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838404409_18413-19021 | The Nanling Mountains , the southernmost of the east - west mountain ranges , overlook areas in China with a tropical climate . The climate allows two crops of rice to be grown per year . Southeast of the mountains lies a coastal , hilly region of small deltas and narrow valley plains . The drainage area of the Pearl River and its associated network of rivers occupies much of the region to the south . West of the Nanling , the Yunnan - Guizhou Plateau rises in two steps , averaging 1,200 and 1,800 m in elevation , respectively , toward the precipitous mountain regions of the eastern Qingzang Plateau . | Geography of China |
838404409_19090-19496 | China has substantial mineral reserves and is the world 's largest producer of antimony , natural graphite , tungsten , and zinc . Other major minerals are aluminum , bauxite , coal , crude petroleum , diamonds , gold , iron ore , lead , magnetite , manganese , mercury , molybdenum , natural gas , phosphate rock , tin , uranium , and vanadium . China 's hydropower potential is the largest in the world . | Geography of China |
838404409_19515-20080 | Based on 2005 estimates , 14.86 % ( about 1,400,000 km ( 540,000 sq mi ) ) of China 's total land area is arable . About 1.3 % ( some 116,580 km2 ) is planted to permanent crops and the rest planted to temporary crops . With comparatively little land planted to permanent crops , intensive agricultural techniques are used to reap harvests that are sufficient to feed the world 's largest population and still have surplus for export . An estimated 544,784 km2 of land were irrigated in 2004 . 42.9 % of total land area was used as pasture , and 17.5 % was forest . | Geography of China |
838404409_20132-20861 | China lies in two of the world 's major ecozones , the Palearctic and the Indomalaya . In the Palearctic zone mammals such as the horse , camel , and jerboa are found . Among the species found in the Indomalaya region are the leopard cat , bamboo rat , treeshrew , and various other species of monkeys and apes . Some overlap exists between the two regions because of natural dispersal and migration , and deer or antelope , bears , wolves , pigs , and rodents are found in all of the diverse climatic and geological environments . The famous giant panda is found only in a limited area along the Yangtze . There is a continuing problem with trade in endangered species , although there are now laws to prohibit such activities . | Geography of China |
838404409_20947-22222 | Chinese history is often explained in terms of several strategic areas , defined by particular topographic limits . Starting from the Chinese central plain , the former heart of the Han populations , the Han people expanded militarily and then demographically toward the Loess Plateau , the Sichuan Basin , and the Southern Hills ( as defined by the map on the left ) , not without resistance from local populations . Pushed by its comparatively higher demographic growth , the Han continued their expansion by military and demographic waves . The far - south of present - day China , the northern parts of today 's Vietnam , and the Tarim Basin were first reached and durably subdued by the Han dynasty 's armies . The Northern steppes were always the source of invasions into China , which culminated in the 13th century by Mongolian conquest of the whole China and creation of Mongolian Yuan dynasty . Manchuria , much of today 's Northeast China , and Korean Peninsula were usually not under Chinese control , with the exception of some limited periods of occupation . Manchuria became strongly integrated into the Chinese empire during the late Qing dynasty , while the west side of the Changbai Mountains , formerly the home of Korean tribes , thus also entered China . | Geography of China |
838404409_22538-22900 | The demographic occupation follows the topography and availability of former arable lands . The Heihe -- Tengchong Line , running from Heihe , Heilongjiang to Tengchong County , Yunnan divides China into two roughly equal sections -- in terms of geographic area , with areas west of the line being sparsely settled and areas east densely populated , in general . | Geography of China |
838404409_23078-23478 | China 's economy has recently become export - oriented . The coastal provinces got the greatest benefits from the recent development of China 's economy , becoming the new economic center of China . The Chinese leadership has long encouraged a move inland , and in 2012 , following the western crisis , the top leadership announced a policy to redirect Chinese production toward the national market . | Geography of China |
838404409_23503-23572 | This section is empty . You can help by adding to it . ( March 2013 ) | Geography of China |
838404409_23607-24004 | Chinese administrative geography was drawn mainly during the 1949 and 1954 administrative reorganizations . These reorganizations have been the source of much debate within China . In addition , a parcel of land was ceded from Guangdong to Guangxi to grant the latter immediate access to the Gulf of Tonkin , while Hainan was split from Guangdong in 1988 and Chongqing from Sichuan in 1997 . Click | Geography of China |
838404409_24193-25857 | China 's borders have more than 20,000 km ( 12,000 mi ) of land frontier shared with nearly all the nations of mainland East Asia , and have been disputed at a number of points . In the western sector , China claimed portions of the 41,000 km ( 16,000 sq mi ) Pamir Mountains area , a region of soaring mountain peaks and glacier - filled valleys where the borders of Afghanistan , Pakistan , the former Soviet Union , and China meet in Central Asia . North and east of this region , some sections of the border remained undemarcated in 1987 . The 6,542 kilometres ( 4,065 mi ) frontier with the Soviet Union has been a source of continual friction . In 1954 China published maps showing substantial portions of Soviet Siberian territory as its own . In the northeast , border friction with the Soviet Union produced a tense situation in remote regions of Inner Mongolia and Heilongjiang along segments of the Argun River , Amur River , and Ussuri River . Each side had massed troops and had exchanged charges of border provocation in this area . In a September 1986 speech in Vladivostok , the Soviet leader Mikhail S. Gorbachev offered the Chinese a more conciliatory position on Sino - Soviet border issues . In 1987 the two sides resumed border talks that had been broken off after the 1979 Soviet invasion of Afghanistan ( see Sino - Soviet relations ) . Although the border issue remained unresolved as of late 1987 , China and the Soviet Union agreed to consider the northeastern sector first . In October 2004 , China signed an agreement with Russia on the delimitation of their entire 4,300 km ( 2,700 mi ) - long border , which had long been in dispute . | Geography of China |
838404409_25874-27070 | Eastward from Bhutan and north of the Brahmaputra River ( Yarlung Zangbo Jiang ) lies a large area controlled and administered by India but claimed by the Chinese . The area was demarcated by the British McMahon Line , drawn along the Himalayas in 1914 as the Sino - Indian border ; India accepts and China rejects this boundary . In June 1980 China made its first move in twenty years to settle the border disputes with India , proposing that India cede the Aksai Chin area in Jammu and Kashmir to China in return for China 's recognition of the McMahon Line ; India did not accept the offer , however , preferring a sector - by - sector approach to the problem . In July 1986 China and India held their seventh round of border talks , but they made little headway toward resolving the dispute . Each side , but primarily India , continued to make allegations of incursions into its territory by the other . Most of the mountainous and militarized boundary with India is still in dispute , but Beijing and New Delhi have committed to begin resolution with discussions on the least disputed middle sector . India does not recognize Pakistan 's ceding lands to China in a 1964 boundary agreement . | Geography of China |
838404409_27071-27270 | The China - Burma border issue was settled October 1 , 1960 , by the signing of the Sino - Burmese Boundary Treaty . The first joint inspection of the border was completed successfully in June 1986 . | Geography of China |
838404409_27276-27807 | China is involved in a complex dispute with Malaysia , Philippines , Vietnam , and possibly Brunei over the Spratly ( Nansha ) Islands in the South China Sea . The 2002 `` Declaration on the Conduct of Parties in the South China Sea '' eased tensions but fell short of a legally binding code of conduct desired by several of the disputants . China also controls the Paracel ( Xisha ) Islands , which are also claimed by Vietnam , and asserts a claim to the Japanese - administered Senkaku ( Diaoyu ) Islands in the East China Sea . | Geography of China |
838404409_27929-28802 | Owing to tremendous differences in latitude , longitude , and altitude , the climate of China is extremely diverse , ranging from tropical in the far south to subarctic in the far north and alpine in the higher elevations of the Tibetan Plateau . Monsoon winds , caused by differences in the heat - absorbing capacity of the continent and the ocean , dominate the climate . During the summer , the East Asian Monsoon carries warm and moist air from the south and delivers the vast majority of the annual precipitation in much of the country . Conversely , the Siberian anticyclone dominates during winter , bringing cold and comparatively dry conditions . The advance and retreat of the monsoons account in large degree for the timing of the rainy season throughout the country . Although most of the country lies in the temperate belt , its climatic patterns are complex . | Geography of China |
838404409_28803-29460 | The northern extremities of both Heilongjiang and Inner Mongolia have a subarctic climate ; in contrast , most of Hainan Island and parts of the extreme southern fringes of Yunnan have a tropical climate . Temperature differences in winter are considerable , but in summer the variance is considerably less . For example , Mohe County , Heilongjiang has a 24 - hour average temperature in January approaching − 30 ° C ( − 22 ° F ) , while the corresponding figure in July exceeds 18 ° C ( 64 ° F ) . By contrast , most of Hainan has a January mean in excess of 17 ° C ( 63 ° F ) , while the July mean there is generally above 28 ° C ( 82 ° F ) . | Geography of China |
838404409_29461-29938 | Precipitation is almost invariably concentrated in the warmer months , though annual totals range from less than 20 millimetres ( 0.8 in ) in northwestern Qinghai and the Turpan Depression of Xinjiang to easily exceeding 2,000 millimetres ( 79 in ) in Guangdong , Guangxi , and Hainan . Only in some pockets of the Dzungaria region of Xinjiang is the conspicuous seasonal variation in precipitation that defines Chinese ( and , to a large extent , East Asian ) climate absent . | Geography of China |
838404409_29939-30229 | Annual sunshine duration ranges from less than 1,100 hours in parts of Sichuan and Chongqing to over 3,400 hours in northwestern Qinghai . Seasonal patterns in sunshine vary considerably by region , but overall , the north and the Tibetan Plateau are sunnier than the south of the country . | Geography of China |
838404409_30230-30916 | The average annual precipitation in China and Taiwan Early - season snow covering part of the North China Plain near Shijiazhuang , Hebei Snow encircling the area around the Bo Hai The first day of spring 2010 brought a massive sandstorm blowing from Inner Mongolia On November 11 , 2010 , a wall of sand blew across northern China , covering much of the North China Plain and Shandong Peninsula . Smog from Eastern China spread over neighboring areas in February 2004 . Haze over the North China Plain and the Lüliang Mountains of Shanxi Natural color satellite image of a smog event in the heart of northern China Dense smog settled over the North China Plain on February 20 , 2011 . | Geography of China |
838404409_30974-32262 | Air pollution ( sulfur dioxide particulates ) from reliance on coal is a major issue , along with water pollution from untreated wastes and use of debated standards of pollutant concentration rather than Total Maximum Daily Load . There are water shortages , particularly in the north . The eastern part of China often experiences smoke and dense fog in the atmosphere as a result of industrial pollution . Heavy deforestation with an estimated loss of one - fifth of agricultural land since 1949 to soil erosion and economic development is occurring with resulting desertification . China is a party to the Antarctic - Environmental Protocol , the Antarctic Treaty , the Convention on Biological Diversity , the Climate Change treaty , the United Nations Convention to Combat Desertification , the Endangered Species treaty , the Hazardous Wastes treaty , the Law of the Sea , the International Tropical Timber Agreements of 1983 and 1994 , the International Convention for the Regulation of Whaling , and agreements on Marine Dumping , Ozone Layer Protection , Ship Pollution , and Wetlands protection . China has signed , but not ratified , the Kyoto Protocol ( but is not yet required to reduce its carbon emission under the agreement , as is India ) and the Nuclear Test Ban treaty . | Geography of China |
838404409_32282-32458 | Most of coastal and eastern central China , including Beijing , are antipodal to Argentina and Chile . Wuhai , Inner Mongolia , for example , is antipodal to Valdivia , Chile . | Geography of China |
837105854_156-830 | A modem ( modulator -- demodulator ) is a network hardware device that modulates one or more carrier wave signals to encode digital information for transmission and demodulates signals to decode the transmitted information . The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data . Modems can be used with any means of transmitting analog signals , from light - emitting diodes to radio . A common type of modem is one that turns the digital data of a computer into modulated electrical signal for transmission over telephone lines and demodulated by another modem at the receiver side to recover the digital data . | Modem |
837105854_831-1692 | Modems are generally classified by the maximum amount of data they can send in a given unit of time , usually expressed in bits per second ( symbol bit ( s ) , sometimes abbreviated `` bps '' ) , or bytes per second ( symbol B ( s ) ) . Modems can also be classified by their symbol rate , measured in baud . The baud unit denotes symbols per second , or the number of times per second the modem sends a new signal . For example , the ITU V. 21 standard used audio frequency - shift keying with two possible frequencies , corresponding to two distinct symbols ( or one bit per symbol ) , to carry 300 bits per second using 300 baud . By contrast , the original ITU V. 22 standard , which could transmit and receive four distinct symbols ( two bits per symbol ) , transmitted 1,200 bits by sending 600 symbols per second ( 600 baud ) using phase - shift keying . | Modem |
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837105854_2747-3198 | News wire services in the 1920s used multiplex devices that satisfied the definition of a modem . However , the modem function was incidental to the multiplexing function , so they are not commonly included in the history of modems . Modems grew out of the need to connect teleprinters over ordinary phone lines instead of the more expensive leased lines which had previously been used for current loop -- based teleprinters and automated telegraphs . | Modem |
837105854_3199-3427 | In 1941 , the Allies developed a voice encryption system called SIGSALY which used a vocoder to digitize speech , then encrypted the speech with one - time pad and encoded the digital data as tones using frequency shift keying . | Modem |
837105854_3428-4010 | Mass - produced modems in the United States began as part of the SAGE air - defense system in 1958 ( the year the word modem was first used ) , connecting terminals at various airbases , radar sites , and command - and - control centers to the SAGE director centers scattered around the United States and Canada . SAGE modems were described by AT&T 's Bell Labs as conforming to their newly published Bell 101 dataset standard . While they ran on dedicated telephone lines , the devices at each end were no different from commercial acoustically coupled Bell 101 , 110 baud modems . | Modem |
837105854_4011-4364 | The 201A and 201B Data - Phones were synchronous modems using two - bit - per - baud phase - shift keying ( PSK ) . The 201A operated half - duplex at 2,000 bit / s over normal phone lines , while the 201B provided full duplex 2,400 bit / s service on four - wire leased lines , the send and receive channels each running on their own set of two wires . | Modem |
837105854_4365-4943 | The famous Bell 103A dataset standard was also introduced by AT&T in 1962 . It provided full - duplex service at 300 bit / s over normal phone lines . Frequency - shift keying was used , with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz . The readily available 103A2 gave an important boost to the use of remote low - speed terminals such as the Teletype Model 33 ASR and KSR , and the IBM 2741 . AT&T reduced modem costs by introducing the originate - only 113D and the answer - only 113B / C modems . Acoustic | Modem |
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837105854_5273-6145 | For many years , the Bell System ( AT&T ) maintained a monopoly on the use of its phone lines and what devices could be connected to them . However , the FCC 's seminal Carterfone Decision of 1968 , the FCC concluded that electronic devices could be connected to the telephone system as long as they used an acoustic coupler . Since most handsets were supplied by Western Electric and thus of a standard design , acoustic couplers were relatively easy to build . Acoustically coupled Bell 103A - compatible 300 bit / s modems were common during the 1970s . Well - known models included the Novation CAT and the Anderson - Jacobson , the latter spun off from an in - house project at Stanford Research Institute ( now SRI International ) . An even lower - cost option was the Pennywhistle modem , designed to be built using parts from electronics scrap and surplus stores . | Modem |
837105854_6146-6957 | In December 1972 , Vadic introduced the VA3400 , notable for full - duplex operation at 1,200 bit / s over the phone network . Like the 103A , it used different frequency bands for transmit and receive . In November 1976 , AT&T introduced the 212A modem to compete with Vadic . It was similar in design , but used the lower frequency set for transmission . One could also use the 212A with a 103A modem at 300 bit / s . According to Vadic , the change in frequency assignments made the 212 intentionally incompatible with acoustic coupling , thereby locking out many potential modem manufacturers . In 1977 , Vadic responded with the VA3467 triple modem , an answer - only modem sold to computer center operators that supported Vadic 's 1,200 - bit / s mode , AT&T 's 212A mode , and 103A operation . Carterfone | Modem |
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837105854_7228-7714 | The Hush - a-Phone decision applied only to mechanical connections , but the Carterfone decision of 1968 , led to the FCC introducing a rule setting stringent AT&T - designed tests for electronically coupling a device to the phone lines . This opened the door to direct - connect modems that plugged directly into the phone line rather than via a handset . However , the cost of passing the tests was considerable , and acoustically coupled modems remained common into the early 1980s . | Modem |
837105854_7715-8548 | The rapidly falling prices of electronics in the late 1970s led to an increasing number of direct - connect models around 1980 . In spite of being directly connected , these modems were generally operated like their earlier acoustic versions -- dialing and other phone - control operations were completed by hand , using an attached handset . A small number of modems added the ability to automatically answer incoming calls , or automatically place an outgoing call to a single number , but even these limited features were relatively rare or limited to special models in a lineup . When more flexible solutions were needed , third party `` dialers '' were used to automate calling , normally using a separate serial port to communicate with the dialler , which would then control the modem through a private electrical connection . | Modem |
837105854_8549-9214 | The introduction of microcomputer systems with internal expansion slots made the first software - controllable modems common . Slot connections gave the computer complete access to the modem 's memory or input / output ( I / O ) channels , which allowed software to send commands to the modem , not just data . This led to a series of popular modems for the S - 100 bus and Apple II computers that could directly dial the phone , answer incoming calls , and hang up the phone , the basic requirements of a bulletin board system ( BBS ) . The seminal CBBS was created on an S - 100 machine with a Hayes internal modem , and a number of similar systems followed . The | Modem |
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837105854_9677-10412 | The next major advance in modems was the Hayes Smartmodem , introduced in 1981 . The Smartmodem was an otherwise standard 103A 300 bit / s direct - connect modem , but it was attached to a small microcontroller that watched the data stream for certain character strings representing commands . This allowed both data and commands to be sent through a single serial port . The now - standard Hayes command set included instructions for picking up and hanging up the phone , dialing numbers , and answering calls , among others . This was similar to the commands offered by the internal modems , but unlike them , the Smartmodem could be connected to any computer with an RS - 232 port , which was practically every microcomputer built . | Modem |
837105854_10413-11214 | The introduction of the Smartmodem made communications much simpler and more easily accessed . This provided a growing market for other vendors , who licensed the Hayes patents and competed on price or by adding features . Through the 1980s , a number of new higher - speed modems , first 1,200 and then 2,400 bit / s , greatly improved the responsiveness of the online systems , and made file transfer practical . This led to rapid growth of online services with their large file libraries , which in turn gave more reason to own a modem . The rapid update of modems led to a similar rapid increase in BBS use , which was helped by the fact that BBSs could control the modem simply by sending strings , rather than talking to a device driver that was different for every direct - connect modem . 1200 | Modem |
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837105854_11480-12021 | The 300 bit / s modems used audio frequency - shift keying to send data . In this system , the stream of 1s and 0s in computer data is translated into sounds which can be easily sent on the phone lines . In the Bell 103 system , the originating modem sends 0s by playing a 1,070 Hz tone , and 1s at 1,270 Hz , with the answering modem transmitting its 0s on 2,025 Hz and 1s on 2,225 Hz . These frequencies were chosen carefully ; they are in the range that suffers minimum distortion on the phone system and are not harmonics of each other . | Modem |
837105854_12022-13129 | In the 1,200 bit / s and faster systems , phase - shift keying was used . In this system the two tones for any one side of the connection are sent at similar frequencies as in the 300 bit / s systems , but slightly out of phase . Voiceband modems generally remained at 300 and 1,200 bit / s ( V. 21 and V. 22 ) into the mid-1980s . A V. 22bis 2,400 - bit / s system similar in concept to the 1,200 - bit / s Bell 212 signaling was introduced in the U.S. , and a slightly different one in Europe . The limited available frequency range meant the symbol rate of 1,200 bit / s modems was still only 600 baud ( symbols per second ) . The bit rate increases were achieved by defining four or eight distinct symbols , which allowed the encoding of two or three bits per symbol instead of only 1 . The use of smaller shifts had the drawback of making each symbol more vulnerable to interference , but improvements in phone line quality at the same time helped compensate for this . By the late 1980s , most modems could support all of these standards and 2,400 - bit / s operation was becoming common . Proprietary | Modem |
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837105854_13388-13835 | Many other standards were also introduced for special purposes , commonly using a high - speed channel for receiving , and a lower - speed channel for sending . One typical example was used in the French Minitel system , in which the user 's terminals spent the majority of their time receiving information . The modem in the Minitel terminal thus operated at 1,200 bit / s for reception , and 75 bit / s for sending commands back to the servers . | Modem |
837105854_13836-14592 | Three U.S. companies became famous for high - speed versions of the same concept . Telebit introduced its Trailblazer modem in 1984 , which used a large number of 36 bit / s channels to send data one - way at rates up to 18,432 bit / s . A single additional channel in the reverse direction allowed the two modems to communicate how much data was waiting at either end of the link , and the modems could change direction on the fly . The Trailblazer modems also supported a feature that allowed them to spoof the UUCP g protocol , commonly used on Unix systems to send e-mail , and thereby speed UUCP up by a tremendous amount . Trailblazers thus became extremely common on Unix systems , and maintained their dominance in this market well into the 1990s . | Modem |
837105854_14593-15268 | USRobotics ( USR ) introduced a similar system , known as HST , although this supplied only 9,600 bit / s ( in early versions at least ) and provided for a larger backchannel . Rather than offer spoofing , USR instead created a large market among Fidonet users by offering its modems to BBS sysops at a much lower price , resulting in sales to end users who wanted faster file transfers . Hayes was forced to compete , and introduced its own 9,600 bit / s standard , Express 96 ( also known as Ping - Pong ) , which was generally similar to Telebit 's PEP . Hayes , however , offered neither protocol spoofing nor sysop discounts , and its high - speed modems remained rare . | Modem |
837105854_15269-16168 | A common feature of these high - speed modems was the concept of fallback , or speed hunting , allowing them to communicate with less - capable modems . During the call initiation , the modem would transmit a series of signals and wait for the remote modem to respond . They would start at high speeds and get progressively slower until there was a response . Thus , two USR modems would be able to connect at 9,600 bit / s , but , when a user with a 2,400 bit / s modem called in , the USR would fall back to the common 2,400 bit / s speed . This would also happen if a V. 32 modem and a HST modem were connected . Because they used a different standard at 9,600 bit / s , they would fall back to their highest commonly supported standard at 2,400 bit / s . The same applies to V. 32bis and 14,400 bit / s HST modem , which would still be able to communicate with each other at 2,400 bit / s . Echo | Modem |
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837105854_16558-17367 | Local telephone lines use the same wires to send and receive data , which results in a small amount of the outgoing signal being reflected back . This is useful for people talking on the phone , as it provides a signal to the speaker that their voice is making it through the system . However , this reflected signal causes problems for the modem , which is unable to distinguish between a signal from the remote modem and the echo of its own signal . This was why earlier modems split the signal frequencies into `` answer '' and `` originate '' ; the modem could then ignore any signals in the frequency range it was using for transmission . Even with improvements to the phone system allowing higher speeds , this splitting of available phone signal bandwidth still imposed a half - speed limit on modems . | Modem |
837105854_17368-17992 | Echo cancellation eliminated this problem . During the call setup and negotiation period , both modems send a series of unique tones and then listen for them to return through the phone system . They measure the total delay time and then set up a local delay loop to the same time . Once the connection is completed , they send their signals into the phone lines as normal , but also into the delay , which is inverted . The signal returning through the echo meets the inverted version coming from the delay line , and cancels out the echo . This allowed both modems to use the full spectrum available , doubling the speed . | Modem |
837105854_17993-18697 | Additional improvements were introduced via the quadrature amplitude modulation ( QAM ) encoding system . Previous systems using phase shift keying ( PSK ) encoded two bits ( or sometimes three ) per symbol by slightly delaying or advancing the signal 's phase relative to a set carrier tone . QAM used a combination of phase shift and amplitude to encode four bits per symbol . Transmitting at 1,200 baud produced the 4,800 bit / s V. 27ter standard , the same working at a base rate of 2,400 baud produced the 9,600 bit / s V. 32 . The carrier frequency was 1,650 Hz in both systems . For many years , most engineers considered this rate to be the limit of data communications over telephone networks . | Modem |
837105854_18698-19115 | The introduction of these higher - speed systems also led to the digital fax machine during the 1980s . Digital faxes are simply an image format sent over a high - speed ( commonly 14.4 kbit / s ) modem . Software running on the host computer can convert any image into fax format , which can then be sent using the modem . Such software was at one time an add - on , but has since become largely universal . Breaking | Modem |
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837105854_19406-19519 | The first 9,600 bit / s modem was developed in 1968 , and sold for more than $20,000 , but had high error rates . | Modem |
837105854_19520-20175 | In 1980 , Gottfried Ungerboeck from IBM Zurich Research Laboratory applied channel coding techniques to search for new ways to increase the speed of modems . His results were astonishing but only conveyed to a few colleagues . In 1982 , he agreed to publish what is now a landmark paper in the theory of information coding . By applying parity check coding to the bits in each symbol , and mapping the encoded bits into a two - dimensional diamond pattern , Ungerboeck showed that it was possible to increase the speed by a factor of two with the same error rate . The new technique was called mapping by set partitions , now known as trellis modulation . | Modem |
837105854_20176-20573 | Error correcting codes , which encode code words ( sets of bits ) in such a way that they are far from each other , so that in case of error they are still closest to the original word ( and not confused with another ) can be thought of as analogous to sphere packing or packing pennies on a surface : the further two bit sequences are from one another , the easier it is to correct minor errors . | Modem |
837105854_20574-20802 | Dave Forney introduced the trellis diagram in a landmark 1973 paper that popularized the Viterbi algorithm . Practically all modems operating faster than 9600 bit / s decode trellis - modulated data using the Viterbi algorithm . | Modem |
837105854_20803-21492 | V. 32 modems operating at 9600 bit / s were expensive and were only starting to enter the market in the early 1990s when V. 32bis was standardized . Rockwell International 's chip division developed a new driver chip set incorporating the standard and aggressively priced it . Supra , Inc. arranged a short - term exclusivity arrangement with Rockwell , and developed the SupraFAXModem 14400 based on it . Introduced in January 1992 at $399 ( or less ) , it was half the price of the slower V. 32 modems already on the market . This led to a price war , and by the end of the year V. 32 was dead , never having been really established , and V. 32bis modems were widely available for $250 . | Modem |
837105854_21493-21776 | V. 32bis was so successful that the older high - speed standards had little to recommend them . USR fought back with a 16,800 bit / s version of HST , while AT&T introduced a one - off 19,200 bit / s method they referred to as V. 32ter , but neither non-standard modem sold well . V. | Modem |
837105854_21967-22457 | Any interest in these proprietary improvements was destroyed during the lengthy introduction of the 28,800 bit / s V. 34 standard . While waiting , several companies decided to release hardware and introduced modems they referred to as V. FAST . In order to guarantee compatibility with V. 34 modems once the standard was ratified ( 1994 ) , the manufacturers were forced to use more flexible parts , generally a DSP and microcontroller , as opposed to purpose - designed ASIC modem chips . | Modem |
837105854_22458-23390 | The ITU standard V. 34 represents the culmination of the joint efforts . It employs the most powerful coding techniques including channel encoding and shape encoding . From the mere four bits per symbol ( 9.6 kbit / s ) , the new standards used the functional equivalent of 6 to 10 bits per symbol , plus increasing baud rates from 2,400 to 3,429 , to create 14.4 , 28.8 , and 33.6 kbit / s modems . This rate is near the theoretical Shannon limit . When calculated , the Shannon capacity of a narrowband line is bandwidth × log 2 ( 1 + P u / P n ) ( \ displaystyle ( \ text ( bandwidth ) ) \ times \ log _ ( 2 ) ( 1 + P_ ( u ) / P_ ( n ) ) ) , with P u / P n ( \ displaystyle P_ ( u ) / P_ ( n ) ) the ( linear ) signal - to - noise ratio . Narrowband phone lines have a bandwidth of 3,000 Hz so using P u / P n = 1000 ( \ displaystyle P_ ( u ) / P_ ( n ) = 1000 ) ( SNR = 30 dB ) , the capacity is approximately 30 kbit / s . | Modem |
837105854_23391-23632 | Without the discovery and eventual application of trellis modulation , maximum telephone rates using voice - bandwidth channels would have been limited to 3,429 baud × 4 bit / symbol = approximately 14 kbit / s using traditional QAM . Using | Modem |
837105854_23700-25625 | During the late 1990s , Rockwell - Lucent and USRobotics introduced competing technologies based upon the digital transmission used in telephony networks . The standard digital transmission in modern networks is 64 kbit / s but some networks use a part of the bandwidth for remote office signaling ( e.g. to hang up the phone ) , limiting the effective rate to 56 kbit / s DS0 . This new technology was adopted into ITU standards V. 90 and is common in modern computers . The 56 kbit / s rate is only possible from the central office to the user site ( downlink ) . In the United States , government regulation limits the maximum power output , resulting in a maximum data rate of 53.3 kbit / s . The uplink ( from the user to the central office ) still uses V. 34 technology at 33.6 kbit / s . USRobotics began work on the technology first , calling theirs X2 because 56k was twice the speed of 28k modems . USRobotics held a 40 - percent share of the retail modem market , and Rockwell International held an 80 - percent share of the modem chipset market . Concerned with being shut out of the market , Rockwell began work on a rival 56k technology and joined with Lucent and Motorola on what it called K56Flex or Flex . Both technologies reached the market around February 1997 ; although problems with K56Flex modems were noted in product reviews through July , within six months they worked equally well with variations dependent on local connection characteristics . The retail price of the 56K modems was about US $200 , compared to $100 for 33K modems . Separate equipment was required by Internet service providers ( ISPs ) to support the incompatible technologies , with costs varying depending on whether their current equipment could be upgraded . About half of all ISPs offered 56K support by October 1997 . Consumer sales were relatively low , which USRobotics and Rockwell attributed to conflicting standards . | Modem |
837105854_25626-26046 | The International Telecommunication Union ( ITU ) announced the draft of a new 56 kbit / s standard , V. 90 , in February 1998 , with strong industry support . Incompatible with either existing standard , it was an amalgam of both which was designed to allow both types of modem to be converted to it by a firmware upgrade . This V. 90 standard was approved in September 1998 , and widely adopted by ISPs and consumers . | Modem |
837105854_26047-26950 | Later in V. 92 , the digital PCM technique was applied to increase the upload speed to a maximum of 48 kbit / s , but at the expense of download rates . A 48 kbit / s upstream rate would reduce the downstream as low as 40 kbit / s due to echo on the telephone line . To avoid this problem , V. 92 modems offer the option to turn off the digital upstream and instead use a 33.6 kbit / s analog connection , in order to maintain a high digital downstream of 50 kbit / s or higher . V. 92 also adds two other features . The first is the ability for users who have call waiting to put their dial - up Internet connection on hold for extended periods of time while they answer a call . The second feature is the ability to quickly connect to one 's ISP . This is achieved by remembering the analog and digital characteristics of the telephone line , and using this saved information when reconnecting . Using | Modem |
837105854_26994-27584 | V. 42 , V. 42bis and V. 44 standards allow the modem to transmit data faster than its basic rate would imply . For instance , a 53.3 kbit / s connection with V. 44 can transmit up to 53.3 × 6 = 320 kbit / s using pure text . However , the compression ratio tends to vary due to noise on the line , or due to the transfer of already - compressed files ( ZIP files , JPEG images , MP3 audio , MPEG video ) . At some points the modem will be sending compressed files at approximately 50 kbit / s , uncompressed files at 160 kbit / s , and pure text at 320 kbit / s , or any value in between . | Modem |
837105854_27585-28153 | In such situations , a small amount of memory in the modem , a buffer , is used to hold the data while it is being compressed and sent across the phone line , but in order to prevent overflow of the buffer , it sometimes becomes necessary to tell the computer to pause the datastream . This is accomplished through hardware flow control using extra lines on the modem -- computer connection . The computer is then set to supply the modem at some higher rate , such as 320 kbit / s , and the modem will tell the computer when to start or stop sending data . Compression | Modem |
837105854_28174-28410 | This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . ( March 2013 ) ( Learn how and when to remove this template message ) | Modem |
837105854_28411-29162 | As telephone - based 56k modems began losing popularity , some Internet service providers such as Netzero / Juno , Netscape , and others started using pre-compression to increase the throughput and maintain their customer base . The server - side compression operates much more efficiently than the on - the - fly compression done by modems because these compression techniques are application - specific ( JPEG , text , EXE , etc . ) . The website text , images , and Flash executables are compacted to approximately 4 % , 12 % , and 30 % , respectively . The drawback of this approach is a loss in quality , which causes image content to become pixelated and smeared . ISPs employing this approach often advertise it as `` accelerated dial - up '' . | Modem |
837105854_29163-29324 | These accelerated downloads are now integrated into the Opera and Amazon Silk web browsers , using their own server - side text and image compression . Softmodem | Modem |
837105854_29359-29599 | This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . ( October 2017 ) ( Learn how and when to remove this template message ) A | Modem |
837105854_29690-30395 | A Winmodem or softmodem is a stripped - down modem that replaces tasks traditionally handled in hardware with software . In this case the modem is a simple interface designed to act as a digital - to - analog and an analog - to - digital converter . Softmodems are cheaper than traditional modems because they have fewer hardware components . However , the software generating and interpreting the modem tones to be sent to the softmodem uses many system resources . For online gaming , this can be a real concern . Another problem is the lack of cross-platform compatibility , meaning that non-Windows operating systems ( such as Linux ) often do not have an equivalent driver to operate the modem . List | Modem |
837105854_30422-30705 | These values are maximum values , and actual values may be slower under certain conditions ( for example , noisy phone lines ) . For a complete list see the companion article list of device bandwidths . A baud is one symbol per second ; each symbol may encode one or more data bits . | Modem |
837105854_30706-31761 | Connection Modulation Bitrate ( kbit / s ) Year released 110 baud Bell 101 modem FSK 0.1 1958 300 baud ( Bell 103 or V. 21 ) FSK 0.3 1962 1200 modem ( 1200 baud ) ( Bell 202 ) FSK 1.2 1200 modem ( 600 baud ) ( Bell 212A or V. 22 ) QPSK 1.2 1980 2400 modem ( 600 baud ) ( V. 22bis ) QAM 2.4 1984 2400 modem ( 1200 baud ) ( V. 26bis ) PSK 2.4 4800 modem ( 1600 baud ) ( V. 27ter ) PSK 4.8 9600 modem ( 2400 baud ) ( V. 32 ) QAM 9.6 1984 14.4 k modem ( 2400 baud ) ( V. 32bis ) trellis 14.4 1991 19.2 k modem ( 2400 baud ) ( V. 32terbo ) trellis 19.2 28.8 k modem ( 3200 baud ) ( V. 34 ) trellis 28.8 33.6 k modem ( 3429 baud ) ( V. 34 ) trellis 33.6 56k modem ( 8000 / 3429 baud ) ( V. 90 ) digital 56.0 / 33.6 1998 56k modem ( 8000 / 8000 baud ) ( V. 92 ) digital 56.0 / 48.0 2000 Bonding modem ( two 56k modems ) ( V. 92 ) 112.0 / 96.0 Hardware compression ( variable ) ( V. 90 / V. 42bis ) 56.0 -- 220.0 Hardware compression ( variable ) ( V. 92 / V. 44 ) 56.0 -- 320.0 Server - side web compression ( variable ) ( Netscape ISP ) 100.0 -- 1,000.0 Various | Modem |
837105854_31886-32373 | A 1994 Software Publishing Association found that although 60 % of computers in US households had a modem , only 7 % of households went online . A CEA study in 2006 found that dial - up Internet access is declining in the U.S. In 2000 , dial - up Internet connections accounted for 74 % of all U.S. residential Internet connections . The United States demographic pattern for dial - up modem users per capita has been more or less mirrored in Canada and Australia for the past 20 years . | Modem |
837105854_32374-33001 | Dial - up modem use in the U.S. had dropped to 60 % by 2003 , and in 2006 , stood at 36 % . Voiceband modems were once the most popular means of Internet access in the U.S. , but with the advent of new ways of accessing the Internet , the traditional 56K modem is losing popularity . The dial - up modem is still widely used by customers in rural areas , where DSL , cable , satellite , or fiber optic service is not available , or they are unwilling to pay what these companies charge . In its 2012 annual report , AOL showed it still collects around US $700 million in fees from dial - up users : about three million people . | Modem |
837105854_33021-33263 | This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . ( October 2017 ) ( Learn how and when to remove this template message ) DSL | Modem |
837105854_33270-33831 | ADSL ( asymmetric digital subscriber line ) modems , a more recent development , are not limited to the telephone 's voiceband audio frequencies . Standard twisted - pair telephone cable can , for short distances , carry signals with much higher frequencies than the cable 's maximum frequency rating . ADSL broadband takes advantage of this capability . However , ADSL 's performance gradually declines as the telephone cable 's length increases . This limits ADSL broadband service to subscribers within a relatively short distance of the telephone exchange . | Modem |
837105854_33832-34208 | Cable modems use infrastructure originally intended to carry television signals and therefore designed from the outset to carry higher frequencies . A single cable can carry radio and television signals at the same time as broadband internet service without interference . Newer types of broadband modems are also available , including satellite modems and power line modems . | Modem |
837105854_34209-34903 | Most consumers did not know about networking and routers when broadband became available . However , many people knew that a modem connected a computer to the Internet over a telephone line . To take advantage of consumers ' familiarity with modems , companies called these devices broadband modems rather than using less familiar terms such as adapter , interface , transceiver , or bridge . In fact , broadband modems fit the definition of modem because they use complex waveforms to carry digital data . They use more advanced technology than dial - up modems : typically they can modulate and demodulate hundreds of channels simultaneously or use much wider channels than dial - up modems . | Modem |
837105854_34919-35157 | This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . ( October 2017 ) ( Learn how and when to remove this template message ) | Modem |
837105854_35158-35582 | Direct broadcast satellite , WiFi , and mobile phones all use modems to communicate , as do most other wireless services today . Modern telecommunications and data networks also make extensive use of radio modems where long distance data links are required . Such systems are an important part of the PSTN , and are also in common use for high - speed computer network links to outlying areas where fibre is not economical . | Modem |
837105854_35583-36338 | Even where a cable is installed , it is often possible to get better performance or make other parts of the system simpler by using radio frequencies and modulation techniques through a cable . Coaxial cable has a very large bandwidth , but signal attenuation becomes a major problem at high data rates if a baseband digital signal is used . By using a modem , a much larger amount of digital data can be transmitted through a single wire . Digital cable television and cable Internet services use radio frequency modems to provide the increasing bandwidth needs of modern households . Using a modem also allows for frequency - division multiple access to be used , making full - duplex digital communication with many users possible using a single wire . | Modem |
837105854_36339-36648 | Wireless modems come in a variety of types , bandwidths , and speeds . Wireless modems are often referred to as transparent or smart . They transmit information that is modulated onto a carrier frequency to allow many simultaneous wireless communication links to work simultaneously on different frequencies . | Modem |
837105854_36649-37100 | Transparent modems operate in a manner similar to their phone line modem cousins . Typically , they were half duplex , meaning that they could not send and receive data at the same time . Typically , transparent modems are polled in a round robin manner to collect small amounts of data from scattered locations that do not have easy access to wired infrastructure . Transparent modems are most commonly used by utility companies for data collection . | Modem |
837105854_37101-37481 | Smart modems come with media access controllers inside , which prevents random data from colliding and resends data that is not correctly received . Smart modems typically require more bandwidth than transparent modems , and typically achieve higher data rates . The IEEE 802.11 standard defines a short range modulation scheme that is used on a large scale throughout the world . | Modem |
837105854_37504-37742 | This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . ( October 2017 ) ( Learn how and when to remove this template message ) | Modem |
837105854_37743-37893 | DC - BUS powerline modem provide communication over noisy power lines at speeds up to 1.3 Mbit / s using ordinary UART , LIN , SPI and CAN protocols . | Modem |
837105854_37918-38156 | This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . ( October 2017 ) ( Learn how and when to remove this template message ) | Modem |
837105854_38157-38259 | The WiFi and WiMax standards use wireless mobile broadband modems operating at microwave frequencies . | Modem |
837105854_38432-39609 | Modems which use a mobile telephone system ( GPRS , UMTS , HSPA , EVDO , WiMax , etc . ) , are known as mobile broadband modems ( sometimes also called wireless modems ) . Wireless modems can be embedded inside a laptop or appliance , or be external to it . External wireless modems are connect cards , USB modems for mobile broadband and cellular routers . A connect card is a PC Card or ExpressCard which slides into a PCMCIA / PC card / ExpressCard slot on a computer . USB wireless modems use a USB port on the laptop instead of a PC card or ExpressCard slot . A USB modem used for mobile broadband Internet is also sometimes referred to as a dongle . A cellular router may have an external datacard ( AirCard ) that slides into it . Most cellular routers do allow such datacards or USB modems . Cellular routers may not be modems by definition , but they contain modems or allow modems to be slid into them . The difference between a cellular router and a wireless modem is that a cellular router normally allows multiple people to connect to it ( since it can route data or support multi-point to multi-point connections ) , while a modem is designed for one connection . | Modem |
837105854_39610-39987 | Most of GSM wireless modems come with an integrated SIM cardholder ( i.e. , Huawei E220 , Sierra 881 , etc . ) and some models are also provided with a microSD memory slot and / or jack for additional external antenna such as Huawei E1762 and Sierra Wireless Compass 885 . The CDMA ( EVDO ) versions do not use R - UIM cards , but use Electronic Serial Number ( ESN ) instead . | Modem |
837105854_39988-40473 | The cost of using a wireless modem varies from country to country . Some carriers implement flat rate plans for unlimited data transfers . Some have caps ( or maximum limits ) on the amount of data that can be transferred per month . Other countries have plans that charge a fixed rate per data transferred -- per megabyte or even kilobyte of data downloaded ; this tends to add up quickly in today 's content - filled world , which is why many people are pushing for flat data rates . | Modem |
837105854_40474-40668 | The faster data rates of the newest wireless modem technologies ( UMTS , HSPA , EVDO , WiMax ) are also considered to be broadband wireless modems and compete with other broadband modems below . | Modem |
837105854_40669-40992 | Until the end of April 2011 , worldwide shipments of USB modems surpassed embedded 3G and 4G modules by 3 : 1 because USB modems can be easily discarded , but embedded modems could start to gain popularity as tablet sales grow and as the incremental cost of the modems shrinks , so by 2016 , the ratio may change to 1 : 1 . | Modem |
837105854_40993-41186 | Like mobile phones , mobile broadband modems can be SIM locked to a particular network provider . Unlocking a modem is achieved the same way as unlocking a phone , by using an ' unlock code ' . | Modem |
837105854_41217-41455 | This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . ( October 2017 ) ( Learn how and when to remove this template message ) | Modem |
837105854_41456-41922 | Some devices referred to as `` broadband modems '' are residential gateways , integrating the functions of a modem , network address translation ( NAT ) router , Ethernet switch , WiFi access point , DHCP server , firewall , among others . Some residential gateway offer a so - called `` bridged mode '' , which disables the built - in routing function and makes the device function similarly to a plain modem . This bridged mode is separate from RFC 1483 bridging . | Modem |
837105854_41947-42136 | Fiber optic systems can be upgraded by the use of quadrature amplitude modulation . The modulator and demodulator are separate components rather than a single assembly as with most modems . | Modem |
837105854_42152-42288 | 16QAM uses a 16 - point constellation to send four bits per symbol . Speeds are usually on the order of 200 or 400 gigabits per second . | Modem |
837105854_42304-42513 | 64QAM uses a 64 - point constellation to send six bits per symbol . Although suppliers have announced components , announcements of installation are rare . Speeds of 65 terabits per second have been observed . | Modem |
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