通信英文文獻(xiàn)翻譯

1、<p><b>　　通信英文文獻(xiàn)翻譯 </b></p><p><b>　　通信英文文獻(xiàn)翻譯</b></p><p>　　切換電路的連接設(shè)置的結(jié)果是保留了從發(fā)送端到接收端的包所通過的線路帶寬。其它的特性這一就是所有的包都沿同一個(gè)線路意味著如果包傳輸超過了時(shí)序那它將不能到達(dá)接收端.由于沒有路徑用來進(jìn)行包交換，所以要想不同的包沿不同的路徑傳

2、送就是能依靠網(wǎng)絡(luò)的本文源自六維論文網(wǎng)條件了。這樣包即便是超過了時(shí)序它們也有可能到達(dá)的。</p><p>　　包交換技術(shù)比線路交換技術(shù)更能容錯(cuò)。事實(shí)上，這也是它被發(fā)明的理由！當(dāng)一個(gè)交換開頭被拉下，則所有使用它的線路將被切斷并且其余的包將不能再被傳送。此時(shí)如果在進(jìn)行包交換，包將在已選擇的路徑上停止交換。</p><p>　　預(yù)先建立一條路徑也開創(chuàng)預(yù)先保留帶寬的可能性。如果帶寬被保留，那么當(dāng)一個(gè)

3、包到達(dá)時(shí)，通過保留的帶寬它可以被立即發(fā)送出去。假如沒有帶寬被保留用于包交換，那么所有的包將不得不等待著被轉(zhuǎn)發(fā)。</p><p>　　預(yù)先保留帶寬意條線路那么當(dāng)這個(gè)用戶沒有數(shù)據(jù)傳送時(shí)，則這些保留下來的帶寬將被浪費(fèi)而不能用于其它用戶的傳輸。而包交換則不會(huì)浪費(fèi)帶寬，因此它在系統(tǒng)寬帶方面的應(yīng)用更有前景。理解電路交換和包交換的這種商業(yè)差別是極其重要的。這種商業(yè)差別是在質(zhì)保服務(wù)和浪費(fèi)資源相對不質(zhì)保服務(wù)和不浪費(fèi)資源的。<

4、/p><p>　　包交換是利用存儲器并向前發(fā)送的。包是先存儲在路由器的內(nèi)存里然后再發(fā)送到下一個(gè)路由器的。和電路交換一樣信息位也是連續(xù)不斷地通過導(dǎo)線傳輸?shù)?。存儲轉(zhuǎn)發(fā)技術(shù)增加了延遲時(shí)間。</p><p>　　另一個(gè)不同是電路交換是完全透明的。發(fā)送者和接收者可以使用他們想要的任一比特速率，格式和幀同步.這通信公司不知道也不關(guān)心這的。而在包交換中載體決定了基本的參數(shù)的。用一個(gè)簡單比喻它們就像一個(gè)是公路

5、一個(gè)是鐵路。在以前，用戶決定介質(zhì)的大小，速度和特性。而后來這些都由通信公司做了的。這些都是透明的它允許聲音，數(shù)據(jù)和傳真共同存在于電話系統(tǒng)里。</p><p>　　電路交換和包交換的最后一個(gè)不同點(diǎn)是計(jì)費(fèi)方式。電路交換的計(jì)費(fèi)是按照傳統(tǒng)的距離和時(shí)間的。比如移動(dòng)電話除打國際電話外距離是不列入計(jì)算的，而時(shí)間也只是列入一個(gè)簡單的計(jì)算(例:拔打2000分鐘電話比1000分鐘電話花費(fèi)的多并且晚上或周末也比平時(shí)來得便宜)。時(shí)間對于

6、包交換來說是沒有問題的，但是傳輸流量卻是個(gè)問題。ISPs為家庭用戶的計(jì)費(fèi)方式基于按一個(gè)月的流量的一半的，這是因?yàn)樗鼈冇玫孟鄬^少并且他們的顧客也很好理解這種計(jì)費(fèi)方式。但是主干網(wǎng)絡(luò)中心的收費(fèi)是基于它們的流量的。這些不同列在圖2-40里。</p><p>　　項(xiàng)目 電路交換 包交換</p><p>　　呼叫設(shè)置 需要 不需要</p><

7、;p>　　專用物理線路 是 否</p><p>　　每個(gè)包沿相同路徑 是  否</p><p>　　包的到達(dá)中有規(guī)則的 是 否</p><p>　　是否會(huì)致命的崩潰 是 否</p><p>　　帶寬利用 固定 動(dòng)態(tài)分配</p>

8、<p>　　可能的時(shí)間堵塞 在設(shè)置時(shí) 在每個(gè)包時(shí)</p><p>　　可能浪費(fèi)帶寬 是 否</p><p>　　以存儲轉(zhuǎn)發(fā)進(jìn)行發(fā)送 否 是</p><p><b>　　透明性 是 否</b></p><p><b>　

9、　收費(fèi) 按分鐘 按包</b></p><p>　　圖2－40：比較電路交換網(wǎng)絡(luò)和包交換網(wǎng)絡(luò)的不同</p><p>　　電路交換和包交換是兩者都是非常重要的，我們將立即在詳細(xì)的資料里提及并描述它們之間各種不同的技術(shù)。</p><p>　　2.6 移動(dòng)電話系統(tǒng)</p><p>　　傳統(tǒng)的電話的系統(tǒng)將不能滿足用戶的大

10、量增長（即有一天實(shí)現(xiàn)了終端到終端的光纖連接）。人類在進(jìn)步，人們期望能在飛機(jī)上，汽車?yán)?，船上，足球場甚至在公園里跑步的時(shí)候也能打電話。在近幾年人們更是期望能在更多的地方發(fā)電子郵件和上網(wǎng)沖浪。因此有很多的人關(guān)心無線電話技術(shù)。在下面的章節(jié)我們將用詳細(xì)的資料來學(xué)習(xí)這些主題。</p><p>　　無線電話可以分為基本的兩種：無繩電話和移動(dòng)電話（有時(shí)也叫蜂窩電話）。無繩電話設(shè)備是由痤機(jī)和聽筒組成它們被安裝在用戶的家里。它們從

11、不被應(yīng)用于網(wǎng)絡(luò)，所以我們沒有辦法做進(jìn)一步的檢測。取而代之，我們將把焦點(diǎn)放在在聲音和數(shù)據(jù)通信上有廣泛應(yīng)用的移動(dòng)系統(tǒng)上。</p><p>　　移動(dòng)電話經(jīng)歷了三個(gè)不同的發(fā)展階段，每個(gè)階段都有不同的技術(shù)：</p><p><b>　　1． 模擬聲音</b></p><p><b>　　2． 數(shù)字聲音</b>&l

12、t;/p><p>　　3． 數(shù)字聲音和數(shù)據(jù)（互聯(lián)網(wǎng)，電子郵件，etc.）</p><p>　　雖然我們主要討論這系統(tǒng)技術(shù)，但是我們也有必要注意政府和市場對它的巨大影響。第一個(gè)移動(dòng)電話系統(tǒng)是在美國由受美國國家聯(lián)邦通訊委員會(huì)(FCC)委托的美國電話電報(bào)公司(AT&T)設(shè)計(jì)的。結(jié)果在整個(gè)美國就有了一個(gè)簡單的(模擬)系統(tǒng)并且移動(dòng)電話開始在加利福尼亞上市同時(shí)也在紐約得到應(yīng)用。相反的，當(dāng)

13、移動(dòng)電話來到歐洲時(shí)，由于每個(gè)國家都設(shè)計(jì)了自己的系統(tǒng)，所以結(jié)局很慘敗。</p><p>　　當(dāng)數(shù)字技術(shù)到來時(shí)，歐洲從前面的失敗中吸收教訓(xùn)，政府和郵政制定了簡單系統(tǒng)(GSM)的統(tǒng)一標(biāo)準(zhǔn)，因此移動(dòng)電話在歐洲的任何一個(gè)地方都是通用的。此時(shí)，美國政府決定不把標(biāo)準(zhǔn)商業(yè)化因而它失去了數(shù)字市場。這個(gè)決定導(dǎo)致不同的設(shè)備生產(chǎn)商生產(chǎn)出不同的移動(dòng)電話。結(jié)果現(xiàn)在美國有兩不可兼容的數(shù)字移動(dòng)電話在使用(加強(qiáng)了一家就會(huì)削弱另一家)。</p

14、><p>　　即使美國擁有移動(dòng)電話的所有權(quán)，但是在應(yīng)用上歐洲要遠(yuǎn)遠(yuǎn)超過美國的。原因之一就是整個(gè)歐洲擁有相同的簡單系統(tǒng)。然而更多的是由于美國和歐洲的基本電話數(shù)量不同。在美國移動(dòng)電話和固定電話是共用的，因此對于用戶沒有辦法區(qū)分(212)234-5678到底是固定電話(撥打是便宜的或者是免費(fèi)的)還是移動(dòng)電話(撥打是昂貴的)。為了使人們從使用電話中獲益，電話公司決定為引入呼叫而花自己的錢制造移動(dòng)電話。結(jié)果導(dǎo)致，很多人怕由于呼

15、叫而收到一大筆帳單而不愿意買移動(dòng)電話。在歐洲移動(dòng)電話有特殊的區(qū)號(如800,900之類的數(shù)字)，所以它很快就得到了認(rèn)可。因而在一般的“叫方付費(fèi)”的原則在歐洲也被應(yīng)用于移動(dòng)電話了(除國際電話分開收費(fèi)外)。需要很多手續(xù)。誰付費(fèi)誰使用，他們預(yù)先存入，比如20或50 euro并且在快用完的時(shí)候可以用PIN碼進(jìn)行再充值。結(jié)果在歐洲幾乎所有的青年和小孩都有一個(gè)(預(yù)付費(fèi)的)移動(dòng)電話，這樣他們的父母就可以準(zhǔn)確的定位他們而不用去擔(dān)心孩子們會(huì)用掉一大筆帳的

16、。如果僅在偶而使用移動(dòng)電話，那么它的使用是基本免費(fèi)的因?yàn)檫@里有月租或者預(yù)先支付呼叫。</p><p>　　2.6.1 第一代移動(dòng)電話：模擬聲音</p><p>　　移動(dòng)電話充分體現(xiàn)了政治和市場的特征。讓我們來看一下它的技術(shù)吧。它是從簡單的系統(tǒng)開始的。在20世紀(jì)的初，移動(dòng)無線電話被偶而應(yīng)用于海上或軍事通信中。在1946年，第一個(gè)基站被安置在St.Louis。這個(gè)系統(tǒng)被安置在高層建筑上，它使用

17、一個(gè)簡單的大的發(fā)送機(jī)并且有了用于發(fā)送和接收的簡單的通道。像這樣的系統(tǒng)稱做被動(dòng)式系統(tǒng)，從1950開始它就被安裝在很多的城市了。可較驗(yàn)的收音機(jī)，出租車，警車還有電視也廣泛采用這種技術(shù)。 在1960年，IMTS（改良的移動(dòng)電話系統(tǒng)）被發(fā)明。它仍然使用高達(dá)200瓦的發(fā)送器，發(fā)送器被安置在山上，不過此時(shí)已經(jīng)有了兩各不同的頻率了，一種用來發(fā)送，另一種用來接收，所以這個(gè)呼叫按鈕就可不要了。相比信號出站移動(dòng)電話的信號入站是通過不同的通道的，因此移動(dòng)用戶

18、不會(huì)互相聽到(不像應(yīng)用在出租車上被叫系統(tǒng))。</p><p>　　IMTS支持從150 MHz到 450 MHz的23條通道。由于通道太少，用戶經(jīng)常要等很長時(shí)間才能聽到拔號音。也由于小山上的發(fā)送器的功率太大，為了不互相干擾鄰近的系統(tǒng)不得不與它相距幾百千米。總而言之，這有限的容量使用這系統(tǒng)有點(diǎn)不合實(shí)際應(yīng)用。</p><p>　　The result of the connection set

19、up with circuit switching is the reservation of bandwidth all the way from the sender to the receiver. All packets follow this path. Among other properties, having all packets follow the same path means that they cannot

20、arrive out of order. With packet switching there is no path , so different packets can follow different paths, depending on network conditions at the time they are sent. They may arrive out of order.</p><p>

21、　　Packet switching is more fault tolerant than circuit switching. In fact, that is why it was invented. If a switch goes down, all of the circuits using it are terminated and no more traffic can be sent on any of them. W

22、ith Packet switching, packets can be routed may have to wait their turn to be forwarded.</p><p>　　Having bandwidth reserved in advance means that no congestion can occur when a packet shows up (unless more p

23、ackets show up than expected).On the other hand, when an attempt is made to establish a circuit, the attempt can fail due to congestion. Thus, congestion can occur at different times with circuit switching(at setup time)

24、 and packet switching(when packets are sent).</p><p>　　If a circuit has been reserved for a particular user and there is no traffic to send, the bandwidth of that circuit is wasted. It cannot be used for oth

25、er traffic. Packet switching does not waste bandwidth and thus is more efficient form a system-wide perspective. Understanding this trade-off is crucial for comprehending the difference between circuit switching and pack

26、et switching. The trade-off is between guaranteed service and wasting resources versus not guaranteeing service an本文源自六維論文網(wǎng)d not</p><p>　　Packet switching uses store-and-forward transmission. A packet is acc

27、umulated. in a router’s memory, then sent on to the next router. With circuit switching, the bits just flow through the wire continuously. The store-and-forward technique adds delay.</p><p>　　Another differe

28、nce is that circuit switching is completely transparent. The sender and receiver can use any bit rate, format, or framing method they want to. The carrier does not know or care. With packet switching, the carrier determi

29、nes the basic parameters. A rough analogy is a road versus a railroad. In the former, the user determines the size, speed, and nature of the vehicle; in the latter, the carrier does. It is this transparency that allows v

30、oice, data, and fax to coexist within the pho</p><p>　　A final difference between circuit and packet switching is the charging algorithm. With circuit switching, charging has historically been based on dista

31、nce and time. For mobile phones, distance usually does not play a role, except for international calls, and time plays only a minor role(e.g., a calling plan with 2000 free minutes costs more than one with 1000 free minu

32、tes and sometimes night or weekend calls are cheaper than normal).With packet switching, connect time is not an issue, but the v</p><p>　　Item Circuit Switched Packet Switched</p><p>　

33、　Call setup Required Not needed</p><p>　　Dedicated physical path Yes No</p><p>　　Each packet follows the same route Yes No</p><p>　　Packets arrive in o

34、rder Yes No</p><p>　　Is a switch crash fatal Yes No</p><p>　　Bandwidth available Fixed Dynamic</p><p>　　Time of possible congestion At setup time&

35、#160;On every packet</p><p>　　Potentially wasted bandwidth Yes No</p><p>　　Store-and-forward transmission No Yes</p><p>　　used in detail.</p><p>　　2

36、.6 THE MOBILE TELEPHONE SYSTEM</p><p>　　The traditional telephone system(even if it some day gets multi-gigabit end-to-end fiber) will still not be able to satisfy a growing group of users :people on the go

37、.People now expect to make phone calls from airplanes ,cars ,swimming pools ,and while jogging in the park .Within a few years they will also expect to send e-mail and surf the Web from all these locations and more conse

38、quently ,there is a tremendous amount of interest in wireless telephony .In the following sections we will study </p><p>　　Wireless telephones come in two basic varieties :cordless phones and mobile phones(s

39、ometimes called cell phones),Cordless phones are devices consisting of a base station and a handset sold as a set for use within the home. These are never used for networking ,so we will not examine them further .Instead

40、 we will concentrate on the mobile system ,which is used for wide area voice and data communication.</p><p>　　Mobile phones have gone through three distinct generations ,with different technologies:</p>

41、;<p>　　1. Analog voice</p><p>　　2. Digital voice</p><p>　　3. Digital voice and data(Internet ,e-mail ,etc.).</p><p>　　Although most of our discussion will be a

42、bout the technology of these systems ,it is interesting to note how political and tiny marketing decisions can have a huge impact .The first mobile system was devised in the U.S. by AT&T and mandated for the whole co

43、untry by the FCC .As a result ,the entire U.S. had a single(analog) system and a mobile phone purchased in California also worked in New York .In contrast ,when mobile came to Europe ,every country devised its own syste本

44、文源自六維論文網(wǎng)m ,which result</p><p>　　Europe learned from its mistake and when digital came around, the government-run PTTs got together and standardized on a single system(GSM),so any European mobile phone will

45、word anywhere in Europe .By then ,the U.S. had decided that government should not be in the standardization business ,so it left digital to the marketplace .This decision resulted in different equipment manufacturers pro

46、ducing different kinds of mobile phones .As a consequence ,the U.S. now has two major incompatible digital</p><p>　　Despite an initial lead by the U.S. , mobile phone ownership and usage in Europe is now far

47、 greater than in the U.S. Having a single system for all of Europe is part of the reason ,but there is more .A second area where the U.S. and Europe differed is in the humble matter of phone numbers .In the U.S. mobile p

48、hones are mixed in with regular (fixed) telephones .Thus ,there is no way for a caller to see if ,say ,(212)234-5678 is a fixed telephone (cheap or free call) or a mobile phone (expensive c</p><p>　　pay for

49、incoming calls .As a consequence ,many people hesitated to buy a mobile phone for fear of running up a big bill by just receiving calls .In Europe ,mobile phones have a special area code (analogous to 800 and 900 numbers

50、) so they are instantly recognizable .Consequently , the usual rule of “caller pays” also applies to mobile phones in Europe (except for international calls where costs are split).</p><p>　　A third issue tha

51、t has had a large impact on adoption is the widespread use of prepaid mobile phones in Europe (up to 75% in some areas ).These can be purchased in many stores with no more formality than buying a radio .You pay and you g

52、o .They are preloaded with ,for example ,20 or 50 euro and can be recharged (using a secret PIN code ) when the balance drops to zero . As a consequence , practically every teenager and many small children in Europe have

53、 (usually prepaid ) mobile phones so their </p><p>　　2.6.1 First-Generation Mobile Phones :Analog Voice</p><p>　　Enough about the politics and marketing aspects of mobile phones .Now let us look

54、 at the technology ,starting with the earliest system .Mobile radiotelephones were used sporadically for maritime and military communication during the early decades of the 20th century .In 1946, the first system for car

55、-based telephones was set up in St.Louis .This system used a single large transmitter on top of a tall building and had a single channel ,used for both sending and receiver .Such systems ,programs of</p><p>

56、　　In the 1960s,IMTS(Improved Mobile Telephone System) was installed .It ,too ,used a high-powered (200-watt) transmitter ,on  top of a hill , but now had two frequencies ,one for sending and one for receiving , so t

57、he push-to-talk button was no longer needed . Since all communication from the mobile telephones went inbound on a different channel than the outbound signals ,the mobile users could not hear each other (unlike the push-

58、to-talk system used in taxis).</p><p>　　IMTS supported 23 channel s spread out from 150 MHz to 450 MHz .Due to the small number of channels ,users often had to wait a long time before getting a dial tone .Al