视频流报文优化的实时视频传输带宽探测方法

传统无线网络呈现高度波动变化的可用带宽和延迟,当视频流报文的发送速率超过可用带宽时,可能会使得视频帧延迟变大,从而导致视频质量下降。本文提出了一种基于视频流报文优化的带宽测量方法,不会导致添加额外流量,而是通过将视频帧作为探测序列传入网络中,主动实时测量可用带宽,使用线性滤波器对未来容量进行预测。为保证视频传输的低时延,避免发生拥塞,本文进一步提出了主动拥塞控制的速率自适应方法,解决了在视频传输时测量带宽导致的链路拥塞问题。经过半实物平台的仿真测试,本文提出的算法在实时视频传输时可以进行有效的带宽探测,颗粒度更小。     

一种新的IPv6网络带宽测量方法

该文提出一种新的应用于IPv6网络的带宽测量方法PTTS(Packet Train Time Stamp)。源端主动向网络发送报文序列(Mh-L-Mt packets train),序列中负载报文反应网络流量特征,测试报文带有IPv6时间戳扩展报文头,逐跳记录路由器的当前时间,获得报文序列通过链路所花费时间,得到可用带宽。同时利用IPv6基本报文头中流标签字段定义测试流,确保报文列中测试报文和负载报文路径一致;利用流量类型字段,增设用于网络测量的测试级消除其与背景业务的相互影响。仿真证明,报文设计合理,测量方法可行。     

一种网络总可用带宽测量方法

可用带宽测量对于网络行为分析、网络业务质量保证、网络负载均衡、流媒体的速率控制、服务器的动态选择、覆盖网络（overlaynetwork）的路由选择、拥塞控制等网络应用有很重要的意义。现有的可用带宽测量方法主要对端到端路径进行测量。由于网络总可用带宽不能由路径的可用带宽简单相加获得,而且路径上的瓶颈链路不一定是网络的瓶颈链路,文章提出了一种网络总可用带宽测量方法,仿真实验验证了方法的有效性。     

Internet上实时视频流的拥塞控制策略

分析了Intemet上实时视频传输的特点．提出了基于Intemet的实时视频流的拥塞控制策略,主要包括速率控制和速率整形,速率控制主要是根据网络运行状态预测当前可用的带宽,并根据预测值调整视频速率．以达到与可用带宽匹配：速率整形则是迫使发送端以码率控制算法规定的码率发送视频流。拥塞控制技术应用于终端系统并不需要路由器和网络的Qos支持．可以最大限度地提高视频质量。     

有线/无线Ad Hoc混合网络的路径带宽测量

Ad Hoc Probe是目前测量无线网络路径带宽的主要方法,由于发送探测分组数目固定,当负载较轻时,仅需要几组探测报文就能够获得正确的样本,所以限制了无线Ad Hoc网络的动态性。针对Ad Hoc Probe算法的不足,提出了AdHoc TP算法,一次发送三个报文,组成两个报文对,通过比较两个报文对的时间间隔和最小时延和提高样本的过滤速度,从而使测量速度更快。Qual Net仿真的结果表明,Ad Hoc TP算法能够正确地测量出有线/无线Ad Hoc混合网络的带宽,而且测量速度较快。     

网络可用带宽的高精度测量算法

精确的网络带宽测量对于许多Internet应用和协议都有很重要的作用.本文在分析网络可用带宽测量算法IGI的基础上,提出了改进算法.改进算法在准确性和跟踪可用带宽变化的能力等方面和原来算法相比有很大的改善.另外,针对PTR算法测量结果不稳定以及在网络路径的瓶颈链路的利用率较高时测量不准确的问题,提出改进算法.实验表明,改进的PTR算法在测量时间以及测量负载增加量不大的情况下,提高了测量的准确性,使得测量结果更平稳,并且能很好地跟随网络路径可用带宽的变化.     

基于可用带宽测量的分层组播拥塞控制机制

针对现有的组播拥塞控制机制对接收端可用带宽估计精度较低的问题,提出了一种基于可用带宽测量的分层组播拥塞控制机制ABM-LMCC.在分析了现有可用带宽估计方法不足的基础上,提出一种适用于组播的可用带宽测量算法,并设计了分层组播拥塞控制机制的具体操作规程.通过调节组播数据包的发送间隔,使其呈现降速率的指数分布,从而实现各接收端对可用带宽的准确测量,并根据其测量值迅速调节期望速率,从而达到组播拥塞控制的目的.仿真表明,ABM-LMCC能够有效避免拥塞,提高链路利用率,显著降低丢包率,具有良好的响应性、稳定性.     

基于Internet的实时视频流的应用层QoS控制策略

分析了Internet上实时视频传输的特点，提出了基于Internet的实时视频流的应用层QoS控制策略，主要包括拥塞控制策略和差错控制策略以及相应的控制技术。在拥塞控制中，讨论速率控制和速率整形，速率控制主要是根据网络运行状态预测当前可用的带宽，并根据预测值调整视频速率，达到与可用带宽匹配；速率整形则是迫使发送端以码率控制算法规定的码率发送视频流。在差错控制中，则讨论了编码器差错复原、解码器错误隐藏和编码器／解码器交互的差错控制等控制策略。这些控制技术应用于终端系统并不需要路由器和网络的QoS支持，可以最大限度地提高视频质量。     

基于排队系统的最佳拥塞控制比例研究

对控制报文和网络拥塞间的平衡问题进行研究。通过一个单服务队列模型来描述拥塞控制策略,利用排队系统中的马尔可夫过程,提出一种两阈值的流量控制算法使其控制报文速率能满足最好的拥塞概率。通过分析发现排队系统中拥塞概率随缓冲区大小变化发生指数衰变,并定义该衰变指数为大偏差指数用来描述控制报文与拥塞概率间的比例。最后通过带宽共享模型,模拟并分析不同带宽情况下控制报文与拥塞概率间的最佳比例及其大偏差指数。     

一种基于优化可变间隔的视频流传输的带宽探测方法

为解决视频传输中的带宽探测问题,本文提出一种基于可变包间隔的视频流传输带宽探测方法。这种方法利用传输的视频流进行带宽测量,通过视频流发送过程中设置不同的报文,组合可变的包间隔,探测时变的网络传输带宽。由于利用视频流传输进行带宽测量可能导致丢包、拥塞等情况,降低视频接收质量,因此本文在提出利用视频流进行带宽测量的基础上,进一步提出优化视频包发送间隔的方法,解决了带宽探测过程中带宽变化导致的网络拥塞问题。经过测试,证明了所提算法在实时视频的传输过程中可以进行有效的带宽探测,相较传统算法依然可以保持2%以下的逾期丢失率以及良好的PSNR。     

Evaluation and characterization of available bandwidth probing techniques

The packet pair mechanism has been shown to be a reliable method to measure the bottleneck link capacity on a network path, but its use for measuring available bandwidth is more challenging. In this paper, we use modeling, measurements, and simulations to better characterize the interaction between probing packets and the competing network traffic. We first construct a simple model to understand how competing traffic changes the probing packet gap for a single-hop network. The gap model shows that the initial probing gap is a critical parameter when using packet pairs to estimate available bandwidth. Based on this insight, we present two available bandwidth measurement techniques, the initial gap increasing (IGI) method and the packet transmission rate (PTR) method. We use extensive Internet measurements to show that these techniques estimate available bandwidth faster than existing techniques such as Pathload, with comparable accuracy. Finally, using both Internet measurements and ns simulations, we explore how the measurement accuracy of active probing is affected by factors such as the probing packet size, the length of probing packet train, and the competing traffic on links other than the tight link.     

Performance of uncoordinated coexistence mechanisms in adhoc networks

Dynamic spectrum sharing between uncoordinated devices is impaired by interference. Simple coexistence mechanism can reduce this interference and improve network performance. We analyze performance of some simple coexistence mechanisms in detail, where the decision to transmit a packet by a given device to its intended receiver is taken solely by the transmitter receiver pair without any central control. Accurate interference models are developed assuming a large number of transmitter-receiver pairs that are randomly distributed according to a Poisson spatial point process. These are used to derive accurate expressions for packet error rates in the case of direct sequence code division multiple access physical layer model and slotted packet transmission schemes. These results are then used to study the performance of the coexistence mechanisms and compare them with each other.     

Ring Mesh Based Multicast Routing Scheme in MANET Using Bandwidth Delay Product

Quality of Service (QoS) support in Mobile Ad Hoc Networks (MANETs) for group communication necessitates design of reliable networks with multicast support mechanisms. Reliable network connectivity among MANET nodes require high quality links that have much less packet drops and reliable nodes considering node mobility and failures. Reliability of a network can be enhanced by designing an end-to-end network pipe that satisfies the required QoS in terms of in-flight packets from source to a destination as well as by using a path comprising of reliable nodes. In-flight packets may be computed by using bandwidth delay product (BDP) of a network pipe. To meet the QoS requirements of an application, BDP should be maintained stable irrespective of vibrant network conditions. In this paper, we propose a BDP based multicast routing scheme in MANET using reliable ring mesh backbone. The scheme operates in the following sequence. (1) Reliable node pairs are computed based on mobility, remaining battery power and differential signal strength. The node pairs also compute BDP between them. BDP of a reliability pair is assessed using available bandwidth and delay experienced by a packet between them. (2) Backbone ring mesh is constructed using reliable pair nodes and convex hull algorithm. Reliable ring mesh is constructed at an arbitrary distance from the centroid of the MANET area. (3) Multicast paths are found by discovering a path from source to each destination of the group with concatenated set of reliability pairs that satisfy the BDP requirement. (4) The ring mesh maintains high BDP on ring links and can recover in case of node mobility and failures. Results show that there is an improvement in terms of end-to-end delay, packet delivery ratio, control overhead, memory overhead and application rejection ratio as compared to the Enhanced On Demand Multicast Routing Protocol.     

Packet-dispersion techniques and a capacity-estimation methodology

The packet-pair technique aims to estimate the capacity of a path (bottleneck bandwidth) from the dispersion of two equal-sized probing packets sent back to back. It has been also argued that the dispersion of longer packet bursts (packet trains) can estimate the available bandwidth of a path. This paper examines such packet-pair and packet-train dispersion techniques in depth. We first demonstrate that, in general, packet-pair bandwidth measurements follow a multimodal distribution and explain the causes of multiple local modes. The path capacity is a local mode, often different than the global mode of this distribution. We illustrate the effects of network load, cross-traffic packet-size variability, and probing packet size on the bandwidth distribution of packet pairs. We then switch to the dispersion of long packet trains. The mean of the packet-train dispersion distribution corresponds to a bandwidth metric that we refer to as average dispersion rate (ADR). We show that the ADR is a lower bound of the capacity and an upper bound of the available bandwidth of a path. Putting all of the pieces together, we present a capacity-estimation methodology that has been implemented in a tool called pathrate. We report on our experiences with pathrate after having measured hundreds of Internet paths over the last three years.     

Accurate estimation of capacities and cross-traffic of all links in a path using ICMP timestamps

The estimation of the link capacity and its available bandwidth in an end-to-end path is crucial for network management, admission control and flow control for adaptive applications. This paper introduces an estimation mechanism able to accurately estimate the available bandwidth of all links in an end-to-end path, through its capacity and cross-traffic estimation. The estimation procedures resort to the dispersions of packet pairs and trains using the concepts of ICMP Timestamp and Traceroute that, efficiently used together, enable the measurement of the dispersions in all links in the path. These mechanisms were evaluated through simulation experiments, where we analyzed the influence of the several network parameters on each estimation mechanism. The results show that these estimation methods are able to accurately estimate both the capacity and the cross-traffic of all links in a path with moderate length and with low overhead.     

Design and optimization of strained-layer-multiquantum-well lasersfor high-speed analog communications

We report on how the contributions from spatial hole burning, gain suppression, and relaxation oscillations to the chirp and harmonic distortion of SL-MQW DFB lasers can be calculated and minimized. It is shown how, by taking into account the specific properties of strained-layer-multiquantum-well (SL-MQW) lasers, simple solutions of the rate equations point the way to a chirp reduction and an increase of the useful bandwidth for analog communications. In such lasers, the absorption is only weakly dependent on the carrier density and therefore the harmonic distortion at lower modulation frequencies is mainly caused by spatial hole burning. Our numerical simulations indicate that in many cases this distortion is seduced by the same measures that reduce the chirp and increase the bandwidth     

End-to-end rate-based congestion control: convergence properties and scalability analysis

We study several properties of binary-feedback congestion control in rate-based applications. We first derive necessary conditions for generic binary-feedback congestion control to converge to fairness monotonically (which guarantees asymptotic stability of the fairness point) and show that AIMD is the only TCP-friendly binomial control with monotonic convergence to fairness. We then study the steady-state behavior of binomial controls with n competing flows on a single bottleneck. Our main result here shows that combined probing for new bandwidth by all flows results in significant overshoot of the available bandwidth and rapid (often super-linear as a function of n) increase in packet loss. We also show that AIMD has the best scalability and lowest packet-loss increase among all TCP-friendly binomial schemes. We conclude the paper by deriving the conditions necessary to achieve constant packet loss regardless of the number of competing flows, n, and, in both simulation and streaming experiments, examine one new scheme, called ideally scalable congestion control, with such constant packet loss.     

分组网络上的视频流传输面临的挑战及对策

视频已成为通信、娱乐中十分重要的媒介，视频流传输在因特网上得到了快速发展及广泛应用。分组网络所呈现出的异构性、时变性等本质属性，使得基于分组网络的视频流传输面临许多挑战。首先，当分组从一个节点传输到另一个节点时，由于其所经历的实际网络路径不同，路由器及通信信道的性能和表示这些分组连接的参数(如带宽、分组丢失概率及分组延迟)也会彼此不同。甚至会出现几个数量级大的差异。此外，上述参数还会随着时间发生变化。本从端主机的角度出发，分析在分组网络视频流传输过程中面临的挑战，并提出相应对策。     

Load balancing and resource reservation in mobile ad hoc networks

To ensure uninterrupted communication in a mobile ad hoc network, efficient route discovery is crucial when nodes move and/or fail. Hence, protocols such as Dynamic Source Routing (DSR) precompute alternate routes before a node moves and/or fails. In this paper, we modify the way these alternate routes are maintained and used in DSR, and show that these modifications permit more efficient route discovery when nodes move and/or fail. Our routing protocol also does load balancing among the number of alternate routes that are available. Our simulation results show that maintenance of these alternate routes (without affecting the route cache size at each router) increases the packet delivery ratio. We also show that our approach enables us to provide QoS guarantees by ensuring that appropriate bandwidth will be available for a flow even when nodes move. Towards this end, we show how reservations can be made on the alternate routes while maximizing the bandwidth usage in situations where nodes do not move. We also show how the load of the traffic generated due to node movement is shared among several alternate routes. In addition, we adaptively use Forward Error Correction techniques with our protocol and show how it can improve the packet delivery ratio.     

A frequency-domain approach to crosstalk identification in xDSLsystems

Crosstalk between multiple services transmitting through the same telephone cable is the primary limitation to digital subscriber line services. From a spectrum management point of view, it is important to have an accurate map of all the services that generate crosstalk into a given pair. If crosstalk is measured via modem-based methods, i.e., while a digital subscriber line (DSL) system is running, what is measured is the crosstalk in the bandwidth of the considered DSL system. For this reason, DSL services running on adjacent pairs may not be detected if their bandwidth is not significantly overlapping with the bandwidth of the disturbed system. This is a major drawback of modem-based system identification techniques since, from a spectrum management point of view, it is important to be able to identify all crosstalkers. We address the important problem of crosstalk identification when the pair under test does not bear DSL services, i.e., via a non-modem-based approach. Crosstalk sources are identified in the frequency domain by finding the maximum correlation with a “basis set” of representative measured crosstalk couplings. The effectiveness of the proposed technique is also verified on the basis of real crosstalk measurements performed on actual cables. Finally, new techniques based on multiple regression and best basis selection are also discussed