Currently, OTT TV has two approaches of realization: HTTPProgressive Download (HPD) and HTTP Adaptive Streaming (HAS). Generally, thetraditional OTT TV applies HPD technology. HPD-based clients only need to wait fora while to download and buffer the starting data of media files before playing, and then the clients can simultaneouslydownload and play the files. HPD OTT TV also has some limitations. Forinstance, it’s not suitable for transmission of live programs which have highrequirements on timeliness; the waiting delay for initial play is generallylong; the video smoothness is poor if the network bandwidth is instable; thedownloaded files may cause (bandwidth resources) waste because the client keepsdownloading video files, even if the user stopped watching the program.
To overcome the limitations of HPD OTT TV technology,HAS-based OTT TV technology is widely used and promoted in the industry inrecent years. HAS OTT TV adopts video fragment and adaptive bit rate (ABR)technologies. In HAS system, the media stream separator divides the videostream output by encoder into a series of continuous and small fragmented filesin equal length, and stores them in Web content distribution server. Based onthe available bandwidth, HAS client devices can automatically send requests toWeb server for fragmented files with appropriate video quality (i.e., differentresolutions and bit rates), so as to provide users with the best visualexperience. To enable HAS clients fast and real-time switches betweenfragmented files with different bit rates, HAS videos generally applies smallerfragment length if possible (e.g., 10 seconds). HAS system provides videofragmented files with appropriate resolution for terminals in different screensizes, and achieves smooth video play under different network bandwidthconditions, therefore, HAS is recognized by the industry as the core technologyof multi-screen interactive video in the future.