實(shí)驗(yàn)室儀器管理系統(tǒng)外文翻譯-無(wú)憂無(wú)慮畢設(shè)網(wǎng)

無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： 本文配套程序下載地址 ： 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ()-大學(xué)生畢業(yè)設(shè)計(jì)站 ,免費(fèi)畢業(yè)設(shè)計(jì)論文 ,無(wú)憂無(wú)慮畢設(shè)網(wǎng) 大學(xué)生畢業(yè)設(shè)計(jì) ,出售各類畢業(yè)設(shè)計(jì)源碼 ,論文 ,程序源碼 ,網(wǎng)站源碼 ,免費(fèi)視頻教程 ,我們將竭誠(chéng)為您服務(wù)！ 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： 外文資料譯文（一） 電路交換網(wǎng)與 vb 調(diào)用數(shù)據(jù)庫(kù) 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(guò)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(guò)了時(shí)序那它將不能到達(dá)接收端 .由于沒(méi)有路徑用來(lái)進(jìn)行包交換，所以要想 不同的包沿不同的路徑傳送就是能依靠網(wǎng)絡(luò)的條件了。這樣包即便是超過(guò)了時(shí)序它們也有可能到達(dá)的。 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。 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)包到達(dá)時(shí)， 通過(guò) 保留的帶寬 它可以被立即發(fā)送出去。假如沒(méi)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)。 預(yù)先保留帶寬意味著當(dāng)一個(gè)包出現(xiàn)時(shí) 不會(huì)發(fā)生 擁塞 (除非 出現(xiàn)的包比預(yù)期的還 多 )。 在另一方面 ,當(dāng)嘗試建立一條 電 路時(shí) ， 由于擁塞 ， 嘗試 將會(huì) 失敗 。因此，在線路交換 (在設(shè)置時(shí) )和包交換 (在包發(fā)送時(shí) )的不同時(shí)間擁塞都可能發(fā)生。 如果為特定的用戶建立一條線路那么當(dāng)這個(gè)用戶沒(méi)有數(shù)據(jù)傳送時(shí)，則這些保留下來(lái)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(duì)不質(zhì)保服務(wù)和不浪費(fèi)資源的。 包交換是利用存儲(chǔ)器并向前發(fā)送的。包是先存儲(chǔ)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(guò)導(dǎo)線傳輸?shù)?。存?chǔ)轉(zhuǎn)發(fā)技術(shù)增加了延遲時(shí)間。 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(jiǎn)單比喻它們就像一個(gè)是公路一個(gè)是鐵路。在以無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： 前，用戶決定介質(zhì)的大小，速度和特性。而后來(lái)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。 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(guó)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(jiǎn)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(lái)得便宜 )。時(shí)間對(duì)于包交換來(lái)說(shuō)是沒(méi)有問(wèn)題的，但是傳輸流量卻是個(gè)問(wèn)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?duì)較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。 項(xiàng)目 電路交換 包交換 呼叫設(shè)置 需要 不需要 專用物理線路 是 否 每個(gè)包沿相同路徑 是 否 包的到達(dá)中有規(guī)則的 是 否 是否會(huì)致命的崩潰 是 否 帶寬利用 固定 動(dòng)態(tài)分配 可能的時(shí)間堵塞 在設(shè)置時(shí) 在每個(gè)包時(shí) 可能浪費(fèi)帶寬 是 否 以存儲(chǔ)轉(zhuǎn)發(fā)進(jìn)行發(fā)送 否 是 透明性 是 否 收費(fèi) 按分鐘 按包 圖 2 40：比較電路交換網(wǎng)絡(luò)和包交換網(wǎng)絡(luò)的不同 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)。 2.6 移動(dòng)電話系統(tǒng) 傳統(tǒng)的電話的系統(tǒng)將不能滿足用戶的大量增長(zhǎng)（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?chǎng)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)電子郵無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： 件和上網(wǎng)沖浪。因此有很多的人關(guān)心無(wú)線電話技術(shù)。在下面的章節(jié)我們將用詳細(xì)的資料來(lái)學(xué)習(xí)這些主題。 無(wú)線電話可以分為基本的兩種：無(wú)繩電話和移動(dòng)電話（有時(shí)也叫蜂窩電話）。無(wú)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。它們從不被應(yīng)用于網(wǎng)絡(luò)，所以我們沒(méi)有辦法做進(jìn)一步的檢測(cè)。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。 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)： 1 模擬聲音 2 數(shù)字聲音 3 數(shù)字聲音和數(shù)據(jù)（互聯(lián)網(wǎng)，電子郵件， etc.） 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(chǎng)對(duì)它的巨大影響。第一個(gè)移動(dòng)電話系統(tǒng)是在美國(guó)由受美國(guó)國(guó)家聯(lián)邦通訊委員會(huì) (FCC)委托的美國(guó)電話電報(bào)公司 (AT&T)設(shè)計(jì)的。結(jié)果在整個(gè)美國(guó)就有了一個(gè)簡(jiǎn)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)移動(dòng)電話來(lái)到歐洲時(shí)，由于每個(gè)國(guó)家都設(shè)計(jì)了自己的系統(tǒng)，所以結(jié)局很慘敗。 當(dāng)數(shù)字技術(shù)到來(lái)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(jiǎn)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(guó)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(chǎng)。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(guó)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 )。 即使美國(guó)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(guò)美國(guó)的。原因之一就是整個(gè)歐洲擁有相同的簡(jiǎn)單系統(tǒng)。然而更多的是由于美國(guó)和歐洲的基本電話數(shù)量不同。在美國(guó)移動(dòng)電話和固定電話是共用的，因此對(duì)于用戶沒(méi)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(hào) (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(guó)際電話分開收費(fèi)外 )。 在歐洲采用頒布廣泛的已經(jīng)付費(fèi)的移動(dòng)電話的影響巨大 (超過(guò)同地區(qū)的 75%)。無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： 移動(dòng)電話可以在很多的商店里購(gòu)買而不像購(gòu)買收音機(jī)那樣需要很多手續(xù)。誰(shuí)付費(fèi)誰(shuí)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。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。 2.6.1 第一代移動(dòng)電話：模擬聲音 移動(dòng)電話充分體現(xiàn)了政治和市場(chǎng)的特征。讓我們來(lái)看一下它的技術(shù)吧。它是從簡(jiǎn)單的系統(tǒng)開始的。在 20世紀(jì)的初，移動(dòng)無(wú)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，它使用一個(gè)簡(jiǎn)單的大的發(fā)送機(jī)并且有了用于發(fā)送和接收的簡(jiǎn)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(guò)此時(shí)已經(jīng)有了兩各不同的頻率了，一種用來(lái)發(fā)送，另一種用來(lái)接收，所以這個(gè)呼叫按鈕就可不要了。相比信號(hào)出站移動(dòng)電話的信號(hào)入站是通過(guò)不同的通道的，因此移動(dòng)用戶不會(huì)互相聽到 (不像應(yīng)用在出租車上被叫系 統(tǒng) )。 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(zhǎng)時(shí)間才能聽到拔號(hào)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?？偠灾?，這有限的容量使用這系統(tǒng)有點(diǎn)不合實(shí)際應(yīng)用。 VB6.0中的 MSChart 控件是一個(gè)功能強(qiáng)大的高級(jí)圖表工具，擁有豐富的圖表繪制功能，可顯示二維和三維的棒圖、區(qū)域圖、線形圖、餅圖等多種常用圖表。近日我為了在雙擊圖表的某個(gè)區(qū)域時(shí)將與該區(qū)域相對(duì)應(yīng)的數(shù)據(jù)庫(kù)的內(nèi)容在 DataGrid控件上顯示出來(lái)，遇到了許多困 難，最后用一個(gè)自定義的變量 SelectSeries 解決了問(wèn)題，具體方法如下： 設(shè)數(shù)據(jù)庫(kù)名稱為 “ 學(xué)生信息 ” ，有一 Access表 “ 學(xué)生成績(jī) ” ，其內(nèi)容為一個(gè)班學(xué)生的考試成績(jī)，包括學(xué)號(hào)、姓名、成績(jī) 3個(gè)字段，成績(jī)字段格式是字符型，無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： 值為 “ 優(yōu) ” 、 “ 良 ” 、 “ 中 ” 、 “ 差 ” 中的一個(gè)。 窗體 Form1包括一個(gè) MSChart控件 McScore，類型為二維餅圖，用于顯示每種成績(jī)的學(xué)生數(shù)；一個(gè) ADO控件 AdScore 用于連接數(shù)據(jù)庫(kù)；一個(gè) DataGrid控件DgScore用于以表格形式顯示數(shù)據(jù)庫(kù)內(nèi)容。 工作過(guò)程為：雙擊餅圖的 某個(gè)區(qū)域，則 DgScore顯示相應(yīng)成績(jī)的學(xué)生名單。 代碼如下： Option Explicit Dim SelectedSeries as Integer 自定義變量 Dim Rs() as String 提取記錄集用的字符串?dāng)?shù)組 Private Sub Form_Load() 設(shè)定 DataGrid 控件的數(shù)據(jù)源 DgScore.DataSource=AdScore 設(shè)定 ADO控件的連接字串和初始的記錄源，即顯示內(nèi)容 AdScore.ConnectString=Provider=Microsoft.OLEDB.3.51；Persist_Security Info=False；Data Source=學(xué)生信息 AdScore.RecordSource=selet from學(xué)生成績(jī) order by 成績(jī) AdScore.Refresh 預(yù)設(shè)好提取記錄用的 SQL語(yǔ)句 Rs(1)=select from學(xué)生成績(jī) where 成績(jī) = Chr(34) 優(yōu) _Chr(34) order by 成績(jī) Rs(2)=select from學(xué)生成 績(jī) where 成績(jī) = Chr(34) 良 _Chr(34) order by 成績(jī) Rs(3)=select from學(xué)生成績(jī) where成績(jī) = Chr(34) 中 _Chr(34) order by 成績(jī) Rs(4)=select from學(xué)生成績(jī) where 成績(jī) = Chr(34) 差 _Chr(34) order by 成績(jī) End Sub Private Sub McScore_SeriesSelected(Series as Integer,MouseFlags as _Integer,Cancel as Integer) 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： SelectedSeries=Series End Sub Private Sub McScore_PointSelected(Series as Integer,DataPoint as_Integer,MouseFlags as Integer,Cancel as Integer) SelectedSeries=Series End Sub Private Sub McScore_Db1Click() 改變 ADO控件的 記錄源并刷新 Adoc1.RecordSource=Rs(SeletedSeries) Adoc1.Refresh End Sub - VB做為快速應(yīng)用開發(fā)（ RAD）工具越來(lái)越得到開發(fā)人員的認(rèn)可和接受。它對(duì)許多 API（如 ODBC API、 SOCKET API等等）的封裝使得編程變得簡(jiǎn)單起來(lái)。同時(shí)，它支持集成開發(fā)環(huán)境下的可視化、事件驅(qū)動(dòng)、面向?qū)ο蟮染幊烫攸c(diǎn)。下面，我們談?wù)勗?VB中調(diào)用存儲(chǔ)過(guò)程的實(shí)現(xiàn)方法及其注意事項(xiàng)。 - 我們知道， VB的數(shù)據(jù)庫(kù)編程有許多種方法，比如直接用 ODBC API編程，這種方法靈活、高效，程序員可以實(shí)現(xiàn)對(duì)數(shù)據(jù)庫(kù)復(fù)雜的控制；也可以用 VB中的數(shù)據(jù)對(duì)象，如 RDO（遠(yuǎn)程數(shù)據(jù)對(duì)象）、 DAO（數(shù)據(jù)訪問(wèn)對(duì)象）、 ADO（ ActiveX 數(shù)據(jù)對(duì)象），這種方法實(shí)現(xiàn)起來(lái)方便、快捷，但靈活性較差一些。由于存儲(chǔ)過(guò)程在實(shí)現(xiàn)數(shù)據(jù)封裝、隱藏以及代碼的預(yù)編譯、減少網(wǎng)絡(luò)負(fù)載、維護(hù)方便等優(yōu)點(diǎn)，所以被許多 RDBMS和編程工具做支持。 VB中的各類數(shù)據(jù)對(duì)象也提供對(duì)存儲(chǔ)過(guò)程的支持。 - 我們以 ADO 為例來(lái)說(shuō)明其實(shí)現(xiàn)的步驟 - 1 創(chuàng)建、調(diào)試存儲(chǔ)過(guò)程。你可以在數(shù)據(jù)庫(kù)中也可以在其他外掛程 序的支持下進(jìn)行存儲(chǔ)過(guò)程的創(chuàng)建和調(diào)試工作。本例中的存儲(chǔ)過(guò)程代碼如下（使用 PUBS的MS SQL中的例子庫(kù) ）： 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： CREATE PROCEDURE myprocedure job_id smallint, job_lvl tinyint AS SELECT * FROM employee WHERE job_id job_lvl - 2 在 VB中生成一個(gè)新的工程，工程有一窗體，一個(gè) COMMAND（ NAME： COMMAND1） 按鈕，一個(gè) MSFlexGrid（ NAME： MSFlexGrid1）控件。 - 3 創(chuàng)建連接 ADO connection； - 4 創(chuàng)建命令 ADO command； - 5 創(chuàng)建參數(shù)并設(shè)置各個(gè)參數(shù)的屬性； - 6 執(zhí)行 ADO command； - 7 對(duì)數(shù)據(jù)進(jìn)行處理； MSFlexGrid 顯示查詢到的數(shù)據(jù) - 8 釋放連接，退出程序。 - 其中代碼如下： 在窗體中聲明以下變量： Dim cnn1 As ADODB.Connection 連 接 Dim mycommand As ADODB.Command 命令 Dim parm_jobid As ADODB.Parameter 參數(shù) 1 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： Dim parm_joblvl As ADODB.Parameter 參數(shù) 2 Dim rstByQuery As ADODB.Recordset 結(jié)果集 Dim strCnn As String 連接字符串 在窗體的 LOAD事件中加入如下代碼： Set cnn1 = New ADODB.Connection 生成一個(gè)連接 strCnn = DSN=MYDSN；uid=sa；pwd= 創(chuàng)建的系統(tǒng)數(shù)據(jù)源 MYDSN指向 PUBS數(shù)據(jù)庫(kù) cnn1.Open strCnn 打開連接 在窗體的 UNLOAD 中的加入代碼如下： cnn1.Close 關(guān)閉連接 Set cnn1 = Nothing 釋放連接 在按鈕中的代碼如下： Dim i As integer Dim j as integer Set parm_jobid = New ADODB.Parameter Set mycommand = New ADODB.Command parm_jobid.Name = name1 this line can be ommited parm_jobid.Type = adInteger 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： 外 文 原 文（二） 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： Electric circuit exchange network and vb transfer database The result of the connection setup 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 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. 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. With Packet switching, packets can be routed around dead switches. Setting up a path in advance also opens up the possibility of reserving bandwidth in advance. If bandwidth is reserved, then when a packet arrives, it can be sent out immediately over the reserved bandwidth. With packet switching, no bandwidth is reserved, so packets may have to wait their turn to be forwarded. Having bandwidth reserved in advance means that no congestion can occur when a packet shows up (unless more packets 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) and packet switching(when packets are sent). 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 other traffic. Packet switching does not waste bandwidth and thus is more efficient form a system-wide perspective. Understanding this 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： trade-off is crucial for comprehending the difference between circuit switching and packet switching. The trade-off is between guaranteed service and wasting resources versus not guaranteeing service and not wasting resources. Packet switching uses store-and-forward transmission. A packet is accumulated. 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. Another difference 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 determines 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 voice, data, and fax to coexist within the phone system. A final difference between circuit and packet switching is the charging algorithm. With circuit switching, charging has historically been based on distance 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 minutes and sometimes night or weekend calls are cheaper than normal).With packet switching, connect time is not an issue, but the volume of traffic sometimes is. For home users, ISPs usually charge a flat monthly rate because it is less work for them and their customers can understand this model easily, but backbone carriers charge regional networks based on the volume of their traffic. The differences are summarized in Fig.2-40. 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： Item Circuit Switched Packet Switched Call setup Required Not needed Dedicated physical path Yes No Each packet follows the same route Yes No Packets arrive in order Yes No Is a switch crash fatal Yes No Bandwidth available Fixed Dynamic Time of possible congestion At setup time On every packet Potentially wasted bandwidth Yes No Store-and-forward transmission No Yes Transparency Yes No Charging Per minute Per packet Both circuit switching and packet switching are important enough that we will come back to them shortly and describe the various technologies used in detail. 2.6 THE MOBILE TELEPHONE SYSTEM 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 .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 consequently ,there is a tremendous amount of interest in wireless telephony .In the following sections we will study this topic in some detail. 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： Wireless telephones come in two basic varieties :cordless phones and mobile phones(sometimes 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 we will concentrate on the mobile system ,which is used for wide area voice and data communication. Mobile phones have gone through three distinct generations ,with different technologies: 1. Analog voice 2. Digital voice 3. Digital voice and data(Internet ,e-mail ,etc.). Although most of our discussion will be about 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 country 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 system ,which resulted in a fiasco. 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 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 producing different kinds of mobile phones .As a consequence ,the U.S. now has two major incompatible digital mobile phone systems in operation(plus one minor one). Despite an initial lead by the U.S. , mobile phone ownership and usage in Europe is now far greater than in the U.S. Having a single system for 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： 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 phones 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 call).To keep people from getting nervous about using the telephone ,the telephone companies decided to make the mobile phone owner pay for 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) 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). A third issue that 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 go .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 (usually prepaid ) mobile phones so their parents can locate them ,without the danger of the child running up a huge bill .If the mobile phone is used only occasionally ,its use is essentially free since there is no monthly charge or charge for incoming calls . 2.6.1 First-Generation Mobile Phones :Analog Voice Enough about the politics and marketing aspects of mobile phones .Now let us look 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-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 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： channel ,used for both sending and receiver .Such systems ,known as push-to-talk systems ,were installed in several cities beginning in the late 1950s.CB-radio,taxis,and police cars on television programs often use this technology. 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 the 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-to-talk system used in taxis). IMTS supported 23 channels 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 .Also ,due to the large power of the hilltop transmitter ,adjacent systems had to be several hundred kilometers apart to avoid interference . All in all ,the limited capacity made the system impractical. In VB6.0 MSChart controls is one function formidable high-level graph tool, has the rich graph plan function, may demonstrate two-dimensional and the three dimensional good chart, the linear chart, the cake chart and so on the many kinds of commonly used graph. Recently I in order to in double struck the graph when some region will the database content which corresponded with this region controlled on in DataGrid demonstrates, encountered many difficulties, finally used one to solve the problem from definition variable SelectSeries, concrete method as follows: Supposes the database name is the student information , has one Access table the student result , its content is one class students test result, includes the student number, the name, the result 3 fields, the result field form is the character, the value is is superior , is good , center , the difference center one. 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： Window Form1 includes one MSChart controls MC$score, the type for the two-dimensional cake chart, uses in to demonstrate each kind of result the student counts； One ADO controls AdScore uses in to connect the database； One DataGrid controls DgScore uses in by the form form demonstration database content. The work process is: Double strikes the cake chart some region, then DgScore demonstrates the corresponding result the student name list. Code as follows: Option Explicit Dim SelectedSeries as Integer from definition variable Dim Rs () as String withdraws the character string array which the record compendium uses Private Sub Form_Load () Establishes DataGrid controls the data pool DgScore.DataSource= AdScore Establishes ADO controls the connection character string and the initial record source, namely demonstration content AdScore.ConnectString= Provider=Microsoftc.oledb.3.51； Persist_Security Info=False； Data Source= student information AdScore.RecordSource= selet * from student result order by result AdScore.Refresh Supposes in advance withdraws the SQL sentence which the recording uses Rs (1) = select * from the student result where result = + Chr (34) + is superior + _Chr (34) + order the by result Rs (2) = select * from the student result where result = + Chr (34) + is good + _Chr (34) + order the by result Rs (3) = select * from student result where result = + Chr (34) + center + _Chr (34) + order by result Rs (4) = select * from student result where result = + Chr (34) + difference _Chr (34) + order by result 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： End Sub Private Sub MC$score_SeriesSelected (Series as Integer, MouseFlags as _Integer, Cancel as Integer) SelectedSeries=Series End Sub Private Sub MC$score_PointSelected (Series as Integer, DataPoint as_Integer, MouseFlags as Integer, Cancel as Integer) SelectedSeries=Series End Sub Private Sub MC$score_Db1Click () Changes ADO controls the record source and renovates Adoc1.RecordSource=Rs (SeletedSeries) Adoc1.Refresh End Sub - VB does for the rapid application development (RAD) the tool more and more obtains the development personnels approval and accepts. It to many API (for example ODBC API, SOCKET API and so on) the seal causes the programming to change is simple. At the same time, it supports under the integrated development environment visible, the event actuates, to be object-oriented and so on the programming characteristic. Under, we chat in VB transfer the memory process the realization method and its the matters needing attention. - We know, the VB database programming has permits the many kinds of method, for instance straight takes over the use of ODBC the API programming, this method is flexible, is highly effective, the programmer may realize to the database complex control； Also may use in VB the data object, like RDO (long-distance data object), DAO (data visit object), ADO (ActiveX data object), this method realizes conveniently, quickly, but 無(wú)憂無(wú)慮畢設(shè)網(wǎng) ():畢業(yè)設(shè)計(jì)源碼下載 畢業(yè)設(shè)計(jì)源碼下載： www.