1 Introduction
With the rapid development and maturity of video and broadband access technologies, it has become a reality to watch various live and on-demand programs through the IP broadband network. Time-shifted TV services combine technical features such as live video, video-on-demand, and video recording, allowing users to perform operations such as pause, bounce delayed viewing, and switching from delayed viewing to live broadcast status while watching live TV. It also allows users to Look back at the TV program that has been broadcast. Time-shifted TV completely frees users from the traditional passive TV watching mode of "you broadcast me", which has been listed as one of the basic services by the CCSA IPTV standard. At present, there are two main ideas to realize the time-shifted TV service on the broadband network, namely the IPTV construction scheme based on the C / S mode and the P2P overlay network scheme based on the P2P technology.
In IPTV, time-shifting TV is achieved through the combination of live broadcast and on-demand. The difficulty is similar to video on demand. In the traditional video-on-demand mode. Each user needs to establish a connection with the video server, so even a limited number of users will quickly exhaust the server's resources. In this way, how to reduce the pressure on the server becomes the key to the system design. In this regard, many flow scheduling algorithms have been proposed in the industry, such as Pyramid algorithm [1,2], Skyscraper algorithm [3], Batching technology [4], Patching technology [5 ~ 7] and hierarchical multicast stream merging (hierarchICal mulTIcast stream merging, HMSM) [8] technology, etc., the basic starting point of most of the proposed algorithms is to use the multicast method to merge multiple on-demand of the same file into one Channel service. However, these strategies are difficult to be used in actual commercial operations. The reason is that the current network does not support full-network IP multicast, and this resource-saving strategy is at the cost of delaying user response. The gains outweigh the gains. Reference [3] proposes a P2P method to achieve the transmission strategy of time-shifted TV, but it is also based on the premise of live streaming using IP multicast transmission, and requires the client to be able to receive both multicast and patch streams. This transmission strategy can be applied to small local area network systems, but not to existing wide area networks. The reason is that the existing wide area network does not support full network IP multicast, and the most widely used ADSL line bandwidth is not enough to support the simultaneous transmission of two streams.
On the other hand, in P2P video systems, live video services are widely used, such as Cool Streaming, PPlive, etc., but P2P video-on-demand systems with large-scale applications are rare, and P2P systems with time-shifting TV functions are basically not seen. To. But from the user's perspective, what really attracts users to use the P2P video system are competition programs, such as sports games, super girl games, etc. Moreover, users have a strong demand for such programs. For example, there is something unexpected during the viewing, which needs to be paused. I hope that I can continue watching after returning, or if a certain wonderful shot is not clear, I hope to jump back to watch, or I missed the broadcast program because of something, I hope to be able to watch it again. If you can add the time shift function based on the existing P2P live broadcast, it will be greatly welcomed by users.
Previously, IPTV and P2P basically developed independently of each other, and the research literature on the combination of IPTV and P2P is rare. Recently, some research literatures on the combination of IPTV and P2P have appeared [8-11], emphasizing the complementary advantages of the two, and discussed how to integrate at the technical level.
This article will propose a time-shifted TV system implemented using P2P technology. This system not only utilizes P2P technology to discretely record and store live programs, but also does not need to rely on IP multicast technology for live broadcasting, so it can solve time-shifted TV services in IPTV systems. The problem of high construction cost and poor scalability. In addition, each client only needs to record and store some program segments while playing programs, and can provide other clients with video services and stored program services. Therefore, the system proposed in this paper not only reduces The cost of each client, and the more participants can be reached, the more resources are available, and the better the scale of service quality is.
2. System solution
2.1 System architecture
Figure 1 is a P2P-based time-shift TV system architecture. As can be seen from Figure 1, the system includes a slice processor, a media locator, and a number of peer nodes (Peer). The fragmentation processor performs segmentation and segmentation processing on the input live stream to form media segmentation and segmentation. A media segment includes a fixed number of sequentially numbered media segments, and the beginning and end of the segment are identified by the flag bits in the block header. Media segmentation is the basic unit of system positioning and storage media, and media segmentation is the basic unit of system transmission media. Peer-to-peer nodes can obtain block data from multiple other nodes for decoding and playback. For the convenience of narrative, a number of media segments that are constantly scrolling around the playback point are defined as a logical special segment—live segment.
Figure 1 P2P-based time-shift TV system architecture
The media locator manages the distribution of media segments (including live segments) among peer nodes and determines whether they are in a serviceable state, and provides media segment positioning services for peer nodes. In addition, the media locator also receives the start time and end time information of each segment from the fragment processor, which is used for the translation service from the time information to the segment information. For example, if a node needs to watch a program on a certain channel at a certain time, the node requests the source node from the media locator. The media locator can obtain the corresponding segment number from the segment time information and return the service with the segment The source node of the capability.
After the peer node receives the media data, it can be cached in memory and disk. The node reports the online and offline events, cache segment data increase and decrease events, and the node's external service capability jump events to the media locator through a message. According to this, the media locator can accurately maintain the availability of each media segment on each node. service status. Data transmission takes place directly between peer nodes.
2.2 Data encapsulation
The media segmentation and segmentation format processed by the fragmentation processor is shown in Figure 2. The block consists of a block header and a load area. The load area stores chronological audio and video frames. In addition to describing these frames, the block header also includes the channel number, segment number, block number, and segment logo. The segment flag is used to identify the location of the segment in the segment. It can take three values: segment start, segment middle and segment end. With this flag, peer nodes can conveniently segment segments from the segment flow.
Figure 2 Media segmentation and block encapsulation format
After sharding, peer nodes can obtain different blocks from multiple nodes in the network, splice and restore media streams. Therefore, nodes can use flexible and robust multi-source transmission strategies for live broadcast and time-shifted service transmission.
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