基于NS-2的IEEE802.11n聚合算法的仿真

IEEE802.11n是下一代无线局域(WLAN)的标准协议,它为上层业务提供至少100Mbit/s的数据速率,可以有效保障VoIP、视频等业务的带宽、Qos需求.该文详细介绍了IEEE802.11n中MAc层引入的聚合算法A-MSDU、A-MPDU,分析聚合算法对数据传输速率的影响;引入网络仿真工具NS-2,使用作者开发的模块对新算法进行仿真,验证了新的聚合算法具有的良好的性能.     

基于新一代Wi-Fi的MAC层关键技术的仿真与分析

分析了新一代Wi-Fi(即IEEE 802.11ac)的MAC层技术,重点研究了IEEE 802.11ac的分布式协调功能和增强的分布式信道接入技术带来的系统增益,并对IEEE 802.11ac MAC层是否采用帧聚合和块应答机制时的数据吞吐效率、在不同信道接入技术下用户业务的公平性进行了仿真和分析。仿真和分析表明增强的分布式信道接入技术相对于传统的分布式协同功能可以显著提升系统吞吐量,并且MAC层通过采用帧聚合和块应答机制,提升了MAC层相对物理层的吞吐率,降低了MAC层协议固有的系统开销所带来的“短板效应”。     

IEEE 802.11n标准的提高吞吐量技术

IEEE802.11系列是目前业界普遍认可的主流无线局域网协议。但高速增长的业务和不断增加的应用种类对无线局域网的性能和服务质量扩展性提出了更高要求。文章介绍了IEEE802.11n协议的LongNAV、MTBA、PSMP等MAC层新技术,它们从设定传输时间、简化帧结构和改进应答传输方式等方面优化帧结构和提高网络吞吐量,整体提高无线局域网数据速率,从而达到性能优化的目的。     

一种基于802.11n的帧长自适应二级帧聚合方法

针对无线局域网中MAC层吞吐量受限的问题,分析了802.11n帧聚合机制和差错信道下采用DCF接入机制时系统饱和吞吐量,讨论了帧长度、误码率对系统吞吐量的影响。在此基础上,提出一种帧长自适应的二级帧聚合方法。该方法根据误码率的不同,自适应地调节第一级聚合数目,以使聚合长度在该误码率下达到最佳,从而使吞吐量达到最优。理论分析与仿真结果表明,在误码率多变的信道中,该方法与现有802.11n中的聚合方法相比,可以通过选择合适的帧聚合长度使吞吐量有较大提高。     

一种基于预测式公平队列调度算法的802.11e MAC层机制

在共享媒体的通信系统中,队列调度的公平性是很重要的。该文在研究了IEEE WLAN已有的各种队列调度算法的基础上,提出了一种预测式公平队列调度算法 (Predict Fair Queuing, PFQ),并结合IEEE 802.11e的EDCF(Enhanced Distributed Coordination Function)机制提出一种基于PFQ的新的MAC层协议P-EDCF(PFQ-based EDCF)。该协议通过引进PFQ算法来修改EDCF的优先级控制方式,提供一种公平、高效的接入机制。仿真结果表明,该机制很好地改善了EDCF的性能,为各种类型业务提供了公平的服务。     

IEEE802.11n技术标准及其在无线局域网中的应用

介绍了无线局域网通信技术标准IEEE802.11n,重点探讨了IEEE 802.11n标准中采用的MIMO、OFDM、信道绑定技术、短保护间隔技术、聚合帧、块应答、向后兼容性等关键技术。在此基础上,针对其在无线局域中的应用,重点介绍了其在无线网用户空间守护程序Hostapd中的配置使用方法。     

无线视频传输的跨层动态映射算法

在IEEE 802.11e EDCA机制和基于场景的马尔可夫链模型基础上,提出跨层动态映射算法来改善无线视频传输的服务质量（QoS）。根据视频帧的重要性和网络的负荷情况,将来自应用层的视频帧信息动态地映射到MAC层中合适的队列中去,并且采用网络仿真工具NS2进行仿真,来比较IEEE 802.11e EDCA机制以及跨层动态映射算法。仿真结果表明跨层动态映射算法能明显提高传输后的视频质量。     

IEEE 802.11n 自适应帧聚合机制研究

分析了传统无线局域网中网络吞吐量受限的原因,研究了802.11n帧聚合机制的原理,提出了一种自适应帧聚合机制.仿真结果表明:采用帧聚合机制降低了帧头额外开销,降低了网络时延,有效地提高了信道利用率;而自适应帧聚合机制可根据信道状况通过动态改变聚合帧中子帧帧长以及重传次数来优化系统性能.     

基于IEEE 802.11高速无线局域网的速率自适应MAC协议研究

目前的IEEE 802.11标准在物理层提供了对多种发送速率的支持,然而在MAC层却没有规定速率自适应的方法。该文研究了高速IEEE 802.11 无线局域网中的速率自适应方案。首先,提出了EACK协议,EACK使用基本速率发送MAC头,并在ACK帧中携带信道信息,因而能够较快速地响应信道的变化,同时具有少的开销;其次,在EACK基础上,提出了一种恒定发送时间(CEACK)的策略,CEACK能够克服传统IEEE 802.11 DCF MAC协议的理论吞吐量上限,并且具有更好的时间公平性能,能够应用于高速的无线局域网。     

高速无线局域网主要技术

无线局域网（WLAN）技术发展迅速，但传输速度慢的缺点始终是阻碍WLAN进步发展的”瓶颈”。实现更高的传输速率，取得更可靠的性能，需要全面采用下一代移动通信的关键技术。IEEE 802.11n标准全面改进了802.11标准，在物理层引入了正交频分复用与多入多出相结合的技术，使传输速度成倍提高；在媒体访问控制（MAC）层采用帧聚合机制、传输机会与拥塞确认技术，使MAC层的性能得到提升，数据帧结构得到优化，网络吞吐能力得到提高；在WLAN中采用新的纠错编码的方法——低密度奇偶校验码，使接收机在较低的信噪比情况下仍然可以拥有较低的误码率，使覆盖范围得到提升。     

Adaptive A-MPDU retransmission scheme with two-level frame aggregation compensation for IEEE 802.11n/ac/ad WLANs

The aggregate MAC protocol data unit (A-MPDU) is one of the significant frame aggregation schemes to improve the performance for high-rate IEEE 802.11n/ac/ad wireless local area networks (WLANs). However, the performance of the A-MPDU scheme does not meet the user expectations because the frame length of the retransmitted A-MPDU will be inevitably and sharply reduced due to the effect of the lost subframe on the number of the aggregatable subframes (i.e., the aggregation level). To overcome this problem, an adaptive A-MPDU retransmission scheme with the two-level frame aggregation compensation is proposed. In this scheme, when the aggregation level of the retransmitted A-MPDU frame dramatically decreases, one of the appropriate two-level aggregation strategies is adaptively employed to compensate the length of the retransmitted A-MPDU frames according to the theoretical analysis of the throughput performance for the conventional A-MPDU scheme and two strategies of the two-level aggregate frame respectively. Simulations using ns-3 platform are performed and the results demonstrate that the proposed adaptive A-MPDU retransmission scheme can achieve higher throughput and medium access control (MAC) layer efficiency.     

An Enhanced A-MSDU Frame Aggregation Scheme for 802.11n Wireless Networks

The main goal of the IEEE 802.11n standard is to achieve a minimum throughput of 100 Mbps at the MAC service access point. This high throughput has been achieved via many enhancements in both the physical and MAC layers. A key enhancement at the MAC layer is frame aggregation in which the timing and headers overheads of the legacy MAC are reduced by aggregating multiple frames into a single large frame before being transmitted. Two aggregation schemes have been defined by the 802.11n standard, aggregate MAC service data unit (A-MSDU) and aggregate MAC protocol data unit (A-MPDU). As a consequence of the aggregation, new aggregation headers are introduced and become parts of the transmitted frame. Even though these headers are small compared to the legacy headers they still have a negative impact on the network performance, especially when aggregating frames of small payload. Moreover, the A-MSDU is highly influenced by the channel condition due mainly to lack of subframes sequence control and retransmission. In this paper, we have proposed an aggregation scheme (mA-MSDU) that reduces the aggregation headers and implements a retransmission control over the individual subframes at the MSDU level. The analysis and simulations results show the significance of the proposed scheme, specifically for applications that have a small frame size such as VoIP.     

Aggregation With Fragment Retransmission for Very High-Speed WLANs

In upcoming very high-speed wireless LANs (WLANs), the physical (PHY) layer rate may reach 600 Mbps. To achieve high efficiency at the medium access control (MAC) layer, we identify fundamental properties that must be satisfied by any CSMA-/CA-based MAC layers and develop a novel scheme called aggregation with fragment retransmission (AFR) that exhibits these properties. In the AFR scheme, multiple packets are aggregated into and transmitted in a single large frame. If errors happen during the transmission, only the corrupted fragments of the large frame are retransmitted. An analytic model is developed to evaluate the throughput and delay performance of AFR over noisy channels and to compare AFR with similar schemes in the literature. Optimal frame and fragment sizes are calculated using this model. Transmission delays are minimized by using a zero-waiting mechanism where frames are transmitted immediately once the MAC wins a transmission opportunity. We prove that zero-waiting can achieve maximum throughput. As a complement to the theoretical analysis, we investigate the impact of AFR on the performance of realistic application traffic with diverse requirements by simulations. We have implemented the AFR scheme in the NS-2 simulator and present detailed results for TCP, VoIP, and HDTV traffic. The AFR scheme described was developed as part of the IEEE 802.11n working group work. The analysis presented here is general enough to be extended to proposed schemes in the upcoming 802.11n standard. Trends indicated in this paper should extend to any well-designed aggregation schemes.     

Optimal frame aggregation level for IEEE 802.11 PCF protocol

In this paper, we analyze the effect of the frame aggregation level on the PCF (Point Coordination Function) MAC performance in IEEE 802.11 wireless LANs and analytically derive the optimal frame aggregation level for maximizing the PCF MAC performance. For various values of unit data frame size and transmission error probability, we propose the optimal frame aggregation levels. By computer simulations, we show that the derived optimal frame aggregation level significantly enhances the PCF MAC performance in IEEE 802.11 wireless LANs.     

IEEE 802.11n MAC Enhancement and Performance Evaluation

The IEEE 802.11-based WiFi wireless technology is one of the most promising technologies to provide ubiquitous networking access. The IEEE 802.11 working group has always strived to improve this wireless technology through creating new amendments to the base 802.11 standard. Recently, IEEE 802.11n amendment was created to enhance 802.11 for higher throughput operation. Not only new Physical Layer enhancements are standardized, but new Medium Access Control Layer mechanism are also defined. In this paper, we examine the network performance enhancement by the proposed 802.11n MAC layer features: aggregation, block acknowledgement, and reverse direction mechanism. We implemented a new 802.11n module in the NS-2 simulation platform. The simulation results demonstrated the effectiveness of 802.11n MAC layer enhancement. VoIP performance is effectively improved with 802.11n MAC enhancement.     

Enhancing IEEE 802.11 MAC via a Sender-Initiated Reservation

The IEEE 802.11 MAC adopted a collision avoidance mechanism in which contending stations should wait a random backoff time before sending a frame. While the algorithm reduces the collision probability in general, a large number of stations may still experience heavy collisions thus decrease the throughput. In this paper, we propose a simple reservation scheme for enhancing the performance of multiple access in 802.11 MAC: when a transmitter sends a frame, if it has another frame to send in its output queue, it may reserve an additional time that is needed to send the next frame and receive an ACK for the frame. Thus a sender can occupy the medium for two data frames, while reducing the collision probability and improving channel utilization via the reservation. We develop a mathematical model to analyze the performance of proposed scheme, and perform simulations to evaluate its performance compared with the original MAC.     

Design of MAC-defined aggregated ARQ schemes for IEEE 802.11n networks

Based on the IEEE 802.11n standard, frame aggregation is considered one of the major factors to improve system performance of wireless local area networks (WLANs) from the medium access control (MAC) perspective. In order to fulfill the requirements of high throughput performance, feasible design of automatic repeat request (ARQ) mechanisms becomes important for providing reliable data transmission. In this paper, two MAC-defined ARQ schemes are proposed to consider the effect of frame aggregation for the enhancement of network throughput. An aggregated selective repeat ARQ (ASR-ARQ) algorithm is proposed, which incorporates the conventional selective repeat ARQ scheme with the consideration of frame aggregation. On the other hand, the aggregated hybrid ARQ (AH-ARQ) protocol is proposed to further enhance throughput performance by adopting the Reed-Solomon block code as the forward error correction (FEC) scheme. Novel analytical models based on the signal flow graph are established in order to realize the retransmission behaviors of both schemes. Simulations are conducted to validate and compare the proposed ARQ mechanisms with existing schemes based on service time distribution. Numerical results show that the proposed AH-ARQ protocol outperforms the other retransmission schemes owing to its effective utilization of FEC mechanism.     

An accurate two dimensional Markov chain model for IEEE 802.11n DCF

According to the amendment 5 of the IEEE 802.11 standard, 802.11n still uses the distributed coordination function (DCF) access method as mandatory function in access points and wireless stations (essentially to assure compatibility with previous 802.11 versions). This article provides an accurate two dimensional Markov chain model to investigate the throughput performance of IEEE 802.11n networks when frame aggregation and block acknowledgements (Block-ACK) schemes are adopted. Our proposed model considered packet loss either from collisions or channel errors. Further, it took anomalous slots and the freezing of backoff counter into account. The contribution of this work was the analysis of the DCF performance under error-prone channels considering both 802.11n MAC schemes and the anomalous slot in the backoff process. To validate the accuracy of our proposed model, we compared its mathematical simulation results with those obtained using the 802.11n DCF in the network simulator (NS-2) and with other analytical models investigating the performance of 802.11n DCF. Simulation results proved the accuracy of our model.     

Providing air-time usage fairness in IEEE 802.11 networks with the deficit transmission time (DTT) scheduler

Wireless systems based on the IEEE 802.11 standard are known to suffer a performance degradation when just a single station in the network experiences bad channel conditions toward the Access Point (AP). This phenomenon, known as the “performance anomaly”, is mainly due to the max-min throughput fairness of the CSMA/CA algorithm of the 802.11 MAC. The simple FIFO scheduling policy usually implemented in the AP also contributes to this problem. In order to overcome the performance anomaly, we propose the Deficit Transmission Time (DTT) scheduler. The aim of DTT is guaranteeing each station a fair medium usage in terms of transmission time. This feature, directly related to the proportional fairness concept, allows to ideally achieve exact isolation among the traffic flows addressed to different stations. DTT achieves this goal taking advantage of measurements of actual frame transmission times. Experiments carried out using a prototype implementation of DTT are compared with analogous tests performed with a classic FIFO queue of a commercial AP and a recently proposed traffic shaping scheme aimed at solving the same 802.11 performance anomaly.