A control theoretic approach to achieve proportional fairness in 802.11e EDCA WLANs

This paper considers proportional fairness amongst ACs in an EDCA WLAN for provision of distinct QoS requirements and priority parameters. A detailed theoretical analysis is provided to derive the optimal station attempt probability which leads to a proportional fair allocation of station throughputs. The desirable fairness can be achieved using a centralised adaptive control approach. This approach is based on multivariable statespace control theory and uses the Linear Quadratic Integral (LQI) controller to periodically update CWmin till the optimal fair point of operation. Performance evaluation demonstrates that the control approach has high accuracy performance and fast convergence speed for general network scenarios. To our knowledge this might be the first time that a closed-loop control system is designed for EDCA WLANs to achieve proportional fairness

Thwarting Selfish Behavior in 802.11 WLANs

The 802.11e standard enables user configuration of several MAC parameters, making WLANs vulnerable to users that selfishly configure these parameters to gain throughput. In this paper we propose a novel distributed algorithm to thwart such selfish behavior. The key idea of the algorithm is for honest stations to react, upon detecting a selfish station, by using a more aggressive configuration that penalizes this station. We show that the proposed algorithm guarantees global stability while providing good response times. By conducting a game theoretic analysis of the algorithm based on repeated games, we also show its effectiveness against selfish stations. Simulation results confirm that the proposed algorithm optimizes throughput performance while discouraging selfish behavior. We also present an experimental prototype of the proposed algorithm demonstrating that it can be implemented on commodity hardware.Comment: 14 pages, 7 figures, journa

A control theoretic approach to achieve proportional fairness in 802.11e EDCA WLANs

This paper considers proportional fairness amongst ACs in an EDCA WLAN redfor provision of distinct QoS requirements and priority parameters. A detailed theoretical analysis is provided to derive the optimal station attempt probability which leads to a proportional fair allocation of station throughputs. The desirable fairness can be achieved using a centralised adaptive control approach. This approach is based on multivariable statespace control theory and uses the Linear Quadratic Integral (LQI) controller to periodically update CWmin till the optimal fair point of operation. Performance evaluation demonstrates that the control approach has high accuracy performance and fast convergence speed for general network scenarios. To our knowledge this might be the first time that a closed-loop control system is designed for EDCA WLANs to achieve proportional fairness

Control-theoretic approaches for efficient transmission on IEEE 802.11e wireless networks

With the increasing use of multimedia applications on the wireless network, the functionalities of the IEEE 802.11 WLAN was extended to allow traffic differentiation so that priority traffic gets quicker service time depending on their Quality of Service (QoS) requirements. The extended functionalities contained in the IEEE Medium Access Control (MAC) and Physical Layer (PHY) Specifications, i.e. the IEEE 802.11e specifications, are recommended values for channel access parameters along traffic lines and the channel access parameters are: the Minimum Contention Window CWmin, Maximum Contention Window CWmax, Arbitration inter-frame space number, (AIFSN) and the Transmission Opportunity (TXOP). These default Enhanced Distributed Channel Access (EDCA) contention values used by each traffic type in accessing the wireless medium are only recommended values which could be adjusted or changed based on the condition of number of associated nodes on the network. In particular, we focus on the Contention Window (CW) parameter and it has been shown that when the number of nodes on the network is small, a smaller value of CWmin should be used for channel access in order to avoid underutilization of channel time and when the number of associated nodes is large, a larger value of CWmin should be used in order to avoid large collisions and retransmissions on the network. Fortunately, allowance was made for these default values to be adjusted or changed but the challenge has been in designing an algorithm that constantly and automatically tunes the CWmin value so that the Access Point (AP) gives out the right CWmin value to be used on the WLAN and this value should be derived based on the level of activity experienced on the network or predefined QoS constraints while considering the dynamic nature of the WLAN. In this thesis, we propose the use of feedback based control and we design a controller for wireless medium access. The controller will give an output which will be the EDCA CWmin value to be used by contending stations/nodes in accessing the medium and this value will be based on current WLAN conditions. We propose the use of feedback control due to its established mathematical concepts particularly for single-input-single-output systems and multi-variable systems which are scenarios that apply to the WLAN

A fair access mechanism based on TXOP in IEEE 802.11e wireless networks

IEEE 802.11e is an extension of IEEE 802.11 that provides Quality of Service (QoS) for the applications with different service requirements. This standard makes use of several parameters such as contention window; inter frame space time and transmission opportunity to create service differentiation in the network. Transmission opportunity (TXOP), that is the focus point of this paper, is the time interval, during which a station is allowed to transmit packets without any contention. As the fixed amounts of TXOPs are allocated to different stations, unfairness appears in the network. And when users with different data rates exist, IEEE 802.11e WLANs face the lack of fairness in the network. Because the higher data rate stations transfer more data than the lower rate ones. Several mechanisms have been proposed to solve this problem by generating new TXOPs adaptive to the network's traffic condition. In this paper, some proposed mechanisms are evaluated and according to their evaluated strengths and weaknesses, a new mechanism is proposed for TXOP determination in IEEE 802.11e wireless networks. Our new algorithm considers data rate, channel error rate and data packet lengths to calculate adaptive TXOPs for the stations. The simulation results show that the proposed algorithm leads to better fairness and also higher throughput and lower delays in the network.

Setting the parameters right for two-hop IEEE 802.11e ad hoc networks

Two-hop ad-hoc networks, in which some nodes forward traffic for multiple sources, with which they also compete for channel access suffer from large queues building up in bottleneck nodes. This problem can often be alleviated by using IEEE 802.11e to give preferential treatment to bottleneck nodes. Previous results have shown that differentiation parameters can be used to allocate capacity in a more efficient way in the two-hop scenario. However, the overall throughput of the bottleneck may differ considerably, depending on the differentiation method used. By applying a very fast and accurate analysis method, based on steady-state analysis of an QBD-type infinite Markov chain, we find the maximum throughput that is possible per differentiation parameter. All possible parameter settings are explored with respect to the maximum throughput conditioned on a maximum buffer occupancy. This design space exploration cannot be done with network simulators like NS2 or Opnet, as each simulation run simply takes to long.\ud The results, which have been validated by detailed simulations, show that by differentiating TXOP it is possible to achieve a throughput that is about 50% larger than when differentiating AIFS and CW_min.\u

A control theoretic approach for throughput optimization in IEEE 802.11e EDCA WLANs

The MAC layer of the 802.11 standard, based on the CSMA/CA mechanism, specifies a set of parameters to control the aggressiveness of stations when trying to access the channel. However, these parameters are statically set independently of the conditions of the WLAN (e.g. the number of contending stations), leading to poor performance for most scenarios. To overcome this limitation previous work proposes to adapt the value of one of those parameters, namely the CW, based on an estimation of the conditions of the WLAN. However, these approaches suffer from two major drawbacks: i) they require extending the capabilities of standard devices or ii) are based on heuristics. In this paper we propose a control theoretic approach to adapt the CW to the conditions of the WLAN, based on an analytical model of its operation, that is fully compliant with the 802.11e standard. We use a Proportional Integrator controller in order to drive the WLAN to its optimal point of operation and perform a theoretic analysis to determine its configuration. We show by means of an exhaustive performance evaluation that our algorithm maximizes the total throughput of the WLAN and substantially outperforms previous standard-compliant proposals.European Community's Seventh Framework ProgramThe work described in this article has been partially supported by the European Community’s Seventh Framework Programme under the ICT FP7 Integrated Project CARMEN (INFSO-ICT-214994) and by the Spanish Government under the POSEIDON project (TSI2006-12507-C03).Publicad