外文資料翻譯---網絡課堂一個大型互動遠程教學平臺

1、<p>　　網絡課堂：一個大型互動遠程教學平臺</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>　　香港科技大學計算機科學系</p><p><b>　　摘要：</b></p><p>　　傳統(tǒng)上，為了與教員進行更好的交流互動，講座一般在學生所處的教室進行。這種模式對于有些學生并不是很方

3、便和劃算，例如對于那些正在工作的、行動不便的、生活在偏遠地區(qū)的，或者類似在SARS期間被社會隔離的人來說。</p><p>　　伴隨著寬帶互聯(lián)網連接和無線媒體 （Wi-fi 和 3G） 的普遍滲透，我們開發(fā)了一個平臺，讓講座可以通過互聯(lián)網以交互方式進行。該項目被稱為 “網絡課堂”，提供了一種類似于現(xiàn)今的傳統(tǒng)教室，但學生們分布在互聯(lián)網的教學體驗。學生可以在任何時間任何地點使用他們的 PC 或便攜式計算機通過互聯(lián)網和

4、無線媒體向他們的導師提問題。視頻、音頻、以及白板也流向實時的用戶終端。利用現(xiàn)有現(xiàn)成的計算產品和最先進的網絡技術，該系統(tǒng)可擴展到數(shù)百名學生。它具有成本效益，能夠有效地突破地域限制讓學生來聽取講座。使用該系統(tǒng)的學生傳來的最初反饋是積極和鼓舞人心的。</p><p><b>　　關鍵詞：</b></p><p>　　遠距離學習，網絡課堂，互動、 教育、 分布式系統(tǒng)、 多媒體

5、應用和通信網</p><p><b>　　引言</b></p><p>　　隨著計算設備和計算機使用的巨大增長，人與人之間的地理距離已顯著減少。簡單舉幾個目前正在使用的互聯(lián)網軟件的例子：網絡會議，網絡電話，ICQ聊天等等。利用這些，人們可以在世界的各個角落，與他人進行比以往任何時候都更經濟、更有效地溝通。</p><p>　　近來，香港政府一直

6、在努力推行“終身學習”的理念。然而，在香港的工作時間和生活節(jié)奏，讓人們幾乎不可能在一個固定的時間，固定的地點進行學習。在香港科技大學計算機科學系,我們已經開發(fā)了一個名為“網絡課堂"的交互式遠程學習平臺。該項目旨在為廣大學生提供現(xiàn)場講座。它允許學生在任何時間任何地點通過任何一臺計算機來參加課程。</p><p>　　我們在下圖展示所開發(fā)的系統(tǒng)。它由學生的分布組成網絡。教授可以通過互聯(lián)網提供他/她的講座錄音

7、和課件，也可以通過輪詢與學生進行交流。</p><p>　　該平臺有以下幾個獨特的功能和技術項目：</p><p>　　網絡模塊：我們設計并實現(xiàn)了一個傳輸機制，以便容納成千上萬沒有強大的服務器的學生和客戶的需要。我們的網絡協(xié)議在最近召開的重大會議和研討會上得以展示，并引起了廣泛的關注。</p><p>　　2、音頻模塊：在該模塊中音頻能夠被很好地傳遞給學生。在傳遞過

8、程中，我們使用了一個高度壓縮的音頻，并不會占用大量的網絡帶寬，使用的編碼標準稱為G.723。</p><p>　　3、視頻模塊：我們已經實現(xiàn)了最新的H.264視頻壓縮編碼技術。它在保證了視頻的高質量的同時，對于網絡和系統(tǒng)資源的使用顯著減少了，也因此，這個軟件的帶寬的要求大大降低。</p><p>　　4、網絡接口：這一部分的重點是支持一個網絡接口。有了這項功能，學生就可以在任何能找到一臺計

9、算機上網的地方學習。為了進一步提高用戶友好性，網絡課堂項目還提供安裝文件。根據指導進行簡單的鼠標點擊使安裝更方便。完全無需編譯或鏈接，而這在去年的項目里是必要的。</p><p>　　下面，我們來詳細解釋一下模塊。</p><p><b>　　1、網絡模塊</b></p><p>　　由于網絡課堂屬于實時應用，并且聽課人的數(shù)量通常都是很大的，因

10、此穩(wěn)定的和可擴展的網絡層顯得極為重要。網絡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生。這項技術被稱為多播。</p><p>　　傳統(tǒng)上，多播有兩種方式：應用層組播（ALM）和IP組播。應用層組播，組播是沿節(jié)點/用戶邏輯樹序列的單播來做的。它是用來解決IP組播的適應性問題。由于數(shù)據向一組ALM主機的傳輸是通過如TCP和UDP之類的單播協(xié)議來完成的，它支持互聯(lián)網上的任何路由器。然而，所有的數(shù)據傳

11、輸是通過單播，冗余傳輸，從而導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，數(shù)據只需發(fā)送一次，同一組內的每個主機都能收到它。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技術和IP組播技術的基礎上，我們設計并實現(xiàn)了一個新的架構，將其命名為“島組播（IM）”。</p><p>　　島組播，即課堂上教師與學生，分成幾個較小的多播能力的地區(qū)（或者“島”）。當所有這些

12、層組播（ALM）協(xié)議在同一個島上，就使用IP 組播。島組播（IM）是一個通用的應用層框架，它適用于任何層組播協(xié)議。對于網絡課堂項目，我們決定再次使用ALMI為島間通信。</p><p><b>　　島組播</b></p><p>　　采用這種結構，會話中的成員可以是一個父節(jié)點的其他成員，這需要將所有的數(shù)據包從鄰居節(jié)點傳送到到其他所有的鄰居。這種對等結構使系統(tǒng)在多播期也

13、能保證信息的高效傳播。這種結構的主要問題是數(shù)據傳輸在很大程度上取決于內的所有節(jié)點的可靠性。如果一個內節(jié)點突然斷開，那么所有的下游節(jié)點也將在一秒鐘內從主樹中斷開。為了解決這個問題，我們將“代理服務器”引入系統(tǒng)之中。代理服務器必須高度可靠，并且有IM網絡運營商建立。這是一個簡單的構件，提供許多其他成員。系統(tǒng)中的代理服務器越多，那么它所提供的網絡就就越穩(wěn)定，越可靠。</p><p><b>　　音頻模塊<

14、;/b></p><p>　　微軟舉，在Windows平臺上進行多媒體應用程序開發(fā)的一個公認的標準，用于在網絡課堂的桌面版操作：重新編碼，音頻壓縮，解壓縮，傳輸和播放。（圖1和圖2。）。</p><p>　　音頻傳輸實時流媒體技術應用于音頻傳輸。當一個人說，所有的用戶應該能夠聽到。音頻傳輸是一對多的。實時音頻的時間敏感，需要低延遲。同時實時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：音頻流設計（教授方面）</p><p>　　圖2：音頻流設計（學生方面）</p><p>　　帶寬是互聯(lián)網上最有價值的資源，通過音頻壓縮帶寬是該應用程序的主要目的。這個應用程序是專為數(shù)以百計的學生同時使用而開發(fā)，所需的帶寬是如此之大，以致于網絡無法支持它。因此，音頻壓縮是必不可少的。在這個項目中我們采用G.723.

16、1。G.723.1語音壓縮算法是一個標準的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語音聲碼器中的比特率提能夠供任何當前ITU標準最高的壓縮比，而不影響語音質量。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準，如H.261，H.263，MPEG4。他們是被一些國際機構開發(fā)的。在這個項目中，新的視頻壓縮標準H.264標準提供了一個更大的壓縮率，大大降低了網絡資源的利用。</p><p>　　比較不同的視頻編碼標準下比特率的PSNR</p><p>　　工作的實施是基于專家設計的標準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>　　對原有的H.264編解碼器各部分的時間消耗</p><p>　　加快視頻編解碼器從在<時間定時器H>功能調查計算瓶頸的過程。從下面的數(shù)字，可以看出整像素的幀間預測需要的計算能力。</p><p>　　加快整像素的幀間預測。為了提高編碼速度，國際搜索模式禁用其他16×1

19、6忽略低效的編碼處理。同時，一個新的算法應用于運動估計。</p><p>　　在許多快速搜索算法的運動估計,預測運動矢量場自適應搜索技術(PMVFAST)算法,現(xiàn)在基于前者MVFAST算法改進,實現(xiàn)最佳性能方面和加速比的PSNR值減少大多數(shù)搜索點,這些有非常小的概率,當前塊最后引用MMX技術也被應用于加速各種耗時的計算如絕對差之和（SAD）和YUV到RGB的轉換等。</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。在這個項目中，H264編碼器和解碼器是處理文件輸入和文件輸出。在這個項目中，H.264編解碼器必須進行修改，使其能為網絡攝像頭和網絡提供必要的接口。隨著JM軟件用C實現(xiàn)，它必須在C和C++之間建立靜態(tài)庫來保持的編解碼速度。靜態(tài)庫的使用也可以更容易的修改和升級。</p><p>　　下面的圖顯示了在4階段的改進速度的優(yōu)化結果。</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：運動</b></p><p><b>　　階段4：運動技術</b></p><p>　　在壓縮和解壓縮速度，視頻質量也在網絡課

22、堂項目的極大關注。拍攝的圖片說明了視頻質量的測試：</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>　　可以從第一個看到第一個圖，因為在編碼器進行量化，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量是可以接

23、受的。這個項目給予高壓縮率仍然是明確的。</p><p><b>　　網絡接口</b></p><p>　　學生可以參加現(xiàn)場講座通過公共計算機或計算機操作系統(tǒng)比Windows更可能發(fā)生。要解決這些問題，開發(fā)了一個Web界面。Web接口包含一個主頁，BBS系統(tǒng)，部分音頻和部分視頻。</p><p>　　網頁是由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項目和文件。</p><p><b>　　主頁</b></p><p>　　BBS系統(tǒng)是在互聯(lián)網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BBS系統(tǒng)源碼的6K集團，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準（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>　　視頻播放網頁</b>&

26、lt;/p><p><b>　　只有音頻</b></p><p>　　很長一段時間過去了，溝通問題在不同地區(qū)和國家之間都受到極大關注，特別是那些在地理上的障礙和距離使得傳統(tǒng)的面對面教授是不可能的。現(xiàn)代技術和互聯(lián)網已經徹底改變了遠程通訊，因此遠程學習的概念已經能夠實現(xiàn)。</p><p>　　網絡課堂提供了很大的靈活性：任何人誰有權訪問互聯(lián)網都可以生活

27、在遠程教育中。教授們可以進行講座就像在傳統(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>　　結語</b></p><p>　　和傳統(tǒng)的課程交付相比，網絡課堂具有幾乎無限的班級大小，以及在時間和地點上有更大的靈活性。去年，非典的爆發(fā)導致停課。“網絡課堂”，學生可以安全的、有效的在家參加講座，教學調度不需要中斷。</p>

28、;<p>　　今天的因特網是組播的“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，應在一個島上使用組播功能，島嶼間的單播連接的。在我們的項目中，我們使用了一個應用層組播框架，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，IP組播技術在較低的水平內島應用。連同IP組播可擴展性增加。這允許一個更大的潛在的學生，因此網絡課堂提供傳統(tǒng)教學的優(yōu)勢和幾乎無限的可供選擇的班級規(guī)模。</p>

29、<p>　　在音頻部分，壓縮算法G.723.1是用來轉換成IP數(shù)據包的演講。G.723.1在提供比當前任何ITU標準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量。在實踐中，它幾乎不可能在不同IP間檢測聲音的語音質量之間的差異。我們可以實現(xiàn)延遲?。ㄐ∮?.5秒）并且好的音頻質量。</p><p>　　該網絡課堂項目實現(xiàn)了H.264標準的視頻編碼和解碼。它提供了更好的視頻質量的同時大大減少

30、了視頻比特率。在有限的帶寬資源下，給我們一個流暢的視頻流。</p><p>　　至于微軟的安裝，安裝使用（MSI）幫助項目提供了一個新的特征，這是不難實現(xiàn)，但很有幫助。它減少了在編譯源和設置軟件造成的不必要的誤差，同時使我們的產品更加人性化。為了網絡接口，一些技術和軟件被使用，如Macromedia Dreamweaver是為了網頁開發(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</b></p><p>　　K. -w.cheuk，S. H. Chan和J.李；組播島：IP組播和應用層組播相結合。IEEE國際會議上交流，出現(xiàn)。</p><p>　　pendarakis，Dimitrios，sherlia石，Dinesh Verma，馬塞爾瓦爾德福格爾：ALMI：應用層組播

32、的基礎設施，2001</p><p>　　tourapis，亞歷克西斯M.，奧斯卡C.金，明湖劉：預測運動矢量場自適應搜索技術（PMVFAST）提高基于運動估計塊。</p><p>　　附件：外文原文（復印件）</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