摘要
本文探讨在没有QoSs保证的IP网络中，如何根据反向信道反馈的网络状态信息，自适应调整视频编码器的输出码率，实现既合理利用网络资源又不会促使网络拥塞的基于信源的实时视频传输控制策略。它由速率控制、自适应编码和速率整形三部分组成。
基于本课题实时视频的特点，速率控制的条件来自两个方面：1)帧间延迟的限制；2)网络传输速率限制。本文提出了通过联合考虑传输缓冲区和解码缓冲区的状态把这种延迟时间的制约转换为对发送端传输缓冲区有效容量的限制的方法，并采用具有TCP友好性的公式法估算网络带宽。以网络带宽控制发送速率，以缓冲区有效容量和网络带宽控制控制自适应编码器。
在自适应编码方面，本文采用基于Lagrangian代价函数的最优前向自适应编码方法，根据网络带宽和缓冲区状态确定编码参数。提出并实现：
1．根据网络带宽确定的限制条件采用折半搜索法寻找最佳λ。
2．对给定λ设计快速搜索最优Lagrangian量化矢量的算法。
考虑到视频编码自适应系统实质上是一个大延迟控制系统，传输缓冲区的输入输出失配不可避免，本文提出一种智能速率整形算法使传输缓冲区的输出速率与网络带宽相匹配。所提出的算法能为自适应编码选择量化矢量提供初步策略，并实现了：
1．控制传输缓冲区的发送速率，当其存在上溢危险或不符合延迟限制的规定时能智能丢帧。
2．能根据统计数据(平均GOP数据量)和限制条件的信息确定是否需要在编码前跳帧，并制定智能跳帧的方案。
This thesis discusses how to adapt video encoder's output rate to the network states offered by backward channel, under the IP network environment, which supplies no QoSs guarantee. The goal is to design a source-based real-time congestion control policy that not only utilize network resource reasonably, but also to arise no network congestion from transporting real-time video. There are three mechanisms for this congestion control: rate control, rate-adaptive video encoding, and rate shaping.
Based on the characteristics of real-time video, there are two conditions for rate control: 1) inter-frame's delay constraints; 2) the constraints of the transport rate network. In the thesis, a new approach is proposed to convert the constraints of the inter frame delay to the constraints of the effective transport buffer capacity of the sender, by taking states of both transport buffer and decoder buffer into account. Model-based TCP friendly rate control method is used to calculate the available network bandwidth. The transmission rate control is based on the calculated bandwidth, and the throughput of video encoder is adapted to both the available network bandwidth and the effective transport buffer size.
The adaptive strategy of used in this paper is Lagrangian's cost function based optimal forward adaptation, and encoding parameters is determined by the bandwidth and the buffer state.
The paper also proposes and realizes:
1. scheme of looking for the optimal Lagrangian multiplier by using the bisection method according to the rate constraint by the network bandwidth,
2. and a fast searching algorithm for the optimal Lagrangian quantization vector for a given operator .
Consider that the adaptive video encoding system is a long delay feedback control system, the I/O mismatch of the transport buffer is inevitable and a mechanism of rate shaping is required. To this end, an intelligent rate shape algorithm
is suggested in the paper to match the transmitting rate and network bandwidth, which provides the primary strategy of selecting a quantization vector for the adaptive encoder and implements:
1. an algorithm of controlling transmission rate to the network and of discarding certain frames intelligently, while a risk of overflow is forthcoming and the violation of delay constraints is encountered.
2. an intelligent hopping-frame scheme in the encoder according to statistics of average bits of a GOP and the constraint condition.
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