WLAN中基于效用的呼叫接纳控制策略

 为了在802.11的网络中提供服务质量(QoS)支持,IEEE 802.11 Task Group E提出了EDCF协议.然而EDCF只能提供业务区分服务,并不能提供服务质量(QoS)保证.为了能在重负载下提供QoS保证,在WLAN中加入呼叫接纳控制(CAC)机制是非常必要的.本文首先提出了一个新的3维Markov模型对非饱和状态下EDCF的吞吐量和平均接入时延进行了分析.并在此基础上,提出了一种基于效用函数的CAC策略,它可以使网络的总收益达到最大.最后通过大量仿真验证了所提出的CAC策略的有效性.     

Provisioning of multimedia services in 802.11-based networks: facts and challenges

QoS provisioning in IEEE 802.11 networks is a nontrivial task due to a certain degree of randomness in the contention-based medium access control protocol. The problem often resides in the fact that flows belonging to the same service class use the same MAC parameters regardless of their respective bit rates. Assigning static MAC parameters confines WLAN deployment possibilities, and often leads to throughput fairness rather than perceived QoS fairness. This creates situations where network resources as well as their potential profitability are underex-ploited. Restricting the volume of traffic load carried by the network is a primordial task in order to preserve QoS performances of active multimedia services. In this article we review existing approaches to deliver QoS to real-time services in order to gain thorough insight into inhibiting factors inherent to contention-based 802.11 networks. The emphasis is put on studying the possible means to sustain QoS guarantees, which is of utmost importance for network operators willing to commit theirs underlying resources through service level agreements.     

一种用于移动自组织网的多址接入动态时延退避策略

在移动Ad Hoc网络中，信道接入公平性和吞吐率是MAC协议需解决的重要问题，而IEEE802．11等协议采用的二进制指数退避算法BEB难以满足公平性要求。本文提出了一种基于对节点竞争失败次数（无效RTS）进行计数的方法估计信道争用情况，动态地分配退避计数器初值，从而实现移动Ad Hoc网络的公平多址接入。研究表明，该接入方法能够有效地反映源节点特性，接入公平性好，同时在高负荷和低负荷的情况下，都能提高网络吞吐量，提供良好的QoS保障。     

A traffic control system for IEEE 802.11 networks based on available bandwidth estimation

To support Quality of service (QoS)‐sensitive applications like real‐time video streaming in IEEE 802.11 networks, a MAC layer extension for QoS, IEEE 802.11e, has been recently ratified as a standard. This MAC layer solution, however, addresses only the issue of prioritized access to the wireless medium and leaves such issues as QoS guarantee and admission control to the traffic control systems at the higher layers. This paper presents an IP‐layer traffic control system for IEEE 802.11 networks based on available bandwidth estimation. We build an analytical model for estimating the available bandwidth by extending an existing throughput computation model, and implement a traffic control system that provides QoS guarantees and admission control by utilizing the estimated available bandwidth information. We have conducted extensive performance evaluation of the proposed scheme via both simulations and measurements in the real test‐bed. The experiment results show that our estimation model and traffic control system work accurately and effectively in various network load conditions without IEEE 802.11e. The presence of IEEE 802.11e will allow even more efficient QoS provision, as the proposed scheme and the MAC layer QoS support will complement each other. Copyright © 2006 John Wiley & Sons, Ltd.     

An enhancement to the IEEE 802.11e EDCA providing QoS guarantees

One of the challenges that must be overcome to realize the practical benefits of ad hoc networks is quality of service (QoS). However, the IEEE 802.11 standard, which undeniably is the most widespread wireless technology of choice for WLANs and ad hoc networks, does not address this issue. In order to support applications with QoS requirements, the upcoming IEEE 802.11e standard enhances the original IEEE 802.11 MAC protocol by introducing a new coordination function which has both contention-based and contention-free medium access methods. In this paper, we consider the contention-based medium access method, the EDCA, and propose an extension to it such that it can be used to provide QoS guarantees in WLANs operating in ad hoc mode. Our solution is fully distributed, uses admission control to regulate the usage of resources and gives stations with high-priority traffic streams an opportunity to reserve time for collision-free access to the medium.     

Design of MAC protocols with fast collision resolution for wireless local area networks

Development of efficient medium access control (MAC) protocols providing both high throughput performance for data traffic and good quality of service (QoS) support for real-time traffic is the current major focus in distributed contention-based MAC protocol research. In this paper, we propose an efficient contention resolution algorithm for wireless local area networks, namely, the fast collision resolution (FCR) algorithm. The MAC protocol with this new algorithm attempts to provide significantly higher throughput performance for data services than the IEEE 802.11 MAC algorithm and more advanced dynamic tuning backoff (DTB) algorithm. We demonstrate that this algorithm indeed resolves collisions faster and reduces the idle slots more effectively. To provide good fairness performance and to support good QoS for real-time traffic, we incorporate the self-clocked fair queueing algorithm and a priority scheme into the FCR algorithm and come up with the real-time FCR (RT-FCR) algorithm, and show that RT-FCR can simultaneously achieve high throughput and good fairness performance for nonreal-time traffic while maintaining satisfactory QoS support for real-time traffic.     

Adaptive QoS Management for IEEE 802.11 Future Wireless ISPs

Wireless Internet Service Providers (WISPs) are expected to be the new generation of access providers using the emerging IEEE 802.11 technology. Face to the high competition of providing network services, the WISP have to offer the best service to the users. For this purpose, the WISP networks' managers need to provide Quality of Service (QoS) with a minimum cost in their wireless networks. The current link layer IEEE 802.11b provides fair sharing of the radio resource with no service differentiation mechanism; similarly to the Internet best effort service. However, the ongoing standard IEEE 802.11e should implement a priority mechanism at the link layer to differentiate the users' traffic. In order to overcome the lack of differentiated mechanism in the current link layer IEEE 802.11b, hence controlling the utilization of the scarce radio resource, we propose in this article to deploy Diffserv architecture coupled with an adaptive provisioning of QoS to provide better services to the users with minimum WISP cost and improve the utilization of the radio resource. Compliant with the current and future IEEE 802.11 link layer, the proposed adaptive QoS provisioning mechanism reacts to the radio resource fluctuation and improves the number of accepted clients in the IEEE 802.11 wireless cells based on the WISP business policies. The network layer differentiation provided by the Diffserv architecture intends to control the concurrent access of the traffic to the scarce radio resources at the IP layer of the mobile hosts for the uplink traffic on one hand, and at the IP layer of the base stations for the downlink traffic on the other hand.     

Dynamic bandwidth management in IEEE 802.11-based multihop wireless networks

In this paper, we propose a new protocol named dynamic regulation of best-effort traffic (DRBT) which supports quality of service (QoS) throughput guarantees and provides a distributed regulation mechanism for best-effort traffic in multihop wireless networks. By adapting dynamically the rate of best-effort traffic at the link layer, DRBT increases the acceptance ratio of QoS flows and provides a good use of the remaining resources through the network. Our protocol also provides an accurate method to evaluate the available bandwidth in IEEE 802.11-based ad hoc networks which is able to differentiate QoS applications from best-effort traffic. Through extensive simulations, we compare the performance of our proposal scheme with some others protocols like QoS protocol for ad hoc real-time traffic for instance.     

Analysis of IEEE 802.11e for QoS support in wireless LANs

The IEEE 802.11e medium access control protocol is an emerging standard for wireless local area networks providing quality of service. An overview of this standard based on the current draft is presented on this article. We analyze the enhancements in 802.11 standard. The new hybrid coordination function of the IEEE 802.11e with its contention-based and contention-free (controlled) medium access control schemes is evaluated. The capability to provide QoS support is discussed by means of simulations.     

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.     

Improving Delay and Jitter Performance in Wireless Mesh Networks for Mobile IPTV Services

Wireless mesh networking has recently emerged as a promising technology for the next-generation wireless networks. In wireless mesh networks (WMNs), it is practically attractive to support the low-cost quality-of-service (QoS) guaranteed mobile TV service. To meet this need, our study addresses how to improve the delay and jitter performance of mobile IPTV services over IEEE 802.11 based WMN. Particularly, we first discuss the adaptation of IEEE 802.11 MAC layer to construct a WMN with emphasis on mobile IPTV service; we then develop an enhanced version of Guaranteed-Rate (GR) packet scheduling algorithm, namely virtual reserved rate GR (VRR-GR), to further reduce the delay and suppress the jitter in multiservice network environment. Simulation results show that our proposed approach can satisfyingly prioritize mobile IPTV services in WMN, while providing non-IPTV services with what they need as well.      

A survey of QoS enhancements for IEEE 802.11 wireless LAN

Quality‐of‐service (QoS) is a key problem of today's IP networks. Many frameworks (IntServ, DiffServ, MPLS etc.) have been proposed to provide service differentiation in the Internet. At the same time, the Internet is becoming more and more heterogeneous due to the recent explosion of wireless networks. In wireless environments, bandwidth is scarce and channel conditions are time‐varying and sometimes highly lossy. Many previous research works show that what works well in a wired network cannot be directly applied in the wireless environment. Although IEEE 802.11 wireless LAN (WLAN) is the most widely used IEEE 802.11 wireless LAN (WLAN) standard today, it cannot provide QoS support for the increasing number of multimedia applications. Thus, a large number of 802.11 QoS enhancement schemes have been proposed, each one focusing on a particular mode. This paper summarizes all these schemes and presents a survey of current research activities. First, we analyze the QoS limitations of IEEE 802.11 wireless MAC layers. Then, different QoS enhancement techniques proposed for 802.11 WLAN are described and classified along with their advantages/drawbacks. Finally, the upcoming IEEE 802.11e QoS enhancement standard is introduced and studied in detail. Copyright © 2004 John Wiley & Sons, Ltd.     

Quality-of-service provisioning system for multimedia transmission in IEEE 802.11 wireless LANs

IEEE 802.11, the standard of wireless local area networks (WLANs), allows the coexistence of asynchronous and time-bounded traffic using the distributed coordination function (DCF) and point coordination function (PCF) modes of operations, respectively. In spite of its increasing popularity in real-world applications, the protocol suffers from the lack of any priority and access control policy to cope with various types of multimedia traffic, as well as user mobility. To expand support for applications with quality-of-service (QoS) requirements, the 802.11E task group was formed to enhance the original IEEE 802.11 medium access control (MAC) protocol. However, the problem of choosing the right set of MAC parameters and QoS mechanism to provide predictable QoS in IEEE 802.11 networks remains unsolved. In this paper, we propose a polling with nonpreemptive priority-based access control scheme for the IEEE 802.11 protocol. Under such a scheme, modifying the DCF access method in the contention period supports multiple levels of priorities such that user handoff calls can be supported in wireless LANs. The proposed transmit-permission policy and adaptive bandwidth allocation scheme derive sufficient conditions such that all the time-bounded traffic sources satisfy their time constraints to provide various QoS guarantees in the contention free period, while maintaining efficient bandwidth utilization at the same time. In addition, our proposed scheme is provably optimal for voice traffic in that it gives minimum average waiting time for voice packets. In addition to theoretical analysis, simulations are conducted to evaluate the performance of the proposed scheme. As it turns out, our design indeed provides a good performance in the IEEE 802.11 WLAN's environment, and can be easily incorporated into the hybrid coordination function (HCF) access scheme in the IEEE 802.11e standard.     

A survey of quality of service in IEEE 802.11 networks

Developed as a simple and cost-effective wireless technology for best effort services, IEEE 802.11 has gained popularity at an unprecedented rate. However, due to the lack of built-in quality of service support, IEEE 802.11 experiences serious challenges in meeting the demands of multimedia services and applications. This article surveys 802.11 QoS schemes, including service differentiation in the MAC layer, admission control and bandwidth reservation in MAC and higher layers, and link adaptation in the physical layer, designed to meet these challenges by providing the necessary enhancements for the required QoS. Furthermore, the article addresses issues that arise when end-to-end QoS has to be guaranteed in today's pervasive heterogeneous wired-cum-wireless networks. Among these challenges, protocol interoperability, multihop scheduling, full mobility support, and seamless vertical handoff among multiple mobile/wireless interfaces are specifically addressed.     

On the unfairness in IEEE 802.11e networks: Desynchronization of its medium access mechanism

Since the advent of the first IEEE 802.11 standard, several papers have proposed means of providing QoS to IEEE 802.11 networks and evaluate various traffic-prioritization mechanisms. Nevertheless, studies on the assignment of AIFS times defined in IEEE 802.11e reveal that the various priority levels work in a synchronized manner. The studies show that, under large loads of high-priority traffic, EDCA starves low-priority frames, which is undesirable. We argue that QoS traffic needs to be prioritized, but users sending best-effort frames should also obtain the expected service. High-priority traffic can also suffer performance degradation when using EDCA because of heavy loads of low-priority frames. Thus, we have proposed a mechanism based on desynchronizing the IEEE 802.11e working procedure. It prevents stations that belong to different priority classes from attempting simultaneous transmission, prioritizes independent collision groups and achieves better short-term and long-term channel access fairness. We have evaluated the proposal based on extensive analytical and simulation results. It prevents the strangulation of low-priority traffic, and, moreover, reduces the degradation of high-priority traffic with the increased presence of low-priority frames.     

Sticky CSMA/CA: Implicit synchronization and real-time QoS in mesh networks

We propose a novel approach to QoS for real-time traffic over wireless mesh networks, in which application layer characteristics are exploited or shaped in the design of medium access control. Specifically, we consider the problem of efficiently supporting a mix of Voice over IP (VoIP) and delay-insensitive traffic, assuming a narrowband physical layer with CSMA/CA capabilities. The VoIP call carrying capacity of wireless mesh networks based on classical CSMA/CA (e.g., the IEEE 802.11 standard) is low compared to the raw available bandwidth, due to lack of bandwidth and delay guarantees. Time Division Multiplexing (TDM) could potentially provide such guarantees, but it requires fine-grained network-wide synchronization and scheduling, which are difficult to implement. In this paper, we introduce Sticky CSMA/CA, a new medium access mechanism that provides TDM-like performance to real-time flows without requiring explicit synchronization. We exploit the natural periodicity of VoIP flows to obtain implicit synchronization and multiplexing gains. Nodes monitor the medium using the standard CSMA/CA mechanism, except that they remember the recent history of activity in the medium. A newly arriving VoIP flow uses this information to grab the medium at the first available opportunity, and then sticks to a periodic schedule, providing delay and bandwidth guarantees. Delay-insensitive traffic fills the gaps left by the real-time flows using novel contention mechanisms to ensure efficient use of the leftover bandwidth. Large gains over IEEE 802.11 networks are demonstrated in terms of increased voice call carrying capacity (more than 100% in some cases). We briefly discuss extensions of these ideas to a broader class of real-time applications, in which artificially imposing periodicity (or some other form of regularity) at the application layer can lead to significant enhancements of QoS due to improved medium access.     

UPCF: a new point coordination function with QoS and power management for multimedia over wireless LANs

In this paper, we propose a new novel polling-based medium access control protocol, named UPCF (Unified Point Coordination Function), to provide power conservation and quality-of-service (QoS) guarantees for multimedia applications over wireless local area networks. Specifically, UPCF has the following attractive features. First, it supports multiple priority levels and guarantees that high-priority stations always join the polling list earlier than low-priority stations. Second, it provides fast reservation scheme such that associated stations with real-time traffic can get on the polling list in bounded time. Third, it employs dynamic channel time allocation scheme to support CBR/VBR transportation and provide per-flow probabilistic bandwidth assurance. Fourth, it employs the power management techniques to let mobile stations save as much energy as possible. Fifth, it adopts the mobile-assisted admission control technique such that the point coordinator can admit as many newly flows as possible while not violating QoS guarantees made to already-admitted flows. The performance of UPCF is evaluated through both analysis and simulations. Simulation results do confirm that, as compared with the PCF in IEEE 802.11, UPCF not only provides higher goodput and energy throughput, but also achieves lower power consumption and frame loss due to delay expiry. Last but not least, we expect that UPCF can pass the current Wi-Fi certification and may coexist with the upcoming IEEE 802.11e standard.     

QoS provisioning for large-scale multi-ap WLANs

Large-scale deployment of IEEE 802.11 wireless LANs (WLANs) with a high density of access points (APs) has become commonplace due mainly to its potential for numerous benefits, such as ubiquitous service coverage, seamless handover, and improved link quality. However, the increased AP density can induce significant channel contention among neighboring cells, thus causing severe performance degradation and throughput imbalance between cells. There have been a plethora of research efforts to improve the WLAN performance, but most of them focused only on single WLAN environments without accounting for inter-cell contention. The de facto QoS-provisioning mechanism for WLANs, i.e., the Enhanced Distributed Channel Access (EDCA), is no exception to this. The EDCA focuses only on inter-flow priority distinction and has not considered the effect of inter-cell contention which significantly restricts its efficiency. This paper presents an enhanced QoS provisioning framework that takes into account inter-cell level differentiation as well as inter-flow level priority, which may be viewed as extension of QoS provisioning from a single-WLAN domain to a multi-WLAN domain. We also propose an architecture for managing multi-AP systems in which a central controller regulates the wireless channel occupancy of APs by adaptively configuring the cell-level QoS parameters. Our extensive simulation results show that the proposed inter-AP cooperative QoS scheme overcomes the limit of legacy 802.11e and provides a high level of fairness in large-scale densely-deployed WLANs.     

A bandwidth allocation/sharing/extension protocol for multimediaover IEEE 802.11 ad hoc wireless LANs

We propose a novel bandwidth allocation/sharing/extension (DBASE) protocol to support both asynchronous traffic and multimedia traffic with the characteristics of variable bit rate (VBR) and constant bit rate (CBR) over IEEE 802.11 ad hoc wireless local area networks. The overall quality of service (QoS) will be guaranteed by DBASE. The designed DBASE protocol will reserve bandwidth for real-time stations based on a fair and efficient allocation. Besides, the proposed DBASE is still compliant with the IEEE 802.11 standard. The performance of DBASE is evaluated by analysis and simulations. Simulations show that the DBASE is able to provide almost 90% channel utilization and low packet loss due to delay expiry for real-time multimedia services