On Adaptive Forward Error Correction for Real Time Traffic

Most of the real-time applications use user data protocol (UDP) as their transport protocol. The reason is that UDP does not provide flow control or error recovery and does not require connection management. Consequently it is a fast protocol suitable for applications that only need to transmit little data or for delay sensitive applications. Nevertheless, UDP has a major drawback, if some packets are dropped then there is no way to recover them. Some applications as video or audio could accept lower quality and most of the times the lost of some packets is less critic than the delay introduced by error recovery methods. Since more applications with real-time constraints such as video image and audio are introduced both over the wired Internet and over wireless some improvements should be made in order to obtain better performance.The main contribution of this thesis is to study an intermediate solution providing more reliability to the communication between applications running on top of UDP and at the same time support its fast connection quality using already existing protocols. In order to obtain it, real-time transport protocol (RTP) has been chosen as upper level protocol (to provide ”flowcontrol”) and an adaptive forward error correction (AFEC) technique has been studied (to provide error management). The idea of AFEC is to inject an adaptive amount of redundancy packets in every sent block (or datagram) in order to achieve a desired recovery rate at the receiver without using any retransmission mechanism. The sender dynamically uses feedback information from the receiver to decide the optimal amount of redundancy to introduce in every sent block. This decision task is managed by a control system at the sender side. Using the network simulator, ns-2, the performance of three different controllers using AFEC is evaluated. The results show in various scenarios that the amount of discarded blocks due to the corruption of some of its packets (block loss probability after decoding) decrease considerably when the AFEC mechanism is introduced.