一种提高802.11无线Ad Hoc网络公平性的新机制-FFMA

实现多个数据流对无线信道的公平共享是802.11无线Ad Hoc网络中的一个重要议题,但802.11DCF机制在无线Ad Hoc网络中存在严重的公平性问题,甚至有可能出现单个节点或数据流独占信道而其他节点和数据流处于"饥饿"状态的情况.论文提出了一种新颖的保证数据流间公平性的MAC层接入机制FFMA(Flow rate-based Fair Medium Access),通过公平调度和公平竞争的方式,FFMA能够在数据流间公平地分配信道带宽资源.仿真结果表明,在无线Ad Hoc网络中,FFMA可以在保证信道吞吐量的前提下取得远优于802.11 DCF的数据流间的公平性.     

Enhanced binary exponential backoff algorithm for fair channel access in the ieee 802.11 medium access control protocol

The medium access control protocol determines system throughput in wireless mobile ad hoc networks following the ieee 802.11 standard. Under this standard, asynchronous data transmissions have a defined distributed coordination function that allows stations to contend for channel usage in a distributed manner via the carrier sensing multiple access with collision avoidance protocol. In distributed coordination function, a slotted binary exponential backoff (BEB) algorithm resolves collisions of packets transmitted simultaneously by different stations. The BEB algorithm prevents packet collisions during simultaneous access by randomizing moments at stations attempting to access the wireless channels. However, this randomization does not eliminate packet collisions entirely, leading to reduced system throughput and increased packet delay and drop. In addition, the BEB algorithm results in unfair channel access among stations. In this paper, we propose an enhanced binary exponential backoff algorithm to improve channel access fairness by adjusting the manner of increasing or decreasing the contention window based on the number of the successfully sent frames. We propose several configurations and use the NS2 simulator to analyze network performance. The enhanced binary exponential backoff algorithm improves channel access fairness, significantly increases network throughput capacity, and reduces packet delay and drop. Copyright © 2013 John Wiley & Sons, Ltd.     

Enhancing Fairness for Short-Lived TCP Flows in 802.11b WLANs

The problem of providing throughput fairness in a wired-cum-wireless network where the wireless portion is an 802.11 wireless local area network (WLAN) is addressed. Due to the distributed nature of the primary 802.11 media access control protocol and the unpredictability of the wireless channel, quality of service guarantees in general and fairness in particular are hard to achieve in WLANs. This fact seriously compromises the interaction between 802.11-based networks and well-established architectures such as DiffServ. The focus of this paper is on transmission control protocol (TCP) traffic, and two fundamental problems related to throughput fairness are identified. First, the basic requirement of providing fair access to all users conflicts with the nature of TCP, which is fair only under certain conditions and hardly met by 802.11b WLANs. Second, short-lived TCP flows that are sensitive to losses during the early stages of TCP window growth need to be protected. To address these issues, a logical-link-control-layer algorithm that can be implemented at both access points and wireless stations is proposed. The algorithm aims at guaranteeing fair access to the medium to every user, independent of their channel conditions. At the same time, the proposed scheme protects short-lived flows, while they strive to get past the critical "small window regime." A simulation study that shows the effectiveness of the new algorithm in comparison to the standard 802.11b implementation is presented     

Improving Quality of Service and Assuring Fairness in WLAN Access Networks

As public deployment of wireless local area networks (WLANs) has increased and various applications with different service requirements have emerged, fairness and quality of service (QoS) are two imperative issues in allocating wireless channels. This study proposes a fair QoS agent (FQA) to simultaneously provide per-class QoS enhancement and per-station fair channel sharing in WLAN access networks. FQA implements two additional components above the 802.11 MAC: a dual service differentiator and a service level manager. The former is intended to improve QoS for different service classes by differentiating service with appropriate scheduling and queue management algorithms, while the latter is to assure fair channel sharing by estimating the fair share for each station and dynamically adjusting the service levels of packets. FQA assures (weighted) fairness among stations in terms of channel access time without decreasing channel utilization. Furthermore, it can provide quantitative service assurance in terms of queuing delay and packet loss rate. FQA neither resorts to any complex fair scheduling algorithm nor requires maintaining per-station queues. Since the FQA algorithm is an add-on scheme above the 802.11 MAC, it does not require any modification of the standard MAC protocol. Extensive ns-2 simulations confirm the effectiveness of the FQA algorithm with respect to the per class QoS enhancement and per-station fair channel sharing     

Distributed fair scheduling in a wireless LAN

Fairness is an important issue when accessing a shared wireless channel. With fair scheduling, it is possible to allocate bandwidth in proportion to weights of the packet flows sharing the channel. This paper presents a fully distributed algorithm for fair scheduling in a wireless LAN. The algorithm can be implemented without using a centralized coordinator to arbitrate medium access. The proposed protocol is derived from the Distributed Coordination Function in the IEEE 802.11 standard. Simulation results show that the proposed algorithm is able to schedule transmissions such that the bandwidth allocated to different flows is proportional to their weights. An attractive feature of the proposed approach is that it can be implemented with simple modifications to the IEEE 802.11 standard.     

一种提高多速率WLAN公平性的MAC协议

DCF应用在多速率无线局域网中存在严重的不公平性：高速率站点占用信道的时间远小于低速率站点,只获得与低速率站点相同的吞吐量。该文利用区分服务的思想,提出接入时间公平的MDCF。MDCF通过为不同速率的站点设置不同的初始竞争窗口,来实现与发送速率成正比的接入概率,从而保证接入时间的公平性,理论分析和仿真结果都表明MDCF能够较大地提高多速率无线局域网中的时间公平性及吞吐量。     

Airtime Fairness for IEEE 802.11 Multirate Networks

Under a multirate network scenario, the IEEE 802.11 DCF MAC fails to provide airtime fairness for all competing stations since the protocol is designed for ensuring max-min throughput fairness. As such, the maximum achievable throughput by any station gets bounded by the slowest transmitting peer. In this paper, we present an analytical model to study the delay and throughput characteristics of such networks so that the rate anomaly problem of IEEE DCF multirate networks could be mitigated. We call our proposal time fair CSMA (TFCSMA) which utilizes an interesting baseline property for estimating a target throughput for each competing station so that its minimum contention window could be adjusted in a distributed manner. As opposed to the previous work in this area, TFCSMA is ideally suited for practical scenarios where stations frequently adapt their data rates to changing channel conditions. In addition, TFCSMA also accounts for packet errors due to the time varying properties of the wireless channel. We thoroughly compare the performance of our proposed protocol with IEEE 802.11 and other existing protocols under different network scenarios and traffic conditions. Our comprehensive simulations validate the efficacy of our method toward providing high throughput and time fair channel allocation.     

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.     

Relay-volunteered multi-rate cooperative MAC protocol for IEEE 802.11 WLANs

In IEEE 802.11, the rate of a station (STA) is dynamically determined by link adaptation. Low-rate STAs tend to hog more channel time than high-rate STAs due to fair characteristics of carrier sense multiple access/collision avoidance, leading to overall throughput degradation. It can be improved by limiting the transmission opportunities of low-rate STAs by backoff parameters. This, however, may cause unfair transmission opportunities to low-rate STAs. In an attempt to increase overall throughput by volunteer high-rate relay STAs while maintaining fairness, we propose a new cooperative medium access control (MAC) protocol, relay-volunteered multi-rate cooperative MAC (RM-CMAC) based on ready to send/clear to send in multi-rate IEEE 802.11. In the RM-CMAC protocol, we show that the effect of hogging channel time by low-rate STAs can be remedied by controlling the initial backoff window size of low-rate STAs and the reduced transmission opportunity of low-rate STAs can be compensated by the help of volunteer high-rate relay STAs. We analyze the performance of RM-CMAC, i.e., throughput and MAC delay, by a multi-rate embedded Markov chain model. We demonstrate that our analysis is accurate and the RM-CMAC protocol enhances the network throughput and MAC delay while maintaining the fairness of low-rate STAs.     

Next Generation Wireless Mobile System Efficient, Fair, Class Based Packet Scheduling Algorithm

Efficient utilization of network resources is a key goal for emerging broadband wireless access systems (BWAS). This is a complex goal to achieve due to the heterogeneous service nature and diverse quality of service (QoS) requirements of various applications that BWAS support. Packet scheduling is an important activity that affects BWAS QoS outcomes. This paper proposes a novel packet scheduling mechanism that improves QoS in mobile wireless networks which exploit IP as a transport technology for data transfer between BWAS base stations and mobile users at the radio transmission layer. In order to improve BWAS QoS the new packet algorithm makes changes at both the IP and the radio layers. The new packet scheduling algorithm exploits handoff priority scheduling principles and takes into account buffer occupancy and channel conditions. The packet scheduling mechanism also incorporates the concept of fairness. Performance results were obtained by computer simulation and compared to the well known algorithms. Results show that by exploiting the new packet scheduling algorithm, the transport system is able to provide a low handoff packet drop rate, low packet forwarding rate, low packet delay and ensure fairness amongst the users of different services.     

Wireless Packet Scheduling Algorithm for OFDMA System Based on Time‐Utility and Channel State

In this paper, we propose an urgency‐ and efficiencybased wireless packet scheduling (UEPS) algorithm that is able to schedule real‐time (RT) and non‐real‐time (NRT) traffics at the same time while supporting multiple users simultaneously at any given scheduling time instant. The UEPS algorithm is designed to support wireless downlink packet scheduling in an orthogonal frequency division multiple access (OFDMA) system, which is a strong candidate as a wireless access method for the next generation of wireless communications. The UEPS algorithm uses the time‐utility function as a scheduling urgency factor and the relative status of the current channel to the average channel status as an efficiency indicator of radio resource usage. The design goal of the UEPS algorithm is to maximize throughput of NRT traffics while satisfying quality‐of‐service (QoS) requirements of RT traffics. The simulation study shows that the UEPS algorithm is able to give better throughput performance than existing wireless packet scheduling algorithms such as proportional fair (PF) and modifiedlargest weighted delay first (M‐LWDF), while satisfying the QoS requirements of RT traffics such as average delay and packet loss rate under various traffic loads.     

MiFi: a framework for fairness and QoS assurance for current IEEE 802.11 networks with multiple access points

In this paper, we present a framework for providing fair service and supporting quality of service (QoS) requirements in IEEE 802.11 networks with multiple access points (APs). These issues becomes critical as IEEE 802.11 wireless LAN are widely deployed in nationwide networks, linking tens of thousands of "hot-spots" for providing both real-time (voice) and non real-time (data) services to a large population of mobile users. However, both fairness and QoS guarantees cannot be supported in the current 802.11 standard. Our system, termed MiFi, relies on centralized coordination of the APs. During any given time of the "contention-free" period only a set of non-interfering APs is activated while the others are silenced. Moreover, the amount of service granted to an AP is proportional to its load and the system's performance is optimized by employing efficient scheduling algorithms. We show that such a system can be implemented without requiring any modification of the underlying MAC protocol standard or the behavior of the mobile stations. Our scheme is complementary to the emerging 802.11e standard for QoS and guarantees to overcome the hidden node and the overlapping cell problems. Our simulations establish that the system supports fairness and hence can provide QoS guarantees for real-time traffic, while maintaining a relative high throughput.     

一种具有小尺度公平的无线分组调度算法

本文在调度判决时考虑到用户的访问时延限制,比例公平调度算法基础上提出了M-PF算法。本文建立了无线分组调度系统模型,通过仿真对新算法在小尺度服务时间保证、大尺度服务时间公平和系统吞吐量等方面的性能进行分析,研究了系统参数对算法性能的影响。结果证明,新算法在保证系统吞吐量和大尺度公平性的同时可以提供更好的小尺度服务时间保证。     

Distributed Proportional Fair Scheduling for IEEE802.11 Wireless LANs

In the time varying wireless channel, opportunistic scheduling is one of the important techniques to achieving the rich diversities inherent in wireless communications. However, most existing scheduling schemes require centralized scheduling and little work has been done on developing distributed algorithms The proportional fair scheduling is one of the representative opportunistic scheduling for centralized networks. In this paper, we propose distributed proportional fair scheduling (DPFS) scheme for wireless LAN network. In the proposed DPFS scheme, each receiver estimates channel condition and calculates independently its own priority with probabilistic manner, which can reduce excessive probing overhead required to gather the channel conditions of all receivers. We evaluate the proposed DPFS using extensive simulation and simulation results show that DPFS obtains higher network throughput than conventional scheduling schemes while maintaining fairness among users.     

Contention-Based Airtime Usage Control in Multirate IEEE 802.11 Wireless LANs

In a multirate wireless LAN, wireless/mobile stations usually adapt their transmission rates to the channel condition. It is difficult to control each station's usage of network resources since the shared channel can be overused by low transmission-rate stations. To solve this problem, we propose a distributed control of stations' airtime usage which 1) always guarantees each station to receive a specified share of airtime, and 2) keeps service for individual stations unaffected by other stations' transmission rates. Such airtime control enables service differentiation or quality of service (QoS) support. Moreover, it can achieve a higher overall system throughput. The proposed airtime usage control exploits the Enhanced Distributed Channel Access (EDCA) of the IEEE 802.11e standard . Two control mechanisms are proposed: one based on controlling the station's arbitration inter-frame space (AIFS) and the other based on the contention window size. We show how the stations' airtime usage is related to the AIFS and contention window size parameters. Using this relation, two analytical models are developed to determine the optimal control parameters. Unlike the other heuristic controls or analytical models, our model provides handles or parameters for quantitative control of stations' airtime usage. Our evaluation results show that a precise airtime usage control can be achieved in a multirate wireless LAN     

Self-coordinating localized fair queueing in wireless ad hoc networks

Distributed fair queueing in a multihop, wireless ad hoc network is challenging for several reasons. First, the wireless channel is shared among multiple contending nodes in a spatial locality. Location-dependent channel contention complicates the fairness notion. Second, the sender of a flow does not have explicit information regarding the contending flows originated from other nodes. Fair queueing over ad hoc networks is a distributed scheduling problem by nature. Finally, the wireless channel capacity is a scarce resource. Spatial channel reuse, i.e., simultaneous transmissions of flows that do not interfere with each other, should be encouraged whenever possible. In this paper, we reexamine the fairness notion in an ad hoc network using a graph-theoretic formulation and extract the fairness requirements that an ad hoc fair queueing algorithm should possess. To meet these requirements, we propose maximize-local-minimum fair queueing (MLM-FQ), a novel distributed packet scheduling algorithm where local schedulers self-coordinate their scheduling decisions and collectively achieve fair bandwidth sharing. We then propose enhanced MLM-FQ (EMLM-FQ) to further improve the spatial channel reuse and limit the impact of inaccurate scheduling information resulted from collisions. EMLM-FQ achieves statistical short-term throughput and delay bounds over the shared wireless channel. Analysis and extensive simulations confirm the effectiveness and efficiency of our self-coordinating localized design in providing global fair channel access in wireless ad hoc networks.     

The Design and Evaluation of Unified Cellular and Ad-Hoc Networks

In third-generation (3G) wireless data networks, providing service to low data-rate users is required for maintaining fairness, but at the cost of reducing the cell's aggregate throughput. In this paper, we propose the unified cellular and ad hoc network (UCAN) architecture for enhancing cell throughput while maintaining fairness. In UCAN, a mobile client has both 3G interface and IEEE 802.11 -based peer-to-peer links. The 3G base station forwards packets for destination clients with poor channel quality to proxy clients with better channel quality. The proxy clients then use an ad hoc network composed of other mobile clients and IEEE 802.11 wireless links to forward the packets to the appropriate destinations, thereby improving cell throughput. We refine the 3G base station scheduling algorithm so that the throughput gains are distributed in proportion to users' average channel rates, thereby maintaining fairness. With the UCAN architecture in place, we propose novel greedy and on-demand protocols for proxy discovery and ad hoc routing that explicitly leverage the existence of the 3G infrastructure to reduce complexity and improve reliability. We further propose secure crediting mechanisms to motivate users that are not actively receiving to participate in relaying packets for others. Through both analysis and extensive simulations with HDR and IEEE 802.11b, we show that the UCAN architecture can increase individual user's throughput by more than 100 percent and the aggregate throughput of the HDR downlink by up to 50 percent.     

OAR: An Opportunistic Auto-Rate Media Access Protocol for Ad Hoc Networks

The IEEE 802.11 wireless media access standard supports multiple data rates at the physical layer. Moreover, various auto rate adaptation mechanisms at the medium access layer have been proposed to utilize this multi-rate capability by automatically adapting the transmission rate to best match the channel conditions. In this paper, we introduce the Opportunistic Auto Rate (OAR) protocol to better exploit durations of high-quality channels conditions. The key mechanism of the OAR protocol is to opportunistically send multiple back-to-back data packets whenever the channel quality is good. As channel coherence times typically exceed multiple packet transmission times for both mobile and non-mobile users, OAR achieves significant throughput gains as compared to state-of-the-art auto-rate adaptation mechanisms. Moreover, over longer time scales, OAR ensures that all nodes are granted channel access for the same time-shares as achieved by single-rate IEEE 802.11. We describe mechanisms to implement OAR on top of any existing auto-rate adaptation scheme in a nearly IEEE 802.11 compliant manner. We also analytically study OAR and characterize the delay jitter and the gains in throughput as a function of the channel conditions. Finally, we perform an extensive set of ns-2 simulations to study the impact of such factors as node velocity, channel conditions, and topology on the throughput of OAR.     

Efficient Packet Scheduling Using Channel Adaptive Fair Queueing in Distributed Mobile Computing Systems

In a distributed mobile computing system, an efficient packet scheduling policy is a crucial component to achieve a high utilization of the precious bandwidth resources while satisfying users' QoS (quality of service) demands. An important class of scheduling techniques, namely, the wireless fair queueing algorithms, have been extensively studied recently. However, a major drawback in existing approaches is that the channel model is overly simplified – a two-state channel (good or bad) is assumed. While it is relatively easy to analyze the system using such a simple model, the algorithms so designed are of a limited applicability in a practical environment, in which the level of burst errors is time-varying and can be exploited by using channel adaptive coding and modulation techniques. In this paper, we first argue that the existing algorithms cannot cater for a more realistic channel model and the traditional notion of fairness is not suitable. We then propose a new notion of fairness, which bounds the actual throughput normalized by channel capacity of any two data connections. Using the new fairness definition, we propose a new fair queueing algorithm called CAFQ (Channel Adaptive Fair Queueing), which, as indicated in our numerical studies, outperforms other algorithms in terms of overall system throughput and fairness among error prone connections.