Throughput analysis of the IEEE 802.11 distributed coordination function considering erroneous channel and capture effects

This paper presents a performance study of the distributed coordination function (DCF) of 802.11 networks considering erroneous channel and capture effects under non-saturated traffic conditions employing a basic access method. The aggregate throughput of a practical wireless local area network (WLAN) strongly depends on the channel conditions. In a real radio environment, the received signal power at the access point from a station is subjected to deterministic path loss, shadowing, and fast multipath fading. The binary exponential backoff (BEB) mechanism of IEEE 802.11 DCF severely suffers from more channel idle time under high bit error rate (BER). To alleviate the low performance of IEEE 802.11 DCF, a new mechanism is introduced, which greatly outperforms the existing methods under a high BER. A multidimensional Markov chain model is used to characterize the behavior of DCF in order to account both non-ideal channel conditions and capture effects.     

Derivative backoff: The other saturation problem for PID controllers

The PID controller is by far the most common controller, and to ensure good behavior under all circumstances, a PID implementation must consider many practical aspects such as integrator windup, derivative kick, and bumpless parameter and mode changes.In this paper, an entirely new issue for the derivative part is introduced and has been named derivative backoff. It occurs when the process value measurement saturates and is a general issue that affects all PID implementations.Derivative backoff is described in detail and it is shown that it can cause considerable, undesired, effects on industrially relevant processes even when they are controlled by well-tuned PID controllers. An anti-backoff strategy for PID controllers with integral action is proposed and evaluated, and a simple patch for existing PID implementations is presented. Possible strategies for PID controllers without integral action are also discussed.     

A multi-objective scatter search for a bi-criteria no-wait flow shop scheduling problem

The flow shop problem as a typical manufacturing challenge has gained wide attention in academic fields. This article considers a bi-criteria no-wait flow shop scheduling problem (FSSP) in which weighted mean completion time and weighted mean tardiness are to be minimized simultaneously. Since a FSSP has been proved to be NP-hard in a strong sense, a new multi-objective scatter search (MOSS) is designed for finding the locally Pareto-optimal frontier of the problem. To prove the efficiency of the proposed algorithm, various test problems are solved and the reliability of the proposed algorithm, based on some comparison metrics, is compared with a distinguished multi-objective genetic algorithm (GA), i.e. SPEA-II. The computational results show that the proposed MOSS performs better than the above GA, especially for the large-sized problems.     

基于多门限估计节点个数的自适应退避算法

针对现有接入协议的性能随着WLAN中节点个数增加而显著下降的问题,本文提出一种新的基于多门限估计激活节点个数的自适应退避算法.该算法优化了时延模型,针对最优退避竞争窗口,分析给出了竞争窗口是激活节点数目的线性函数以及准确的窗系数计算方法;通过更公平地计算空闲时隙间隔,使用三个门限来识别节点个数变化的四种情况,实现了节点个数的估计.理论分析表明,该退避算法使得碰撞概率较小并能获得较高的总吞吐量,同时节点个数的变化对性能影响较小.仿真验证了分析的正确性和算法较好的可扩展性,在公平性方面也优于其它算法.     

DTN中概率选择的散发等待路由

目前的Internet体系结构和许多协议无法很好地适应延迟较高和间歇性链接的网络,这些网络有它们自己的专有协议,即聚束层协议,而不采用TCP/IP协议。为了实现这样的网络之间的互联,国际上提出了一种在端到端链接和节点资源都受限的新型网络体系和应用接口,即容迟网络（DTN）。文中主要讨论DTN网络的路由算法,在DTN路由算法中,蔓延路由和概率路由能提供较高的报文投递率,较小的投递延迟,但是开销很大。散发等待路由虽然具有较小的开销,但是投递率低、延迟较大,提出了投递效用的概念。在融合概率选择路由和散发等待路由的基础上,提出了具有概率选择的散发等待路由。通过仿真,与其它已有算法相比,显著提高了报文投递效用。     

一种新的紫外光自组织通信网络MAC层避退算法

紫外光自组织通信网络是把紫外光与自组织网络相结合的,能够利用自组织网络的多跳特性来克服紫外光距离有限性的通信网络。为了使紫外光自组织通信网络中各个节点能够对有限的信道资源进行充分与公平的利用,需要对网络协议栈中的第二层——媒介接入控制层上的传统算法进行优化。本文以媒质接入控制层(MAC,Media Access Control)传统上采用的二进制指数退避算法为基础,以提高公平性为目的,提出了依据退避计数器值的随机选取是否合理,继而采取相应的奖励惩罚机制的新算法。研究结果表明:新算法提高了无线紫外光自组织通信网络中各节点接入信道的公平性,并在信道吞吐量和信道接入公平性之间达到了一个较好的折衷。     

An adaptive random access strategy for multi-channel relaying networks

In this paper, an adaptive random access strategy is presented for multi-channel relaying networks to address the issue of random access of the non-real-time (NRT) services. In the proposed scheme, NRT services access the base station (BS) by first accessing the nearest relay node (RN). When collision occurs, for the sake of fast and efficient access, the user will begin a frequency domain backoff rather than randomly retry in time domain. A remarkable feature of this scheme is that the RN will adaptively determine the maximum allowed frequency backoff window at each access period. This is achieved according to the new arrival rate as well as the number of available access channels. Moreover, to alleviate the interference caused by sub-channel reuse among RNs, a fractional frequency reuse scheme is also considered. The analysis and numerical results demonstrate that our scheme achieves higher throughput, lower collision probability and lower access delay than conventional slotted Aloha as well as the scheme without frequency backoff window adaptation.     

操作系统同步研究——Linux内核同步机制

本文将Linux内核中用于同步的几种机制集中起来分析，强调了它们之间在实现和使用上的不同。     

车载自组织网络中基于邻居节点数估计的最小竞争窗口调整算法

针对基于IEEE 802.11p的车载自组织网络(VANET)中控制信道上周期性广播的可扩展性问题,提出了一种基于邻居节点估计的最小竞争窗口调整算法.首先,在IEEE 802.11广播退避的马尔可夫模型的基础上,以最小化碰撞概率为目的,推导出最小竞争窗口(CWmin)和活跃节点数n之间的关系;然后,利用周期性广播的beacon消息的特点对节点的邻居节点数进行实时估计,并根据估计的邻居节点数动态调整最小竞争窗口;最后,对提出的算法和IEEE 802.11p固定CWmin方法进行仿真比较分析.结果表明,提出的算法在广播接收率上优于原始方法.     

一种基于IEEE802.11的无线自组网MAC协议

基于IEEE802.11DCF提出了一种应用于无线自组网的媒质接入控制协议，该协议包括网络适应性退避算法和拥塞反馈两个关键机制。协议的主要思想是根据节点周围实际竞争状况和网络拥塞情况进行包调度。仿真结果表明，提出的机制有效地降低了平均端到端时延和数据丢包率,提高了信道接入公平性。