Analysis of the 802.11e enhanced distributed channel access function

The IEEE 802.11e standard has recently been ratified. This standard provides quality-of-service (QoS) guarantees in the medium access control (MAC) layer of 802.11 wireless local area networks (WLANs). In other words, the 802.11e standard updates the MAC layer of the former 802.11 standard for QoS. In particular, the enhanced distributed channel access (EDCA) function of 802.11e is an enhancement of the distributed coordination function (DCF) of 802.11. In this paper, we present an analytical model for EDCA in saturation mode. Our model incorporates an accurate contention window (CW) and arbitration interframe space (AIFS) differentiation. In addition, it provides virtual collision treatment for an access category (AC). Validating the theoretical results via simulations, we show that i) the proposed model accurately captures EDCA saturation throughput, and ii) this model is substantially more accurate than the models previously proposed in the literature.

Most of the recent analytical studies for IEEE802.11e quality of service (QoS)-aware enhanced distributed coordination function require a large degree of complexity that are difficult to apply to a wide range of parameter settings for the evaluation of service differentiation mechanisms supported in enhanced distributed channel access (EDCA), including the contention window (CW) and arbitrary inter-frame space (AIFS) mechanisms. The authors propose a novel analytical model to analyse the throughput of EDCA with AIFS and CW differentiation schemes. The model is simplified by decomposing the problem into two easily solved Markov chains that can jointly be solved by numerical method. The authors present simulation and analytical results over a broad range of system parameters to demonstrate the accuracy of the proposed model. Compared to the previous models, our model is simple to implement and can be applied to a general configuration circumstances for the evaluation of EDCA. The results are valuable to facilitate proper design of parameters in 802.11e EDCA for the QoS support required by specific applications.

This paper introduces a simple three-dimensional Markov chain model for the Enhanced Distributed Channel Access (EDCA) mechanism of IEEE 802.11e MAC under saturation condition. This new Markov chain model can be used to evaluate the throughput performance of IEEE 802.11e EDCA. This new model captures all of the major Quality of Service (QoS) specific features, namely Arbitration Interframe Space (AIFS), minimum contention window size, maximum contention window size and virtual collision, for the IEEE 802.11e EDCA mechanism and hence can provide a novel numerical approach to pick the set of EDCA parameter values to meet the QoS requirements. The results of our analytical model are then verified using simulations.

With the onset of multimedia applications, Quality of Service (QoS) in 802.11-based wireless ad hoc networks has been an essential area to look into. Standard wireless protocols such as IEEE 802.11b and IEEE 802.11e have been deemed unsuitable for multimedia traffic transmission in an ad hoc (multihop) network environment due to the unique network characteristics. As an ad hoc network is infrastructureless and distributed in nature, it gives rise to several challenges in QoS provisioning. In this thesis, we focus on the research of three key areas on QoS in ad hoc networks namely, buffer management in the medium access control (MAC) layer, cross-layer design and flow admission control. Firstly, we study MAC layer buffer management in the IEEE 802.11e protocol. The IEEE 802.11e MAC protocol provides QoS for multimedia traffic by probabilistic priority queueing. Ad hoc networks tend to generate a large volume of network control packets for route maintenance and these control packets are typically queued in the highest priority queue. As a result, higher priority control traffic is always being served, consuming most of the bandwidth and results in poor network performance for data traffic. We examine this problem and propose a dynamic buffer management scheme to mitigate it. By setting different threshold levels in different priority queues, we allow lower priority data traffic to be served when the higher priority control traffic is deemed to be excessive. Simulation of our proposed scheme has shown that our approach yielded better network performance compared to the standard protocol in terms of packet delivery ratio and average delay. Secondly, we study the cross-layer interaction between the MAC and Network layers to identify its impact on QoS. We select the IEEE 802.11e MAC protocol and Ad-hoc On Demand Distance Vector (AODV) routing protocol in our study since they are standardized protocols for wireless networks. In IEEE 802.11e MAC protocol, lower priority traffic has a higher probability of being blocked. Hence, the MAC layer will inform the Network layer of a broken link through the link notification function. In this case, AODV will re-route the paths that traverse this link. As a result, higher priority traffic which shares the same link as the lower priority traffic will undergo the same re-routing procedures. We termed this phenomenon path correlation effect. Under high traffic load, the path correlation effect will be more intense and hence degrade the QoS of higher priority traffic. To break the path correlation effect, we propose a cross-layer design called the Bundled Virtual Circuit (BVC). Our proposed BVC scheme not only addresses the path correlation issues, but also provides priority treatment from the Network layer to the MAC layer. Through our simulations, our proposed cross-layer design approach is proven to provide better network performance than the 802.11e/AODV protocols in terms of packet delivery ratio and delay. Finally, we examine flow admission control at the routing layer in ad hoc networks. Standard routing protocols such as AODV performs routing on a shortest path basis. In the presence of multiple routes, hotspots are likely to form among the intermediate nodes and thus lead to network congestion. In addition, existing routes are not well-protected from the introduction of new routes. We propose a flow admission control algorithm to be used with AODV. Our algorithm allows routes to be created through available bandwidth and congestion estimation. During the route discovery phase, each node will be gauged on its estimated available bandwidth and its backlog of packets in the MAC buffer. If a node does not have sufficient bandwidth, or has too many backlogged packets compared to the requested flow rate, it will not participate in the route discovery process. From simulations, our new scheme is able to provide better network performance than the standard AODV protocol in terms of delay. Our work in this thesis focuses on QoS provisioning in ad hoc networks from the MAC and network layer perspectives. By studying the network characteristics and protocols’ behavior, we investigate the various problems of providing QoS in an ad hoc network environment and propose three different schemes to improve the QoS provisioning at the MAC and network layers. We believe that our work is able to provide some insights to the research area on QoS in ad hoc networks.

The IEEE 802.11 legacy standard could not offer QoS support for time-critical applications. Considerable research efforts on medium access control (MAC) layer have been carried out important enhancements on improving QoS for IEEE 802.11 device implemented standard. Among them, IEEE 802.11e standard is the QoS enhanced proposed by the IEEE. The contention based channel access method called enhanced distributed channel access (EDCA) is considered as the mandatory mode for medium access control in IEEE 802.11e as distributed contention function (DFC) in IEEE 802.11. The scope of this paper is to evaluate the performances of IEEE 802.11e EDCA compared to the legacy DCF QoS support on handling multimedia applications. The performance of IEEE 802.11e MAC mechanism was evaluated through computer simulations. Performing ad-hoc and infrastructure scenarios, the study demonstrates the benefits of QoS prioritization for the IEEE 802.11e framework in terms of jitter, delay and corresponding access categories allocation.

Over the past few years, mobile networks have emerged as a promising approach for future mobile IP applications. With limited frequency resources, designing an effective MAC (Medium Access Control) protocol is a hot challenge. IEEE 802.11b/g/a/n networks are currently the most popular wireless LAN products on the market [1]. The conventional IEEE 802.11b and 802.11g/a specification provide up to 11 and 54 Mbps data rates, respectively. However, the MAC protocol that they are based upon is the same and employs a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) protocol with binary exponential back-off. IEEE 802.11 DCF (Distributed Coordination Function) is the de facto MAC protocol for wireless LAN because of its simplicity and robustness [2,3]. Therefore, considerable research efforts have been put on the investigation of the DCF performance over wireless LAN [2]. With the successful deployment of IEEE 802.11a/b/g wireless LAN and the increasing demand for real-time applications over wireless, the IEEE 802.11n Working Group standardized a new MAC and PHY (Physical) layer specification to increase the bit rate to be up to 600 Mbps [3]. The throughput performance at the MAC layer can be improved by aggregating several frames before transmission [3]. Frame aggregation not only reduces the transmission time for preamble and frame headers, but also reduces the waiting time during CSMA/CA random backoff period for successive frame transmissions. The frame aggregation can be performed within different sub-layers. In 802.11n, frame aggregation can be performed either by A-MPDU (MAC Protocol Data Unit Aggregation) or A-MSDU (MAC Service Data Unit Aggregation). Although frame aggregation can increase the throughput at the MAC layer under ideal channel conditions, a larger aggregated frame will cause each station to wait longer before its next chance for channel access. Under errorprone channels, corrupting a large aggregated frame may waste a long period of channel time and lead to a lower MAC efficiency [4]. On the other hand, wireless LAN mobile stations that are defined as the stations that access the LAN while in motion are considered in this chapter. The previous paper analyzed the IEEE 802.11b/g/n MAC performance for wireless LAN with fixed stations, not for wireless LAN with mobile stations [5, 6, 7, 8, 9, 10]. On the contrary, Xi Yong [11] and Ha Cheol Lee [12] analyzed the MAC performance for IEEE 802.11 wireless LAN with mobile stations, but considered only IEEE 802.11 and 802.11g/a wireless LAN specification. So, this chapter summarizes all the reference papers and analyzes the IEEE 802.11b/g/a/n MAC performance for wireless LAN with fixed and mobile stations. In other words, we will present the analytical evaluation of saturation

IEEE 802.11e EDCA(Enhanced Distributed Channel Access)는 서로 다른 AC(Access Category)에 따라서 CW(Contention Window)를 이용하여 Quality-of-Service(QoS)를 지원한다. 그러나 무선 네트워크 환경이 혼잡 (Congested)할 경우 패킷의 충돌 발생 확률을 높일 수 있다는 문제점이 여전히 존재한다. 이를 해결하기 위해 여러 방법들이 제시되었으나 혼잡 네트워크상에서는 동일 우선순위를 가지는 패킷 전송을 위해서 전송 채널을 이용하는 경쟁 과정에서 패킷 충돌(Collision)이 발생할 확률이 여전히 존재한다. 따라서 본 논문에서는 EDCA 포화 상태에서 전송 효율을 높이는 APCA(Advanced Priority Collision Avoidance) 알고리즘을 제안한다. 제안된 알고리즘은 IEEE 802.11e 표준을 기반으로 RTS/CTS (Request to Send / Clear to Send)를 이용하여 패킷 전송 FC(Frame Control) 필드의 예약된 필드(Reserved Field) 비트를 이용하여 데이터 패킷 충돌을 회피하는 것이다. 시뮬레이션의 결과로 제안된 알고리즘이 기존의 EDCA에 비해 패킷 전송 실패율이 감소했음을 보였다. Jain's fairness index를 이용하여 제안된 APCA 알고리즘이 네트워크 혼잡 상황에서 EDCA 방식 보다 공정성을 유지하는 것을 확인하였다. The Enhanced Distributed Channel Access (EDCA) function of IEEE 802.11e standard defines contention window (CW) for different Access Category (AC) limits to support Quality-of-Service (QoS). However, it have been remained the problem that the collision probability of transmission is increasing in congested network. Several different solutions have been proposed but the collision occurs among same priority queue within the same station to compete the channel access. This paper presents an APCA (Advanced Priority Collision Avoidance) algorithm for EDCA that increases the throughput in saturated situation. The proposed algorithm use reserved field's bits of FC(Frame Control) using IEEE 802.11e standard's RTS/CTS (Request to Send / Clear to Send) mechanism to avoid data collision. The simulation results show that the proposed algorithm improves the performance of EDCA in packet loss. Using Jain's fairness index formula, we also prove that the proposed APCA algorithm achieves the better fairness than EDCA method under network congested condition.

11 - Performance Modeling and Analysis over Medium Access Control Layer Wireless Channels

This paper presents a System on a Chip (SoC) Field-Programmable Gate Array (FPGA) implementation for Media Access Control (MAC) layer of a fast industrial Wireless Local Area Network (iWLAN) protocol, which is used to control industrial robots in factory automation environments. This protocol is a potential technique for industrial wireless communication system by using the Low overhead Packet Division Multiple Access (PDMA) scheme controlled by MAC layer. In our previous research, the MAC layer hardware was implemented and tested on simulation software, but the SoC design for the MAC layer has not been implemented and real tested on the FPGA. In this paper, we have successfully implemented the SoC system for MAC layer of this high-speed wireless communication protocol on the FPGA.

Dedicated short range communications (DSRC) has been regarded as one of the most promising technologies to provide robust communications for large scale vehicle networks. It is designed to support both road safety and commercial applications. Road safety applications will require reliable and timely wireless communications. However, as the medium access control (MAC) layer of DSRC is based on the IEEE 802.11 distributed coordination function (DCF), it is well known that the random channel access based MAC cannot provide guaranteed quality of services (QoS). It is very important to understand the quantitative performance of DSRC, in order to make better decisions on its adoption, control, adaptation, and improvement. In this paper, we propose an analytic model to evaluate the DSRC‐based inter‐vehicle communication. We investigate the impacts of the channel access parameters associated with the different services including arbitration inter‐frame space (AIFS) and contention window (CW). Based on the proposed model, we analyze the successful message delivery ratio and channel service delay for broadcast messages. The proposed analytical model can provide a convenient tool to evaluate the inter‐vehicle safety applications and analyze the suitability of DSRC for road safety applications. Copyright © 2009 John Wiley & Sons, Ltd.

With the provisioning of high-speed wireless LAN (WLAN) environments, multi- media services (e.g., VoIP and video-conference) with different QoS requirements will be available in next generation WLANs. Multimedia services could be categorized into multiple traffic classes and different priorities will be applied to access the wireless me- dium. The IEEE 802.11 working group has been developing a new generation distributed access protocol, called enhanced distributed channel access (EDCA), to support service differentiation in the 802.11 MAC layer. Service differentiation is achieved by assigning different values of EDCA access parameters (i.e., minimum contention window, maxi- mum contention window, and arbitration interframe space) to different traffic classes. To investigate the system performance under various network conditions, it is helpful to have a theoretical model for EDCA. In this paper, we introduce an analytical model for EDCA so that the saturation bandwidth can be estimated by closed-form formulas for each traffic class. We use ns-2 simulator to validate the analytical model. Some numeri- cal results are provided to evaluate the performance of EDCA. The numerical results demonstrate the corresponding effects for tuning different EDCA access parameters.

The popular IEEE 802.11 WLAN today does not provide any quality of service (QoS) because of its contention‐based channel access nature of the medium access control (MAC). A simple software upgrade‐based solution, called a dual queue scheme, can be used to provide QoS to real‐time services such as Voice‐over‐IP (VoIP). The dual queue scheme operates on top of the legacy MAC. In this paper, we present a modified dual queue (MDQ) scheme by considering a practical implementation limitation. On the other hand, the emerging 802.11e MAC includes a contention‐based channel access mechanism, called enhanced distributed channel access (EDCA). Prioritized channel accesses are provided to different traffic classes under EDCA.In this paper, we comparatively evaluate the performances of MDQ and EDCA via simulations in terms of delay, jitter and throughput for various scenarios. We also compare the two access schemes when multiple transmission rates are used via rate adaptation mechanism for reliable transmission over erroneous wireless channel. The simulation results show that EDCA apparently provides a better performance in most situations. However, MDQ provides an acceptable performance close to that of EDCA so that it can be used when the EDCA is not available and/or the hardware upgrade to 802.11e is not desirable for the cost reason. Copyright © 2006 AEIT

Voice over Internet protocol (VoIP) is one of the most important applications for IEEE 802.11 wireless local area networks (WLANs). For network planners who deploy VoIP over WLANs, one of the important issues is the VoIP capacity. Directly implementing VoIP over infrastructure WLANs will create the bottleneck problem at the access point (AP). In this paper, we propose the use of the service differentiation provided by the new IEEE 802.11e standard to solve the bottleneck problem and improve voice capacity. In particular, we propose the allocation of a higher priority access category (AC) to the AP while allocating lower priority AC to mobile stations. We develop a simple Markov chain model, which considers the important enhanced distributed channel access (EDCA) parameters and the channel errors under saturation and nonsaturation conditions. Based on the developed analytical model, we analyze the performance of VoIP over EDCA. By appropriately selecting the EDCA parameters, we are able to differentiate the services for the downlink and uplink. The experimental results are very promising. With the adjustment of only one EDCA parameter, we improve the VoIP capacity by 20%-30%.

For the influence of Internet, 802.11 lacks the definition over the MAC (medium access control) layer, user-data are switched via upper protocols and the switching point is located in the network layer. This redundant architecture has shortcomings on efficiency and is not propitious to guarantee QoS (quality of service). In this paper, we propose a novel user-data switching mechanism in the lower layer, viz. the MAC layer in 802.11. This mechanism modifies the traditional finite state machine, the header format of user-data and the communication primitives of MAC layer to achieve the goal of reducing user-data plane. Our simulations show this user-data mechanism can better the relay efficiency and improve the QoS.

The prediction of numbers of newborns is an important issue in hospital management. Relying on the inherent non-aftereffect property, discrete-time Markov chain (DTMC) is a candidate for solving the problem. But the classical DTMC is unable to handle the uncertainty of states, especially when the state space is not discrete, which would lead to instable predicted results. In order to overcome the limitation of the existing DTMC model, a belief Markov chain (BMC) model is proposed by synthesizing the classical DTMC and Dempster-Shafer theory effectively. Depending on the advantages of Dempster-Shafer theory in expressing uncertainty, the proposed BMC model is capable of dealing with various uncertainties, which improves and perfects the classical DTMC model. An illustrative example demonstrates the effectiveness of the proposed model. Moreover, a comparison between the proposed BMC model and the classical and fuzzy states modified DTMC models is given to show the superiority of the proposed model against the other two. Finally, the stability of the proposed model has been proven.