MAC协议冲突概率自适应退避算法的研究与实现

针对退避策略在无线传感器网络介质访问控制(MAC)协议中的作用与要求,分析了典型的无线传感器网络MAC((S-MAC)协议的退避机制,提出网络冲突概率自适应退避算法.并用NS2对其进行了性能仿真测试,仿真结果表明,在网络节点分布较为密集、突发数据流量变化较大的网络环境中,改进后的方案在能量效率和吞吐量方面都有较大的改善.     

基于SMAC的无线传感器网络MAC协议的分析与优化

首先对MAC协议进行了相关介绍。然后重点介绍了一种基于竞争的无线传感器网络MAC层协议S-MAC协议。其核心是提出了一种新的无线传感器网络的MAC协议设计方案。基于动态调整占空比的思想,提出了ATC-SMAC协议。该协议在S-MAC协议的基础上改进了固定占空比的劣势,根据每个节点上的数据包的平均延迟调整占空比。通过动态地调整每个节点的占空比,使不同流量的节点拥有不同的工作时间,协议根据不同节点的流量情况自适应地对其占空比进行调整。经过仿真试验,得到ATC-MAC在网络端对端延迟、能量消耗以及吞吐量方面较S-MAC协议都有比较明显的提高。     

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.     

无线传感器网络S-MAC协议分析

S-MAc协议是weiYe等人在IEEE802．儿协议的基础上,针对WSNs网络能量有效性而提出的一种专用于WSNs网络的节能协议。S-MAC协议主要使用三种新技术来减少能耗：节点周期性的休眠,邻节点形成虚拟簇在休眠制度上自同步,以及利用消息传递机制来发送长包,减少竞争反应时间。本文将对无线传感网络（WSNs）的节点能耗问题进行讨论,并对无线传感器网络MAc层节能协议进行分析。     

基于Ad hoc网络的退避算法研究

在移动Ad hoc网络中,无线信道由多个节点共享,合理协调多个节点访问共享信道的媒体接入控制(MAC,Medium Access Control)协议是移动Ad hoc网络的关键技术之一。退避算法在MAC协议中起着至关重要的作用。针对目前退避算法存在的问题,本文提出了一种改进算法,根据网络节点发送数据包成功与否,预测网络信道空闲或繁忙的趋势,应用趋势合理地调节退避算法中节点发送数据时退避值的大小,并通过Omnet++软件对算法进行了仿真分析对比。     

无线多媒体传感器网络S-MAC协议的改进与仿真

在无线多媒体传感器网络中,基于竞争的S-MAC协议存在无法使节点的活动时间,根据通信负载动态变化以及节点休眠带来的延迟问题。考虑到WMSNs中的数据流量变化的突发性,为提高网络的时延性能以及吞吐量性能,提出了基于流量负载的动态占空比调整机制(QD-MAC)。针对改进的算法在NS2上进行仿真实验,结果表明,QD-MAC协议不仅能使节点的活动时间适应通信负载的动态变化,还能大幅减少节点休眠带来的延迟。     

无线传感器网络S-MAC协议的能耗改进

无线传感器节点由于能量的耗尽而失效或者废弃,造成相应监测区域的空白,能耗问题一直是传感器网络介质访问控制（MAC）协议研究的重点问题之一。在S-MAC协议采用周期性侦听和睡眠机制、自适应侦听机制等方式改善网络能耗的基础上提出选择性睡眠、动态调整占空比、发送功率控制三种改进方法。通过NS2仿真实验,改进后S-MAC协议的能耗明显得到优化。     

基于S-MAC改进的自适应占空比机制研究

S-MAC协议作为经典的基于无线传感器网络的MAC协议引起了人们的普遍关注。本文根据S-MAC固定占空比机制,提出了一种基于负载控制的自适应占空比调整的MAC协议——DSMAC。在DSMAC中,设定一个节点定时器,通过判断单位时间内负载情况,动态调整占空比,该协议旨在改善S-MAC固定占空比不能适应负载变化以及延迟过大的不足。利用NS2仿真工具对DSMAC进行了仿真,结果显示DSMAC协议不仅能够显著地提高吞吐量,减少时延,还能有效提高能量效率,在提高网络性能的同时达到节能的目的。     

无线传感器网络S-MAC协议研究

传感器节点能量受限,节能是传感器网络中媒体访问控制（MAC）协议设计的首要问题。采用周期性睡眠机制、自适应侦听机制、串音避免机制和消息传递机制可使得传感器媒体访问控制（S-MAC）协议在网络能耗和时延方面得到改进。对S-MAC协议的改进主要有两种方式：动态调整、区别控制包与数据包的发送条件进行发送。对无线传感器网络,要想设计出一种满足各方面要求的MAC协议是不现实的,可针对不同应用的要求,灵活采用不同的方式,设计出相应的协议。     

无线Adhoc网络中一种基于网络编码的协同MAC协议

协同通信中,中继节点通常用来转发源节点的数据分组.但是从中继节点的角度出发,帮助其它节点转发数据会降低其自身的性能,导致中继低效率问题的出现.针对此问题,该文基于网络编码设计了一种适用于无线Ad hoc网络的协同MAC协议.该协议将网络编码和协同机制相结合,在需要协同传输时,利用网络编码让中继节点在发送源节点数据的同时也发送自身的数据,利用网络编码来解决协同传输中中继低效率问题,进一步提升协同传输的性能.该文还在2维Markov退避模型的基础上,加入了网络编码和协同机制,对所提协议在多跳网络中进行了吞吐量性能分析,并探讨了网络节点数、多跳和信道质量对吞吐量性能的影响.仿真结果表明,相比一般的协同MAC协议,采用网络编码的协同MAC协议可以显著的改善系统性能,是解决协同传输中的中继低效率问题的有效手段.     

Nonpreemptive priority scheme for the S-MAC protocol in multimedia mobile sensor networks

S-MAC is a well-known medium access control (MAC) protocol designed for wireless sensor networks (WSNs). S-MAC uses a few novel techniques to reduce energy consumption and support self-configuration. Besides, S-MAC also achieves good scalability and collision avoidance by utilizing a combined scheduling and contention scheme. However, S-MAC does not have any priority scheme, making it unsuitable for multimedia applications and suffering from varying degree of starvation and fairness problems. In this paper, we propose a simple, efficient, but yet well-performing sliding contention window control mechanism to modify the S-MAC protocol such that priority can be supported along with the multimedia applications in multimedia mobile sensor networks. Furthermore, the proposed scheme could solve the starvation problem and alleviate the fairness problem. An analytic model is carried out to study the saturation throughput of proposed scheme. In addition, simulations were also conducted to analyze the proposed scheme and the results show that the proposed scheme is able to carry the prioritized traffic and mitigate the starvation and fairness problems.     

Contention resolution with EIED backoff for bandwidth request in IEEE 802.16 networks

In this letter, we suggest contention resolution with exponential increase and exponential decrease (EIED) backoff for bandwidth request in worldwide interoperability for microwave access (WiMAX) networks. In EIED, setting of backoff factor to overcome collision due to contention is very challenging and hence we suggest a method to compute backoff factor with average contention window. Further, to reduce access delay, we estimate the response time based on probability of failure and average contention window. Simulations validate the proposed EIED backoff in terms of contention efficiency, capacity and access delay. The contention efficiency and capacity is improved by 47.50% (for q value of 0.25) and 28.57% (for 25 numbers of transmission opportunity), respectively, when bandwidth request is made with the proposed EIED backoff mechanism.     

A backoff algorithm based on self-adaptive contention window update factor for IEEE 802.11 DCF

The binary exponential backoff (BEB) mechanism is applied to the packet retransmission in lots of wireless network protocols including IEEE 802.11 and 802.15.4. In distributed dynamic network environments, the fixed contention window (CW) updating factor of BEB mechanism can’t adapt to the variety of network size properly, resulting in serious collisions. To solve this problem, this paper proposes a backoff algorithm based on self-adaptive contention window update factor for IEEE 802.11 DCF. In WLANs, this proposed backoff algorithm can greatly enhance the throughput by setting the optimal CW updating factor according to the theoretical analysis. When the number of active nodes varies, an intelligent scheme can adaptively adjust the CW updating factor to achieve the maximal throughput during run time. As a result, it effectively reduces the number of collisions, improves the channel utilization and retains the advantages of the binary exponential back-off algorithm, such as simplicity and zero cost. In IEEE 802.11 distributed coordination function (DCF) protocol, the numerical analysis of physical layer parameters show that the new backoff algorithm performance is much better than BEB, MIMD and MMS algorithm.     

Priority-based service differentiation scheme for IEEE 802.15.4 sensor networks

In sensor networks, data packets transmitted by different devices in home networking and industrial applications maintain different levels of importance. In this paper, we propose two mechanisms for IEEE 802.15.4 sensor networks to provide multi-level differentiated services which are required by each and every device. Mathematical model based on the discrete-time Markov chain is presented and is analyzed to measure the performance of the proposed mechanisms. Numerical results show the effects of the variation of contention window size and backoff exponent for service differentiation on 802.15.4 sensor networks. From the results, we derive that contention window size is more affective than backoff exponent on the saturation throughput while backoff exponent is more affective than contention window size on the average delay of every device. Simulation results are given to verify the accuracy of the numerical model.     

一种基于实时优化思想的多址接入协议性能分析

该文提出了一种基于慢退避和实时优化思想的碰撞减少多址接入CRMA (Collision Reduced Multiple Access)协议。CRMA协议将实时优化的思想同慢退避的思想相结合,有效地解决了IEEE 802.11协议二进制指数退避算法成功发送数据帧后没有记录网络当前繁忙程度的缺点,能够更准确地记录数据帧成功发送后高负荷网络的退避阶数,降低数据帧接入信道的碰撞概率,提高无线信道的利用率。新的多址接入协议能够与现有的IEEE 802.11协议完全兼容,具有简单、无开销、完全分布性和自适应性的特点。分析和仿真结果表明,CRMA协议较之改进前的IEEE 802.11协议和其它最新的多址接入算法能够更有效地利用网络中已有的信息,更准确地估计网络当前的竞争状态,表现出更好的网络性能。     

Investigation on group based contention bandwidth request in LTE-A networks under high delay spread environment

In this paper, contention bandwidth request has been investigated for long term evolution-advanced (LTE-A) networks under extended typical urban based multipath fading channel that displays high delay spread environment. As the choice of preambles has to provide high detection probability under such environments, at the outset, this paper examines various group based preamble selection mechanisms, namely, Type I, Type II and Type III preamble sets. With suitable type of group based preamble, the challenge during contention bandwidth request is the appropriate choice of contention window during contention resolution. The contention window in this paper is chosen based on the indicators of various failure events, namely, probability of collision due to contention, probability of unavailability of bandwidth, probability of channel error and probability of improper detection of Zadoff–Chu sequences. After suggesting a scheme to account the possible failure events, an analytical model for contention-based bandwidth request has been developed for LTE-A networks. In addition, two backoff mechanisms are proposed to resolve contention among user equipment’s effectively and these mechanisms are compared to the existing techniques, namely, binary exponential backoff and uniform backoff. Further, the contention mechanism has been substantiated for varying depth of channel errors. With Type I grouping, the backoff with optimized contention window improves the efficiency by 13.95 %, reduces the access delay by 18.71 % and decreases the dropping probability by 59.33 % than the existing uniform backoff mechanism.     

A Multichain Backoff Mechanism for IEEE 802.11 WLANs

The distributed coordination function (DCF) of IEEE 802.11 standard adopts the binary exponential backoff (BEB) for collision avoidance. In DCF, the contention window is reset to an initial value, i.e., CWmin, after each successful transmission. Much research has shown that this dramatic change of window size may degrade the network performance. Therefore, backoff algorithms, such as gentle DCF (GDCF), multiplicative increase–linear decrease (MILD), exponential increase–exponential decrease (EIED), etc., have been proposed that try to keep the memory of congestion level by not resetting the contention window after each successful transmission. This paper proposes a multichain backoff (MCB) algorithm, which allows stations to adapt to different congestion levels by using more than one backoff chain together with collision events caused by stations themselves as well as other stations as indications for choosing the next backoff chain. The performance of MCB is analyzed and compared with those of 802.11 DCF, GDCF, MILD, and EIED backoff algorithms. Simulation results show that, with multiple backoff chains and collision events as reference for chain transition, MCB can offer a higher throughput while still maintaining fair channel access than the existing backoff algorithms.     

Performance Analysis of S-MAC Protocol under Unsaturated Conditions

In this letter, we present a new analytical model that accurately evaluates the throughput, service delay and energy consumption of S-MAC protocol. Our model takes into account the impact of several factors together, including periodic listen and sleep cycle, various incoming traffic loads, the backoff mechanism in S-MAC protocol, the queueing behavior at the MAC layer, and the non-independent nature of service delay distributions of nodes. Simulations show that our analytical results are highly accurate.     

An Efficient Backoff Algorithm for IEEE 802.15.4 Wireless Sensor Networks

IEEE 802.15.4 is one of the most prominent MAC protocol standard designed to achieve low-power, low-cost, and low-rate wireless personal area networks. The contention access period of IEEE 802.15.4 employs carrier sense multiple access with collision avoidance (CSMA/CA) algorithm. A long random backoff time causes longer average delay, while a small one gives a high collision rate. In this paper, we propose an efficient backoff algorithm, called EBA-15.4MAC that enhances the performance of slotted CSMA/CA algorithm. EBA-15.4MAC is designed based on two new techniques; firstly, it updates the contention window size based on the probability of collision parameter. Secondly, EBA-15.4MAC resolves the problem of access collision via the deployment of a novel Temporary Backoff (TB) and Next Temporary Backoff (NTB). In this case, the nodes not choose backoff exponent randomly as mentioned in the standard but they select TB and NTB values which can be 10–50 % of the actual backoff delay selected by the node randomly. By using these two new methods, EBA-15.4MAC minimizes the level of collision since the probability of two nodes selecting the same backoff period will be low. To evaluate the performance of EBA-15.4MAC mechanism, the network simulator has been conducted. Simulation results demonstrate that the proposed scheme significantly improves the throughput, delivery ratio, power consumption and average delay.     

An analytic model for Throughput Optimal Distributed Coordination Function (TO-DCF)

TO-DCF, a new backoff scheme for 802.11, has the potential to significantly increase throughput in dense wireless LANs while also opportunistically favouring nodes with heavier traffic loads and/or better channel conditions. In this paper we present an analytical model to investigate the behaviour and performance of the TO-DCF protocol with regards to operating parameters such as the number of nodes, the contention window size and the backoff countdown probabilities. We then compare numerical results from an implementation of our model with simulations. Our model shows a high level of accuracy, even when the model assumptions are relaxed, and provides guidance for network operators to correctly configure the weight functions for nodes running TO-DCF given the network’s operating conditions.