Throughput and QoS optimization for EDCA-based IEEE 802.11 WLANs

The enhanced distributed channel access (EDCA) mechanism has been adopted by both the IEEE 802.11e standard and the Multiband OFDM Alliance (MBOA) for quality of service (QoS) provisioning in high speed wireless LANs and UWB-based wireless PANs. Based on an analytical model of EDCA, this paper presents an optimal solution to providing maximum system throughput while maintaining the service differentiation among traffic classes. Contention window sizes are adapted to achieve throughput optimization according to the throughput ratio and number of active stations of each traffic class. To overcome the difficulty of accurate estimation of the number of competing stations we propose to use the method of subrange partitioning. An admission control scheme is also discussed. Simulation results demonstrate the effectiveness of the proposed MAC optimization framework.     

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.     

Throughput Analysis and Measurements in IEEE 802.11 WLANs with TCP and UDP Traffic Flows

There is a vast literature on the throughput analysis of the IEEE 802.11 media access control (MAC) protocol. However, very little has been done on investigating the interplay between the collision avoidance mechanisms of the 802.11 MAC protocol and the dynamics of upper layer transport protocols. In this paper, we tackle this issue from an analytical, simulative, and experimental perspective. Specifically, we develop Markov chain models to compute the distribution of the number of active stations in an 802.11 wireless local area network (WLAN) when long-lived transmission control protocol (TCP) connections compete with finite-load user datagram protocol (UDP) flows. By embedding these distributions in the MAC protocol modeling, we derive approximate but accurate expressions of the TCP and UDP throughput. We validate the model accuracy through performance tests carried out in a real WLAN for a wide range of configurations. Our analytical model and the supporting experimental outcomes show that 1) the total TCP throughput is basically independent of the number of open TCP connections and the aggregate TCP traffic can be equivalently modeled as two saturated flows; and 2) in the saturated regime, n UDP flows obtain about n times the aggregate throughput achieved by the TCP flows, which is independent of the overall number of persistent TCP connections.     

Multiple WNIC-based handoff in IEEE 802.11 WLANs

We propose a novel scanning scheme for IEEE 802.11 by equipping Access Points (APs) with multiple Wireless Network Interface Cards (Multi-WNICs), one of which is set to operate in an exclusively reserved channel for the scanning purpose. In this environment, a STAtion (STA) can easily search neighboring APs by scanning the reserved channel. Our simulation results demonstrate that the proposed scheme ultimately reduces the overall scanning time to improve the handoff latency.     

Enhancing Network Throughput via the Equal Interval Frame Aggregation Scheme for IEEE 802.11ax WLANs

Frame aggregation is fully supported in the newly published IEEE 802.11ax standard to improve throughput. With frame aggregation, a mobile station combines multiple subframes into an aggregate MAC service data unit(A-MSDU) or an aggregate MAC protocol data unit(A-MPDU) for transmission. It is challenging for a mobile station in 802.11ax WLANs to set an appropriate number of subframes being included in an AMSDU or A-MPDU. This problem is solved in this paper by the proposed equal interval frame a...     

IEEE 802.11a Throughput Performance with Hidden Nodes

Analytic performance of IEEE 802.11 distributed coordination function that takes into account hidden node problem and unsaturated traffic condition is presented for symmetric networks. This enables us to estimate the performance of practical wireless local area networks more accurately. It is shown that the presence of hidden nodes barely affects the network performance in low traffic conditions, but it causes 33% performance loss in moderate traffic scenarios. Analytical results presented in the paper are backed by simulation results.     

A Multichain Backoff Mechanism for IEEE 802.11 WLANs

The distributed coordination function (DCF) of IEEE 802.11 standard adopts the binary exponential backoff (BEB) for collision avoidance. In DCF, the contention window is reset to an initial value, i.e., CWmin, after each successful transmission. Much research has shown that this dramatic change of window size may degrade the network performance. Therefore, backoff algorithms, such as gentle DCF (GDCF), multiplicative increase–linear decrease (MILD), exponential increase–exponential decrease (EIED), etc., have been proposed that try to keep the memory of congestion level by not resetting the contention window after each successful transmission. This paper proposes a multichain backoff (MCB) algorithm, which allows stations to adapt to different congestion levels by using more than one backoff chain together with collision events caused by stations themselves as well as other stations as indications for choosing the next backoff chain. The performance of MCB is analyzed and compared with those of 802.11 DCF, GDCF, MILD, and EIED backoff algorithms. Simulation results show that, with multiple backoff chains and collision events as reference for chain transition, MCB can offer a higher throughput while still maintaining fair channel access than the existing backoff algorithms.     

Throughput Analysis and Admission Control for IEEE 802.11a

We propose a new Markov model for the distributed coordination function (DCF) of IEEE 802.11. The model incorporates carrier sense, non-saturated traffic and SNR, for both basic and RTS/CTS access mechanisms. Analysis of the model shows that the throughput first increases, and then decreases with the number of active stations, suggesting the need for an admission control mechanism.We introduce such a mechanism, which tries to maximize the throughput while maintaining a fair allocation. The maximum achievable throughput is tracked by the mechanism as the number of active stations increases. An extensive performance analysis shows that the mechanism provides significant improvements.Mustafa Ergen received the B.S. degree in electrical engineering from Middle East Technical University (METU) and was the METU Valedictorian in 2000. He received the M.S. and Ph.D. degrees in electrical engineering in 2002 and 2004, the MOT certificate of HAAS Business School in 2003, and the M.A. degree in International and Area Studies in 2004 from the University of California, Berkeley.Dr. Ergen has been conducting research in wireless communication networks with an emphasis on sensor networks, wireless LAN and OFDM systems and is the author of many works in the field, including the book (with A.R.S. Bahai and B.R. Saltzberg) Multi-Carrier Digital Communications: Theory and Applications of OFDM (New York: Springer, 2004).He is National Semiconductor Post Doctoral Fellow and was awarded eight times Bulent Kerim Altay Award by department of electrical engineering in METU and received Best Student Paper Award in IEEE ISCC 2003 and has an invited paper in IEEE GLOBECOM CAMAD 200.Pravin Varaiya is Nortel Networks Distinguished Professor in the Department of Electrical Engineering and Computer Sciences at the University of California, Berkeley. From 1975 to 1992, he was also Professor of Economics at Berkeley. His research is concerned with communication networks, transportation, and hybrid systems. He has taught at MIT and the Federal University of Rio de Janeiro, Varaiya has held a Guggenheim Fellowship and a Miller Research Professorship. He received an Honorary Doctorate from L’Institut National Polytechnique de Toulouse, and the Field Medal of the IEEE Control Systems Society. He is a Fellow of IEEE and a member of the National Academy of Engineering. He is on the editorial board of several journals, including “Discrete Event Dynamical Systems” and “Transportation Research-C.” He has co-authored three books and more than 250 technical papers. The second edition of “High-Performance Communication Networks” (with Jean Walrand) was published by Morgan-Kaufmann in 2000. “Structure and interpretation of signals and systems” (with Edward Lee) was published in 2002 by Addison-Wesley.     

Distributed QoE-aware channel assignment algorithms for IEEE 802.11 WLANs

Wireless Networks - One of the challenging problems with deployment of IEEE 802.11WLANs in the same hotspot is assignment of appropriate channels to the Access Points (APs). As the number of...     

基于IEEE 802.11标准的WLAN的物理层方案

作为WLAN第一个成熟的标准，IEEE 802．11成功地解决了无线介质给WLAN造成的困难。本文详细地分析了WLAN在物理层上存在的困难，并给出了IEEE 802．11相应的解决方案。     

Throughput and delay limits of IEEE 802.11

The IEEE 802.11 protocol family provides up to 54-Mbps data rate, whereas the industry is seeking higher data rates. This paper shows that a theoretical throughput upper limit and a theoretical delay lower limit exist for the IEEE 802.11 protocols. The existence of such limits indicates that by simply increasing the data rate without reducing overhead, the enhanced performance, in terms of throughput and delay, is bounded even when the data rate goes into infinitely high. Reducing overhead is vital for good performance.     

Frequency assignments in IEEE 802.11 WLANs with efficient spectrum sharing

As the number of WLAN users grows, the need to perform efficient radio resource management strategies becomes essential due to the fact that most popular technologies, those based on IEEE 802.11 standards, use unlicensed frequency bands. A good channel assignment improves the network performance, producing benefits that are perceived by the users and also by the network administrators. In this paper, we present a new frequency management scheme for IEEE 802.11 WLANs in the 2.4 GHz ISM band that minimizes interference to increase the throughput available to client stations by adapting a weighted DSATUR algorithm for graph coloring. The algorithm takes both co‐channel and adjacent channel interference into account, and makes use of all available channels instead of the traditional non‐overlapping three. In this way, collisions as well as transmission errors are minimized, thus improving the network capacity and the user experience. Different architectures are discussed for the implementation of our approach, including the possibility to incorporate client stations into the management system. Copyright © 2008 John Wiley & Sons, Ltd.     

FastScan: a handoff scheme for voice over IEEE 802.11 WLANs

IEEE 802.11 Wireless LANs are increasingly being used in enterprise environments for broadband access. Such large scale IEEE 802.11 WLAN deployments implies the need for client mobility support; a mobile station has to be “handed off” from one Access Point to another. Seamless handoff is possible for data traffic, which is not affected much by the handoff delay. However, voice traffic has stringent QoS requirements and cannot tolerate more than 50ms net handoff delay. The basic IEEE 802.11 handoff scheme (implemented in Layers 1 & 2) only achieves a handoff delay of 300ms at best, leading to disrupted connectivity and call dropping. The delay incurred in scanning for APs across channels contributes to 90% of the total handoff delay. In this paper, the FastScan scheme is proposed which reduces the scanning delay by using a client-based database. The net handoff delay is reduced to as low as 20 ms for IEEE 802.11b networks. We next suggest “Enhanced FastScan” that uses the direction and relative position of the client with respect to the current AP to satisfy the latency constraint in IEEE 802.11a scenarios, which have significantly higher scanning delays due to the larger number of channels. The proposed schemes do not need any changes in the infrastructure (access points) and require only a single radio and a small cache memory at the client side.     

A novel pseudo differential transconductor for IEEE 802.11 WLANs

A novel pseudo differential transconductor for multi-mode analog baseband channel selection filter is presented. The highly linear transconductor is designed based on the dynamic source degeneration and predistortion cancellation technique. Meanwhile, wide tuning range is achieved with the current division technique. An LC ladder third-order Butterworth low-pass filter implemented with transconductors and capacitors was fabricated by TSMC 0.18-μm CMOS process. The results show that the filter can operate with the cutoff frequency ranging from 4 to 20 MHz. The tuning range is wide enough for the specifications of IEEE 802.11a/b/g/n Wireless LANs under the consideration of low power consumption and linearity requirement. The maximum power consumption is 3.61 mA at the cutoff frequency of 20 MHz.     

Relay-volunteered multi-rate cooperative MAC protocol for IEEE 802.11 WLANs

In IEEE 802.11, the rate of a station (STA) is dynamically determined by link adaptation. Low-rate STAs tend to hog more channel time than high-rate STAs due to fair characteristics of carrier sense multiple access/collision avoidance, leading to overall throughput degradation. It can be improved by limiting the transmission opportunities of low-rate STAs by backoff parameters. This, however, may cause unfair transmission opportunities to low-rate STAs. In an attempt to increase overall throughput by volunteer high-rate relay STAs while maintaining fairness, we propose a new cooperative medium access control (MAC) protocol, relay-volunteered multi-rate cooperative MAC (RM-CMAC) based on ready to send/clear to send in multi-rate IEEE 802.11. In the RM-CMAC protocol, we show that the effect of hogging channel time by low-rate STAs can be remedied by controlling the initial backoff window size of low-rate STAs and the reduced transmission opportunity of low-rate STAs can be compensated by the help of volunteer high-rate relay STAs. We analyze the performance of RM-CMAC, i.e., throughput and MAC delay, by a multi-rate embedded Markov chain model. We demonstrate that our analysis is accurate and the RM-CMAC protocol enhances the network throughput and MAC delay while maintaining the fairness of low-rate STAs.     

Performance analysis of differentiated QoS in IEEE 802.11e WLANs

This paper proposes a multi‐dimensional Markov model to analyse the performance of the IEEE 802.11e EDCF MAC protocol. Based on this model, we present extensive performance evaluation in terms of throughput, throughput ratios, and access delay of flows of distinct priorities under RTS/CTS mode. We also provide quantitative analysis of the impact of prioritized parameters, i.e. Arbitration InterFrame Space (AIFS), Contention Window (CW) on Quality of Service (QoS) differentiation. The accuracy of the proposed model is verified by means of comparing the numerical results obtained from both analytical model and simulations. Our research can be used as a guideline for the prediction of how flows belonging to a certain Traffic Category (TC) perform with their TC‐specific parameters, as well as designing EDCF‐based WLANs and tuning the parameters to achieve the desirable differentiated QoS objectives. Copyright © 2005 John Wiley & Sons, Ltd.     

A Seamless Handoff Mechanism for DHCP-Based IEEE 802.11 WLANs

IEEE 802.11 wireless networks have gained great popularity. However, handoff is always a critical issue in this area. In this paper, we propose a novel seamless handoff mechanism for IEEE 802.11 wireless networks which support IEEE 802.11i security standard. Our approach consists of a dynamic tunnel establishing procedure and a seamless handoff mechanism. Both intra- and inter-subnet handoff cases are considered in our seamless handoff approach. Our work focuses on handoffs in DHCP-based IP networks rather than mobile IP-supported networks, but the proposed scheme can be easily tailored to mobile IP-supported networks.     

Enhancing IEEE 802.11n WLANs using group-orthogonal code-division multiplex

The definition of the next generation of wireless networks is well under way within the IEEE 802.11 High Throughput Task Group committee. The resulting standard, to be called IEEE 802.11n, is expected to be a backward-compatible evolution of the successful IEEE 802.11a/g systems also based on multicarrier techniques. It can be anticipated that 802.11n systems will outperform its predecessors in terms of transmission rate and/or performance, mainly, due to the use of multiple antennae technology for transmission and reception. In this paper we propose to incorporate group-orthogonal (GO) code division multiplex (CDM) into the IEEE 802.11n specifications to further enhance its performance. It is shown how GO-CDM can take full advantage of the diversity offered by the multiple antennae and multicarrier transmission by using an iterative maximum likelihood (ML) joint detector. Furthermore, the use of GO-CDM does not compromise the backward compatibility with legacy systems.     

基于IEEE802.11r的无线局域网快速切换研究

在IEEE802.11网络中高效地切换是多媒体实时应用的一个关键要求。IEEE802.11r快速切换协议使切换时延得到了有效限制。但整个切换过程仍无法满足实时应用的需求。文中综述了无线局域网快速切换的研究,分析了IEEE802.11r对整个切换过程的影响,并介绍了快速切换研究工作。     

Simultaneous handover scheme for IEEE 802.11 WLANs with IEEE 802.21 triggers

Handover performance in wireless networks is important, especially nowadays, when multimedia services are becoming increasingly available over the wireless devices. However, users expect uncompromised mobility when using the service. Thus, the support of multimedia services is not possible if handover is inefficient. At the same time it is clear that a strict separation between IP Layer and the Link Layer results in built-in sources of delay. The paper discusses the IEEE 802.11 and Mobile IPv4 handover performance in practical scenarios. We introduce a new simultaneous handover scheme with IEEE 802.21 triggers. In order to verify the handover performance, simulation experiments have been conducted, whose results are also presented and discussed.