基于应用层组播的远程授课直播系统的设计和实现
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摘要
在现代远程教学中为方便远程学习者较直观快速的学习,以视频会议为主的实时在线远程教学往往成为学习者的首选。这种同步讲授型教学模式在实现时普遍采用点对点实时传输技术,网络资源的开销开始时较小,但是随着接入主机数目的增多,服务器数据传输负荷明显增大,占用越来越多的服务器资源和网络资源,甚至造成网络拥塞,需要不断增加网络带宽,这些都会影响系统的扩展。而随着计算机网络的发展,应用层组播技术作为组播网络的前沿技术得到了广泛的研究并取得一些成熟的理论,其最显著的特点是“时间上集中,空间上分布”,单源发送一个包,多端接收,充分利用带宽和计算机资源,具有较好的数据传输效率,弥补了已有实时网络在可扩展性和伸缩性等方面的不足。应用层组播技术所具有的这些特点较适合远程同步讲授型教学模式。鉴于此,本文提出了基于应用层组播的远程授课直播系统的设计与实现,阐述了系统总体框架的设计及实现过程,通过教师授课直播室用相对有限的网络资源完成屏幕演示、现场直播、文件管理/下载、消息广播四个方面的远程教学应用。
首先本文概述了应用层组播技术的原理和远程教学目前存在的状况,提出了基于应用层组播的远程授课直播系统的必要性和可行性。接着介绍了系统开发中用到的关键技术,阐述了应用层组播平台(MRTP)的通信工作原理,在此基础上设计了系统的体系结构,各模块的功能及其工作流程。
然后系统利用应用层组播平台提高带宽利用率和降低传输延时的优势,详细描述了屏幕图像传输、视频信息传输等模块的设计及其实现过程。对屏幕图像传输模块采用了帧内帧间二次压缩算法,先对相邻的两幅图像逐行比较记录下变化的部分,再对该部分进行zlib压缩,减少数据传输量,最后分块通过MRTP平台进行传输。对于视频信息传输模块,采用directshow技术来实现捕捉,用Xvid Video MPEG-4编码器对视频数据进行编码压缩,对音频采用MP3方式进行压缩,由根服务器设置时间戳Timestamp来完成在同一个通道上传输音频和视频两个信道的内容,从而实现声像的同步传输。文件下载是靠TCP控制通道进行可靠性的验证。消息广播主要是完成对消息的同步传输。
最后将该系统应用在学校网络学院,并对系统使用结果进行实验测评。从测试的结果看,节点间的数据转发延迟在150-200ms之内,整体时延小于5秒,传输的视频画面具有良好的效果,保证了系统的同步性和实时性。
总之,该系统采用对动态获取屏幕图像和捕捉的视频分别压缩处理后再进行分块传输,减轻服务器负荷,进一步提高传输速率,从而使系统具备良好的同步性和实时性。另外,该系统能自适合调整,且不需要额外的硬件设备,操作较简单,是典型的异地同步讲授型模式的应用,为远程实时教学带来极大的便利。
In modern distance learning in order to facilitate rapid long-distance learners to learn, real-time online distance teaching mainly based on video conference are often the first choice .This synchronous teaching-teaching model commonly adopted in the real-time point-to-point transmission technology, so network resources spend less time at the beginning. But with the increase in the number of host access, data transmission load of server increases ,so that more and more the resource of server and network are occupied, and even network congestion resulted from the increasing need of network bandwidth, which affects the expansion of the system. Following the development of computer networks, application-level multicast technology as cutting-edge technology of network multicast has been widely studied and some mature theories come into being . Its most notable feature is that " time focuses and the space distributes ", Single-source sending a package and many terminals receiving it, which adequately makes full use of bandwidth and computer resources to have better data transmission efficiency .It makes up for the shortage of scalability and extencibility etc in real-time network.The features of ALM technology is more suitable for long-range synchronous teaching-teaching mode. In view of this, design and implementation of remote schoolteaching living broadcast system based on application layer Multicast ,which expatiates on the overall framework of system for the design and realization of the process. The medium of teachers live Room uses relatively limited network resources to complete four aspects of distance learning applications, such as presentation of the screen, video live, Document Management / downloads, and news broadcasts.
First , this article summarizes an overview of ALM technology and distance learning principles of the present situation ,and moreover, it provides the necessity and feasibility of remote schoolteaching living broadcast system based on ALM. Then it introduced the key technologies used in the development of the system and expatiates on the principle of communication of the ALM platform (MRTP), which designs the system architecture, the module features and its workflow.
Then the system uses the advantages of MRTP platform which increase bandwidth utilization rate and reduce the transmission delay .The thesis has a detailed description of design and implementation process as to the screen image transmission module and video messaging transmission module etc.On-screen image transmission module uses algorithm of second intra-frame and extra-frame compressions. First , two adjacent images line by line are compared and recorded the part of changes.Next, having changed data is deaded with by the zlib compression, which reduces the volume of data transmission. After that the frame is splited, and finally the little data segments are tranmissed through the MRTP transmission platform. The video messaging module used the following technology,such as directshow technology adapted to catche video data and audio data, Xvid Video MPEG-4 encoder for video data compression coding, the use of the MP3 audio compression method, time-stamp set by the root server which completed to transmit the data of Audio and video of the two transmission channels in the same channel,so as to achieve synchronization of audio and video transmission. File is downloaded on TCP control channels to verify the reliability. Broadcasting news is the completion of the simultaneous transmission of information.
Last, the system is used in college network which gives the evaluation of the results of the experiment. From the test results, data transmitted between the two nodes in the 150-200ms delay, and as a whole the delay is less than 5 seconds, so that the transmission of video images has good results, which ensures real-time synchronization of the system.
In short, the system uses for dynamic capture of the screen images and video.After data was compressed, the little segments splitted were carried out transmission ,which reduces the server load and further improves the transmission rate, so as to have a good system and real-time synchronization. In addition, the system for self-adjustment, and no additional hardware, simple to operate, is a typical mode of teaching in different localities of the application for real-time remote teaching to give a great convenience.
首先本文概述了应用层组播技术的原理和远程教学目前存在的状况,提出了基于应用层组播的远程授课直播系统的必要性和可行性。接着介绍了系统开发中用到的关键技术,阐述了应用层组播平台(MRTP)的通信工作原理,在此基础上设计了系统的体系结构,各模块的功能及其工作流程。
然后系统利用应用层组播平台提高带宽利用率和降低传输延时的优势,详细描述了屏幕图像传输、视频信息传输等模块的设计及其实现过程。对屏幕图像传输模块采用了帧内帧间二次压缩算法,先对相邻的两幅图像逐行比较记录下变化的部分,再对该部分进行zlib压缩,减少数据传输量,最后分块通过MRTP平台进行传输。对于视频信息传输模块,采用directshow技术来实现捕捉,用Xvid Video MPEG-4编码器对视频数据进行编码压缩,对音频采用MP3方式进行压缩,由根服务器设置时间戳Timestamp来完成在同一个通道上传输音频和视频两个信道的内容,从而实现声像的同步传输。文件下载是靠TCP控制通道进行可靠性的验证。消息广播主要是完成对消息的同步传输。
最后将该系统应用在学校网络学院,并对系统使用结果进行实验测评。从测试的结果看,节点间的数据转发延迟在150-200ms之内,整体时延小于5秒,传输的视频画面具有良好的效果,保证了系统的同步性和实时性。
总之,该系统采用对动态获取屏幕图像和捕捉的视频分别压缩处理后再进行分块传输,减轻服务器负荷,进一步提高传输速率,从而使系统具备良好的同步性和实时性。另外,该系统能自适合调整,且不需要额外的硬件设备,操作较简单,是典型的异地同步讲授型模式的应用,为远程实时教学带来极大的便利。
In modern distance learning in order to facilitate rapid long-distance learners to learn, real-time online distance teaching mainly based on video conference are often the first choice .This synchronous teaching-teaching model commonly adopted in the real-time point-to-point transmission technology, so network resources spend less time at the beginning. But with the increase in the number of host access, data transmission load of server increases ,so that more and more the resource of server and network are occupied, and even network congestion resulted from the increasing need of network bandwidth, which affects the expansion of the system. Following the development of computer networks, application-level multicast technology as cutting-edge technology of network multicast has been widely studied and some mature theories come into being . Its most notable feature is that " time focuses and the space distributes ", Single-source sending a package and many terminals receiving it, which adequately makes full use of bandwidth and computer resources to have better data transmission efficiency .It makes up for the shortage of scalability and extencibility etc in real-time network.The features of ALM technology is more suitable for long-range synchronous teaching-teaching mode. In view of this, design and implementation of remote schoolteaching living broadcast system based on application layer Multicast ,which expatiates on the overall framework of system for the design and realization of the process. The medium of teachers live Room uses relatively limited network resources to complete four aspects of distance learning applications, such as presentation of the screen, video live, Document Management / downloads, and news broadcasts.
First , this article summarizes an overview of ALM technology and distance learning principles of the present situation ,and moreover, it provides the necessity and feasibility of remote schoolteaching living broadcast system based on ALM. Then it introduced the key technologies used in the development of the system and expatiates on the principle of communication of the ALM platform (MRTP), which designs the system architecture, the module features and its workflow.
Then the system uses the advantages of MRTP platform which increase bandwidth utilization rate and reduce the transmission delay .The thesis has a detailed description of design and implementation process as to the screen image transmission module and video messaging transmission module etc.On-screen image transmission module uses algorithm of second intra-frame and extra-frame compressions. First , two adjacent images line by line are compared and recorded the part of changes.Next, having changed data is deaded with by the zlib compression, which reduces the volume of data transmission. After that the frame is splited, and finally the little data segments are tranmissed through the MRTP transmission platform. The video messaging module used the following technology,such as directshow technology adapted to catche video data and audio data, Xvid Video MPEG-4 encoder for video data compression coding, the use of the MP3 audio compression method, time-stamp set by the root server which completed to transmit the data of Audio and video of the two transmission channels in the same channel,so as to achieve synchronization of audio and video transmission. File is downloaded on TCP control channels to verify the reliability. Broadcasting news is the completion of the simultaneous transmission of information.
Last, the system is used in college network which gives the evaluation of the results of the experiment. From the test results, data transmitted between the two nodes in the 150-200ms delay, and as a whole the delay is less than 5 seconds, so that the transmission of video images has good results, which ensures real-time synchronization of the system.
In short, the system uses for dynamic capture of the screen images and video.After data was compressed, the little segments splitted were carried out transmission ,which reduces the server load and further improves the transmission rate, so as to have a good system and real-time synchronization. In addition, the system for self-adjustment, and no additional hardware, simple to operate, is a typical mode of teaching in different localities of the application for real-time remote teaching to give a great convenience.
引文
[1]S.Deering.RFC 1112 Host extensions for IP multicasting,Aug 1989
[2]J.Jannotti,D.Gifford,K.Johnson,M.Kaashoek,and J.O'Toole.Overcast:Reliable Multicasting with an Overlay Network[Z].The 4th Symposium on Operating Systems Design and Implementation,October 2000.
[3]Dennis M.Moen.Overview of Overlay Multicast Protocols[S].
[4]李珺晟,余镇危,潘耘,李霞,曹建华,武浦军.应用层组播综述.计算机应用研究,2004年21卷11期.
[5]Y.-H,Rao S.G,and Zhang.H.A Case for End System Multicast.In Proceedings of ACM,SUGNEfRICS,2000
[6]Paul Francis.Yoid:Extending the Intemet Multicast Architecture.White Paper,http://www.icir.org/void
[7]Yatin Chawathe.Scattercast:An Arthitecture for Intemet Broadcast Distribution as an Infrastructure Service.Ph.D.Thesis,University of California,Berkeley,2000
[8]Jannotti.J,Gifford.D,Johnson.K,and etc.Overcast:Realable Multicasting with an Overlay Network,In Proc.of the 4th Symposium on Operating Systems Design and Implementation,Oct,2000.
[9]Dimitrios Pendarakis,Sherlia Shi,Dinesh Verma,and etc.ALMI:An Application Level Multicast Infrastructure.In Proceeding of USENIX Symposium on Internet Technologies and System,2001
[10]Zhang Beichuan,Jamin Sugih,and 7.hang Lixia.Host Multicast:A Framework for Delivering Multicast to End Users.In proceeding of IEEE INFOCOM,2002
[11]陈旭孟,张卫.一个面向实时传输的应用层组播协议的设计与实现[J].计算机应用与软件.2008
[12]Sandra Feldman,Edward J Mcelroy,DISTANCE EDUCATION 2000,5.
[13]Morre,M.G,(1996).Distance Education View[M].Belmont,CA:Wadsworth
[14]刘晓强,陈家训.Internet对远程教学的技术支持分析[J].电化教育研究,2000,(12):40
[15]KaLia Passerini,Marv J.Grander.A Developmental Modal for Distance Learning using the Internet[J].Computer&Education 34,2000:(1-15).
[16]高小玲,吕鹏宇.远程教育系统[M].北京:中国宇航出版社,2004.
[17]王慧芳.网络教育技术基础[M].北京:国防工业出版社,2003
[18]现代远程教育技术标准化项目http://www.edu.cn/html/keyanfz/yuanchenjiaoyu02.shtml,2006-2-11/2006-2-17.
[19]张栓记,何丕廉,胡敏.我国网络远程教学技术平台的研究现状与将来趋势.现代教育技术,2005,(1):15
[20]陈海林,李海霞,李斌.网络课程设计与案例赏析[M].北京:清华在学出版社,2005年6月,P13-P16
[21]彭铁光,一种基于SIP协议的远程教学系统的设计[J].计算机与数字工程,2006,(3):147.
[22]方奕,张卫.一个单源的应用层组播协议的设计和实现[J].计算机应用,2005,(2)
[23]姚玮,张卫.基于应用层组播的实时视频传送协议关键技术研究[J].计算机系统应用.2007,16(8):81-84
[24]康博.Visual C++.NET开发人员指南[M].北京:清华大学出版社,2002.8
[25](美)谢菲德(Shepherd,G,),克鲁格林斯基(Kruglinski,D)著,潘爱民译.Visual C++.NET技术内幕(第6版)[M].北京:清华大学出版社,2004.7
[26]HANTRO,《MPEG-4 Codec Overview》,www.hanto.com
[27]Zebra:Peer To Peer Multicast for Live Streaming Video,Maya Dobuzhskaya,Rose Liu,Jim Roewe,Nidhi Sharma,2004.6
[28]Microsoft DirectX C++ SDK Document[EB/OL],2003
[29]陆其明.DirectShow实务精选[M].北京:科学出版社,2006.9
[30]戴博,周杨,龚涛,李仪.Visual C++程序员成长攻略P344.北京:中国水利电出版社,2007.4
[31]戴博,周杨,龚涛,李仪.Visual C++程序员成长攻略P360.北京:中国水利电出版社,2007.4
[32]朱春喜,吴桂鲜.DirectX高级多媒体程序开发[M].北京:北京航空航天大学出版,1994
[33]崔璐璐.实时交互支持系统的研究与实现[D].华东师范大学,2007.10
[34]Microsoft Corporation,Microsoft Directx9.0 SDK
[35]陆其明.DirectShow开发指南[M].北京:清华大学出版社,2006.12
[36]姚奇富.网络辅助教学理论与设计[M].浙江:浙江大学出版社,2006年8月,P33-P40
[37]http://www.zlib.net
[38]蔡龙华,何道清,李永乐.基于DirectShow技术的视频捕获[J].计算机与 现代化,2003,(8):81-84.
[39]翁肇杰.DirectShow网络视频教育方案的实现[D].华东师范大学,2004.3
[40]邵林,曹汉强.基于DiectShow的视频广播系统设计与实现[J].微型机与应用,2004,4:58-60
[41]求是科技.Visual C++音视频编解码技术及实践[M].北京:人民邮电出版社,2006年6月,P306-P307
[2]J.Jannotti,D.Gifford,K.Johnson,M.Kaashoek,and J.O'Toole.Overcast:Reliable Multicasting with an Overlay Network[Z].The 4th Symposium on Operating Systems Design and Implementation,October 2000.
[3]Dennis M.Moen.Overview of Overlay Multicast Protocols[S].
[4]李珺晟,余镇危,潘耘,李霞,曹建华,武浦军.应用层组播综述.计算机应用研究,2004年21卷11期.
[5]Y.-H,Rao S.G,and Zhang.H.A Case for End System Multicast.In Proceedings of ACM,SUGNEfRICS,2000
[6]Paul Francis.Yoid:Extending the Intemet Multicast Architecture.White Paper,http://www.icir.org/void
[7]Yatin Chawathe.Scattercast:An Arthitecture for Intemet Broadcast Distribution as an Infrastructure Service.Ph.D.Thesis,University of California,Berkeley,2000
[8]Jannotti.J,Gifford.D,Johnson.K,and etc.Overcast:Realable Multicasting with an Overlay Network,In Proc.of the 4th Symposium on Operating Systems Design and Implementation,Oct,2000.
[9]Dimitrios Pendarakis,Sherlia Shi,Dinesh Verma,and etc.ALMI:An Application Level Multicast Infrastructure.In Proceeding of USENIX Symposium on Internet Technologies and System,2001
[10]Zhang Beichuan,Jamin Sugih,and 7.hang Lixia.Host Multicast:A Framework for Delivering Multicast to End Users.In proceeding of IEEE INFOCOM,2002
[11]陈旭孟,张卫.一个面向实时传输的应用层组播协议的设计与实现[J].计算机应用与软件.2008
[12]Sandra Feldman,Edward J Mcelroy,DISTANCE EDUCATION 2000,5.
[13]Morre,M.G,(1996).Distance Education View[M].Belmont,CA:Wadsworth
[14]刘晓强,陈家训.Internet对远程教学的技术支持分析[J].电化教育研究,2000,(12):40
[15]KaLia Passerini,Marv J.Grander.A Developmental Modal for Distance Learning using the Internet[J].Computer&Education 34,2000:(1-15).
[16]高小玲,吕鹏宇.远程教育系统[M].北京:中国宇航出版社,2004.
[17]王慧芳.网络教育技术基础[M].北京:国防工业出版社,2003
[18]现代远程教育技术标准化项目http://www.edu.cn/html/keyanfz/yuanchenjiaoyu02.shtml,2006-2-11/2006-2-17.
[19]张栓记,何丕廉,胡敏.我国网络远程教学技术平台的研究现状与将来趋势.现代教育技术,2005,(1):15
[20]陈海林,李海霞,李斌.网络课程设计与案例赏析[M].北京:清华在学出版社,2005年6月,P13-P16
[21]彭铁光,一种基于SIP协议的远程教学系统的设计[J].计算机与数字工程,2006,(3):147.
[22]方奕,张卫.一个单源的应用层组播协议的设计和实现[J].计算机应用,2005,(2)
[23]姚玮,张卫.基于应用层组播的实时视频传送协议关键技术研究[J].计算机系统应用.2007,16(8):81-84
[24]康博.Visual C++.NET开发人员指南[M].北京:清华大学出版社,2002.8
[25](美)谢菲德(Shepherd,G,),克鲁格林斯基(Kruglinski,D)著,潘爱民译.Visual C++.NET技术内幕(第6版)[M].北京:清华大学出版社,2004.7
[26]HANTRO,《MPEG-4 Codec Overview》,www.hanto.com
[27]Zebra:Peer To Peer Multicast for Live Streaming Video,Maya Dobuzhskaya,Rose Liu,Jim Roewe,Nidhi Sharma,2004.6
[28]Microsoft DirectX C++ SDK Document[EB/OL],2003
[29]陆其明.DirectShow实务精选[M].北京:科学出版社,2006.9
[30]戴博,周杨,龚涛,李仪.Visual C++程序员成长攻略P344.北京:中国水利电出版社,2007.4
[31]戴博,周杨,龚涛,李仪.Visual C++程序员成长攻略P360.北京:中国水利电出版社,2007.4
[32]朱春喜,吴桂鲜.DirectX高级多媒体程序开发[M].北京:北京航空航天大学出版,1994
[33]崔璐璐.实时交互支持系统的研究与实现[D].华东师范大学,2007.10
[34]Microsoft Corporation,Microsoft Directx9.0 SDK
[35]陆其明.DirectShow开发指南[M].北京:清华大学出版社,2006.12
[36]姚奇富.网络辅助教学理论与设计[M].浙江:浙江大学出版社,2006年8月,P33-P40
[37]http://www.zlib.net
[38]蔡龙华,何道清,李永乐.基于DirectShow技术的视频捕获[J].计算机与 现代化,2003,(8):81-84.
[39]翁肇杰.DirectShow网络视频教育方案的实现[D].华东师范大学,2004.3
[40]邵林,曹汉强.基于DiectShow的视频广播系统设计与实现[J].微型机与应用,2004,4:58-60
[41]求是科技.Visual C++音视频编解码技术及实践[M].北京:人民邮电出版社,2006年6月,P306-P307