一种新的用于IEEE 802.11e EDCA中提供QoS的方法

该文提出了一种新的应用于IEEE 802.11e EDCA (Enhanced Distributed Channel Access)中提供QoS(Quality of Service)的方法。这种方法是将几个时隙组合起来构成一个超时隙,每个超时隙的开始分配给不同的业务来进行发包。时隙的分配是根据各种业务的不同优先级来实现的。这种方法可以保证高优先级业务具有较大的吞吐量,较少的MAC延时和较低的丢包率。与802.11e EDCA草案中提出的不同冲突窗口大小的方法相比,这种方法具有可以提高吞吐量,降低丢包率,并能减小站点数目变化对高优先级业务吞吐量的影响等优点。这种新的提供QoS的方法优于不同冲突窗口大小的方法,在IEEE 802.11e EDCA中应用超时隙方法可以大大提高EDCA的性能。     

A unified model for the performance analysis of IEEE 802.11e EDCA

Rapid deployment of IEEE 802.11 wireless local area networks (WLANs) and their increasing quality of service (QoS) requirements motivate extensive performance evaluations of the upcoming 802.11e QoS-aware enhanced distributed coordination function (EDCA). Most of the analytical studies up-to-date have been based on one of the three major performance models in legacy distributed coordination function analysis, requiring a large degree of complexity in solving multidimensional Markov chains. Here, we expose the common guiding principle behind these three seemingly different models. Subsequently, by abstracting, unifying, and extending this common principle, we propose a new unified performance model and analysis method to study the saturation throughput and delay performance of EDCA, under the assumption of a finite number of stations and ideal channel conditions in a single-hop WLAN. This unified model combines the strengths of all three models, and thus, is easy to understand and apply; on the other hand, it helps increase the understanding of the existing performance analysis. Despite its appealing simplicity, our unified model and analysis are validated very well by simulation results. Ultimately, by means of the proposed model, we are able to precisely evaluate the differentiation effects of EDCA parameters on WLAN performance in very broad settings, a feature which is essential for network design.     

A delay-based admission control mechanism for multimedia support in IEEE 802.11e wireless LANs

Multimedia over IEEE 802.11 wireless local area networks (WLANs) has recently been the focus of many researchers due to its rapidly increasing popularity. Unlike their best-effort counterparts, multimedia applications have quality of service (QoS) needs typically expressed in terms of the maximum allowed delay and/or the minimum required throughput. Therefore, prior to accepting a multimedia application, the network must assure the satisfaction of its QoS requirements. In this paper, we develop a mechanism that can be used to control the admissibility of multimedia applications into WLANs. To develop the proposed mechanism, we first derive an analytical approximation of the delay experienced by packets when travelled through these networks. The analytical approximation of the delay is then used to propose an admission control mechanism for the enhanced distributed channel access (EDCA) method used by the hybrid coordination function (HCF) of IEEE 802.11e. The proposed delay-based admission control mechanism is validated via simulations of voice traffic.     

Performance analysis of IEEE 802.11e contention-based channel access

The new standard IEEE 802.11e is specified to support quality-of-service in wireless local area networks. A comprehensive study of the performance of enhanced distributed channel access (EDCA), the fundamental medium access control mechanism in IEEE 802.11e, is reported in this paper. We present our development of an analytical model, in which most new features of the EDCA such as virtual collision, different arbitration interframe space (AIFS), and different contention window are taken into account. Based on the model, we analyze the throughput performance of differentiated service traffic and propose a recursive method capable of calculating the mean access delay. Service differentiation functionality and effectiveness of the EDCA are investigated through extensive numerical and simulation results. The model and the analysis provide an in-depth understanding and insights into the protocol and the effects of different parameters on the performance.     

车用自组网媒体访问控制机制改进

为提升车用自组网传输音频、视频的服务质量,对基于IEEE802.11p的车用无线接入技术MAC机制进行改进,提出竞争窗口自适应EDCA机制。仿真实验表明,竞争窗口自适应EDCA机制有效地降低了车用自组网中音频、视频流的传输时延、时延抖动和丢包率,保证了车用自组网传输VoIP、视频会议、音视频流媒体等多媒体业务的服务质量。     

Model-Based QoS Parameter Control for IEEE 802.11e EDCA

In this paper, we give an analytical justification how to control the QoS parameters using a throughput approximation model for IEEE 802.11e EDCA. Using the model, we propose a new parameter control algorithm which successfully achieves the dynamic parameter assignment under desired throughput ratio constraints.     

Assessing the IEEE 802.11e QoS effectiveness in multi-hop indoor scenarios

The IEEE 802.11e technology is receiving much interest due to the enhancements offered to wireless local area networks in terms of QoS. Other application fields for this technology are wireless ad hoc networks, wireless mesh networks, and vehicular ad hoc networks. In the literature, most of the research works available focusing on the IEEE 802.11e technology offer simulation results alone, being hard to find empirical results of implementations that prove its effectiveness in realistic scenarios. Additionally, we consider that studies of IEEE 802.11e based on simulation platforms have not been thoroughly validated using real-life results. In this work we analyze the performance of the IEEE 802.11e technology in real multi-hop ad hoc networks. With this purpose we first we devise a set of experiments where we compare the results obtained on a small testbed to those from the ns-2 simulation platform. A significant consistency in terms of overall trends is found, although remarkable differences can be appreciated in terms of both delay and throughput results. Afterward we proceed with a full deployment of IEEE 802.11e enabled stations throughout the floor of an university building, performing several experiments using both static and dynamic routing. Experimental results show that QoS can be reasonably sustained for both voice and video traffic in multi-hop ad hoc networks, although dynamic routing protocols can hinder performance by provoking frequent on-off connectivity problems.     

Incomplete cooperation‐based service differentiation in WLANs

In the IEEE 802.11 wireless LAN (WLAN), the fundamental medium access control (MAC) mechanism—distributed coordination function (DCF), only supports best‐effort service, and is unaware of the quality‐of‐service (QoS). IEEE 802.11e enhanced distributed channel access (EDCA) supports service differentiation by differentiating contention parameters. This may introduce the problem of non‐cooperative service differentiation. Hence, an incompletely cooperative EDCA (IC‐EDCA) is proposed in this paper to solve the problem. In IC‐EDCA, each node that is cooperative a priori adjusts its contention parameters (e.g., the contention window (CW)) adaptively to the estimated system state (e.g., the number of competing nodes of each service priority). To implement IC‐EDCA in current WLAN nodes, a frame‐analytic estimation algorithm is presented. Moreover, an analytical model is proposed to analyze the performance of IC‐EDCA under saturation cases. Extensive simulations are also carried out to compare the performances of DCF, EDCA, incompletely cooperative game, and IC‐EDCA, and to evaluate the accuracy of the proposed performance model. The simulation results show that IC‐EDCA performs better than DCF, EDCA, and incompletely cooperative game in terms of system throughput or QoS, and that the proposed analytical model is valid. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance Analysis on Coexistence of EDCA and Legacy DCF Stations in IEEE 802.11 Wireless LANs

The distributed coordination function (DCF) scheme of IEEE 802.11 MAC protocol does not support any concepts of quality of service (QoS) but the enhanced distributed channel access (EDCA) scheme in IEEE 802.11e standard provides QoS according to access categories using different access parameters. However, the legacy DCF stations may be used together with EDCA stations. In this letter, we investigate and analyze the performance discrimination when EDCA and DCF stations operate simultaneously     

Throughput and delay analysis for the IEEE 802.11e enhanced distributed channel access

In this letter, we propose a three-dimensional Markov chain model for the 802.11e enhanced distributed channel access (EDCA) mode. This model can be used to compute the maximum sustainable throughput and service delay distribution for each priority class when under saturation load. The new framework models the performance impact of major quality-of-service (QoS)-specific features (e.g., CWMin, CWMax, AIFS, internal collision resolution) of the 802.11e EDCA mode, and hence can provide an analytical approach to pick the parameter values associated with EDCA to meet the QoS requirements of each priority.     

基于Ad hoc网络的新EDCA参数调整方案

IEEE802.11e标准中业务优先级不同的AC（access category）是通过设置不同的竞争窗口最大、最小值CWmax,CWmin和仲裁帧间隔值来体现的,如高优先级AC设置小的CWmin,CWmax和AIFS值.研究表明,EDCA对每个AC指定的默认参数值只适用于中等负载、节点数目少的网络场景,并不适用于负载较重、节点数目较多且链路动态变化的Ad hoc网络环境.提出了一种根据网络状况动态调整IEEE 802.11e EDCA的QoS参数的新方案I-ED-CA,该方案根据网络状态调整竞争窗口CW,并通过修改退避计数器值调整AIFS参数,使I-EDCA适合动态变化的Ad hoc网络环境,采用NS2仿真软件对EDCA改进协议I-EDCA进行仿真.仿真结果表明,随着网络中负载的增加,I-EDCA的吞吐量表现平稳,而EDCA吞吐量是下降的.另外,在业务公平性方面,对优先级不同的业务I-EDCA比EDCA的表现更公平.     

A Distributed End-to-End Reservation Protocol for IEEE 802.11-Based Wireless Mesh Networks

This paper presents an end-to-end reservation protocol for quality-of-service (QoS) support in the medium access control layer of wireless multihop mesh networks. It reserves periodically repeating time slots for QoS-demanding applications, while retaining the distributed coordination function (DCF) for best effort applications. The key features of the new protocol, called "distributed end-to-end allocation of time slots for real-time traffic (DARE), are distributed setup, interference protection, and scheduling of real-time data packets, as well as the repair of broken reservations and the release of unused reservations. A simulation-based performance study compares the delay and throughput of DARE with those of DCF and the priority-based enhanced distributed channel access (EDCA) used in IEEE 802.11e. In contrast to DCF and EDCA, DARE has a low, nonvarying delay and a constant throughput for each reserved flow     

A distributed MAC scheme to achieve QoS in ad hoc networks

Real-time applications introduce new requirements on wireless networks and impose quality thresholds on parameters like delay, jitter, throughput, and packet loss in order to run smoothly. This paper addresses this issue by presenting a MAC scheme that offers real-time applications the opportunity to reserve transmission time based on their QoS requirements for contention-free medium access. Our scheme, which is called EDCA with Resource Reservation (EDCA/RR), operates in a fully distributed manner, is compatible with IEEE 802.11, and provides both prioritized and parameterized QoS. In this study, we have extended EDCA/RR to handle reservation collisions and, through extensive simulations, we show that our proposal can handle multiple reservations as well as uninformed stations that lie outside the transmission range of both the transmitter and the receiver while providing QoS guarantees. We compare EDCA/RR with EDCA and our results show that, as the traffic in the network increases, EDCA/RR succeeds providing the required service to QoS-demanding applications whereas EDCA fails in this task. In addition, when the medium is lossy we show that, not only does EDCA/RR give better service to real-time traffic, but also to contending non-real-time traffic.     

A Performance Analysis Model for IEEE 802.11e EDCA Under Saturation Condition

An accurate model of IEEE 802.11e EDCA for a performance analysis that captures all major QoS features of the standard is introduced in this paper. Using the model, we derive the saturation throughput, and validate the accuracy of the proposed model through comparisons with other models via simulations.     

Supporting multimedia streaming and best‐effort data transmission over IEEE 802.11e EDCA

As demand for broadband multimedia wireless services increases, improving quality of service (QoS) of the widely deployed IEEE 802.11 wireless LAN (WLAN) has become crucial. To support the QoS required by a wide range of applications, the IEEE 802.11 working group has defined a new standard: IEEE 802.11e. In this paper, we propose a measurement‐based dynamic media time allocation (MBDMTA) scheme combined with a concatenating window scheme to support real‐time variable bit rate (rt‐VBR) video and best‐effort (BE) data transmission using IEEE 802.11e enhanced distributed channel access (EDCA). To provide the QoS guarantee for rt‐VBR video, the proposed MBDMTA scheme dynamically assigns channel time to the rt‐VBR video based on the estimate of the required network resources. On the other hand, the concatenating window scheme controls the contention window (CW) ranges of different priority flows such that real‐time services always have higher channel access probability, thus achieving the capability of preemptive priorities. In addition, the concatenating window scheme preserves fairness among flows of the same class and attains high channel utilization under different network conditions. Simulation results demonstrate that the throughput and delay performance improve significantly for the transmission of rt‐VBR video and BE traffic as compared to those for the 802.11e EDCA specification. It is also revealed that combining the two proposed schemes provides seamless integration and reliable transmission of digital video and data service within the 802.11e EDCA framework. Copyright © 2007 John Wiley & Sons, Ltd.     

A MAC-Layer QoS Provisioning Protocol for Cognitive Radio Networks

Due to the proliferation of diverse network devices with multimedia capabilities, there is an increasing need for Quality of Service (QoS) provisioning in wireless networks. The MAC layer protocol with enhanced distributed channel access (EDCA) in the IEEE 802.11-2007 is able to provide differentiated QoS for different traffic types in wireless networks through varying the Arbitration Inter-Frame Spaces (AIFS) and contention window sizes. However, the performance of high priority traffic can be seriously degraded in the presence of strong noise over the wireless channels. Schemes utilizing adaptive modulation and coding (AMC) technique have also been proposed for the provisioning of QoS. They can provide limited protection in the presence of noise but are ineffective in a high noise scenario. Although multiple non-overlapped channels exist in the 2.4 and 5?GHz spectrum, most IEEE 802.11-based multi-hop ad hoc networks today use only a single channel at anytime. As a result, these networks cannot fully exploit the aggregate bandwidth available in the radio spectrum provisioned by the standards. By identifying vacant channels through the use of cognitive radios technique, the noise problem can be mitigated by distributing network traffic across multiple vacant channels to reduce the node density per transmission channel. In this paper, we propose the MAC-Layer QoS Provisioning Protocol (MQPP) for 802.11-based cognitive radio networks (CRNs) which combines adaptive modulation and coding with dynamic spectrum access. Simulation results demonstrate that MQPP can achieve better performance in terms of lower delay and higher throughput.     

Quality of service support in IEEE 802.11 wireless ad hoc networks

Supporting Quality of Service (QoS) in wireless networks is a challenging problem. The IEEE 802.11 LAN standard was developed primarily for elastic data applications. In order to support the transmission of real-time data, a polling-based scheme called the point coordination function (PCF) was introduced in IEEE 802.11. However, PCF was not able to meet the desired and practical service differentiation requirements to fulfill the need of real-time data. Therefore, Task Group E of the IEEE 802.11 working group released several IEEE 802.11e drafts, whose main task is to support QoS in IEEE 802.11 LANs. The polling scheme of PCF is extended in IEEE 802.11e into the more complex hybrid coordination function (HCF). We found that HCF has several performance issues that may affect its anticipated performance. In this paper, we address these issues and propose a QoS enhancement over PCF, called enhanced PCF (EPCF) that enables Wireless LAN to send a combination of voice, data and isochronous data packets using the current IEEE 802.11 PCF. First, we compare the performance of the proposed model (EPCF) with the HCF function of the IEEE 802.11e through simulation. Second, we extend the proposed model (EPCF) to work in a multihop wireless ad hoc mode and present the advantages and limitations in this case. Simulation results demonstrate an enhanced performance of our scheme over the legacy PCF and a comparable performance to the IEEE 802.11e HCF in terms of the average delay and system throughput. However, EPCF is much simpler than HCF, provides flow differentiation, and is easy to implement in the current IEEE 802.11 standard.     

Admission control in IEEE 802.11e wireless LANs

Although IEEE 802.11 based wireless local area networks have become more and more popular due to low cost and easy deployment, they can only provide best effort services and do not have quality of service supports for multimedia applications. Recently, a new standard, IEEE 802.11e, has been proposed, which introduces a so-called hybrid coordination function containing two medium access mechanisms: contention-based channel access and controlled channel access. In this article we first give a brief tutorial on the various MAC-layer QoS mechanisms provided by 802.11e. We show that the 802.11e standard provides a very powerful platform for QoS supports in WLANs. Then we provide an extensive survey of recent advances in admission control algorithms/protocols in IEEE 802.11e WLANs. Our survey covers the research work in admission control for both EDCA and HCCA. We show that the new MAC-layer QoS schemes and parameters provided in EDCA and HCCA can be well utilized to fulfill the requirements of admission control so that QoS for multimedia applications can be provided in WLANs. Last, we give a summary of the design of admission control in EDCA and HCCA, and point out the remaining challenges.     

Saturation Throughput Analysis of IEEE 802.11e EDCA Through Analytical Model

The IEEE 802.11e EDCA is designed to provide quality of support for real time applications with stringent latency and throughput requirements. Theoretical frameworks for analysis of throughput performance of wireless LAN employing exponential back-off exist extensively. Several models rely on simplification assumptions that preclude their direct applicability to the enhanced distributed coordination access (EDCA) which uses heterogeneous protocol parameters, while other models are exceedingly complex to analyze. In this paper, a tractable analytical model is proposed for saturation throughput of the IEEE 802.11e EDCA. The prioritization through channel access parameters including the AIFS and contention window is catered for within a three dimensional Markov chain. The integration of back-off counter freezing and retry limit enhance the models precision. Its validation is done by simulation on NS-2. Practical applicability of the model is established based on accuracy and computational efficiency. The model is utilized for throughput analysis of the EDCA under saturated traffic loads.     

IEEE802.11e EDCA和CFB下饱和吞吐量分析

IEEE802.11eMAC草案规范对IEEE802.11无线局域网标准在QOS方面加以了补充。IEEE802.11e采用2种协调机制基于控制的综合协调可控信道接入方式HCCA和基于竞争的增强型分布式信道接入方式EDCA。主要评估EDCA和竞争空闲脉冲(contention-freeburst)CFB相结合时,在系统负载过载的情况下,改变性能参数后系统饱和吞吐量的变化,并分析系统性能参数对饱和吞吐量的影响,从而达到系统参数优化的作用。