改善卫星融合网络随机丢包和间歇性中断的方法研究*1

当移动自组织网用户通过网关经卫星网络发送数据时,存在卫星链路的长传播时延、随机丢包以及移动终端的移动性引起链接的间歇性中断,导致TCP触发拥塞控制机制而降低传输性能降低。针对以上卫星网络的特点,提出了TCP M-Veno方法。在发送端对TCP Veno进行优化改进使得它能够缓解卫星网络中长的传播时延和随机丢包的影响。在网关中结合M-TCP算法来解决卫星融合网络移动终端间歇性中断。仿真表明,所提出的TCP M-Veno比NewReno、Veno以及M-TCP有更好的传输性能。     

基于FPGA的突发误码测试仪的设计

传统误码测试仪通过统计误码比特数,采用长期平均误码率为指标,这在分析评估频繁突发误码信道性能时有一定局限性.文章提出了一种基于FPGA的突发误码测试仪设计方案,将突发误码持续时间、误码间隔时间及突发误码起止时刻作为性能评价指标,给出了误码持续长度检测的仿真结果,指出了位同步和序列同步是设计的关键.     

一种新颖实用的交互式视频抗误码方法

本文提出一种新颖的交互式视频抗误码方法.首先,分析了块匹配运动补偿编码方法产生时域误码扩散的机理,然后,相应地提出了一种新颖的误码掩盖和防止误码扩散联合方法.实验结果表明,使用本文提出的交互式视频抗误码方法能够有效地抑制和防止误码的扩散,保证了恢复视频质量.另外,该算法与H.263+标准兼容,具有实用价值.     

低速率视频传输的抗误码技术

误码掩盖是在无线信道进行低速率视频传输的一种抗误码技术。本文在比较和分析了现有几种误码掩盖方法优劣势后,提出了一种混合误码掩盖法。该算法在没有过多增加解码端运算复杂度的同时,大大提高了恢复图像的质量。     

抗突发误码的双重交织Turbo码设计

为了解决无线通信中常见的突发误码问题,提出了一种新的抗突发误码的Turbo码设计方案。该设计方案通过采用双重交织结构,极大地降低了突发误码对接收信息序列连续删除的风险,在不引入额外延迟的前提下,显著地提升了Turbo码抗突发误码的性能。仿真结果表明,当通信中存在较长突发误码时,双重交织Turbo码比一般Turbo码的纠错性能更好,其误比特率甚至可达到10-6数量级。     

范德蒙阵列纠删码在嵌入式系统中的应用

针对范德蒙阵列纠删码算法,介绍了纠删码编译码过程,重点论述了范德蒙码编码算法和译码算法,提出了适合在嵌入式系统实现时的快速算法;在Matlab软件中构建了数字卫星广播系统(DVB-S)模型并进行了信道误码分布仿真,获得了DVB-S系统的误码分布,分析了在系统中使用范德蒙纠删码的可行性;提出了纠删码与系统中的纠错码级联使用模型,并对算法的纠错性能进行仿真,仿真结果表明级联模型能大大提高无线传输系统的可靠性。     

基于H.264的改进时域误码掩盖算法

提出一种模式自适应的误码掩盖算法。对宏块进行误码掩盖时,对宏块的模式进行估计,选择最优的模式进行误码掩盖。这样可以近似地恢复了宏块运动矢量和编码模式,而不是通常算法中仅仅只恢复运动矢量。通过一定的实验测试,该算法在不同的序列和不同的丢包率均稳定的优于单一模式的误码掩盖算法。     

一种甚低速率语音编码的抗误码算法

为满足在高误码率的窄带信道上进行语音通信的需求，本文研究了一种适用于甚低速率语音通信的抗误码参数估值算法。在一定的解码状态下，声码器通过计算最小均方误差（MMSE）估计的方法估计最优参数，充分降低信道误码对重建语音质量的影响。对于解码状态参数，通过计算最大后验转移概率的方法作最佳估计，并给出了一种简化的计算方法。这种抗误码算法复杂度低。计算机仿真结果表明，在不同误码率下该算法恢复出的语音的平均谱失真（ASD）低于帧删除方法的谱失真最大可达0.4dB。     

802.11网络中Eifel算法性能的探讨

相对于有线网络来说,无线局域网的无线链路具有带宽低、误码率高、易受影响和电磁波能量容易被吸收等特点。因此,用户数据分组在无线链路上的传输具有传输时延高、传输时延变化大、链路突发中断的特点。这可能会引起错误的超时,触发TCP重传。分析了传统TCP触发无用重发的原因,介绍了Eifel算法消除错误重传的原理,并在NS-2环境下对Eifel和其他版本的TCP进行了仿真和比较。     

光突发交换网络中TCP Reno的性能分析

该文在分析光突发交换(OBS)网络对TCP性能影响的基础上,研究了单个突发所包含的属于同一TCP/ IP连接的分组数对TCP Reno吞吐量性能的影响,得到了一个吞吐量与突发丢失率、单个突发所包含分组数以及往返时延(RTT)的闭合表达式;并通过仿真验证了分析的正确性;分析和仿真结果表明,在接入链路带宽较大时,突发所包含的分组数存在一个最佳值,使TCP吞吐量达到最大。     

A TCP acceleration algorithm for a wireless link using rate adaptation based on round-trip-time and virtual receiver window information

Fast transmission control protocol (TCP) was previously proposed for high capacity network environments with long delay, and "FAST TCP with Snoop" performs better than conventional TCP enhancements in mobile wireless network environments. However, FAST TCP has limitations when used over a dynamic mobile wireless link with a high frame error ratio (FER) and frequent delay changes due to the variable rate. We propose an enhanced TCP acceleration algorithm at the TCP sender side which efficiently adapts to the maximum transmission rate of a mobile wireless link using the round trip time (RTT) and virtual receiver window (RWND) information. The proposed algorithm provides superior performance over mobile wireless network environments.     

Optimal TCP segment size for mobile server access over wireless links of cellular networks

Internet access from mobile phones over cellular networks suffers from severe bandwidth limitations and high bit error rates over wireless access links. Tailoring TCP connections to best fit the characteristics of this bottleneck link is thus very important for overall performance improvement. In this work, we propose a simple algorithm in deciding the optimal TCP segment size to maximize the utilization of the bottleneck wireless TCP connection for mobile contents server access, taking the dynamic TCP window variation into account. The proposed algorithm can be used when the product of the access rate of the wireless link and the propagation time to mobile contents servers is not large. With some numerical examples, it is shown that the optimal TCP segment size becomes a constant value when the TCP window size (WS) exceeds a threshold. One can set the maximum segment size (MSS) of a wireless TCP connection to this optimal segment size for mobile contents server access for maximum efficiency on the expensive wireless link. Copyright © 2007 John Wiley & Sons, Ltd.     

Advances in Wireless Video Delivery

This paper reviews practical video delivery technologies, examining existing mobile networks, commercialized or standardized transport, and coding technologies. Compression efficiency, power dissipation, and error control are intrinsic issues in wireless video delivery. Among these issues, error control technologies are evaluated at four layers: 1) layer-1/2 transport; 2) end-to-end transport layer such as TCP/IP or RTP/UDP/IP; 3) error-resilience tool and network adaptation layer) and 4) source coder layer. Layer-1/2 transport tends to provide two distinct conditions: one quasi-error-free, in which upper layer error control technologies show a limited improvement, and one with a burst of errors during the fading period, in which the adaptability of error control is essential. Emerging mobile network quality of service will have a large variation of bandwidth and delay. Thus, adaptive rate control and error recovery are identified as more crucial issues for future research.     

TCP SACK突发分组丢失吞吐量模型

利用Gilbert分组丢失模型描述端对端突发分组丢失特性,提出了基于RFC6675的快重传和快恢复模型,推导并基于该模型建立TCP SACK吞吐量模型。数值实验和仿真实验表明,快重传和快恢复模型能准确描述基于RFC6675的快重传和快恢复过程;TCP SACK流吞吐量模型估计的准确性得到提升。     

Improving TCP performance over networks with wireless components using ‘probing devices’

TCP error control mechanism lacks the ability to detect with precision the nature of potential errors during communication. It is only capable of detecting the results of the errors, namely that segments are dropped. As a result, the protocol lacks the ability to implement an appropriate error recovery strategy cognizant of current network conditions and responsive to the distinctive error characteristics of the communication channel. TCP sender always calls for the sending window to shrink. We show that probing mechanisms could enhance the error detection capabilities of the protocol. TCP could then flexibly adjust its window in a manner that permits the available bandwidth to be exploited without violating the requirements of stability, efficiency and fairness that need to be guaranteed during congestion. Our experiments have three distinct goals: First, to demonstrate the potential contribution of probing mechanisms. A simple probing mechanism and an immediate recovery strategy are grafted into TCP‐Tahoe and TCP‐Reno. We show that, this way, standard TCP can improve its performance without requiring any further change. Second, to study the performance of adaptive strategies. An adaptive TCP with probing is used, that is responsive to the detected error conditions by alternating slow start, fast recovery and immediate recovery. An adaptive error recovery strategy can yield better performance. Third, to study the design limitations of the probing device itself. The aggressive or conservative nature of the probing mechanisms themselves can determine the aggressive or conservative behaviour of the protocol and exploit accordingly the energy/throughput trade‐off. Copyright © 2002 John Wiley & Sons, Ltd.     

基于GMSK调制终端的相位误差测试中的算法研究

丈章采用优化最小二乘算法实现基于GMSK调制终端中相位、频率误差的测试.对GSM测试终端的频率和相位误差特性进行了详细分析.在分析罗德施瓦茨综合测试仪CMU200的相位误差测试原理和方法的基础上,结合GSM突发脉冲的特性,提出了采用分段式最小二乘法来计算相位误差的算法.仿真表明分段最小二乘方法是测试射频终端相位误差的快速、准确的方法.     

Using channel state dependent packet scheduling to improve TCPthroughput over wireless LANs

In recent years, a variety of mobile computers equipped with wireless communication devices have become popular. These computers use applications and protocols, originally developed for wired desktop hosts, to communicate over wireless channels. Unlike wired networks, packets transmitted on wireless channels are often subject to burst errors which cause back to back packet losses. In this paper we study the effect of burst packet errors and error recovery mechanisms employed in wireless MAC protocols on the performance of transport protocols such as TCP. Most wireless LAN link layer protocols recover from packet losses by retransmitting lost segments. When the wireless channel is in a burst error state, most retransmission attempts fail, thereby causing poor utilization of the wireless channel. Furthermore, in the event of multiple sessions sharing a wireless link, FIFO packet scheduling can cause the HOL blocking effect, resulting in unfair sharing of the bandwidth. This observation leads to a new class of packet dispatching methods which explicitly take wireless channel characteristics into consideration in making packet dispatching decisions. We compare a variety of channel state dependent packet (CSDP) scheduling methods with a view towards enhancing the performance of transport layer sessions. Our results indicate that by employing a CSDP scheduler at the wireless LAN device driver level, significant improvement in channel utilization can be achieved in typical wireless LAN configurations. This revised version was published online in July 2006 with corrections to the Cover Date.     

A New Analytical Model for TCP Reno with Bursts Error Considered

1　IntroductionInthepastfewyears,moststudiesofTCPpro tocolhaveconcentratedonexperimentsandsimula tions[1～2 ,9～ 1 4 ] .Theprevioustheoreticalstudieson lyconsideredTCP sowncapacity ,butthecharac teristicsofwirelesslinkswerenotcaredaboutenough .Evensomeofthemconsideredwirelesslinks,butmostofthemfailedtoconsidertheeffectsofbursterror[3,1 1 ] whichisanimportantfeatureofwirelesslinks.InRef.[4],ZORZIM proposesaanalyticalmodelforTCP ,buthismodelisverycomplicated ,andalsoatsomeplacesheusesuppe…     

即时恢复--一种新的TCP拥塞控制快速恢复算法

针对TCP连接的网络传输中经常会出现同一窗口多个分组丢失的情况,本文提出了一种新的TCP拥塞控制快速恢复算法:即时恢复算法。即时恢复算法能在恢复阶段探测网络的可用带宽,并将之用于窗口拥塞控制。另外,通过动态设定退出恢复阶段的参数域值,除了能恢复首次丢失的多个分组外,该算法还能很好的解决恢复阶段分组进一步丢失的情况。与SACK、FACK TCP等算法需要对TCP协议的发送、接收部分同时修改不同,该算法只需修改协议的发送部分,而接收部分不需作任何改变。在ns仿真环境的仿真结果表明:在同一窗口有多个分组丢失时,即时恢复算法的性能比TCP New-Reno有较大提高,与 SACK TCP性能相当。     

Congestion window-based adaptive burst assembly for TCP traffic in OBS networks

Burst assembly is one of the key factors affecting the TCP performance in optical burst switching (OBS) networks. When the TCP congestion window is small, the fixed-delay burst assembler waits unnecessarily long, which increases the end-to-end delay and thus decreases the TCP goodput. On the other hand, when the TCP congestion window becomes larger, the fixed-delay burst assembler may unnecessarily generate a large number of small-sized bursts, which increases the overhead and decreases the correlation gain, resulting in a reduction in the TCP goodput. In this paper, we propose adaptive burst assembly algorithms that use the congestion window sizes of TCP flows. Using simulations, we show that the usage of the congestion window size in the burst assembly algorithm significantly improves the TCP goodput (by up to 38.4% on the average and by up to 173.89% for individual flows) compared with the timer-based assembly, even when the timer-based assembler uses the optimum assembly period. It is shown through simulations that even when estimated values of the congestion window size, that are obtained via passive measurements, are used, TCP goodput improvements are still close to the results obtained by using exact values of the congestion window.