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Abstract: With the deployment of heterogeneous networks, mobile users are expecting ubiquitous connectivity when using applications. For bandwidth-intensive applications such as Internet Protocol Television (IPTV), multimedia contents are typically transmitted using a multicast delivery method due to its bandwidth efficiency. However, not all networks support multicasting. Multicasting alone could lead to service disruption when the users move from a multicast-capable network to a non-multicast network. In this paper, we propose a handover scheme called application layer seamless switching (ALSS) to provide smooth real-time multimedia delivery across unicast and multicast networks. ALSS adopts a soft handover to achieve seamless playback during the handover period. A real-time streaming testbed is implemented to investigate the overall handover performance, especially the overlapping period where both network interfaces are receiving audio and video packets. Both the quality of service (QoS) and objective-mapped quality of experience (QoE) metrics are measured. Experimental results show that the overlapping period takes a minimum of 56 and 4 ms for multicast-to-unicast (M2U) and unicast-to-multicast (U2M) handover, respectively. The measured peak signal-to-noise ratio (PSNR) confirms that the frame-by-frame quality of the streamed video during the handover is at least 33 dB, which is categorized as good based on ITU-T recommendations. The estimated mean opinion score (MOS) in terms of video playback smoothness is also at a satisfactory level.

一种针对多媒体应用的单播多播无缝切换技术

研究目的：随着异构网络的快速发展和部署，移动用户期望得到无处不在的网络接入服务。对于带宽需求较高的应用，如互联网协议电视（IPTV），多媒体内容分发等服务通常采用具有较高带宽利用率的组播数据传输方法。然而，在异构网络环境中，并非所有网络都支持多播技术。因此，当用户从多播网络移至单播网络时，服务可能中断。此种情形下，保证实时多媒体应用顺畅传输，具有重要意义。
创新要点：有别于以往的分析和模拟研究，本文建立了一个实时流媒体测试平台，提出了一个标准流式传输方法。本文的设计考虑了实际限制，特别是，音频和视频缓冲器的行为均可能影响用户体验。
方法提亮：对于客户端的操作，本文设计了触发器代理和流媒体代理。触发器代理负责发起一个连接切换。因此，该切换方案属于移动端控制切换。根据可选择的连接类型（单播或多播），触发器代理将传送当前多媒体流的统一资源定位器（URL）或发出组播加入请求信息。流媒体代理从该服务器接收流媒体内容。如果是多媒体新流，它将传入的数据区分为音频和视频数据，分别转发到A/V解码器。对于服务器的操作，本文设计了请求处理器和多媒体流切换代理。请求处理器侦听传入的客户端请求，主要负责提取客户的IP地址，并与多媒体流的URL一起转发至多媒体流切换代理。多媒体流切换代理使用多媒体流的URL查找当前被传递至发出请求的用户的资源，同时获取该多媒体流的当前播放时间。随后，多媒体流切换代理将通知数据流接口，并以交替的方法将多媒体流传送给客户端。
重要结论：实验结果表明，在网络的重叠区域内，多播网络—单播网络和单播网络—多播网络的切换时间分别为56毫秒和4毫秒。依据ITU-T建议书，利用本文方法得到的视频画质被归为"好"。平均意见得分（mean opinion score, MOS）证实了视频播放流畅度。
实验方法；多媒体会话连续性；无缝切换；单播或组播交换；多媒体流

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