| With the rapid development of wireless communication technology and the increase of wireless networks bandwidth, it is possible to transmit multimedia content in wireless circumstance. Video communication is the kernel of multimedia operation because vision is the most important information media of human being. However, due to the serious disturbances in wireless channels, reliable video transmission quality is usually not guaranteed. The prediction coding and entropy coding technologies employed in traditional video encoders speed up the propagation of channel errors along spatial and temporal directions in video sequences. It makes the compressed bit streams more vulnerable to errors, and further deteriorates the video quality. This imposes more challenges on research of robust video transmission over wireless error-prone environment. It becomes an important issue as how to efficiently control the effects of channel noises and to provide optimal video quality under limited bandwidth resources. This is our research object.Due to the inherent characteristics of wireless Channel, e.g. jitter effects, multi-path effects, shadow, the limited bandwidth of wireless networks, and the transmission time-varying environment the following cases appear, e.g. high bit-error rate, serious interference, these make it difficult to provide the QoS requirements of video applications. In order to improve the QoS of Wireless Channel video applications, the appropriate error control methods are essential. This paper firstly introduces the application field of major error control strategy of wireless communications, then analysis and compare the communication protocol of error detection and error control technology in detail, especially the focus on the agreement IEEE802.11 Wireless LAN error control techniques, based on the new technology of current domestic and international wireless video error control technology, research has been done in various parts of video error control in the wireless LAN during the process of transmission, including the error control techniques of video codec terminal, transmission of channel , as well as joint coding of source and channel. Some research of error control techniques in communication channel for video transmission layer has been done in the Wireless LAN in this paper.Wireless LAN, as each wired or wireless node need to send date to other nodes, wireless, the data transferred first arrive the access point (AP)., AP forward this packet to the appropriate node. Therefore, in the paper dynamic FEC mechanism has been added to AP, to improve the quality of video transmission, since static FEC mechanism in the network can not adapt to dynamic changes of the network statusWhen the network error rate is low, static FEC algorithm will reduce the redundant network load, when network traffic is high, the static FEC will lose more packets due to the network congestion. So in this paper the number of FEC redundancy packages has been dynamic adjustment to the network status, and it could not only avoid the loss of packets due to the congestion, but also attain the better quality of video transmission. Based on the block codes theory of error control technology in the Channel transport layer, at AP, to calculate the weighted average queue length and weighted average retransmitted number of the N-(1)packets of video, according to the dynamic algorithm, adjust the number of redundant packet ,k, transmit N + k-packets to the wireless link, at the receiving terminal, arbitrary N packets could restitute the original video packets, thereby the performance of transmission could be improved on the wireless video linkBecause of its unique features of wireless channel, it has formed a very complex, time-varying fiercely nonlinear system; to accurately describe the wireless system in the analytical methods is a very difficult, and often not practical. Ordinarily, it is possible to build a model to research the specific performance of the wireless transmission system. In the meanwhile, the focus has been concentrated on the error characteristics of the wireless transmission system.Its aim is to provide error control (FEC, ARQ) design to the wireless channel transmission system, a wealth of theoretical foundation has been built after Markov model, and this model could be implemented easily, and also easy to extend to other fields, so is has been applied widely. The model is considered a good choice, which has a balance between the complexity (model-building, parameter calculation, and performance analysis) and effectiveness (the burst of Channel). The main advantages of the binary state Markov model is easy to implemented, and it has the obvious physical significance for parameters of state transition probability model, Therefore Based on the binary-state Markov model DFEC algorithm was studied in this paper.Because of the high cost of constructing a real network to experiment and there are many nonideal factors in real network, the real effects hardly be attained from experiments. So in this paper, NS network simulation software has been adopted to do simulation experiments. In the aspect of video traffic simulation, the method of putting Trace file of video streaming into NS simulation environment has been adopted, too, which can reflect the change of video traffic exactly. The network topology used in experiments is the wireless LAN topology, which is the combination of wire network and wireless network. Because wired and wireless links coexist, hierarchical routing of network should be set up and addresses should be allocated for each node.Four groups of simulation are in this paper, the first group of simulation results show the feasibility of actual video simulation of flow Trace document , in the second group simulation results, there is the compare between the error model of indifferent unanimous distribution and the error-Markov models; in the third group results, the comparison between the static FEC and the DFEC done, and the results of comparison shows that, the DFEC can effectively reduce the loss of packets due to congestion; in the fourth group results of simulation have proved that , with the DFEC mechanism, the dynamic characteristics of FEC redundant packet to the changes of network, and through the comparison between the statistical packet-loss and PSNR in the case with static FEC and without FEC, is has proved that, DFEC has improve the quality of video transmission.In this paper, a summary is in the ending, to sum up all the work, and further the programme to be improved. Because is has assumed that, the error occurs only in wireless links, the wire part of the wireless LAN has no errors. That is, video packet can not arrive at the wireless access points without error, so it has been the limited factors of the application. if the wire error could be combined into the dynamic error control, it could make the error control system of wireless video further improved. |
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