Dynamic priority re-allocation scheme for quality of service in IEEE 802.11e wireless networks

In IEEE 802.11e Enhanced Distributed Coordinator Function (EDCF) (In the recently approved IEEE 802.11e standard, EDCF is renamed to enhanced distributed channel access (EDCA). Throughout this paper, we use EDCF for consistency with early work in the literature.), per-flow service differentiation is achieved by maintaining separate queues for different traffic categories (TCs). However, due to its static Quality of Service (QoS) parameter setting, EDCF does not perform adequately under high traffic load (Romdhani et al., Proceedings of IEEE wireless communications and networking conference, 2003). We present an extended performance model of EDCF and analyze conditions for network getting overloaded. With this extended model, we show that the overall throughput of a network can be improved by changing the distribution of the number of active stations (an active station is one that has a pending packet to be sent) over a set of TCs. Hence, we propose to dynamically re-allocate flow priorities evenly in order to maintain high system performance while providing QoS guarantee for individual real-time flows. Our scheme has several interesting features: (1) performance of EDCF is improved; (2) low priority flows are not starved under high traffic load; (3) misuse of priority (misuse of priority means that a flow requests much higher priority than necessary) can be easily handled. Simulations are conducted for both infrastructure-based and Ad hoc models. Results show that dynamic priority re-allocation does not decrease throughput of real-time flows under low to medium loads, while considerable improvement over EDCF is obtained even under high loads, making it easy to support multimedia applications.     

Quality of service support in IEEE 802.11 wireless ad hoc networks

Supporting Quality of Service (QoS) in wireless networks is a challenging problem. The IEEE 802.11 LAN standard was developed primarily for elastic data applications. In order to support the transmission of real-time data, a polling-based scheme called the point coordination function (PCF) was introduced in IEEE 802.11. However, PCF was not able to meet the desired and practical service differentiation requirements to fulfill the need of real-time data. Therefore, Task Group E of the IEEE 802.11 working group released several IEEE 802.11e drafts, whose main task is to support QoS in IEEE 802.11 LANs. The polling scheme of PCF is extended in IEEE 802.11e into the more complex hybrid coordination function (HCF). We found that HCF has several performance issues that may affect its anticipated performance. In this paper, we address these issues and propose a QoS enhancement over PCF, called enhanced PCF (EPCF) that enables Wireless LAN to send a combination of voice, data and isochronous data packets using the current IEEE 802.11 PCF. First, we compare the performance of the proposed model (EPCF) with the HCF function of the IEEE 802.11e through simulation. Second, we extend the proposed model (EPCF) to work in a multihop wireless ad hoc mode and present the advantages and limitations in this case. Simulation results demonstrate an enhanced performance of our scheme over the legacy PCF and a comparable performance to the IEEE 802.11e HCF in terms of the average delay and system throughput. However, EPCF is much simpler than HCF, provides flow differentiation, and is easy to implement in the current IEEE 802.11 standard.     

Statistical characterization of the service time in saturated IEEE 802.11 networks

In this paper, we propose an analytical model for the service time in saturated IEEE 802.11 DCF networks. We derive a closed-form probability generating function for the packet service time of a cluster of IEEE 802.11 terminals, both for the RTC/CTS and the basic access mode. The probability generating function is inverted by numerical methods, providing the probability distribution function of the service time. Interestingly, it catches certain features of the DCF service time, which cannot be revealed with the second order analysis known in the literature. Finally, this analytical model is validated by means of extensive simulation outcomes.     

IEEE802.11无线网络中一种新的切换技术

作为当前无线局域网的主流标准,IEEE802.11网络提供了种类繁多的服务.然而,由于无线网络的移动性本质,切换技术成为IEEES02.11网络的关键技术.对IEEE802.11无线网络中现有的切换技术进行了研究,分析了可行的改进方向,并实现了一种改进的切换方案.     

A distributed cooperative multicast scheduling strategy in IEEE 802.11 networks

In wireless multicast, some users may fail to receive data as a result of bad channel conditions. The throughput of traditional multicast strategy is constrained by the node with the worst channel condition. In this paper, we propose a distributed cooperative multicast scheduling strategy, in which, every user which has successfully received data can decide whether to retransmit data by sensing MNACK packets. The proposed scheme can achieve good throughput by exploiting multi-channel diversity across multiple users’ cooperation. In addition, the constraint of power consumption is also considered. Simulation results show that our scheme can provide good throughput and fairness performance with the constraint of average system power consumption.     

IEEE802.11协议中关联业务的实现

本文着重分绍了802.11协议中关联/再关联的定义,以及关联业务的实现和实现所采用的工具库ACE.     

A scheduling algorithm for QoS support in IEEE802.11 networks

This article presents a scheduling algorithm for the IEEE 802.11e hybrid coordination function under definition by the IEEE 802.11e task group. HCF can be used to provide IP quality of service guarantees in IEEE802.11e infrastructure WLANs. The enhanced distributed coordination function is mainly used for data transmission without QoS guarantees, but can also be used to decrease the transmission delay of QoS-sensitive traffic. Scheduling of queued packets follows a delay-earliest-due-date algorithm. The proposed algorithm is compatible with the link adaptation mechanisms implemented in commercial WLANs, as it limits the amount of time during which the stations control the wireless medium. The performance of the algorithm is evaluated through computer simulation and compared with the reference scheduler presented by the IEEE 802.11e task group.     

IEEE 802.11e enhancement for voice service

Motivated by the promising voice over IP technology and the wide service availability of WLANs, the application of voice over WLAN (VoWLAN) is expected to experience dramatic growth in the near future. Originally designed for high-rate data traffic, WLANs may experience bandwidth inefficiency when supporting delay-sensitive and low-rate voice traffic. This article proposes mechanisms to enhance QoS support capability of IEEE 802.11e for voice service. Unnecessary polling of silent voice stations is avoided, and header and control overheads are suppressed significantly. Compared to IEEE 802.11e, our mechanisms can greatly improve the capacity of WLANs to support voice service.     

Route selection in IEEE 802.11 wireless mesh networks

This paper addresses the problem of route selection in IEEE 802.11 based Wireless Mesh Networks (WMNs). Traditional routing protocols choose the shortest path between two routers. However, recent research reveals that there can be enormous differences between links in terms of quality (link loss ratio, interference, noise etc) and therefore selecting the shortest path (hop count metric) is a poor choice. We propose a novel routing metric—Expected Link Performance (ELP) metric for wireless mesh networks which takes into consideration multiple factors pertaining to quality (link loss ratio, link capacity and link interference) to select the best end-to-end route. Simulation based performance evaluation of ELP against contemporary routing metrics shows an improvement in terms of throughput and delay. Moreover, we propose an extension of the metric called ELP-Gateway Selection (ELP-GS) which is an extension meant for traffic specifically oriented towards the gateway nodes in the mesh network. We also propose a gateway discovery protocol which facilitates the dissemination of ELP-GS in the network. Simulation results for ELP-GS show substantial improvement in performance.     

Propagation delay influence in IEEE 802.11 outdoor networks

Originally, Wireless Local Area Networks served only small indoor areas. Nevertheless, the idea of employing IEEE 802.11 networks in large outdoor environments is a very attractive possibility. IEEE 802.11 technology offers several advantages: the low cost of equipment, its operation in the unlicensed spectrum and its higher data rates. Since the advent of the first IEEE 802.11 standard, a great deal of research has been carried out. So-called Wifi-based Long Distance networks are currently being deployed. In this paper, we study the suitability of employing IEEE 802.11 networks in large outdoor environments without modifying the standard working procedure. In such scenarios, IEEE 802.11 networks should offer coverage ranges of several kilometer, which leads to high propagation delay values. Thus, we analyze the influence of increasing propagation delay in the IEEE 802.11 MAC protocol. To carry out our analysis we present a mathematical model and simulation results. We provide an operating range in which IEEE 802.11 performance is feasible and establish a throughput threshold according to the propagation delay.     

A robust to multi-path ranging technique over IEEE 802.11 networks

In this paper, we present a two-way TOA technique for distance estimation (i.e., ranging) over IEEE 802.11 networks. It is based on repetitively measuring the Round Trip Time (RTT) of IEEE 802.11 MAC frames and carrying out statistical processing in order to identify the multi-path condition of the channel and to improve accuracy. The study demonstrates that a multi-path condition has a negative impact on the ranging accuracy in Non-Dominant Direct Path (NDDP) and Undetectable Direct Path (UDP) situations. In this paper, an approach is presented to detect and overcome this impact. The proposed approach identifies the channel profile (i.e., the multi-path condition) from the statistical features of a set of RTT measurements and uses a ranging algorithm that is sensitive to multi-path. Results demonstrate the feasibility of channel profile identification in real time and a ranging accuracy of around one meter in all possible multi-path situations.     

Performance and fairness enhancement in IEEE 802.11 WLAN networks

The multi-rate transmission mechanism in IEEE 802.11 can improve its reliability and robustness. However, it causes a performance anomaly. After analyzing the reasons for the performance anomaly in multi-rate mechanism, we propose a new scheme to solve the performance anomaly. By adjusting packet size according to the transmission rate, this scheme guarantees that these nodes with different transmit rates can access wireless channel fairly. Theoretical analysis and performance evaluation show that the proposed scheme can well solve the performance anomaly problem.     

QoS enhancement in IEEE 802.11 wireless local area networks

A distributed medium access scheme called EDCF, which is adopted in an upcoming standard IEEE 802.11e to allow prioritized medium access for applications with QoS requirements, is described and discussed. Its performance is also evaluated via simulations.     

Mobility impact in IEEE 802.11p infrastructureless vehicular networks

Vehicular ad hoc networks (VANETs) are an extreme case of mobile ad hoc networks (MANETs). High speed and frequent network topology changes are the main characteristics of vehicular networks. These characteristics lead to special issues and challenges in the network design, especially at the medium access control (MAC) layer. In this paper, we provide a comprehensive evaluation of mobility impact on the IEEE 802.11p MAC performance. The study evaluates basic performance metrics such as packet delivery ratio, throughput, and delay. An unfairness problem due to the relative speed is identified for both broadcast and unicast scenarios. We propose two dynamic contention window mechanisms to alleviate network performance degradation due to high mobility. The first scheme provides dynamic level of service priority via adaptation to the number of neighboring nodes, while the second scheme provides service priority based on node relative speed. Extensive simulation results demonstrate a significant impact of mobility on the IEEE 802.11p MAC performance, the unfairness problem in the vehicle-to-vehicle (V2V) communications, and the effectiveness of the proposed MAC schemes.     

Modeling intra-flow contention problem in IEEE 802.11 wireless multi-hop networks

This letter proposes a new approach to model the intra-flow contention problem in multi-hop networks. The model takes into consideration of neighboring interference, hidden-node collision and multi-rate scenario. It can be easily used to do admission control or to calculate the end-to-end capacity of a given multi-hop route. Simulation results validate its accuracy.     

Distributed throughput optimization for heterogeneous IEEE 802.11 DCF networks

For IEEE 802.11 DCF networks in ad-hoc mode, how to achieve the maximum throughput in a distributed manner draws much attention in previous studies. The problem becomes challenging for partially-saturated heterogeneous networks with multiple groups, as the optimal access parameters not only depend on the group size of saturated groups but also the aggregate input rate of all the unsaturated groups, both of which are hard to obtain without a central controller. In this paper, a novel distributive scheme is proposed for partially-saturated heterogeneous IEEE 802.11 DCF networks to achieve the maximum network throughput. With the proposed scheme, each saturated transmitter can obtain the optimal initial backoff window size distributively by two estimation rounds. In each estimation round, each saturated transmitter only needs to count the number of busy intervals and ACK frames on the channel. For fully-saturated networks, only one estimation round is needed. It is shown by extensive simulations that the proposed scheme can achieve the maximum network throughput in a distributive manner.