITU-T 50 Years of Excellence, Internation Telecommunication Union, 2006.
19. ^ The Electromagnetic Telegraph, J. B. Calvert, 19 May 2004.
20. ^ The Atlantic Cable, Bern Dibner, Burndy Library Inc., 1959
21. ^ Elisha Gray, Oberlin College Archives, Electronic Oberlin Group, 2006.
22. ^ Antonio Santi Giuseppe Meucci, Eugenii Katz. (Retrieved May, 2006 from http://chem.ch.huji.ac.il/~eugeniik/history/meucci.html)
23. ^ Connected Earth: The telephone, BT, 2006.
24. ^ History of AT&T, AT&T, 2006.
25. ^ James Bowman Lindsay, Macdonald Black, Dundee City Council, 1999.
26. ^ Tesla Biography, Ljubo Vujovic, Tesla Memorial Society of New York, 1998.
27. ^ Tesla's Radio Controlled Boat, Twenty First Century Books, 2007.
28. ^ The Pioneers, MZTV Museum of Television, 2006.
29. ^ Philo Farnsworth, Neil Postman, TIME Magazine, 29 March 1999
30. ^ George Stlibetz, Kerry Redshaw, 1996.
31. ^ Hafner, Katie (1998). Where Wizards Stay Up Late: The Origins Of The Internet. Simon & Schuster. ISBN 0-684-83267-4. 
32. ^ Data transmission system, Olof Solderblom, PN 4,293,948, October 1974.
33. ^ Ethernet: Distributed Packet Switching for Local Computer Networks, Robert M. Metcalfe and David R. Boggs, Communications of the ACM (pp 395-404, Vol. 19, No. 5), July 1976.
34. ^ How Telephone Works, HowStuffWorks.com, 2006.
35. ^ Telephone technology page, ePanorama, 2006.
36. ^ Gartner Says Top Six Vendors Drive Worldwide Mobile Phone Sales to 21% Growth in 2005, Gartner Group, 28 February 2006.
37. ^ Africa Calling, Victor and Irene Mbarika, IEEE Spectrum, May 2006.
38. ^ Ten Years of GSM in Australia, Australia Telecommunications Association, 2003.
39. ^ Milestones in AT&T History, AT&T Knowledge Ventures, 2006.
40. ^ Optical fibre waveguide, Saleem Bhatti, 1995.
41. ^ Fundamentals of DWDM Technology, CISCO Systems, 2006.
42. ^ Report: DWDM No Match for Sonet, Mary Jander, Light Reading, 2006.
43. ^ Stallings, William (2004). Data and Computer Communications, 7th edition (intl), Pearson Prentice Hall, pp 337-366. ISBN 0-13-183311-1. 
44. ^ MPLS is the future, but ATM hangs on, John Dix, Network World, 2002
45. ^ Haykin, Simon (2001). Communication Systems, 4th edition, John Wiley & Sons, pp 1-3. ISBN 0-471-17869-1. 
46. ^ How Radio Works, HowStuffWorks.com, 2006.
47. ^ Digital Television in Australia, Digital Television News Australia, 2001.
48. ^ Stallings, William (2004). Data and Computer Communications, 7th edition (intl), Pearson Prentice Hall. ISBN 0-13-183311-1. 
49. ^ HDV Technology Handbook, Sony, 2004.
50. ^ Audio, Digital Video Broadcasting Project, 2003.
51. ^ Status of DAB (USA), World DAB Forum, March 2005.
52. ^ Consumer Corner FAQ, dtv.gov, 2006.
53. ^ GE 72664 Portable AM/FM Radio, Amazon.com, June 2006.
54. ^ DAB Products, World DAB Forum, 2006.
55. ^ Robert E. Kahn and Vinton G. Cerf, What Is The Internet (And What Makes It Work), December 1999. (specifically see footnote xv)
56. ^ How Internet Infrastructure Works, HowStuffWorks.com, 2007.
57. ^ World Internet Users and Population Stats, internetworldstats.com, March 19 2007.
58. ^ OECD Broadband Statistics, Organisation for Economic Co-operation and Development, December 2005.
59. ^ History of the OSI Reference Model, The TCP/IP Guide v3.0, Charles M. Kozierok, 2005.
60. ^ Introduction to IPv6, Microsoft Corporation, February 2006.
61. ^ Stallings, pp 683-702.
62. ^ T. Dierks and C. Allen, The TLS Protocol Version 1.0, RFC 2246, 1999.
63. ^ Martin, Michael (2000). Understanding the Network (The Networker’s Guide to AppleTalk, IPX, and NetBIOS), SAMS Publishing, ISBN 0-7357-0977-7.
64. ^ Ralph Droms, Resources for DHCP, November 2003.
65. ^ Stallings, pp 500-526.
66. ^ Stallings, pp 514-516.
67. ^ Fiber Optic Cable Tutorial, Arc Electronics. (Retrieved June, 2007)

External links

In telecommunications, transmission is the forwarding of signal traffic over distances that are too great to be simply connected by a twisted pair wires. Techniques available now may be microwave link, satellite link, coaxial cable or fibre optic cable.
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signal is any time-varying quantity. Signals are often scalar-valued functions of time (waveforms), but may be vector valued and may be functions of any other relevant independent variable.

The concept is broad, and hard to define precisely.
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Communication is a process that allows organisms to exchange information by several methods. Communication requires that all parties understand a common language that is exchanged with each other.
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Electromagnetic (EM) radiation is a self-propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation, and are in phase with each other.
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A smoke signal is a form of optical communication used over a long distance, developed both in the Americas and in China. By covering a fire with a blanket and quickly removing it, a puff of smoke can be generated.
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drums served as an early form of long distance communication, and were used during ceremonial and religious functions.

In Africa, New Guinea and the tropical America, natives used drum telegraphy to communicate with each other from far away for centuries.
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semaphore or optical telegraph is an apparatus for conveying information by means of visual signals, with towers with pivoting blades or paddles, shutters, in a matrix, or hand-held flags etc.
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heliograph uses a mirror to reflect sunlight to a distant observer. By moving the mirror, flashes of light can be used to send Morse code. The heliograph was a simple but highly effective instrument for instantaneous optical communication over 80km or more in the 19th century.
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Television (often abbreviated to TV, T.V., or more recently, tv; sometimes called telly, the tube, boob tube, or idiot box in British English) is a widely used telecommunication system for broadcasting and receiving moving pictures
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Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space.
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The telephone is a telecommunications device which is used to transmit and receive sound (most commonly speech). Most telephones operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost anyone.
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as a college campus, industrial complex, or a military base. A CAN, may be considered a type of MAN (metropolitan area network), but is generally limited to an area that is smaller than a typical MAN.
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The public switched telephone network (PSTN) is the network of the world's public circuit-switched telephone networks, in much the same way that the Internet is the network of the world's public IP-based packet-switched networks.
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A radio network is a network system which distributes programming to multiple stations simultaneously, or slightly delayed, for the purpose of extending total coverage beyond the limits of a single broadcast signal.
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A television network is a distribution for television content whereby a central operation provides programming for many television stations. Until the mid-1980s, television programming in most countries of the world was dominated by a small number of broadcast networks.
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Internet is a worldwide, publicly accessible series of interconnected computer networks that transmit data by packet switching using the standard Internet Protocol (IP). It is a "network of networks" that consists of millions of smaller domestic, academic, business, and government
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Alexander Graham Bell (3 March 1847 - 2 August 1922) was a Scottish scientist, inventor and innovator. Throughout his early life, Alexander Graham Bell was a British subject but in 1915, he characterized his status as: "I am not one of those hyphenated Americans who claim
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Guglielmo Marconi

Guglielmo Marconi
Born March 25 1874(1874--)
Palazzo Marescalchi, Bologna, Italy
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John Logie Baird (August 13 1888 – June 14 1946) was a Scottish engineer and inventor of the world's first working television system. Although Baird's electromechanical system was eventually displaced by purely electronic systems (such as those of Vladimir Zworykin and Philo
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Gross world product (GWP) is the total gross national product of all the countries in the world. This also equals the total gross domestic product. See measures of national income and output for more details. The per capita GWP in 2000 was approximately $7,200 US dollars (USD).
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Édouard Estaunié (Dijon, February 4 1862 - Paris, February 11942) was a French novelist. Estaunié trained as a scientist and engineer before turning to the novel in 1891. In 1904, he devised the word "telecommunication". He was elected to the Académie française in 1923.
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transmitter (sometimes abbreviated XMTR) is an electronic device which with the aid of an antenna propagates an electromagnetic signal such as radio, television, or other telecommunications.
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Information is the result of processing, gathering, manipulating and organizing data in a way that adds to the knowledge of the receiver. In other words, it is the context in which data is taken.
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signal is any time-varying quantity. Signals are often scalar-valued functions of time (waveforms), but may be vector valued and may be functions of any other relevant independent variable.

The concept is broad, and hard to define precisely.
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A transmission medium (plural transmission media) is a material substance (solid, liquid or gas) which can propagate energy waves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media
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A radio receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through
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Radio masts and towers are, typically, tall structures designed to support antennas (also known as aerials in the UK) for telecommunications and broadcasting, including television. They are among the tallest man-made structures.
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Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space.
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In physics, free space is a concept of electromagnetic theory, corresponding to a theoretical "perfect vacuum".

Definition

Free space simply means that there is no material or other physical phenomenon
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mobile phone or cell phone is a long-range, portable electronic device used for mobile communication. In addition to the standard voice function of a telephone, current mobile phones can support many additional services such as SMS for text messaging, email, packet switching
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