Supporting QoS in IEEE 802.11e wireless LANs over fading channel

Transferring real-time traffic such as voice and video over wireless LAN networks (WLAN) requires stringent delay and jitter requirements. Recently IEEE 802.11e standard has been emerged to support QoS in WLAN. One of the methods to provide QoS in this standard is Enhanced Distributed Channel Access (EDCA) which benefits form the concept of traffic categories. However, EDCA is a contention based method; therefore it can not guarantee strict QoS required by real-time services without proper network control mechanisms. In this paper, we analyze the effect of loss and delay caused by fading channel on EDCA performance. Then, we propose a modification to the media access scheme, called CAFD (Collision Avoidance with Fading Detection) to elevate the performance against channel failures. Moreover an adjustment for the maximum number of retransmissions is proposed to maintain the delay and jitter requirements of the real-time traffic. The performances of the proposed methods are evaluated by simulations.     

Runtime optimization of IEEE 802.11 wireless LANs performance

IEEE 802.11 is the standard for wireless local area networks (WLANs) promoted by the Institute of Electrical and Electronics Engineers. Wireless technologies in the LAN environment are becoming increasingly important and the IEEE 802.11 is the most mature technology to date. Previous works have pointed out that the standard protocol can be very inefficient and that an appropriate tuning of its congestion control mechanism (i.e., the backoff algorithm) can drive the IEEE 802.11 protocol close to its optimal behavior. To perform this tuning, a station must have exact knowledge of the network contention level; unfortunately, in a real case, a station cannot have exact knowledge of the network contention level (i.e., number of active stations and length of the message transmitted on the channel), but it, at most, can estimate it. We present and evaluate a distributed mechanism for contention control in IEEE 802.11 wireless LANs. Our mechanism, named asymptotically optimal backoff (AOB), dynamically adapts the backoff window size to the current network contention level and guarantees that an IEEE 802.11 WLAN asymptotically achieves its optimal channel utilization. The AOB mechanism measures the network contention level by using two simple estimates: the slot utilization and the average size of transmitted frames. These estimates are simple and can be obtained by exploiting information that is already available in the standard protocol. AOB can be used to extend the standard 802.11 access mechanism without requiring any additional hardware. The performance of the IEEE 802.11 protocol, with and without the AOB mechanism, is investigated through simulation. Simulation results indicate that our mechanism is very effective, robust, and has traffic differentiation potentialities.     

IEEE 802.11: moving closer to practical wireless LANs

Although the IEEE 802.11 standard has been around since 1997, work continues to make it more adaptable to the demand for higher data rates and true wireless flexibility. Until recently, few organizations used wireless LANs because they cost too much, their data rates were too low, they posed occupational safety problems because of concerns about the health effects of electromagnetic radiation, and the spectrum used required a license. Today, these problems have largely diminished, and wireless LAN popularity is skyrocketing. Wireless LANs must meet requirements typical of any LAN. They must also meet requirements specific to their intended environment. IEEE 802.11 defines several services that the wireless LAN must provide if its usefulness is to match the functionality inherent in wired LANs. IEEE 802.11 is poised to have a significant impact on the LAN marketplace. As the demand for mobility and freedom from wiring requirements increases, the standard offers a comprehensive yet flexible approach to wireless LAN products     

Performance enhancement and bandwidth guarantee in IEEE 802.11 wireless LANs

This paper first revisits the previously proposed NSAD (New Self-Adapt DCF) mechanism. Some modifications are presented to further enhance the performance of NSAD in the error-prone environment. Then a new MAC mechanism is proposed that can realize bandwidth guarantee by assigning different self-adapt parameters to users at different priority levels. The bandwidth guarantee property of this new mechanism is analyzed and the high priority users are found to have bandwidth guaranteed even in heavy contention condition, which is proved true not only by theoretical analysis but also by simulation results. At the same time the new scheme keeps the self-adapt character of NSAD, so the overall system utilization1 is kept very high in heavy contention condition compared with the previously studied DCF-based QoS mechanisms.     

一种基于IEEE 802.11e的多跳无线网络拥塞控制协议

多跳无线网络中网络拥塞的出现将严重降低网络的性能。基于802.11e提供的区分业务类型的信道接入优先级机制,提出了一种对尽力而为(best-effort)类型业务动态调整其优先级进行网络拥塞控制的协议。该算法的主要思想是对发生网络拥塞的节点提升其业务流传输的优先级使其获得更多的传输机会以缓解拥塞状况,并对严重的网络拥塞状况采取反向施压的策略降低上游节点业务流的转发速率。仿真结果表明,该算法有效地提高了网络重负载情况下的吞吐量。     

Performance modeling of IEEE 802.11 wireless LANs with stochastic Petri nets

In 1997, IEEE standardized the physical layers and the medium access for wireless local area networks. This paper presents a performance study of the distributed coordination function, the fundamental contention-based access mechanism. Most performance studies adopt unchecked simplifying assumptions or do not reveal all details of the simulation model. We develop a stochastic Petri net model, which captures all relevant system aspects in a concise way. Simulation allows to quantify the influence of many mandatory features of the standard on performance, especially the backoff procedure, extended interframe spaces, and the timing synchronization function. We identify conditions when simplifying assumptions commonly used in analytical modeling are justified. Applying these conditions, we derive a more compact and analytically tractable model from the detailed model.     

IEEE 802.11e中基于失真度度量的视频包传输方法

为改善H. 264编码的视频流在802. 11e中的传输性能, 提出了一种结合H. 264/AVC中不同类型的数据分割对视频重建质量的重要性因子和队列状态的视频包映射方法。首先定量分析H. 264/AVC中A、B、C三种分割的丢失对视频重建质量的影响, 得到其重要因子; 然后依据重要因子和队列长度将视频数据包映射到802. 11e的不同EDCA队列中。算法改进了EDCA机制中数据包的静态映射机制, 根据视频分割数据的不等重要性, 提供差异性服务。仿真结果表明, 与目前的视频包静态映射机制相比, 该算法提高了视频重建质量, 最好可提高1 dB以上。     

IEEE 802.11e基于竞争的信道访问机制性能分析

在回顾了IEEE802．11MAC协议的基础上，讨论并分析了即将定稿的IEEE802．11e的基于竞争的信道访问工作机制下MAC协议对QoS的支持。     

On distributed power saving mechanisms of wireless LANs 802.11e U-APSD vs 802.11 power save mode

The integration of wireless LAN technology in mobile devices such as cellular phones, PDAs or laptops has become a user need due to its popularity in providing high speed wireless Internet access at a low cost. Such devices though should meet users’ expectations with regard to QoS, i.e., guarantee a reasonable voice quality when VoIP is used, and power saving efficiency, i.e., standby and calling times should be similar to the ones of cellular phones. The IEEE 802.11e standard, which extends the 802.11 wireless LAN MAC layer with QoS and power saving enhancements, should be the most appropriate solution to address users’ wishes in those devices.In this paper, we focus on the 802.11e functionality likely to be included in mobile devices in the short-term, EDCA for QoS and U-APSD for power saving, and evaluate the performance improvements and associated costs of two possible configurations of U-APSD as compared to the 802.11 power save mode. In addition, the dependency between the QoS and power saving enhancements obtained with U-APSD and the available channel capacity is analyzed considering three different scenarios: 802.11b, 802.11b+g and 802.11g. The evaluation is based on our proposed implementation of U-APSD: Static U-APSD (SU-APSD).The main conclusions that can be drawn from our results are that U-APSD significantly outperforms the 802.11 power save mode in all considered performance metrics and that the performance enhancements obtained with U-APSD are independent of the available channel capacity.     

The performance evaluation of IEEE 802.11 DCF using Markov chain model for wireless LANs

During the past few years the widespread use of the wireless local area networks (WLANs) communication technique is one of the most popular technologies in data telecommunications and networking. The IEEE 802.11 protocol has achieved worldwide acceptance with WLANs with minimum management and maintenance costs. The theoretical performance and numerical results in terms of saturation throughput and delay of distributed coordination function (DCF) were finished by Ziouva and Antonakopoulous. It took into account of the busy medium conditions and how they affected the use of the backoff mechanism. However, the definition of their proposed channel busy probability is not suitable for the operating system architecture. In this paper, the channel busy conditions is modified and the Ziouva and Antonakopoulous's (ZA's) model is improved, and the more accurate analyses of the DCF are presented. Our analysis is also extended to the computation of the delay performance of both the request-to-send/clear-to-send (RTS/CTS) and basic access mechanisms. The numerical results show that the modified model has better performance than the ZA's model under ideal channel scenario.     

基于IEEE 802.11e EDCA的优先级可调机制

针对IEEE 802.11e中不同优先级数据流所获得的服务相差很大、低优先级数据流获得服务量很少的问题,提出一个可以提高低优先级竞争能力的EDCA改进机制.低优先级数据流在重传m次后使其优先级增加1,并用马尔可夫链模型对此改进机制进行了吞吐量性能分析.通过数学分析和仿真结果的对比,验证了此改进机制可提高总体吞吐量性能.     

A simple recursive scheme for adjusting the contention window size in IEEE 802.11e wireless ad hoc networks

The IEEE 802.11e standard has been introduced recently for providing Quality of Service (QoS) capabilities in the emerging wireless local area networks. This standard introduces a contention window based Enhanced Distributed Channel Access (EDCA) technique that provides a prioritized traffic to guarantee the minimum bandwidth needed for time critical applications. However, the EDCA technique resets the Contention Window (CW) of the mobile station statically after each successful transmission. This static behavior does not adapt to the network state since it reduces the network usage and results in bad performance and poor link utilization whenever the demand for link utilization increases. This paper proposes a new adaptive differentiation technique for IEEE 802.11e Wireless Local Area Networks that takes into account the network state before resetting the contention window. In the new technique, the congestion level of the network is sensed by using previous CW values. Three other enhancement techniques that focus on network adaptation are also discussed. Their main limitations are the high complexity of the implemented algorithms and their slow adaptation to the network state when the channel experiences bursty traffic. The proposed technique is compared to the original differentiation techniques of IEEE 802.11a and IEEE 802.11e standards, as well as to the enhancement schemes. Results show that the proposed adaptive technique outperforms IEEE 802.11e and is comparable to the other enhancement schemes while maintaining relatively low complexity requirements.     

Admission control scheme based on priority access for wireless LANs

In order to support the quality of service (QoS) requirements at the medium access control (MAC) layer, the enhanced distributed channel access (EDCA) has been developed in IEEE 802.11e standard. However, it cannot guarantee the stringent real-time constraints of multimedia applications such as voice and video without an efficient method of controlling network loads. In this paper, we propose a measurement-based admission control scheme, which is made up of two parts: priority access and admission control. First, in order to measure the channel status per traffic type, we propose a priority access mechanism in which each priority traffic is distinguished by a busy tone, and separately performs its own packet transmission operation. Then, admission control mechanism protects existing flows from new ones, and maintains the QoS of the admitted flows based on the measured channel status information. Performance of the proposed scheme is evaluated by simulation. Our results show that the proposed scheme is very effective in guaranteeing the QoS of multimedia applications as well as in avoiding the performance starvation of low priority traffics.     

On optimizing backoff counter reservation and classifying stations for the IEEE 802.11 distributed wireless LANs

In this paper, we propose a novel contention-based protocol called backoff counter reservation and classifying stations for the IEEE 802.11 distributed coordination function (DCF). In the proposed scheme, each station has three states: idle, reserved, and contentious. A station is in the idle state if it has no frame ready to transmit. A station is in the reserved state if it has a frame ready to transmit and this frame's backoff counter has been successfully announced through the previous successfully transmitted frame so that other stations know this information. A station is in the contentious state if it has a frame ready to transmit, but this frame's backoff counter has not been successfully announced to other stations. All the stations in the idle state, the reserved state, and the contentious state form an idle group, a reserved group, and a contentious group, respectively. Two backoff schemes are proposed in the BCR-CS protocol based on the number of stations in the contentious group including the optimal pseudo-p-persistent scheme. The proposed schemes are compared with the DCF and the enhanced collision avoidance (ECA) scheme in the literature. Extensive simulations and some analytical analysts are carried out. Our results show that all proposed schemes outperform both the DCF and the ECA, and the BCR-CS with optimal pseudo-p-persistent scheme is the best scheme among the four schemes.     

Integer-multiple-spacing-based scheduling for multimedia applications in IEEE 802.11e HCCA wireless networks

This paper is concerned with improving bandwidth utilization while guaranteeing quality-of-service (QoS) requirements for multimedia applications in IEEE 802.11e HCCA wireless networks. We propose two integer-multiple-spacing-based (IMS-based) designs: the IMS-based-δ and IMS-based-l designs. In the IMS-based designs, the scheduling spacing of any traffic must be an integer multiple of the minimum spacing. The IMS-based designs have more potential to improve the bandwidth utilization because they schedule traffic with longer spacings compared with the prior designs. Besides the improvement of the bandwidth utilization, the IMS-based-δ design can guarantee the packet delay requirements, where a δ factor is introduced to ensure that the bandwidth utilization must benefit from longer spacings, while the IMS-based-l design can guarantee the packet loss requirements. Numerical results show that the improvement of the bandwidth utilization for the IMS-based-δ and -l designs can be up to 7.7% and 56% or more, respectively, compared with the prior designs.     

IEEE802.11e EDCA饱和吞吐量的性能分析

提出了新的饱和状态下EDCA分析模型。与原有模型相比增加了站点内部竞争冲突和传输机遇两个特性。根据提出的模型,估计了多业务在实施不同的传输机遇情况下的吞吐量性能。仿真结果显示提出的新模型比原模型更加准确。     

Slot allocation algorithms in centralized scheduling scheme for IEEE 802.16 based wireless mesh networks

In IEEE 802.16 based wireless mesh networks (WMNs), TDMA (Time Division Multiple Access) is employed as the channel access method and only TDD (Time Division Duplex) is supported and there are no clearly separate downlink and uplink subframes in the physical frame structure. As the uplink and downlink traffic has different characteristics in that the uplink traffic decentralizes in each MSS (Mesh Subscriber Station) and the downlink traffic centralizes in the MBS (Mesh Base Station), different scheduling methods should be taken in the uplink and downlink. This paper presents a uniform slot allocation algorithm which is suitable for both uplinks and downlinks. To achieve higher spatial reuse and greater throughput and to avoid switching frequently between receiving and transmitting within two adjacent time slots when a relay node forwards traffic, different link selection criteria are taken into account when allocating slots for uplinks and downlinks. A combined uplink and downlink slot allocation algorithm is proposed for further improving the spatial reuse and network throughput. The proposed algorithms are evaluated by extensive simulations and the results show that it has good performance in terms of spatial reuse and network throughput. To the best of the authors’ knowledge, this work is the first one that considers combined uplink and downlink slot allocation on the centralized scheduling scheme in IEEE 802.16 based WMNs.     

IEEE 802.11e网络中传输可分级视频流的调度方案

在保证可分级视频流延时要求的情况下,为了有效提高其在无线网络中传输的带宽利用率,提出了一种新的调度方案。该方案利用可分级视频流的分解特性以及不同的流采用不同的调度周期来提高带宽利用率。数值结果表明该方案非常有效,特别是当物理层传输速率较大的时候,其有效性更加明显。     

Adaptive unequal protection for wireless video transmission over IEEE 802.11e networks

Packet loss of video streams cannot be avoided at wireless links for limited wireless bandwidth and frequently changed environments. To provide differentiated Quality of Service (QoS) guarantees between multimedia and data services, IEEE 802.11e was proposed. However, its performance and flexibility need to be further improved. In this paper, after a survey on various modifications of IEEE 802.11e, we formulate the problem of video transmission over IEEE 802.11e networks to help scheme design and performance analysis. Then accompanied with in-depth analysis, an adaptive unequal protection schema is proposed, which is composed of three mechanisms: (1) Insert each video packet into the access category (AC) with the minimum relative queuing delay; (2) Assign each packet dynamically to a proper AC based on several parameters to guarantee the transmission of high priority frames; (3) Apply fuzzy logic controllers to adjust parameters so as to reply quickly to the variation of video data rate, coding structure and network load. Finally, regarding MPEG-4 codec as the example, we perform extensive evaluations and validate the effectiveness and flexibility of proposed scheme. Simulations are divided into WLAN and multihop parts, involving different video sequences and various traffic modes of data streams. Beside performance comparison between proposed scheme and other ones, influence of parameter setting and combination with routing algorithms are also evaluated.     

On Sustained QoS Guarantees in Operated IEEE 802.11 Wireless LANs

Most of QoS-capable IEEE 802.11 MAC protocols are unable to deliver sustained quality of service while maintaining high network utilization, particularly under congested network conditions. The problem often resides in the fact that flows belonging to the same service class are assigned the same MAC parameters regardless theirs respective bitrate, which leads to throughput fairness rather than perceived QoS fairness. Harmonizing MAC parameters of traffic classes's flows may further lead to sub-optimal situations since certain network configurations (in terms of per class traffic load) can not be accommodated without reassigning the basic MAC parameters. In this paper, we propose a new cross-layer MAC design featuring a delay-sensitive backoff range adaptation along with a distributed flow admission control. By monitoring both MAC queue dynamics and network conditions, each traffic class reacts based on the degree to which application QoS metrics (delay) are satisfied. Besides, we use a distributed admission control mechanism to accept new flows while protecting the active one. Simulation results show that compared to the enhanced distributed coordination function (EDCA) scheme of 802.11e, our protocol consistently excels, in terms of network utilization, bounded delays, and service-level fairness.