Recent studies on operational wireless LANs (WLANs) have shown that user load is often unevenly distributed among wireless access points (APs). This unbalanced load results in unfair bandwidth allocation among users. We observe that the unbalanced load and unfair bandwidth allocation can be greatly alleviated by intelligently associating users to APs, termed association control, rather than having users greedily associate APs of best received signal strength.In this study, we present an efficient algorithmic solution to determine the user-AP associations that ensure max-min fair bandwidth allocation. We provide a rigorous formulation of the association control problem that considers bandwidth constraints of both the wireless and backhaul links. Our formulation indicates the strong correlation between fairness and load balancing, which enables us to use load balancing techniques for obtaining near optimal max-min fair bandwidth allocation. Since this problem is NP-hard, we present algorithms that achieve a constant-factor approximate max-min fair bandwidth allocation. First, we calculate a fractional load balancing solution, where users can be associated with multiple APs simultaneously. This solution guarantees the fairest bandwidth allocation in terms of max-min fairness. Then, by utilizing a rounding method we obtain an efficient integral association. In particular, we provide a 2-approximation algorithm for unweighted greedy users and a 3-approximation algorithm for weighted and bounded-demand users. In addition to bandwidth fairness, we also consider time fairness and we show it can be solved optimally. We further extend our schemes for the on-line case where users may join and leave. Our simulations demonstrate that the proposed algorithms achieve close to optimal load balancing and max-min fairness and they outperform commonly used heuristic approaches.

/pdf/fairness-and-load-balancing-in-wireless-lans-using-5by5y0ku7b.pdf

Fairness and load balancing in wireless LANs using association control

This paper describes an adaptive service differentiation scheme for QoS enhancement in IEEE 802.11 wireless ad-hoc networks. Our approach, called adaptive enhanced distributed coordination function (AEDCF), is derived from the new EDCF introduced in the upcoming IEEE 802.11e standard. Our scheme aims to share the transmission channel efficiently. Relative priorities are provisioned by adjusting the size of the contention window (CW) of each traffic class taking into account both applications requirements and network conditions. We evaluate through simulations the performance of AEDCF and compare it with the EDCF scheme proposed in the 802.11e. Results show that AEDCF outperforms the basic EDCF, especially at high traffic load conditions. Indeed, our scheme increases the medium utilization ratio and reduces for more than 50% the collision rate. While achieving delay differentiation, the overall goodput obtained is up to 25% higher than EDCF. Moreover, the complexity of AEDCF remains similar to the EDCF scheme, enabling the design of cheap implementations.

/pdf/adaptive-edcf-enhanced-service-differentiation-for-ieee-802-4o0ijgslp2.pdf

Adaptive EDCF: enhanced service differentiation for IEEE 802.11 wireless ad-hoc networks

One key usage of VANET is to support vehicle safety applications. This use case is characterized by the prominence of broadcasts in scaled settings. In this context, we try to answer the following questions: i) what is the probability of reception of a broadcast message by another car depending on its distance to the sender, ii) how to give priority access and an improved reception rate for important warnings, e.g., sent out in an emergency situation, and iii) how are the above two results affected by signal strength fluctuations caused by radio channel fading? We quantify via simulation the probability of reception for the two-ray-ground propagation model as well as for the Nakagami distribution in saturated environments. By making use of some IEEE 802.11e EDCA mechanisms for priority access, we do not only quantify how channel access times can be reduced but also demonstrate how improved reception rates can be achieved. Our results show that the mechanisms for priority access are successful under the two-way-ground model. However, with a non-deterministic radio propagation model like Nakagami's distribution the benefit is still obvious but the general level of probability of reception is much smaller compared to two-ray-ground model. The results indicate that -- particularly for safety-critical and sensor network type of applications -- the proper design of repetition or multi-hop retransmission strategies represents an important aspect of future work for robustness and network stability of vehicular ad hoc networks.

/pdf/broadcast-reception-rates-and-effects-of-priority-access-in-48wnv4dy84.pdf

Broadcast reception rates and effects of priority access in 802.11-based vehicular ad-hoc networks

*Summary 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 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.

/pdf/a-survey-of-qos-enhancements-for-ieee-802-11-wireless-lan-5d2t9i0lu3.pdf

A survey of QoS enhancements for IEEE 802.11 wireless LAN

Survey A survey of routing protocols for smart grid communications

In this work, we present a Cross-Layer Forwarding Strategy (CLFS), which is based on the cooperation between the new IEEE 802.11e MAC protocol (EDCA) and the On-Demand AODV routing protocol. The proposal aims to minimize the number of Forwarding nodes (FN) by hop, in the network. By this way, we decrease the contention amount and we improve the medium utilization. The selection of FN is based on maximum battery level and queue occupancy. These informations are injected into routing requests and replies crossing nodes in the network. Then, each node is able to select the FN that will participate in path establishment. In order to maintain a fair node capability, the forwarding procedure is dynamically distributed and assigned to nodes in the network. Moreover, an Adaptive Transmission Opportunity (ATXOP) mechanism, is derived from the EDCA. It aims to share the transmission channel fairly according to traffic load of nodes. We demonstrate that CLFS has good network performance, specially in term of throughput, that can be significantly improved. Moreover, it achieves a high degree of fairness among applications.

A cross-layer feature for an efficient forwarding strategy in wireless ad hoc networks

One of the key challenges in multimedia networks is video delivery over wireless channels. MRC (Multi-Resolution Coding) Layered video, divides video into a base layer and multiple enhancement layers. In this paper, we aim to improve video quality, impacted by high channel contention, through mapping individual video layers to EDCA (Enhanced Distributed Channel Access) access categories in order to maximize the average number of reconstructed video layers. We propose an adaptive cross layer video layers mapping technique that optimally enhances the QoS of wireless video transmission over IEEE 802.11e EDCA priority queues. The optimization is based on a dynamic program that takes into account the EDCA parameters and the layered dependency nature of layered video delivery. Our proposed technique makes use of a channel delay estimation model and an estimation of average video useful layers delivered. The optimal mapping strategies are selected by an optimization module based on the information from the analytical model. The accuracy of our optimized mapping technique performance is verified through extensive simulations. The obtained results illustrate significant trade-off between complexity and delivered video quality for canonical mapping schemes.

An analytic study of a distributed EDCA-based QoS mapping for layered video delivery in WLAN

Multi-layer video transmission over wireless net- works poses numerous challenges due to symantec nature of the encoded video and the intrinsic dependency between the video layers, which, if not considered, may cause severe quality degradation. In this paper, we present a new analytical model leveraging cross layer design for distributed video layers mapping that optimizes the QoS of wireless video transmission over IEEE 802.11e EDCA (Enhanced Distributed Channel Access) priority queues. MRC (Multi-Resolution Coding) Layered video, divides video into a base layer and multiple enhancement layers. We aim to improve the perceived video quality, impacted by high channel contention, through mapping individual video layers into EDCA access categories in order to maximize the average number of reconstructed video layers. The proposed technique is based on a dynamic program that takes into account the EDCA parameters and the dependency nature of layered video delivery, leveraging an application-based metric that measures the symantec video quality. Our proposed technique makes use of a channel delay statistical model to estimate the expected useful video layers delivered. The performance of our optimized mapping tech- nique is verified through extensive simulations. The obtained results illustrate significant trade-off between complexity and delivered video quality for canonical mapping schemes. Index Terms—EDCA, Layered video, mapping strategy, analytical model.

/pdf/sequential-qos-mapping-model-for-multi-layervideo-delivery-19morqfgfc.pdf

Sequential QoS Mapping Model for Multi-layerVideo Delivery in 802.11e Wireless Networks

The link delivery probability in a wireless mesh network is most accurately determined only by experiment, as it depends on many factors, including environment characteristics, transmission power, distance between transmitter and receiver, channel fading, and background noise. This paper provides an insightful framework for characterizing link performance results produced at Qatar University (QU) wireless mesh test-bed under harsh environmental conditions, including heavy buildings structure, mixed indoor/outdoor architecture, weather conditions, and coexistence with high interfering wireless transmission. Indeed, we are interested in looking at the link quality behavior for indoor and outdoor environments. In the outdoor environment, the effect of different weather conditions such as high humidity and sandstorms, which are typical in the Qatari weather, will be analyzed. Regarding the indoor environment, we investigate the effect of unique cell structure, and heavy lab machinery of the engineering building on the link performance. Furthermore, we study different wireless mesh node configurations and equipments' effect on performance and network connectivity. The causes behind link instability and performance shortcoming of the QU wireless mesh network (QUMESH) are identified. Thus, the content of this paper provides an input for developing the criteria for the test-bed and performing the evaluation of new proposed adaptive mechanisms.

QUMESH: Wireless mesh network deployment and configuration in harsh environment

The cross-layer design approach is an important concept in mobile ad-hoc networks which is adopted to solve several open issues. It aims to overcome MANET performance problems by allowing protocols belonging to different layers to cooperate and share network status informations while still maintaining separated layers. While cross-layer model could enhance the performance of the applications and achieve better QoS support, there is a lot of proposed models that have to be compared and optimized. In this paper, we investigate the different requirements to achieve a good cooperation. Then, we compare two cross -layer models dealing with delay sensitive application constraints and stability issues in MANET. The simulation results demonstrate that the performance of the proposals is strongly depending on the selective shared parameters.

Lamia Romdhani

Papers

A cross-layer feature for an efficient forwarding strategy in wireless ad hoc networks

An analytic study of a distributed EDCA-based QoS mapping for layered video delivery in WLAN

Sequential QoS Mapping Model for Multi-layerVideo Delivery in 802.11e Wireless Networks

QUMESH: Wireless mesh network deployment and configuration in harsh environment

Achieving a good trade-off between complexity and enhancement in cross-layer architectures.