Ilenia Tinnirello

Kalman filter estimation of the number of competing terminals in an IEEE 802.11 network

Throughput performance of the IEEE 802.11 distributed coordination function (DCF) is very sensitive to the number n of competing stations. The contribute of this paper is threefold. First, we show that n can be expressed as function of the collision probability encountered on the channel; hence, it can be estimated based on run-time measurements. Second, we show that the estimation of n, based on exponential smoothing of the measured collision probability (specifically, an ARMA filter), results to be a biased estimation, with poor performance in terms of accuracy/tracking trade-offs. Third, we propose a methodology to estimate n, based on an extended Kalman filter coupled with a change detection mechanism. This approach shows both high accuracy as well as prompt reactivity to changes in the network occupancy status. Numerical results show that, although devised in the assumption of saturated terminals, our proposed approach results effective also in non-saturated conditions, and specifically in tracking the average number of competing terminals.

Refinements on IEEE 802.11 Distributed Coordination Function Modeling Approaches

With the popularity of the IEEE 802.11 standards, many analytical saturation throughput studies for the distributed coordination function (DCF) have been reported. In this paper, we outline a number of issues and criticalities raised by previously proposed models. In particular, a careful look at backoff counter decrement rules allows us to conclude that, under saturation conditions, the slot immediately following a successful transmission can be accessed only by the station (STA) that has successfully transmitted in the previous channel access. Moreover, due to the specific acknowledgment (ACK) timeout setting adopted in the standard, the slot immediately following a collision cannot be accessed by any STA. Thus, the hypothesis of uncorrelation between consecutive channel slots and statistical homogeneity is not generally true. We propose a new backoff decrement model that retains the simplicity of traditional DCF models while being able to take into account such a correlation, and we compare the accuracy of our model with that of previously proposed approaches.

Temporal fairness provisioning in multi-rate contention-based 802.11e WLANs

The IEEE 802.11e extensions for QoS support in WLAN define the transmission opportunity (TXOP) concept, in order to limit the channel holding times of the contending stations in the presence of delay-sensitive traffic. We evaluate the use of TXOP for a different purpose: temporal fairness provisioning among stations employing different data rates. We show that the equalization of the channel access times allows each station to obtain its throughput basically (1) proportional to its transmission rate, and (2) independent of the transmitted frame length. This also improves the aggregate throughput of the overall WLAN. For a given TXOP limit, i.e., a granted channel access time, a station is required to fragment its pending frame if the TXOP limit is too short, and is allowed to transmit multiple frames back-to-back in a burst if the TXOP limit is long enough, while different fragmentation and bursting rules are possible. Based on the analytical and simulation results, we demonstrate the advantages of TXOP operations over the legacy 802.11 DCF, and compare different TXOP managing policies to find the optimal one.

Analysis of priority mechanisms based on differentiated inter frame spacing in CSMA-CA

A number of service differentiation mechanisms have been proposed for, in general, CSMA-CA systems, and, in particular, the 802.11 enhanced distributed coordination function. An effective way to provide prioritized service support is to use different inter frame spaces (IFS) for stations belonging to different priority classes. This paper proposes a novel analytical approach to evaluate throughput and delay performance of IFS based priority mechanisms.

Rethinking the IEEE 802.11e EDCA performance modeling methodology

Analytical modeling of the 802.11e enhanced distributed channel access (EDCA) mechanism is today a fairly mature research area, considering the very large number of papers that have appeared in the literature. However, most work in this area models the EDCA operation through per-slot statistics, namely probability of transmission and collisions referred to slots. In so doing, they still share a methodology originally proposed for the 802.11 Distributed Coordination Function (DCF), although they do extend it by considering differentiated transmission/ collision probabilities over different slots.We aim to show that it is possible to devise 802.11e models that do not rely on per-slot statistics. To this purpose, we introduce and describe a novel modeling methodology that does not use per-slot transmission/collision probabilities, but relies on the fixed-point computation of the whole (residual) backoff counter distribution occurring after a generic transmission attempt. The proposed approach achieves high accuracy in describing the channel access operations, not only in terms of throughput and delay performance, but also in terms of low-level performance metrics.

Improving load balancing mechanisms in wireless packet networks

This paper provides a comparative performance evaluation of various load balancing schemes in cellular packet networks. With respect to circuit switched networks, wireless packet technology adds the further issue of quality of service of accepted connections. In fact, with packet technology, transmission error performance does not uniquely depend on the perceived channel quality, but it can be improved by adopting a scheduling mechanism enforcing fast retransmission of corrupted packets. The result is that throughput can be traded off with QoS experienced by an admitted flow. This paper proposes new packet-level load balancing mechanisms. In addition to the number of calls admitted in a cell, our schemes use supplementary packet level information, expressed in terms of effective resource consumption of each individual call when retransmission mechanisms are employed. Simulation results prove the superiority of our proposed schemes with respect to traditional load balancing schemes.

On the side-effects of proprietary solutions for fading and interference mitigation in IEEE 802.11b/g outdoor links

Experimental results are typically envisioned as the ultimate validation reference for any theoretical and/or simulation modelling assumptions. However, in the case of Wireless LANs, the situation is not as straightforward as it might seem. In this paper, we discuss to what (large) extent measurement results may depend on proprietary undocumented algorithms implemented in the vendor-specific card/driver employed. Specifically, we focus on the experimental study of IEEE 802.11b/g outdoor links based on the widely used Atheros/MadWiFi chipset/driver pair. We show that significant and unexpected performance degradation may occur as a consequence of two Atheros proprietary algorithms: Transmit Antenna Diversity and Ambient Noise Immunity. Our findings appear quite critical in sight of the fact that part of the WLAN research community is often unaware of the existence of these proprietary mechanisms. Such lack of awareness is critical as it can lead to biased experimental trials and/or to erroneous interpretation of experimental results.

Revisit of RTS/CTS exchange in high-speed IEEE 802.11 networks

IEEE 802.11 medium access control (MAC), called distributed coordination function (DCF), provides two different access modes, namely, 2-way (basic access) and 4-way (RTS/CTS) handshaking. The 4-way handshaking has been introduced in order to combat the hidden terminal phenomenon. It has been also proved that such a mechanism can be beneficial even in the absence of hidden terminals, because of the collision time reduction. We analyze the effectiveness of the RTS/CTS access mode, in current 802.11b and 802.11a networks. Since the rates employed for control frame transmissions can be much lower than the rate employed for data frames, the assumption on the basis of the 4-way handshaking introduction, i.e., a short transmission time for the RTS control frame, is no longer valid. As a consequence, the basic access mode results in the optimal access solution in most cases, even in heavy load conditions with hidden nodes. We compare the 2-way and 4-way access performances through both analytical and simulation tools. We also discuss the operating conditions at which the switch from one access mode to another is desired for the cases of uniform and heterogeneous data rates among the stations. We conclude that, for the heterogeneous data rate environments, the RTS/CTS threshold should be redefined as a frame transmission time rather than as a frame size.

An Explanation for Unexpected 802.11 Outdoor Link-level Measurement Results

This paper provides experimental evidence that weird/poor outdoor link-level performance measurements may be caused by driver/card-specific antenna diversity algorithms unexpectedly supported/activated at the WLAN transmitter side. We focus our analysis on the Atheros/MADWiFi card/driver case, and we observe that the transmit antenna diversity mechanisms remain by default enabled when the available antennas are not homogeneous in terms of gain or, even worse, when only a single antenna is connected. This may cause considerable performance impairments (large frame loss ratio), in conditions frequently encountered in outdoor link deployments. The negative impact of transmit antenna diversity is not limited to the transmission of broadcast frames (where a cyclic shift between the two assumed antennas is performed), but under certain circumstances it can severely affect the delivery of unicast frames as well, and despite the fact that in this case the ACK receptions may provide a feedback about the best receiving antenna. While, as obvious, driver developers are expectedly fully aware of the existence of such mechanisms, we believe that the scientific research community has very limited awareness of the implications these mechanisms have on the measured link-level performance. Indeed, to the best of our knowledge, ours is the first research paper which explicitly raises this issue.

Analysis of the IEEE 802.11e EDCA Under Statistical Traffic

Many models have been proposed to analyze the performance of the IEEE 802.11 distributed coordination function (DCF) and the IEEE 802.11e enhanced distributed coordination function (EDCA) under saturation condition. To analyze DCF under statistical traffic, Foh and Zukerman introduce a model that uses Markovian Framework to compute the throughput and delay performance. In this paper, we analyze the protocol service time of EDCA mechanism and introduce a model to analyze EDCA under statistical traffic using Markovian Framework. Using this model, we analyze the throughput and delay performance of EDCA mechanism under statistical traffic.