外文資料翻譯---網(wǎng)絡(luò)課堂一個(gè)大型互動(dòng)遠(yuǎn)程教學(xué)平臺(tái)

1、<p>　　網(wǎng)絡(luò)課堂：一個(gè)大型互動(dòng)遠(yuǎn)程教學(xué)平臺(tái)</p><p>　　林，SW1，程，K L2，王，R3，周，H4和H5陳，S</p><p>　　1cs_lswaa@stu.ust.hk</p><p>　　2cs_cklaa@stu.ust.hk</p><p>　　3cs_wrx@stu.ust.hk</p>&l

2、t;p>　　4cs_zhxaa@stu.ust.hk</p><p>　　5gchan@cs.ust.hk</p><p>　　香港科技大學(xué)計(jì)算機(jī)科學(xué)系</p><p><b>　　摘要：</b></p><p>　　傳統(tǒng)上，為了與教員進(jìn)行更好的交流互動(dòng)，講座一般在學(xué)生所處的教室進(jìn)行。這種模式對(duì)于有些學(xué)生并不是很方

3、便和劃算，例如對(duì)于那些正在工作的、行動(dòng)不便的、生活在偏遠(yuǎn)地區(qū)的，或者類似在SARS期間被社會(huì)隔離的人來說。</p><p>　　伴隨著寬帶互聯(lián)網(wǎng)連接和無(wú)線媒體 （Wi-fi 和 3G） 的普遍滲透，我們開發(fā)了一個(gè)平臺(tái)，讓講座可以通過互聯(lián)網(wǎng)以交互方式進(jìn)行。該項(xiàng)目被稱為 “網(wǎng)絡(luò)課堂”，提供了一種類似于現(xiàn)今的傳統(tǒng)教室，但學(xué)生們分布在互聯(lián)網(wǎng)的教學(xué)體驗(yàn)。學(xué)生可以在任何時(shí)間任何地點(diǎn)使用他們的 PC 或便攜式計(jì)算機(jī)通過互聯(lián)網(wǎng)和

4、無(wú)線媒體向他們的導(dǎo)師提問題。視頻、音頻、以及白板也流向?qū)崟r(shí)的用戶終端。利用現(xiàn)有現(xiàn)成的計(jì)算產(chǎn)品和最先進(jìn)的網(wǎng)絡(luò)技術(shù)，該系統(tǒng)可擴(kuò)展到數(shù)百名學(xué)生。它具有成本效益，能夠有效地突破地域限制讓學(xué)生來聽取講座。使用該系統(tǒng)的學(xué)生傳來的最初反饋是積極和鼓舞人心的。</p><p><b>　　關(guān)鍵詞：</b></p><p>　　遠(yuǎn)距離學(xué)習(xí)，網(wǎng)絡(luò)課堂，互動(dòng)、 教育、 分布式系統(tǒng)、 多媒體

5、應(yīng)用和通信網(wǎng)</p><p><b>　　引言</b></p><p>　　隨著計(jì)算設(shè)備和計(jì)算機(jī)使用的巨大增長(zhǎng)，人與人之間的地理距離已顯著減少。簡(jiǎn)單舉幾個(gè)目前正在使用的互聯(lián)網(wǎng)軟件的例子：網(wǎng)絡(luò)會(huì)議，網(wǎng)絡(luò)電話，ICQ聊天等等。利用這些，人們可以在世界的各個(gè)角落，與他人進(jìn)行比以往任何時(shí)候都更經(jīng)濟(jì)、更有效地溝通。</p><p>　　近來，香港政府一直

6、在努力推行“終身學(xué)習(xí)”的理念。然而，在香港的工作時(shí)間和生活節(jié)奏，讓人們幾乎不可能在一個(gè)固定的時(shí)間，固定的地點(diǎn)進(jìn)行學(xué)習(xí)。在香港科技大學(xué)計(jì)算機(jī)科學(xué)系,我們已經(jīng)開發(fā)了一個(gè)名為“網(wǎng)絡(luò)課堂"的交互式遠(yuǎn)程學(xué)習(xí)平臺(tái)。該項(xiàng)目旨在為廣大學(xué)生提供現(xiàn)場(chǎng)講座。它允許學(xué)生在任何時(shí)間任何地點(diǎn)通過任何一臺(tái)計(jì)算機(jī)來參加課程。</p><p>　　我們?cè)谙聢D展示所開發(fā)的系統(tǒng)。它由學(xué)生的分布組成網(wǎng)絡(luò)。教授可以通過互聯(lián)網(wǎng)提供他/她的講座錄音

7、和課件，也可以通過輪詢與學(xué)生進(jìn)行交流。</p><p>　　該平臺(tái)有以下幾個(gè)獨(dú)特的功能和技術(shù)項(xiàng)目：</p><p>　　網(wǎng)絡(luò)模塊：我們?cè)O(shè)計(jì)并實(shí)現(xiàn)了一個(gè)傳輸機(jī)制，以便容納成千上萬(wàn)沒有強(qiáng)大的服務(wù)器的學(xué)生和客戶的需要。我們的網(wǎng)絡(luò)協(xié)議在最近召開的重大會(huì)議和研討會(huì)上得以展示，并引起了廣泛的關(guān)注。</p><p>　　2、音頻模塊：在該模塊中音頻能夠被很好地傳遞給學(xué)生。在傳遞過

8、程中，我們使用了一個(gè)高度壓縮的音頻，并不會(huì)占用大量的網(wǎng)絡(luò)帶寬，使用的編碼標(biāo)準(zhǔn)稱為G.723。</p><p>　　3、視頻模塊：我們已經(jīng)實(shí)現(xiàn)了最新的H.264視頻壓縮編碼技術(shù)。它在保證了視頻的高質(zhì)量的同時(shí)，對(duì)于網(wǎng)絡(luò)和系統(tǒng)資源的使用顯著減少了，也因此，這個(gè)軟件的帶寬的要求大大降低。</p><p>　　4、網(wǎng)絡(luò)接口：這一部分的重點(diǎn)是支持一個(gè)網(wǎng)絡(luò)接口。有了這項(xiàng)功能，學(xué)生就可以在任何能找到一臺(tái)計(jì)

9、算機(jī)上網(wǎng)的地方學(xué)習(xí)。為了進(jìn)一步提高用戶友好性，網(wǎng)絡(luò)課堂項(xiàng)目還提供安裝文件。根據(jù)指導(dǎo)進(jìn)行簡(jiǎn)單的鼠標(biāo)點(diǎn)擊使安裝更方便。完全無(wú)需編譯或鏈接，而這在去年的項(xiàng)目里是必要的。</p><p>　　下面，我們來詳細(xì)解釋一下模塊。</p><p><b>　　1、網(wǎng)絡(luò)模塊</b></p><p>　　由于網(wǎng)絡(luò)課堂屬于實(shí)時(shí)應(yīng)用，并且聽課人的數(shù)量通常都是很大的，因

10、此穩(wěn)定的和可擴(kuò)展的網(wǎng)絡(luò)層顯得極為重要。網(wǎng)絡(luò)的主要功能是將演講材料，即，視頻和音頻，分發(fā)給所有參加講座的學(xué)生。這項(xiàng)技術(shù)被稱為多播。</p><p>　　傳統(tǒng)上，多播有兩種方式：應(yīng)用層組播（ALM）和IP組播。應(yīng)用層組播，組播是沿節(jié)點(diǎn)/用戶邏輯樹序列的單播來做的。它是用來解決IP組播的適應(yīng)性問題。由于數(shù)據(jù)向一組ALM主機(jī)的傳輸是通過如TCP和UDP之類的單播協(xié)議來完成的，它支持互聯(lián)網(wǎng)上的任何路由器。然而，所有的數(shù)據(jù)傳

11、輸是通過單播，冗余傳輸，從而導(dǎo)致帶寬利用效率低下，這將成為層組播使用的主要問題。相比之下，利用IP組播技術(shù)，數(shù)據(jù)只需發(fā)送一次，同一組內(nèi)的每個(gè)主機(jī)都能收到它。與多播通信相比，IP組播能夠更有效的消除冗余傳輸，節(jié)省帶寬。在ALM技術(shù)和IP組播技術(shù)的基礎(chǔ)上，我們?cè)O(shè)計(jì)并實(shí)現(xiàn)了一個(gè)新的架構(gòu)，將其命名為“島組播（IM）”。</p><p>　　島組播，即課堂上教師與學(xué)生，分成幾個(gè)較小的多播能力的地區(qū)（或者“島”）。當(dāng)所有這些

12、層組播（ALM）協(xié)議在同一個(gè)島上，就使用IP 組播。島組播（IM）是一個(gè)通用的應(yīng)用層框架，它適用于任何層組播協(xié)議。對(duì)于網(wǎng)絡(luò)課堂項(xiàng)目，我們決定再次使用ALMI為島間通信。</p><p><b>　　島組播</b></p><p>　　采用這種結(jié)構(gòu)，會(huì)話中的成員可以是一個(gè)父節(jié)點(diǎn)的其他成員，這需要將所有的數(shù)據(jù)包從鄰居節(jié)點(diǎn)傳送到到其他所有的鄰居。這種對(duì)等結(jié)構(gòu)使系統(tǒng)在多播期也

13、能保證信息的高效傳播。這種結(jié)構(gòu)的主要問題是數(shù)據(jù)傳輸在很大程度上取決于內(nèi)的所有節(jié)點(diǎn)的可靠性。如果一個(gè)內(nèi)節(jié)點(diǎn)突然斷開，那么所有的下游節(jié)點(diǎn)也將在一秒鐘內(nèi)從主樹中斷開。為了解決這個(gè)問題，我們將“代理服務(wù)器”引入系統(tǒng)之中。代理服務(wù)器必須高度可靠，并且有IM網(wǎng)絡(luò)運(yùn)營(yíng)商建立。這是一個(gè)簡(jiǎn)單的構(gòu)件，提供許多其他成員。系統(tǒng)中的代理服務(wù)器越多，那么它所提供的網(wǎng)絡(luò)就就越穩(wěn)定，越可靠。</p><p><b>　　音頻模塊<

14、;/b></p><p>　　微軟舉，在Windows平臺(tái)上進(jìn)行多媒體應(yīng)用程序開發(fā)的一個(gè)公認(rèn)的標(biāo)準(zhǔn)，用于在網(wǎng)絡(luò)課堂的桌面版操作：重新編碼，音頻壓縮，解壓縮，傳輸和播放。（圖1和圖2。）。</p><p>　　音頻傳輸實(shí)時(shí)流媒體技術(shù)應(yīng)用于音頻傳輸。當(dāng)一個(gè)人說，所有的用戶應(yīng)該能夠聽到。音頻傳輸是一對(duì)多的。實(shí)時(shí)音頻的時(shí)間敏感，需要低延遲。同時(shí)實(shí)時(shí)音頻可以忍受一些損失。因此，播放音頻的最合適

15、的方法是使用傳輸層協(xié)議。</p><p>　　圖1：音頻流設(shè)計(jì)（教授方面）</p><p>　　圖2：音頻流設(shè)計(jì)（學(xué)生方面）</p><p>　　帶寬是互聯(lián)網(wǎng)上最有價(jià)值的資源，通過音頻壓縮帶寬是該應(yīng)用程序的主要目的。這個(gè)應(yīng)用程序是專為數(shù)以百計(jì)的學(xué)生同時(shí)使用而開發(fā)，所需的帶寬是如此之大，以致于網(wǎng)絡(luò)無(wú)法支持它。因此，音頻壓縮是必不可少的。在這個(gè)項(xiàng)目中我們采用G.723.

16、1。G.723.1語(yǔ)音壓縮算法是一個(gè)標(biāo)準(zhǔn)的ITU。G.723.1在5.3和6.3千位/秒的DUL編碼速率。該聲碼器信號(hào)處理30毫秒的幀和低失真。該算法的輸入/輸出是16位線性PCM樣品。G.723.1語(yǔ)音聲碼器中的比特率提能夠供任何當(dāng)前ITU標(biāo)準(zhǔn)最高的壓縮比，而不影響語(yǔ)音質(zhì)量。此外，我們的聲碼器的編碼延遲是非常低的（小于0.5秒）。</p><p><b>　　視頻模塊</b></p&

17、gt;<p>　　有許多不同類型的視頻壓縮編碼標(biāo)準(zhǔn)，如H.261，H.263，MPEG4。他們是被一些國(guó)際機(jī)構(gòu)開發(fā)的。在這個(gè)項(xiàng)目中，新的視頻壓縮標(biāo)準(zhǔn)H.264標(biāo)準(zhǔn)提供了一個(gè)更大的壓縮率，大大降低了網(wǎng)絡(luò)資源的利用。</p><p>　　比較不同的視頻編碼標(biāo)準(zhǔn)下比特率的PSNR</p><p>　　工作的實(shí)施是基于專家設(shè)計(jì)的標(biāo)準(zhǔn)H264 ITU下的軟件JM 5.0（最新版本7.6

18、）。加快視頻壓縮，修改也將是基于JM 5.0現(xiàn)有的代碼。</p><p>　　對(duì)原有的H.264編解碼器各部分的時(shí)間消耗</p><p>　　加快視頻編解碼器從在<時(shí)間定時(shí)器H>功能調(diào)查計(jì)算瓶頸的過程。從下面的數(shù)字，可以看出整像素的幀間預(yù)測(cè)需要的計(jì)算能力。</p><p>　　加快整像素的幀間預(yù)測(cè)。為了提高編碼速度，國(guó)際搜索模式禁用其他16×1

19、6忽略低效的編碼處理。同時(shí)，一個(gè)新的算法應(yīng)用于運(yùn)動(dòng)估計(jì)。</p><p>　　在許多快速搜索算法的運(yùn)動(dòng)估計(jì),預(yù)測(cè)運(yùn)動(dòng)矢量場(chǎng)自適應(yīng)搜索技術(shù)(PMVFAST)算法,現(xiàn)在基于前者M(jìn)VFAST算法改進(jìn),實(shí)現(xiàn)最佳性能方面和加速比的PSNR值減少大多數(shù)搜索點(diǎn),這些有非常小的概率,當(dāng)前塊最后引用MMX技術(shù)也被應(yīng)用于加速各種耗時(shí)的計(jì)算如絕對(duì)差之和（SAD）和YUV到RGB的轉(zhuǎn)換等。</p><p>　　原

20、來的H.264編碼器和解碼器是使用文件輸入和文件輸出。在這個(gè)項(xiàng)目中，H264編碼器和解碼器是處理文件輸入和文件輸出。在這個(gè)項(xiàng)目中，H.264編解碼器必須進(jìn)行修改，使其能為網(wǎng)絡(luò)攝像頭和網(wǎng)絡(luò)提供必要的接口。隨著JM軟件用C實(shí)現(xiàn)，它必須在C和C++之間建立靜態(tài)庫(kù)來保持的編解碼速度。靜態(tài)庫(kù)的使用也可以更容易的修改和升級(jí)。</p><p>　　下面的圖顯示了在4階段的改進(jìn)速度的優(yōu)化結(jié)果。</p><p&

21、gt;<b>　　階段1：原始</b></p><p><b>　　階段2：配置修改</b></p><p><b>　　階段3：運(yùn)動(dòng)</b></p><p><b>　　階段4：運(yùn)動(dòng)技術(shù)</b></p><p>　　在壓縮和解壓縮速度，視頻質(zhì)量也在網(wǎng)絡(luò)課

22、堂項(xiàng)目的極大關(guān)注。拍攝的圖片說明了視頻質(zhì)量的測(cè)試：</p><p>　　原foreman.qcif序列（左）和解碼序列（右）</p><p>　　原container.qcif序列（左）和解碼序列（右）</p><p>　　可以從第一個(gè)看到第一個(gè)圖，因?yàn)樵诰幋a器進(jìn)行量化，工頭的臉模糊。出于同樣的原因，在湖的表面的波變得不明顯。然而，該框架在整體上和他們的質(zhì)量是可以接

23、受的。這個(gè)項(xiàng)目給予高壓縮率仍然是明確的。</p><p><b>　　網(wǎng)絡(luò)接口</b></p><p>　　學(xué)生可以參加現(xiàn)場(chǎng)講座通過公共計(jì)算機(jī)或計(jì)算機(jī)操作系統(tǒng)比Windows更可能發(fā)生。要解決這些問題，開發(fā)了一個(gè)Web界面。Web接口包含一個(gè)主頁(yè)，BBS系統(tǒng)，部分音頻和部分視頻。</p><p>　　網(wǎng)頁(yè)是由Dreamweaver MX，F(xiàn)la

24、sh MX開發(fā)的。用戶可以找到所有的一般信息的有關(guān)項(xiàng)目和文件。</p><p><b>　　主頁(yè)</b></p><p>　　BBS系統(tǒng)是在互聯(lián)網(wǎng)上廣泛使用的在線討論。所有的用戶都可以通過發(fā)布和回答后互相溝通。通過提供免費(fèi)BBS系統(tǒng)源碼的6K集團(tuán)，我們使用ASP建立了自己的BBS系統(tǒng)。</p><p><b>　　BBS</b&

25、gt;</p><p>　　幾種不同的實(shí)現(xiàn)可以考慮使用視頻和音頻部分。但我們的軟件使用特定的視頻和音頻標(biāo)準(zhǔn)（H. 264，G.723），因此很難用java寫的匹配的解碼器，我們選擇了Windows Media系列Windows Media編碼器9.0的視頻和音頻編碼器和媒體流可以通過Windows媒體播放器輕松解碼。</p><p><b>　　視頻播放網(wǎng)頁(yè)</b>&

26、lt;/p><p><b>　　只有音頻</b></p><p>　　很長(zhǎng)一段時(shí)間過去了，溝通問題在不同地區(qū)和國(guó)家之間都受到極大關(guān)注，特別是那些在地理上的障礙和距離使得傳統(tǒng)的面對(duì)面教授是不可能的?，F(xiàn)代技術(shù)和互聯(lián)網(wǎng)已經(jīng)徹底改變了遠(yuǎn)程通訊，因此遠(yuǎn)程學(xué)習(xí)的概念已經(jīng)能夠?qū)崿F(xiàn)。</p><p>　　網(wǎng)絡(luò)課堂提供了很大的靈活性：任何人誰(shuí)有權(quán)訪問互聯(lián)網(wǎng)都可以生活

27、在遠(yuǎn)程教育中。教授們可以進(jìn)行講座就像在傳統(tǒng)的教室，有輕微的變化，利用功能如PRS，白板，視頻和音頻傳輸。</p><p><b>　　結(jié)語(yǔ)</b></p><p>　　和傳統(tǒng)的課程交付相比，網(wǎng)絡(luò)課堂具有幾乎無(wú)限的班級(jí)大小，以及在時(shí)間和地點(diǎn)上有更大的靈活性。去年，非典的爆發(fā)導(dǎo)致停課?！熬W(wǎng)絡(luò)課堂”，學(xué)生可以安全的、有效的在家參加講座，教學(xué)調(diào)度不需要中斷。</p>

28、;<p>　　今天的因特網(wǎng)是組播的“島嶼”，不能由組播路由器互連。為了使全球多播組播功能有效，應(yīng)在一個(gè)島上使用組播功能，島嶼間的單播連接的。在我們的項(xiàng)目中，我們使用了一個(gè)應(yīng)用層組播框架，稱為島組播（IM），其中組播為兩層組織：在上層島間覆蓋建立的同時(shí)，IP組播技術(shù)在較低的水平內(nèi)島應(yīng)用。連同IP組播可擴(kuò)展性增加。這允許一個(gè)更大的潛在的學(xué)生，因此網(wǎng)絡(luò)課堂提供傳統(tǒng)教學(xué)的優(yōu)勢(shì)和幾乎無(wú)限的可供選擇的班級(jí)規(guī)模。</p>

29、<p>　　在音頻部分，壓縮算法G.723.1是用來轉(zhuǎn)換成IP數(shù)據(jù)包的演講。G.723.1在提供比當(dāng)前任何ITU標(biāo)準(zhǔn)都高的壓縮比的前提下，盡管巨大的壓縮，仍然保留非常高的聲音質(zhì)量。在實(shí)踐中，它幾乎不可能在不同IP間檢測(cè)聲音的語(yǔ)音質(zhì)量之間的差異。我們可以實(shí)現(xiàn)延遲?。ㄐ∮?.5秒）并且好的音頻質(zhì)量。</p><p>　　該網(wǎng)絡(luò)課堂項(xiàng)目實(shí)現(xiàn)了H.264標(biāo)準(zhǔn)的視頻編碼和解碼。它提供了更好的視頻質(zhì)量的同時(shí)大大減少

30、了視頻比特率。在有限的帶寬資源下，給我們一個(gè)流暢的視頻流。</p><p>　　至于微軟的安裝，安裝使用（MSI）幫助項(xiàng)目提供了一個(gè)新的特征，這是不難實(shí)現(xiàn)，但很有幫助。它減少了在編譯源和設(shè)置軟件造成的不必要的誤差，同時(shí)使我們的產(chǎn)品更加人性化。為了網(wǎng)絡(luò)接口，一些技術(shù)和軟件被使用，如Macromedia Dreamweaver是為了網(wǎng)頁(yè)開發(fā)；ASP編程建立的BBS系統(tǒng)和Windows Media編碼器9系列解碼和播放

31、多媒體流。</p><p><b>　　參考文獻(xiàn)</b></p><p>　　K. -w.cheuk，S. H. Chan和J.李；組播島：IP組播和應(yīng)用層組播相結(jié)合。IEEE國(guó)際會(huì)議上交流，出現(xiàn)。</p><p>　　pendarakis，Dimitrios，sherlia石，Dinesh Verma，馬塞爾瓦爾德福格爾：ALMI：應(yīng)用層組播

32、的基礎(chǔ)設(shè)施，2001</p><p>　　tourapis，亞歷克西斯M.，奧斯卡C.金，明湖劉：預(yù)測(cè)運(yùn)動(dòng)矢量場(chǎng)自適應(yīng)搜索技術(shù)（PMVFAST）提高基于運(yùn)動(dòng)估計(jì)塊。</p><p>　　附件：外文原文（復(fù)印件）</p><p>　　Cyberclassroom : A Large-Scale Interactive </p><p>　　Di

33、stance-Learning Platform</p><p>　　Lin，SW1,Cheng,K L2,Wang, R3,Zhou, H4 and Chan, S H5</p><p>　　1cs_lswaa@stu.ust.hk </p><p>　　2cs_cklaa@stu.ust.hk</p><p>　　3cs_wrx@stu.

34、ust.hk</p><p>　　4cs_zhxaa@stu.ust.hk</p><p>　　5gchan@cs.ust.hk</p><p>　　Department of Computer Science</p><p>　　The Hong Kong University of Science and Technology</

35、p><p><b>　　ABSTRACT</b></p><p>　　Traditionally, lectures are held in classrooms where students are co-located in order to interact with the instructors. This model is not convenient or

36、cost-effective for some students, e.g., those who are working, have access/mobility difficulties, live in remote areas, or experience some social isolation such as during the SARS period.</p><p>　　With the p

37、ervasive penetration of broadband Internet connections and wireless medium (Wi-Fi and 3G ), we have developed a platform so that lectures can be conducted interactively over the Internet. The project, termed 'CyberC

38、lassroom ,' offers an experience similar to the traditional classroom today but with the students distributed in the Internet. Students anywhere may raise questions to their instructors at any time using their PCs or

39、 laptops over the Internet and wireless medium. Video, aud</p><p>　　Key words </p><p>　　Distance-learning, cyberclassroom, interactive, education, distributed,system, multimedia applications and

40、 communication networks</p><p>　　INTRODUCTION</p><p>　　With the tremendous growth in computing devices and computer usage, geographical distances between people have been significantly reduced.

41、It is easy to name a few examples of Internet software that are currently in use: Net Meeting ,Internet telephony, ICQ, and so on.Using these,people in different pares of the world can communicate with each other more ec

42、onomically and efficiently than ever before.</p><p>　　Recently, the Hong Kong Government has been promoting and putting great effort into the idea of ' life-long learning '.However, working schedules

43、 and the pace of life in Hong Kong make it nearly impossible for people to learm at a fixed time in a fixed place. In the HKUST Computer Science Department，we have been developing a ' cyberclassroom ' platform fo

44、r interactive distance-learning. This project aims at delivering live lectures to large groups of students. It allows students to participate i</p><p>　　We show in the following figure the system we develope

45、d. It consists of students distributed in the network. The professor may deliver his/her lectures via the Internet with audiovisual streams and Power Point. He/She may also interact with the students through polling.<

46、/p><p>　　There are several unique features and technologies in the project:</p><p>　　Network module: We have designed and implemented a delivery mechanism so as to accommodate hundreds to thousands

47、 of students without the need for powerful servers and clients. Our network protocol has been presented in major conferences and seminars and has received wide interest.</p><p>　　Audio module: The audio has

48、to be delivered to the students in good quality. We have used a highly compressed audio which does not take up much network bandwidth. The encoding standard we use is called G.723.</p><p>　　Video module: We

49、have implemented the most recent encoding technology H.264, for video compression. It guarantees good quality of video while greatly minimizing the use of network and system resources. As a result, bandwidth requirement

50、 for this software is greatly reduced.</p><p>　　Web interface: The focus of this part is to support a web interface. With this feature, students can have lessons anywhere they can find a computer with acces

51、s to the web. To further increase user-friendliness, the CyberClassroom project also provides installation files. Simple mouse clicks according to the guidelines make installation much more convenient. No compilation or

52、linking, as required in last year's project, is necessary.</p><p>　　In the following, we explain the modules in detail.</p><p>　　NETWORK MODULE</p><p>　　Since the Cyberclassroom

53、 is a real-time application and the number of people attending a class is usually quite large, a stable and scalable network layer is important. The main function of the network is to distribute the lecture materials, n

54、amely, video and audio, to all students attending the lecture. This technique is called multicasting.</p><p>　　Traditionally, there are two ways of multicasting. Application Layer Multicast(ALM) and IP-mult

55、icast. In ALM, the multicast is done by a sequence of unicasts along a logical tree of nodes/users. It is used to solve the adaptability problem of IP-multicasting. Since the transmission of the data to a group of ALM ho

56、sts is done by unicast protocols such as TCP and UDP, it is supported by any router on the Internet. However, as all data transmissions are done by unicast, redundant transmission, whi</p><p>　　In Island Mul

57、ticast, a group of classroom members namely a teacher and students, is divided into several smaller multicat-capable regions(or'islands'). All of these layer multicast(ALM) protocol, while within an island, IPmul

58、ticast is used. Island Multicast(IM) is a generic application-layer framework that works with any ALM protocol. For the Cyber Classroom project, we decide to use ALMI again for inter island communication.</p><

59、;p>　　Using this structure, a menber in the session can be a ' parent node ' of other members, which need to forward all packets from a neighbor node to all other neighbors. This peer-to-peer structure makes

60、the system highly scalable in term of message multicasting. The major problem of this structure is that the data delivery depends to a large extent on the session ungracefully,then all of the downstream nodes will be dis

61、connected from the main tree for a second. To solve this problem, we introdu</p><p>　　AUDIO MODULE</p><p>　　Microsoft DirectX, a recognized standard for multimedia application development on the

62、 Windows platform, is used in the Cyber Classroom desktop version for operations on audio: recoding, compression, decompression, transmission and playing. (Fig 1 and Fig 2.).</p><p>　　Audio is transmitted in

63、 real-time so streaming technology was used in audio transmission. When one person speaks, all users should be able to hear. The audio transmission is one-to-many. Real-time audio is time-sensitive and requires low delay

64、. Meanwhile teal-time audio can tolerate some loss. Therefore, the most suitable way to broadcast audio is using transport layer protocol UDP.</p><p>　　As bandwidth is the most valuable resource on the Inter

65、net, minimizing bandwidth by audio compression is one of the main aims of this application. As this application is designed for hundreds of students using simultaneously, the required bandwidth is so large that the netw

66、ork cannot support it. Therefore, audio compression is essential. In this project we adapted G.723.1. G723.1 is a speech-compression algorithm standardized by ITU. G.723.1 has dul coding rates at 5.3 and 6.3 kbps. The

67、vocod</p><p>　　VIDEO MODULE</p><p>　　There are many different kinds of video compression standard, such as H.261, H263, and MPEG4, developed by several international institutes. In this project,

68、 the new video compression standard H264 standard provides a much larger compression rate and greatly reduces the use of network resources. </p><p>　　The implementation work was based on software JM5.0 (

69、the most updated version 7.6 ) of standard H264 implemented by ITU experts. To speed up the video compression,modification will be made based on the existing code of JM 5.0.</p><p>　　The process of speeding

70、up the video codec started from investigating compitational bottlenecks by timer functions in<time.h>. From the figure below, Integer Pixel Inter-Prediction takes most of computational power.</p><p>　　

71、To speed up the Integer Pixel Inter-Predictions. Inter-search mode other than 16×16 was disabled to ignore inefficient encoding processes in order to increase the encoding speed. Also, a new algorithm was applied fo

72、r motion estimation.</p><p>　　In many fast search algorithms for motion estimation, the Predictive Motion Vector Field Adaptive Search Technique（PMVFAST）algorithm, now improved based on the former MVFAST Alg

73、orithm, achieves best performance in terms of PSNR and speed-up ratio by reducing most of search points, These have very small probabilities that the current block finally reference to MMX technology was also applied to

74、speed up various time-consuming calculations such as Sum of Absolute Difference (SAD) and YUV to RGB co</p><p>　　The original H.264 encoder and decoder work with file input and file output. In this project,

75、the H264 encoder and decoder work with file input and file output. In this project, the H,264 codec must be modified so that it can work with web cam and network by providing necessary interfaces. As the JM software was

76、implemented in C, it became necessary to build static libraries in order to achieve capability between C and C++, as well as to maintain the speed of the codec . The use of static librar</p><p>　　The followi

77、ng graph shows the speed optimization result during 4 stages of improvement.</p><p>　　Stage 1 ：Original</p><p>　　Stage 2 : Configuration Modification</p><p>　　Stage3 : PMVFAST</p

78、><p>　　Stage 4: PMVFAST with MMX</p><p>　　Performance in terms of FPS for Video Codec (Tested in P4 2.4G Machine)</p><p>　　Beside the compression and decompression speed,video quality

79、is also a great concern in the CyberClassroom project.The captured pictures below illustrate the testing of video quality:</p><p>　　The original foreman.qcif sequence(Left) and the decoded sequence (Right)&l

80、t;/p><p>　　The original container.qcif sequence (Left) and the decoded sequence (Right)</p><p>　　As can be seen from the first figure , the face of the foreman blurs because of the quantization pe

81、rformed at the encoder. For the same reason, the waves on the surface of the lake became less apparent . However, the frames are still clear in their entirety and their quality is acceptable for this project given the hi

82、gh compression rate.</p><p>　　WEB INTERFACE</p><p>　　It may happen that students may have to attend live lectures through public computers or computers with operating systems other than MS Windo

83、ws. To tackle those problems, a web interface was developed. The web interface consists of a homepage ,a BBS system and a section for video and audio displaying.</p><p>　　The homepage was developed by Dreamw

84、eaver MX and Flash MX. Users can find all the general information of this project and the documentations.</p><p><b>　　Homepage</b></p><p>　　The BBS system is widely used in the Inter

85、net to make online discussions. All users can communicate with each other by posting and answering to the post. By using the free BBS system source developed by the 6k Group ,we built our own BBS system using ASP.</p&

86、gt;<p><b>　　BBS</b></p><p>　　Several different implementations were considered for the video and audio part. But as our software uses specific video and audio standards (H. 264, G.723) and

87、 are thus difficult to write matching decoders in Java , we chose Windows Media Series Windows Media Encoder 9.0 as the video and audio encoder and the media stream can be easily decoded by the Windows Media Player.</

88、p><p>　　Webpage with video player</p><p>　　Audio only </p><p>　　For a long time historically, the communication problems between different regions and countries were of great concern,e

89、specially for those where geographical barriers and distances made the traditional face-to-face delivery of instructions impossible.Modern technology and the Internet have revolutionized long-distance communications and

90、thus the concept of distance learning has been able to be realized.</p><p>　　The CyberClassroom project provides great flexibilities for beaching:anyone who has access to the Internet can take live lessons r

91、emotely. Professors can conduct lectures as if in traditional classrooms, with minor changes utilizing functionalities such as PRS, whiteboard ,video and audio transmission.</p><p>　　CONCLUSIONS</p>&

92、lt;p>　　Advantages of a CyberClassroom over traditional course delivery include virtually unlimited class size, and greater flexibility in terms of time and place to study. Last year, the outbreak of SARS resulted in t

93、he suspension of classes. With the ‘CyberClassroom’,students can attend lectures at home safely and effectively, and teaching sched ules need not be disrupted.</p><p>　　The Internet today consists of multica

94、st ‘islands’ interconnected by multicast-incapable routers. In order to enable global multicast efficiently,multicast features should be used within an island while the islands are inter connected by unicast connections.

95、 In our project,we made use of an application layer framework called Island Multicast (IM),</p><p>　　Which organizes multicast delivery into two levels: at the upper level inter-island overlay is established

96、 while, at the lower intra-island level,IP multicast is used. Together with IP multicast, the scalability increased. This allows a larger pool of potential students and hence CyberClassroom offers advantages of virtually

97、 unlimited class size and greater choice over traditional course delivery.</p><p>　　In audio part, a compression algorithm G723.1 is used to convert speech into IP packets. G.723.1 delivers the highest compr

98、ession ratio of any of the current ITU standards without compromising speech quality and retains very high voice quality,in spite of the huge compression.In practice,it is nearly impossible to detect a difference in voic

99、e quality between voices over IP. We can achieve a good audio quality with small delay (smaller than 0.5 second).</p><p>　　The CyberClassroom project implements H.264 standard for video encoding and decoding

100、. It provides better video quality while reducing the video bit rate significantly. This promises a smooth video stream under limited bandwidth resources.</p><p>　　As regards installation,the use of Microsof

101、t Installer(MSI) helped the project to provide with a new feature , which is not difficult to implement but very helpful . It reduced the unnecessary errors caused in compiling the source and setting up the software , an

102、d at the same time made our produce more user-friendly . For the web interface , several techniques and software packages were used , such as Macromedia Dreamweaver , in order to develop the web pages ; ASP programming t

103、o build the BBS s</p><p>　　REFERENCES</p><p>　　K.-W.CHeuk, S.-H. Chan and J.Lee;Island Multicast: The Combination of IP Multicast with Application-Level Multicast. IEEE International Conference