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While online lectures has been a mature application for desktop PC, enabling access of live online lectures from smartphones posses some unique challenges which can not be solved with existing desktop-oriented online lecture systems. We designed a new live online lectures system called PPClass-M which is optimized for mobile scenario. It adopts an enhanced HTTP-streaming based streaming protocol which can elegantly handle the interim coverage outage and network switch commonly found in of mobile network as well as better support of the dynamic switch of consumed element streams needed to meet the resource constraints of smartphones. It also features a semi-3D based client GUI design which can deliver better user experience on smartphones with constrained screen size Index Terms-http streaming, online lecture, smart phone, streaming media

In traditional video broadcast systems, the encoders have to insert periodical I frames (a.k.a. key frame) in order to allow newly arrived receivers to be able to start the decoding process in a timely manner. However, such insertion of periodical I frames will result a loss of encoding efficiency as these frames could actually have been encoded into more bit-rate efficient P/B frames instead. This issue is even more relevant for video sequences with very slow scene changes like that in an online lecture. In our p2p streaming based live lecture broadcast system named PPClass, we designed a novel scheme to address this issue.The key idea is that when a new receiver joins the channel, it will try to retrieve the necessary reference frames from peers which have already been watching the channel. Moreover, a new type of frame named Sync-P-frame is introduced in the encoder in place of the periodical I frame, which has similar coding efficiency as normal P frame but also remain the semantic as normal I frame in the sense of providing random-access points. The initial experiments proved the effectiveness of this mechanism.