TCP fairness in. 802.11e WLANs

We investigate the use of the 8.02.11e MAC EDCF to address transport layer unfairness in WLANs. A simple solution is developed that uses the 802.11e AIFS, TXOP and CW/sub min/ parameters to ensure fairness between competing TCP uploads and downloads.     

On the capabilities of IEEE 802.11e for multimedia communications over heterogeneous 802.11/802.11e WLANs

Multimedia communications over WLAN is widely acknowledged as one of the key, emerging applications for wireless LANs. As with any multi-service network, there is the need to provision the WLANs with the QoS mechanisms capable of guaranteeing the requirements of various services. The upcoming IEEE 802.11e (EDCA) standard is a proposal defining the mechanisms for wireless LANs aiming to provide QoS support to time-sensitive applications such as voice and video communications. Due to the fact that the IEEE 802.11e interface cards will take over the WLAN market, replacing the use of legacy IEEE 802.11 interface cards in most WLAN applications, an important number of networking scenarios will consist of a hybrid configuration comprising legacy IEEE 802.11-based stations and IEEE 802.11e-based stations. For this reason, in this paper we carry out a performance analysis on the effectiveness of the IEEE 802.11e (EDCA) upcoming standard when supporting different services, such as, voice, video, best-effort, background and in the presence of traffic generated by legacy 802.11 (DCF) based stations.     

Buffer Sizing for TCP Flows in 802.11e WLANs

We consider the task of sizing buffers for TCP flows in 802.11e WLANs. A number of fundamental new issues arise compared to wired networks. These include that the mean service rate is dependent on the level of channel contention and packet inter-service times vary stochastically due to the random nature of CSMA/CA operation. We find that these considerations lead naturally to a requirement for adaptation of buffer sizes in response to changing network conditions.     

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.     

Analytical Study of TCP Performance over IEEE 802.11e WLANs

IEEE 802.11 Wireless LAN (WLAN) has become a prevailing solution for broadband wireless Internet access while the Transport Control Protocol (TCP) is the dominant transport-layer protocol in the Internet. Therefore, it is critical to have a good understanding of the TCP dynamics over WLANs. In this paper, we conduct rigorous and comprehensive modeling and analysis of the TCP performance over the emerging 802.11e WLANs, or more specifically, the 802.11e Enhanced Distributed Channel Access (EDCA) WLANs. We investigate the effects of minimum contention window sizes and transmission opportunity (TXOP) limits (of both the AP and stations) on the aggregate TCP throughput via analytical and simulation studies. We show that the best aggregate TCP throughput performance can be achieved via AP’s contention-free access for downlink packet transmissions and the TXOP mechanism. We also study the effects of some simplifying assumptions used in our analytical model, and simulation results show that our model is reasonably accurate, particularly, when the wireline delay is small and/or the packet loss rate is low.

Collision-aware design of rate adaptation for multi-rate 802.11 WLANs

One of the key challenges in designing a rate adaptation scheme for IEEE 802.11 wireless LANs (WLANs) is to differentiate bit errors from link-layer collisions. Many recent rate adaptation schemes adopt the RTS/CTS mechanism to prevent collision losses from triggering unnecessary rate decrease. However, the RTS/CTS handshake incurs significant overhead and is rarely activated in today's infrastructure WLANs. In this paper we propose a new rate adaptation scheme that mitigates the collision effect on the operation of rate adaptation. In contrast to previous approaches adopting fixed rate-increasing and decreasing thresholds, our scheme varies threshold values based on the measured network status. Using the "retry" information in 802.11 MAC headers as feedback, we enable the transmitter to gauge current network state. The proposed rate adaptation scheme does not require additional probing overhead incurred by RTS/CTS exchanges and can be easily deployed without changes in firmware. We demonstrate the effectiveness of our solution by comparing with existing approaches through extensive simulations.     

The IEEE 802.11g standard for high data rate WLANs

Continuous WLAN public acceptance comes with increasing demand for provision of higher data rates. Building on this context, the IEEE published the IEEE 802.11g standard for providing data rates of up to 54 Mb/s at the 2.4 GHz band. This article presents the new features of IEEE 802.11g and, using an open source C++-based simulation tool, evaluates both the performance and effectiveness of these features compared to the older IEEE 802.11 standard versions.     

Performance analysis and comparison of burst transmission schemes in unsaturated 802.11e WLANs

Contention free bursting (CFB) and block acknowledgement (BACK) are two innovative burst transmission schemes specified in the IEEE 802.11e standard for reducing the contention overheads and further improving the channel utilization of wireless local area networks (WLANs). Existing studies on performance analysis of the CFB and BACK schemes have been primarily focused on the system throughput and have not taken into account the realistic factors, such as unsaturated traffic loads and finite buffer capacity. To fill this gap, this paper proposes a new and comprehensive analytical model for evaluating the Quality‐of‐Service (QoS) metrics including throughput, end‐to‐end delay, and frame loss probability of both burst transmission schemes under unsaturated traffic conditions. The proposed model is validated through extensive simulation experiments and then is used to conduct performance analysis and comparison of the burst transmission schemes under various working conditions. The analytical results reveal that (1) both CFB and BACK schemes can substantially improve the QoS performance; (2) BACK scheme outperforms the CFB scheme when the transmission opportunity (TXOP) limit exceeds a threshold; (3) the analytical model can be used to identify the optimal configuration of system parameters for the burst transmission schemes subject to QoS constraints. Copyright © 2010 John Wiley & Sons, Ltd.     

Bandwidth Sharing Schemes for Multimedia Traffic in the IEEE 802.11e Contention-Based WLANs

Bandwidth allocation schemes have been well studied for mobile cellular networks. However, there is no study about this aspect reported for IEEE 802.11 contention-based distributed wireless LANs. In cellular networks, bandwidth is deterministic in terms of the number of channels by frequency division, time division, or code division. On the contrary, bandwidth allocation in contention- based distributed wireless LANs is extremely challenging due to its contention-based nature, packet-based network, and the most important aspect: only one channel is available, competed for by an unknown number of stations. As a consequence, guaranteeing bandwidth and allocating bandwidth are both challenging issues. In this paper, we address these difficult issues. We propose and study nine bandwidth allocation schemes, called sharing schemes, with guaranteed Quality of Service (QoS) for integrated voice/video/data traffic in IEEE 802.11e contention-based distributed wireless LANs. A guard period is proposed to prevent bandwidth allocation from overprovisioning and is for best-effort data traffic. Our study and analysis show that the guard period is a key concept for QoS guarantees in a contention-based channel. The proposed schemes are compared and evaluated via extensive simulations.     

Two-level protection and guarantee for multimedia traffic in IEEE 802.11e distributed WLANs

In order to support multimedia applications such as voice and video over the wireless medium, a contention-based channel access function, called Enhanced Distributed Channel Access (EDCA), has been developed in the emerging standard IEEE 802.11e. In the EDCA, differentiated channel access is provided for different traffic classes. In this paper, we propose a two-level protection and guarantee mechanism for voice and video traffic in the EDCA-based distributed wireless LANs. In the first-level protection, the existing voice and video flows are protected from the new and other existing voice and video flows via a distributed admission control with tried-and-known and early-protection enhancements. In the second-level protection, the voice and video flows are protected from the best-effort data traffic by adopting frame-based and limit-based data control mechanisms. Performance evaluations are conducted in terms of throughput, delay, transmission limit, number of collisions, and throughput square relative difference. Extensive simulation results demonstrate that the proposed two-level protection and guarantee mechanism is very effective in terms of the protection and guarantee of existing voice and video flows as well as the utilization of the channel capacity. An early version of this paper was presented at IEEE INFOCOM 2004.     

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.     

Detecting selfish configurations in 802.11 WLANs

Lately, there has been an increase in the number of IEEE 802.11 devices that provide users with the ability to modify the MAC parameters or do not conform to the standard specification. This increases the risk of having a WLAN with selfish stations that, through the CSMA/CA parameters, obtain a larger share of the resources at the expense of well-behaved users. In this letter we propose a mechanism to detect these selfish stations that, unlike previous approaches, is not based on heuristics nor makes any assumption about radio conditions.     

On centralized schedulers for 802.11e WLANs distribution versus grouping of resources allocation

Wireless LAN is becoming a pervasive wireless access technology that can be found in almost any mobile device such as laptops, PDAs, portable game consoles and mobile phones. Each of these groups of devices have a different set of requirements according to their intended use and applications but most of them share two main requirements: QoS support to satisfy applications' demands and power saving functionality to achieve an operating time according to users' expectations. IEEE 802.11e defines two centralized solutions in order to address these problems: Hybrid Coordination Channel Access (HCCA) for QoS and Scheduled Automatic Power Save Delivery (S‐APSD) for power saving. The focus of our work in this paper is the analysis and evaluation of a proposed centralized scheduler that makes use of both aforementioned IEEE 802.11e QoS and power saving solutions. Our contributions are as follows: (i) Design and analytical modeling of a proposed centralized scheduler (DRA) that maximizes the minimum distance between the resource allocations with pseudo‐polynomial complexity, (ii) Extensive performance evaluation of the QoS and power saving benefits of the Distribution proposal (DRA) as compared to a generic Grouping one (GRA), and (iii) Evaluation of the complexity and scalability of the proposal to assess its feasibility in practice. Copyright © 2010 John Wiley & Sons, Ltd.     

Reducing the Impact of Legacy Stations on Voice Traffic in 802.11e EDCA WLANs

The EDCA access mechanism of the 802.11e standard supports legacy DCF stations, but the performance of high priority traffic is substantially degraded in their presence. In a previous letter we proposed the ACKS technique to mitigate the impact of legacy stations and studied this technique under high priority data traffic. In this letter we analyze the suitability of the ACKS technique for protecting voice traffic. Results show that voice performance is significantly improved as a result of using this technique     

Retry Limit Based ULP for Scalable Video Transmission over IEEE 802.11e WLANs

We investigate the packet loss behavior in the IEEE 802.11e wireless local area networks (WLANs) under various retry limit settings. Considering scalable video traffic delivery over the IEEE 802.11e WLANs, our study shows the importance of adaptiveness in retry limit settings for the unequal loss protection (ULP) design. Based on the study, we present a simple yet effective retry limit based ULP which adaptively adjusts the retry limit setting of the IEEE 802.11e medium access control protocol to maintain a strong loss protection for critical video traffic transmission. The simulation results illustrate significant advantages in the delivered video quality for our proposed design.     

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.     

Analysis of Cross-Layer Interaction in Multirate 802.11 WLANs

Recent works in empirical 802.11 wireless LAN performance evaluation have shown that cross-layer interactions in WLANs can be subtle, sometimes leading to unexpected results. Two such instances are: (i) significant throughput degradation resulting from automatic rate fallback (ARF) having difficulty distinguishing collision from channel noise, and (ii) scalable TCP over DCF performance that is able to mitigate the negative performance effect of ARF by curbing multiple access contention even when the number of stations is large. In this paper, we present a framework for analyzing complex cross-layer interactions in 802.11 WLANs, with the aim of providing effective tools for understanding and improving WLAN performance. We focus on cross-layer interactions between ARF, DCF, and TCP, where ARF adjusts coding at the physical layer, DCF mediates link layer multiple access control, and TCP performs end-to-end transport. We advance station-centric Markov chain models of ARF, ARF-DCF with and without RTS/CTS, and TCP over DCF that may be viewed as multi-protocol extensions of Bianchi's IEEE 802.11 model. We show that despite significant increase in complexity the analysis framework leads to tractable and accurate performance predictions. Our results complement empirical and simulation-based findings, demonstrating the versatility and efficacy of station-centric Markov chain analysis for capturing cross-layer WLAN dynamics.     

ACKS: a technique to reduce the impact of legacy stations in 802.11e EDCA WLANs

The EDCA access mechanism of the upcoming 802.11e standard supports legacy DCF stations, but with substantially degraded performance. The reason being that DCF stations typically compete for access with overly small contention windows (CWs). In this letter we propose a new technique that, implemented at the access points (AP's), mitigates the impact of legacy stations on EDCA. The key idea of the technique is that, upon receiving a frame from a legacy station, the AP skips the ACK frame reply with a certain probability. When missing the ACK, the legacy station increases its CW and thus our technique allows us to have some control over the CW's of the legacy stations. We show by means of an example that this technique improves the overall performance of the WLAN.     

Saving Energy during Channel Contention in 802.11 WLANs

We focus on energy saving in 802.11-based WLANs. Previous work has shown that, on the one hand, 802.11 wireless interfaces consume a significant amount of energy, on the other hand the use of current power management schemes can severely degrade the QoS performance of several Internet-based applications. Furthermore, the energy spent by wireless devices may even increase when the standard 802.11 power-saving mode (PSM) is implemented. These facts suggest that other solutions to energy saving are highly needed. In this paper, we consider the 802.11 distributed access scheme and we propose a novel approach that enables a station to enter a low-power operational state during channel contention. More specifically, our technique exploits the virtual carrier sense mechanism and the backoff function specified in the IEEE 802.11 DCF, so that a station can dramatically reduce its energy consumption without significant degradation of the QoS performance. To efficiently implement our mechanism, a low-power state with negligible transition time into the active state must be identified. This can be any of the non-standard, low-power states defined by proprietary solutions in the current or next-generation products [7,15,22]. By using the network simulator ns2, we evaluate the performance improvement that is obtained when the proposed mechanism is implemented, against the results attained through the standard DCF. The results show that we can achieve a reduction in energy consumption as large as 80% and 28% under, respectively, UDP and TCP traffic. This work was supported by the Italian Ministry of University and Research through the PATTERN and the PRIMO projects. Valeria Baiamonte graduated from Politecnico di Torino with a degree in Telecommunications Engineering in 2002, with 110/110. Between March 2002 and November 2002, she was supported by TiLab (Italian Public Telephone Research Company) while developing her thesis work on packet scheduling algorithms for WCDMA systems networks. From January 2003 to December 2003, she was a CNIT researcher at the Electronics Department of Politecnico working on the VICOM project. Currently she is a PhD student at the Electronics Department of Politecnico di Torino. Carla-Fabiana Chiasserini graduated with a summa cum laude degree in Electrical Engineering from the University of Florence in 1996. She did her graduate work at the Politecnico di Torino, Italy, receiving the Ph.D. degree in 1999. Since then she has been with the department of Electrical Engineering at Politecnico di Torino, where she is currently an assistant professor. Since 1999, she has worked as a visiting researcher at the University of California, San Diego, California. Her research interests include architectures, protocols and performance analysis of wireless networks for integrated multi-media services. She is a member of the editorial board of the Ad Hoc Networks Journal (Elsevier), and has served as an associate editor of the IEEE Communications Letters since 2004.