![]() ![]() Applied to Tetrys, it means that a data packet should stay in the “sliding encoding window” at most for a certain predefined time that depends on its real-time validity period. In that case it’s wiser to drop it anyway. If this frame arrives after the moment it should have been displayed, it’s of no use. Indeed, a data packet may be useful only for a given time span. Applied to Tetrys, it means that a data packet stays in the encoding window until it is acknowledged.īut full reliability is not necessarily what an application is looking for. Usually, full reliability is only feasible if data is acknowledged by the receiver (or all the receivers with multicast transmissions), even lately. "On the fly encoding" is the key technology of Tetrys: only non-acknowledged and useful data packets are protected. Then the encoding window is updated to contain only P9 and P10,and the R(9,10) redundant packet is then calculated. In the absence of acknowledgment, the encoding window keeps on growing… until an acknowledgment of packets P1 to P8 arrives at the sender. Since the acknowledgment of P1 has been lost, the encoding window grows and encompasses packets P1 to P4 the next time a redundant packet is calculated, R(1,4). In this example, R(1,2) is a redundant packet calculated with the two packets of the encoding window, P1 and P2. This sliding window contains a number of packets that varies over the time, depending on what packets have been received or decoded so far at the receiver, and on the arrival of acknowledgements at the sender (they may be lost too). The key innovation of Tetrys is the way encoding is performed: only non-acknowledged but useful packets are considered during encoding, using a sliding window over the data packets. We assume that a feedback channel exists, also prone to losses. In the figure below, the sender periodically adds a redundant packet (noted R(.), in red) after two data packets (noted P., in black). TETRYS enables a new reliability algorithm that we call “on-the-fly encoding”. ![]() What about Tetrys and "On-the-Fly coding"? Tetrys is a solution that goes beyond these two techniques, taking the best of them, thanks to a unified encoding approach. That’s efficient and can be used even if the feedback channel is limited and in case of real-time content. The number of redundant packets can be either fixed or can depend on reception feedbacks. However the usual approach consists in calculating redundant packets from a predetermined set of packets. This FEC code is a smart component that can calculate redundant packets from a set of data packets at a sender, and reciprocally that uses redundant packets to recover the missing data packets in case of losses. However retransmitting requires both that a feedback channel exists (to know what is lost) and that data arrive in due time (it’s not necessarily the case with real-time video transmissions).Īdding redundancy to the flow of data sent, thanks to an FEC code, is another solution. with the various “adaptive streaming over HTTP” techniques heavily used in video over the Internet services). TCP, the dominant protocol over the Internet, heavily relies on TCP, even for video streaming (e.g. Retransmitting lost packets is a solution. The result is the same and a certain number of data packets are missing. The origin of losses does not matter: it may be caused by bad reception conditions in case of wireless networks, or by IP datagram erasures in congested link. ![]() in case of a file transfer) or a highly desired feature, even if a limited amount of losses may be tolerated (e.g. Reliable transmissions over lossy channels are often either a must (e.g. Thanks to its "on-the-fly encoding" approach, Tetrys enables a fully or partially reliable delivery of contents, potentially with real-time constraints, better and faster than the TCP protocol or than traditional block FEC solutions would enable. ![]()
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