A Cross-Layer Approach for Per-Station Fairness in TCP over WLANs

In this paper, we investigate the issue of per-station fairness in TCP over IEEE 802.11-compliant wireless local area networks (WLANs), especially in Wi-Fi hot spot. It is asserted that the hot spot suffers from the unfairness among stations in exploiting the wireless medium. The source of this unfairness is analyzed from two aspects, TCP-induced asymmetry and MAC-induced asymmetry; the former causes TCP congestion control with a cumulative acknowledgment mechanism to prefer the sending stations to receiving stations, while the later exacerbates the unfairness problem in the hot spots. We investigate the interaction between TCP congestion control and MAC contention control, and propose a cross-layer feedback approach to assure per-station fairness and to ensure high channel utilization. In this approach, we introduce the notion of channel access cost to quantify the system-wide traffic load and per-station channel usage. The access cost is estimated at the MAC in an access point and conveyed to the TCP sender. Then, the TCP sender adjusts its sending rate based on the access cost, so as to assure per-station fairness. The simulation results indicate that the proposed approach can provide both per-station fairness and high channel utilization, irrespective of network configurations.     

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.     

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.     

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.     

Arbitration Interframe Space‐controlled Medium Access Control: a medium access control protocol guaranteeing absolute priority in wireless local area networks

The demand for multimedia services, such as voice over Internet Protocol, video on demand, information dissemination, and ?le sharing, is increasing explosively in wireless local area networks. These multimedia services require a certain level of QoS. Thus, it is important to provide QoS for multimedia applications. IEEE 802.11e tries to meet the QoS requirement of multimedia services by using Enhanced Distributed Channel Access. This gives more weights to high‐priority tra?c than low‐priority tra?c in accessing the wireless channel. However, Enhanced Distributed Channel Access suffers from many problems such as low aggregate throughput, high collision rates, and ineffective QoS differentiation among priority classes. In this paper, we propose a new medium access scheme, the Arbitration Interframe Space‐controlled Medium Access Control (AC‐MAC), that guarantees absolute priority in 802.11 wireless networks. In AC‐MAC, the AIFS and contention window values are controlled, so that a higher‐priority tra?c can preferentially access and effectively utilize the channel. Extensive simulations show that AC‐MAC can perfectly provide absolute priority and good throughput performance regardless of the number of contending nodes. In the simulation of voice over Internet Protocol service, AC‐MAC provides effective QoS differentiation among services and also meets the high level of QoS requirements. AC‐MAC also adapts quickly in a dynamic environment and provides good fairness among the nodes belonging to the same priority class. Copyright © 2011 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     

A two-phase utility maximization framework for wireless medium access control

In multi-hop wireless networks, the optimal medium access control (MAC) design is challenging, partially due to the time-varying nature of the PHY-layer communication channels and the network topology. In this paper, we take a utility maximization approach to study fair MAC design towards QoS provisioning. To this end, we first identify two key challenges of wireless access control, namely the topology dependency and the channel dependency, therein. Based on the observation that the topology change and channel variation occur on different time scales, we decompose the utility maximization to two phases: a "global" optimization phase addresses the topology dependency, and arbitrates fair channel access across the links by adapting the persistence probability to achieve long-term fairness, and a "local" optimization phase deals with the channel dependence, and determines the transmission duration based on local channel conditions while maintaining short-term fairness. Observing that the MAC throughput depends on the realizations of channel contention in random access networks, we use stochastic approximation to investigate in depth the MAC design with the adaptive persistence mechanism in the global phase. Using Lyapunov's Stability Theorems and LaSalle's Invariance Theorem, we establish the stability of the proposed algorithm for the global phase and analyze the fairness under (omegaoarr, kappa)-fair utility functions. Our findings reveal that under the large network assumption, there exists a single equilibrium point for the proposed (omegaoarr, kappa)-fair MAC algorithm provided that kappa > 1. We also present the solution to the local optimization phase under general fairness constraints.     

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.     

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.     

Sequential Coordination Function for Throughput and Fairness Enhancement of Wireless LANs

High throughput and fair resource sharing are two of the most important objectives in designing a medium access control (MAC) protocol. Currently, most MAC protocols including IEEE 802.11 DCF adopt a random access based approach in a distributed manner in order to coordinate the wireless channel accesses among competing stations. In this paper, we first identify that a random access?Cbased MAC protocol may suffer from MAC protocol overhead such as a random backoff for data transmission and a collision among simultaneously transmitting stations. Then, we propose a new MAC protocol, called sequential coordination function (SCF), which coordinates every station to send a data frame sequentially one after another in a distributed manner. By defining a service period and a joining period, the SCF eliminates unnecessary contentions during the service period, and by explicitly determining the sequence of frame transmission for each stations, it reduces collision occurrences and ensures fairness among stations in the service period. The performance of SCF is investigated through intensive simulations, which show that the SCF achieves higher throughput and fairness performances than other existing MAC protocols in a wide range of the traffic load and the number of stations.     

Fair round robin binary countdown to achieve QoS guarantee and fairness in WLANs

How to guarantee both quality of service (QoS) and fairness in wireless local area networks (WLANs) is a challenging issue. To touch this issue, a fair medium access control (MAC) scheme called fair round robin binary countdown (FRRBC) adopting the eminent concepts of allowance and binary countdown is proposed in this paper. FRRBC can guarantee QoS for both audio and video with the aid of adaptive adjustment on system parameters and some extra rules designed according to delay bounds. Using multiple mapping functions from allowances to fixed-bit binary numbers to indicate different priorities, FRRBC not only provides guaranteed system performance but also achieves good fairness. Finally, we demonstrate that FRRBC can significantly outperform the enhanced distributed channel access (EDCA) (IEEE 802.11 WG in IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements, 2005) and the synchronized medium access control (SYN-MAC) (Wu HY et al. in Mobile Netw Appl 11:627–637, 2005) because of its superiority to offer guaranteed QoS for both audio and video, low delay jitter, low blocking ratio, and good fairness. Of course, FRRBC can be illustrated to be a better choice than the enhanced distributed elastic round robin (EDERR) (Ferng HW, Liau HY in IEEE Trans Mobile Comput 8(7):880–894, 2009).     

A Hybrid Collision Avoidance Scheme for Ad Hoc Networks

A novel hybrid collision avoidance scheme that combines both sender-initiated and receiver-initiated collision-avoidance handshake is proposed for multi-hop ad hoc networks. The new scheme is compatible with the popular IEEE 802.11 MAC protocol and involves only some additional queue management and book-keeping work. Simulations of both UDP- and TCP-based applications are conducted with the IEEE 802.11 MAC protocol, a measurement-based fair scheme and the new scheme. It is shown that the new scheme can alleviate the fairness problem with almost no degradation in throughput. More importantly, it is shown that without explicit information exchange among nodes, the fairness problem cannot be solved conclusively if reasonable throughput is to be maintained. Hence it calls for further work to integrate the new collision avoidance scheme with other schemes that approximate fair queueing and use more contention information in channel access to achieve some QoS assurances in ad hoc networks.     

A cross-layer scheduling algorithm with QoS support in wireless networks

Scheduling plays an important role in providing quality of service (QoS) support to multimedia communications in various kinds of wireless networks, including cellular networks, mobile ad hoc networks, and wireless sensor networks. The authors propose a scheduling algorithm at the medium access control (MAC) layer for multiple connections with diverse QoS requirements, where each connection employs adaptive modulation and coding (AMC) scheme at the physical (PHY) layer over wireless fading channels. Each connection is assigned a priority, which is updated dynamically based on its channel and service status; the connection with the highest priority is scheduled each time. The authors' scheduler provides diverse QoS guarantees, uses the wireless bandwidth efficiently, and enjoys flexibility, scalability, and low implementation complexity. Its performance is evaluated via simulations.     

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.     

A fuzzy logic cooperative MAC for MANET

In both wireless local area networks（WLAN） and mobile ad hoc networks（MANET）, the 1EEE 802.11e medium access control （MAC） protocol is proposed for an effective quality of service （QoS） solution. A number of studies have been done to enhance the performance of 802.11e in MANET by independently adjusting contention window （CW） size of each access category （AC） in every node. However, without the cooperation between the high priority flows and lower priority flows, the QoS goal of high priority flows cannot achieve effectively. In this article, a fuzzy logic based cooperative MAC protocol （FLCMAC） is proposed to cooperate amongst network flows and dynamically adjust access probability of each low priority flow affecting the high priority flows to satisfy their QoS requirement. The simulation results indicate that compared to the enhanced distributed channel access （EDCA） scheme of 802.11e, the FLCMAC consistently excels, in terms of throughput and delay under moderate and heavy background traffic both in single-hop and multi-hop scenarios.     

WTRP - wireless token ring protocol

The wireless token ring protocol (WTRP) is a novel medium access control (MAC) protocol for wireless local area networks (WLANs). In contrast with IEEE 802.11 networks, WTRP guarantees quality of service (QoS) in terms of bounded latency and reserved bandwidth, which are critical in many real-time applications. Compared to 802.11, WTRP improves efficiency by reducing the number of retransmissions due to collisions, and it is more fair as all stations use the channel for the same amount of time. Stations take turns transmitting and give up the right to transmit after a specified amount of time. WTRP is a distributed protocol that supports many topologies, as not all stations need to be connected to each other or to a central station. WTRP is robust against single node failures, and recovers gracefully from multiple simultaneous faults. WTRP is suitable for interaccess point coordination in ITS DSRC, safety-critical vehicle-to-vehicle communications, and home networking, and provides extensions to other networks and Mobile IP.     

Physical layer capture aware MAC for WLANs

The physical layer capture (PLC) effect occurs frequently in the real wireless deployment; when two or more nodes transmit simultaneously, a receiver can successfully decode the collided frame if the signal strength of one frame is sufficiently high enough. Although the PLC effect increases the channel utilization, it results in an unfair channel access among the wireless nodes. In this paper, we propose a PLC-aware media access control (MAC) algorithm that employs the average waiting time as a common control reference. It enables the nodes to converge to a fair channel access by changing one of the IEEE 802.11 enhanced distributed channel access parameters: contention window, arbitration interframe space, or transmission opportunity. We then find multiple control references that meet the fair channel access constraint and obtain the near-optimal reference that maximizes the overall throughput. Through ns-2 simulations and real in-door experiments using the universal software radio peripheral platform, we evaluate the fairness and throughput performance of the PLC-aware MAC algorithm.     

Analysis of M-LWDF fairness and an enhanced M-LWDF packet scheduling mechanism

Modified largest weighted delay first(M-LWDF)is a typical packet scheduling algorithm for supporting hybrid real-time services over wireless networks.However,so far,there is little literature available regarding the theoretic analysis of M-LWDF fairness.This paper gives a theoretic analysis of M-LWDF fairness,which shows that M-LWDF fairness is related to channel condition,packet’s arrival process and the ratio of quality of service(QoS)requirements of different service queues.Given service QoS requirements and other parameters related to channel model and packet’s arrival process,the fairness is merely related to the ratio of the number of users in the service queues.Based on the analysis,an enhanced M-LWDF algorithm(EM-LWDF)is proposed and demonstrated in this paper.EM-LWDF is strictly designed in light of the fairness criteria of QoS requirements,so its fairness is almost not related to the ratio of the number of users in the service queues,and the theoretical value of fairness index is equal to 1.Simulation results validate the theoretic analysis and show the effectiveness of EM-LWDF in improving fairness.     

Provisioning QoS controlled media access in vehicular to infrastructure communications

The emerging IEEE 802.11p standard adopts the enhanced distributed channel access (EDCA) mechanism as its Media Access Control (MAC) scheme to support quality-of-service (QoS) in the rapidly changing vehicular environment. While the IEEE 802.11 protocol family represents the dominant solutions for wireless local area networks, its QoS performance in terms of throughput and delay, in the highly mobile vehicular networks, is still unclear. To explore an in-depth understanding on this issue, in this paper, we develop a comprehensive analytical model that takes into account both the QoS features of EDCA and the vehicle mobility (velocity and moving directions). Based on the model, we analyze the throughput performance and mean transmission delay of differentiated service traffic, and seek solutions to optimally adjust the parameters of EDCA towards the controllable QoS provision to vehicles. Analytical and simulation results are given to demonstrate the accuracy of the proposed model for varying EDCA parameters and vehicle velocity and density.     

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.