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۴۷ کرښه:
==== چینلونه ====
 
A [[Channel (communications)|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 has received a separate frequency to broadcast on. Alternatively, one could allocate each channel a recurring segment of time over which to broadcast &mdash; this is known as [[time-division multiplexing]] and is sometimes used in digital communication.<ref name="glossary" />
 
==== Modulation ====
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==== سولګریز جال او انټرنیټ ====
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]].<ref>[http://www.kerryr.net/pioneers/stibitz.htm George Stlibetz], Kerry Redshaw, 1996.</ref> This configuration of a centralized computer or [[Mainframe computer|mainframe]] with remote dumb terminals remained popular throughout the 1950s. However, it was not until the 1960s that researchers started to investigate [[packet switching]] &mdash; 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.<ref>{{cite book | last = Hafner | first = Katie | title = Where Wizards Stay Up Late: The Origins Of The Internet | publisher = Simon & Schuster | year = 1998 | id = ISBN 0-684-83267-4 }}</ref>
 
[[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) &mdash; 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 network]]s (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]]''.<ref>[http://patft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&co1=AND&d=PTXT&s1=4293948.PN.&OS=PN/4293948&RS=PN/4293948 Data transmission system], Olof Solderblom, PN 4,293,948, October 1974.</ref> <ref>[http://www.acm.org/classics/apr96/ 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.</ref>
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In an analogue telephone network, the [[calling party|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 [[pulse dialling|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 [[loudspeaker|speaker]] in that person's handset. There is a separate electrical connection that works in reverse, allowing the users to converse.<ref>[http://electronics.howstuffworks.com/telephone1.htm How Telephone Works], HowStuffWorks.com, 2006.</ref><ref>[http://www.epanorama.net/links/telephone.html Telephone technology page], ePanorama, 2006.</ref>
 
The [[land line|fixed-line]] telephones in most residential homes are analogue &mdash; 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).<ref>[http://www.gartner.com/press_releases/asset_145891_11.html Gartner Says Top Six Vendors Drive Worldwide Mobile Phone Sales to 21% Growth in 2005], Gartner Group, 28 February 2006.</ref> In terms of new subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth.<ref>[http://www.spectrum.ieee.org/may06/3426 Africa Calling], Victor and Irene Mbarika, [[IEEE Spectrum]], May 2006.</ref> 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]].<ref>[http://www.amta.org.au/default.asp?Page=142 Ten Years of GSM in Australia], Australia Telecommunications Association, 2003.</ref>
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In a broadcast system, a central high-powered [[radio masts and towers|broadcast tower]] transmits a high-frequency [[electromagnetic wave]] to numerous low-powered receivers. The high-frequency wave sent by the tower is [[modulation|modulated]] with a signal containing visual or audio information. The [[Antenna (radio)|antenna]] of the receiver is then [[antenna tuner|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).<ref>{{cite book | last = Haykin | first = Simon | edition= 4th edition | title = Communication Systems | publisher = John Wiley & Sons | year = 2001 | pages = pp 1-3 | id = ISBN 0-471-17869-1 }}</ref> <ref>[http://www.howstuffworks.com/radio.htm How Radio Works], HowStuffWorks.com, 2006.</ref>
 
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 circuit]]s. 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 [[noise (video)|snowy pictures]], [[television interference (ghosting)|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 &mdash; 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.<ref>[http://www.digitaltv.com.au/ Digital Television in Australia], Digital Television News Australia, 2001.</ref> <ref>{{cite book | last = Stallings | first = William | edition= 7th edition (intl) | title = Data and Computer Communications | publisher = Pearson Prentice Hall | year = 2004 | id = ISBN 0-13-183311-1 }}</ref>
 
In digital television broadcasting, there are three competing standards that are likely to be adopted worldwide. These are the [[ATSC Standards|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|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|MPEG-1 Part 3 Layer 2]].<ref>[http://www.dynamix.ca/doc/HDVhandbook1.pdf HDV Technology Handbook], [[Sony]], 2004.</ref> <ref>[http://www.dvb.org/technology/standards_specifications/audio/ Audio], [[DVB|Digital Video Broadcasting Project]], 2003.</ref> 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 [[8VSB|vestigial sideband modulation]] (VSB). OFDM should offer better resistance to multipath interference and the [[Doppler effect]] (which would impact reception using moving receivers).<ref>[http://www.mrcbroadcast.com/tech_services/COFDM%20vs%20VSB.html COFDM versus VSB in ENG/HD-ENG], Microwave Radio Communications, 2006.</ref> 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.<ref>[http://www.hdtvmagazine.com/archives/mstvtestsum.html 8VSB/COFDM Comparison Report], VSB/COFDM Project, December 2000 (preface by Dale Cripps of HDTV Magazine).</ref>--> 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.<ref>[http://www.worlddab.org/cstatus.aspx Status of DAB (USA)], World DAB Forum, March 2005.</ref> <!-- This avoids 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. --><!-- 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]].<ref>[http://www.ibiquity.com/licensing/index.htm Licensing], iBiquity Digital, 2005.</ref> 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|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|MPEG-1 Part 3 Layer 2]]. HD Radio uses a codec called [[High-Definition Coding]].-->
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Despite the growth of the Internet, the characteristics of [[local area network]]s (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.<ref>Martin, Michael (2000). ''Understanding the Network'' ([http://www.samspublishing.com/content/images/0735709777/samplechapter/0735709777.pdf The Networker’s Guide to AppleTalk, IPX, and NetBIOS]), SAMS Publishing, ISBN 0-7357-0977-7.</ref> 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 &mdash; a functionality that came standard with the AppleTalk/IPX/NetBIOS protocol suites.<ref>Ralph Droms, [http://www.dhcp.org/ Resources for DHCP], November 2003.</ref>
 
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 [[IBM token ring|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|Quality of Service]] guarantees) and offer [[carrier sense multiple access with collision detection|collision prevention]]. Both of these differences allow for more economic set-ups.<ref>Stallings, pp 500-526.</ref>
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