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

An analytical model of the virtual collision handler of 802.11e

A number of analytical models have been proposed to describe the priority schemes of the Enhanced Distributed Channel Access (EDCA) mechanism of the IEEE 802.11e standard. EDCA provides a class-based differentiated Quality of Service (QoS) to IEEE 802.11 WLANs. Many have used a multiple number of nodes to study the differentiation behaviour of the model. However, in many real-life usage scenarios Internet traffic is often asymmetric with much downlink traffic from the access point to the stations and little traffic in the reverse direction. Hence, most of the overall traffic differentiation will happen in the Virtual Collision Handler (VCH) of the access point. If the access point uses EDCA, it should know the characteristics of the VCH to be able to control the differentiation of the downlink traffic. The main contribution of this paper opposed to other works is that it demonstrates how a generic channel model of 802.11e can be modified to predict the behaviour of the VCH with a remarkably good accuracy. In doing so, we first introduce virtual collision handling into the generic model. We observe good match between the analytical model and simulations

Optimal Configuration of 802.1e EDCA for Real-Time and Data Traffic

The enhanced distributed channel access (EDCA) mechanism of the IEEE 802.11e standard provides quality-of-service (QoS) support through service differentiation by using different medium-access-control (MAC) parameters for different stations. The configuration of these parameters, however, is still an open research challenge, as the standard provides only a set of fixed recommended values that do not take into account the current wireless local area network (WLAN) conditions and, therefore, lead to suboptimal performance. In this paper, we propose a novel algorithm for EDCA that, given the throughput and delay requirements of the stations that are present in the WLAN, computes the optimal configuration of the EDCA parameters. We first present a throughput and delay analysis that provides the mathematical foundation upon which our algorithm is based. This analysis is validated through simulations of different traffic sources (both data and real time) and EDCA configurations. We then propose a mechanism to derive the optimal configuration of the EDCA parameters, given a set of performance criteria for throughput and delay. We assess the effectiveness of the configuration provided by our algorithm by comparing it against 1) the recommended values by the standard, 2) the results from an exhaustive search over the parameter space, and 3) previous configuration proposals, which are both standard and nonstandard compliant. Results show that our configuration outperforms all other approaches.European Community's Seventh Framework ProgramThis work was supported in part by the European Community’s Seventh Framework Program (FP7/2007-2013) under Grant Agreement 214994.Publicad

VoIP Call Admission Control in WLANs in Presence of Elastic Traffic

VoIP service over WLAN networks is a promising alternative to provide mobile voice communications. However, several performance problems appear due to i) heavy protocol overheads, ii) unfairness and asymmetry between the uplink and downlink flows, and iii) the coexistence with other traffic flows. This paper addresses the performance of VoIP communications with simultaneous presence of bidirectional TCP traffic, and shows how the presence of elastic flows drastically reduces the capacity of the system. To solve this limitation a simple solution is proposed using an adaptive Admission and Rate Control algorithm which tunes the BEB (Binary Exponential Backoff) parameters. Analytical results are obtained by using an IEEE 802.11e user centric queuing model based on a bulk service M=G[1;B]=1=K queue, which is able to capture the main dynamics of the EDCA-based traffic differentiation parameters (AIFS, BEB and TXOP). The results show that the improvement achieved by our scheme on the overall VoIP performance is significant

Advanced Wireless LAN

The past two decades have witnessed starling advances in wireless LAN technologies that were stimulated by its increasing popularity in the home due to ease of installation, and in commercial complexes offering wireless access to their customers. This book presents some of the latest development status of wireless LAN, covering the topics on physical layer, MAC layer, QoS and systems. It provides an opportunity for both practitioners and researchers to explore the problems that arise in the rapidly developed technologies in wireless LAN

Traffic Control and Quality of Service in Wireless LANs

The thesis deals with two aspects of the IEEE 802.11 standard. The first is the so-called “performance anomaly”: the variable bandwidth of the links and the use of multiple transmission rates push the throughput of all stations to align to the slowest one. To tackle this problem we designed and developed a simple channel-aware scheduling algorithm, called DTT, which actualises the proportional fairness concept, thus leading to noteworthy improvements, and in particular to flow isolation. This is achieved by measuring link quality as the time needed to deliver a frame. The resource to share is no longer capacity, but the time the channel is in use. DTT has then been integrated into a prototype Access Point, which is the first working implementation of a scheduler based on proportional fairness. Secondly, we focused on 802.11e networks, which, though enhancing QoS support, still offer scarce reliability of QoS guarantees and suffer from network congestion. We devised two admission control algorithms to assess the maximum number of users allowable to the services while satisfying QoS requirements. Following the studies on DTT, both algorithms centre the admission test on the time occupancy of the medium. The first algorithm builds on an analytical model of the EDCA mode in non-saturation conditions. This closely matches the real behaviour of a network carrying time-sensitive applications, thus overcoming the limits of all previous works, based on saturation models. The second algorithm uses and extends to 802.11e the NUC, a parameter defined and proved effective for 802.11b systems. This scheme needs measures of the actual state of the network. Simulations run within the E-model framework show good accuracy performance for both models

Throughput quantitative analysis of EDCA 802.11e in different scenarios

This document presents a quantitative analysis of the direct and relative throughput of IEEE 802.11e. The global throughput of an 802.11e WLAN is determined by EDCA (Enhanced Distributed Channel Access) parameters, among other aspects, that are usually configured with predetermined and static values. This study carefully evaluates the Quality of Service (QoS) of Wi-Fi with EDCA in several realistic scenarios with noise and a blend of wireless traffic (e.g., voice, video, and best effort, with Pareto distribution). The metrics of the benefits obtained in each case are compared, and the differentiated impact of network dynamics on each case is quantified. The results obtained show that the default settings are not optimal, and that with an appropriate selection, can be achieved improvements of the order of 25 %, according to the type of traffic. In addition, it could be shown the quantitative impact of each parameter EDCA on the overall performance. This study proposes a new experimental scenario based on the relative proportion of traffic present in the network. Stations have been simulated using the Möbius tool, which supports an extension of SPN (Stochastic Petri Networks), known as HSAN (Hierarchical Stochastic Activity Networks).Facultad de Informátic