326 research outputs found
Measuring Transmission Opportunities in 802.11 Links
We propose a powerful MAC/PHY cross-layer approach
to measuring IEEE 802.11 transmission opportunities in
WLAN networks on a per-link basis. Our estimator can operate
at a single station and it is able to: 1) classify losses caused by
noise, collisions, and hidden nodes; and 2) distinguish between
these losses and the unfairness caused by both exposed nodes and
channel capture. Our estimator provides quantitative measures of
the different causes of lost transmission opportunities, requiring
only local measures at the 802.11 transmitter and no modification
to the 802.11 protocol or in other stations. Our approach is suited
to implementation on commodity hardware, and we demonstrate
our prototype implementation via experimental assessments. We
finally show how our estimator can help the WLAN station to
improve its local performance
MAC design for WiFi infrastructure networks: a game-theoretic approach
In WiFi networks, mobile nodes compete for accessing a shared channel by
means of a random access protocol called Distributed Coordination Function
(DCF). Although this protocol is in principle fair, since all the stations have
the same probability to transmit on the channel, it has been shown that unfair
behaviors may emerge in actual networking scenarios because of non-standard
configurations of the nodes. Due to the proliferation of open source drivers
and programmable cards, enabling an easy customization of the channel access
policies, we propose a game-theoretic analysis of random access schemes.
Assuming that each node is rational and implements a best response strategy, we
show that efficient equilibria conditions can be reached when stations are
interested in both uploading and downloading traffic. More interesting, these
equilibria are reached when all the stations play the same strategy, thus
guaranteeing a fair resource sharing. When stations are interested in upload
traffic only, we also propose a mechanism design, based on an artificial
dropping of layer-2 acknowledgments, to force desired equilibria. Finally, we
propose and evaluate some simple DCF extensions for practically implementing
our theoretical findings.Comment: under review on IEEE Transaction on wireless communication
Towards reliable geographic broadcasting in vehicular networks
In Vehicular ad hoc Networks (VANETs), safety-related messages are broadcasted amongst cars, helping to improve drivers' awareness of the road situation. VANETs’ reliability are highly affected by channel contention. This thesis first addresses the issue of channel use efficiency in geographical broadcasts (geocasts). Constant connectivity changes inside a VANET make the existing routing algorithms unsuitable. This thesis presents a geocast algorithm that uses a metric to estimate the ratio of useful to useless packet received. Simulations showed that this algorithm is more channel-efficient than the farthest-first strategy. It also exposes a parameter, allowing it to adapt to channel load. Second, this thesis presents a method of estimating channel load for providing feedback to moderate the offered load. A theoretical model showing the relationship between channel load and the idle time between transmissions is presented and used to estimate channel contention. Unsaturated stations on the network were shown to have small but observable effects on this relationship. In simulations, channel estimators based on this model show higher accuracy and faster convergence time than by observing packet collisions. These estimators are also less affected by unsaturated stations than by observing packet collisions. Third, this thesis couples the channel estimator to the geocast algorithm, producing a closed-loop load-reactive system that allows geocasts to adapt to instantaneous channel conditions. Simulations showed that this system is not only shown to be more efficient in channel use and be able to adapt to channel contention, but is also able to self-correct suboptimal retransmission decisions. Finally, this thesis demonstrates that all tested network simulators exhibit unexpected behaviours when simulating broadcasts. This thesis describes in depth the error in ns-3, leading to a set of workarounds that allows results from most versions of ns-3 to be interpreted correctly
Channel quality estimation and impairment mitigation in 802.11 networks
Wireless communication has been boosted by the adoption of 802.11 as standard de facto for WLAN
transmission. Born as a niche technology for providing wireless connectivity in small office/enterprise
environments, 802.11 has in fact become a common and cheap access solution to the Internet, thanks to
the large availability of wireless gateways (home modems, public hot-spots, community networks, and
so on). Nowdays, the trend towards increasingly dense 802.11 wireless deployments is creating a real
need for effective approaches for channel allocation/hopping, power control, etc. for interference mitigation
while new applications such mesh networks in outdoor contexts and media distribution within
the home are creating new quality of service demands that require more sophisticated approaches to
radio resource allocation.
The new framework of WLAN deployments require a complete understanding of channel quality
at PHY and MAC layer. Goal of this thesis is to assess the MAC/PHY channel quality and mitigate
the different channel impairments in 802.11 networks, both in dense/controlled indoor scenarios
and emerging outdoor contexts. More specifically, chapter 1 deals with the necessary background
material and gives insight into the different channel impairments/quality it can be encountered in
WLAN networks. Then the thesis pursues a down/top approach: chapter 2, 3 and 4 aim at affording
impairments/quality at PHY level, while chapter 5 and 6 analyse channel impairments/quality from
a MAC level perspective. An important contribution of this thesis is to undisclose that some PHY layer parameters, such
as the transmission power, the antenna selection, and interference mitigation scheme, have a deep
impact on network performance. Since the criteria for selecting these parameters is left to the vendor
specific implementations, the performance spread of most experimental results about 802.11 WLAN
could be affected by vendor proprietary schemes. Particularly, in chapter 2 we find that switching
transmit diversity mechanisms implemented in off-the-shelf devices with two antenna connectors can
dramatically affect both performance and link quality probing mechanisms in outdoor medium-range
WLAN deployments, whenever one antenna deterministically works worse than the other one. A second physical algorithm with side-effects is shown in chapter 3. Particulary the chapter shows that
interference mitigation algorithms may play havoc with the link-level testbeds, since they may erroneously
lower the sensitivity threshold, and thus not detect the 802.11 transmit sources. Finally, once
disabled the interference mitigation algorithm — as well as any switching diversity scheme described
in the previous chapter — link-level experimental assessment concludes that, unlike 802.11b, which
appears a robust technology in most of the operational conditions, 802.11g may lead to inefficiencies
when employed in an outdoor scenario, due to the lower multi-path tolerance of 802.11g. Since multipath
is hard to predict, a novel mechanism to improve the link-distance estimation accuracy — based
on CPU clock information — is outlined in chapter 4. The proposed methodology can not only be
applied in localization context, but also for estimating the multi-path profile. The second part of the thesis moves the perspective to the MAC point of view and its impairments.
Particularly, chapter 5 provides the design of a MAC channel quality estimator to distinguish the
different types of MAC impairments and gives separate quantitative measures of the severity of each
one. Since the estimator takes advantage of the native characteristics of the 802.11 protocol, the
approach is suited to implementation on commodity hardware and makes available new measures
that can be of direct use for rate adaptation, channel allocation, etc. Then, chapter 6 introduces a
previous unknown phenomenon, the Hidden ACK, that may cause frame losses into multiple WLAN
networks when a node replies with an ACK frame. Again, a solution is provided without requiring
any modification to the 802.11 protocol. Whenever possible, the quantitative analysis has been led through experimental assessments with
implementation on commodity hardware. This was the adopted methodology in chapter 2, 3, 4 and 5.
Particularly, this has required an accurate investigation of two brands of WLAN cards, particularly
the Atheros and Intel cards, and their driver/firmware, respectively MADWiFi and IPW2200, which
are currently the most adopted, respectively, by researchers and layman users
State-of-the-art in Power Line Communications: from the Applications to the Medium
In recent decades, power line communication has attracted considerable
attention from the research community and industry, as well as from regulatory
and standardization bodies. In this article we provide an overview of both
narrowband and broadband systems, covering potential applications, regulatory
and standardization efforts and recent research advancements in channel
characterization, physical layer performance, medium access and higher layer
specifications and evaluations. We also identify areas of current and further
study that will enable the continued success of power line communication
technology.Comment: 19 pages, 12 figures. Accepted for publication, IEEE Journal on
Selected Areas in Communications. Special Issue on Power Line Communications
and its Integration with the Networking Ecosystem. 201
Assessing the fidelity of COTS 802.11 sniffers
Proceedings of: 2009 IEEE INFOCOM, 19 – 25 April 2009, Rio de Janeiro, BrazilRecent measurement studies have analyzed WLAN performance by means of wireless sniffers that passively capture transmitted frames. Also, for relatively large (enterprise) WLAN scenarios, previous work has investigated multi-sniffer deployments with devices placed far apart in order to capture all traffic in the network (even frames transmitted simultaneously by different nodes at non-interfering locations). However, for both these single- and multi-sniffer scenarios, little attention has been given to the fidelity of an individual device, i.e., the ability of a given sniffer to capture all frames that could have been captured by a more faithful device. We assess this fidelity (a term we make precise in this paper) by running controlled experiments inside an anechoic chamber and analyzing the similarities and differences between the trace file from the device under study and those of additional "shadow" devices placed in its close proximity. Our results show that fidelity varies significantly across sniffers, both quantitatively and qualitatively, and that performance may also depend on the nature of the experiment under study and on slight changes of the sniffer position.European Community's Seventh Framework ProgramThis work was funded in part by the National Science Foundation under grants, EEC-0313747 001, ANI-0325868, and EIA-0080119, and by the Ministry of Education and Science of Spain, under a José Castillejo grant, and POSEIDON project (TSI2006-12507-C03-01)Publicad
Rigorous and Practical Proportional-fair Allocation for Multi-rate Wi-Fi
Recent experimental studies confirm the prevalence of the widely known performance anomaly
problem in current Wi-Fi networks, and report on the severe network utility degradation caused by
this phenomenon. Although a large body of work addressed this issue, we attribute the refusal of
prior solutions to their poor implementation feasibility with off-the-shelf hardware and their impre-
cise modelling of the 802.11 protocol. Their applicability is further challenged today by very high
throughput enhancements (802.11n/ac) whereby link speeds can vary by two orders of magnitude.
Unlike earlier approaches, in this paper we introduce the first rigorous analytical model of 802.11
stations’ throughput and airtime in multi-rate settings, without sacrificing accuracy for tractability.
We use the proportional-fair allocation criterion to formulate network utility maximisation as a con-
vex optimisation problem for which we give a closed-form solution. We present a fully functional
light-weight implementation of our scheme on commodity access points and evaluate this extensively
via experiments in a real deployment, over a broad range of network conditions. Results demonstrate
that our proposal achieves up to 100% utility gains, can double video streaming goodput and reduces
TCP download times by 8x
An adaptable fuzzy-based model for predicting link quality in robot networks.
It is often essential for robots to maintain wireless connectivity with other systems so that commands, sensor data, and other situational information can be exchanged. Unfortunately, maintaining sufficient connection quality between these systems can be problematic. Robot mobility, combined with the attenuation and rapid dynamics associated with radio wave propagation, can cause frequent link quality (LQ) issues such as degraded throughput, temporary disconnects, or even link failure. In order to proactively mitigate such problems, robots must possess the capability, at the application layer, to gauge the quality of their wireless connections. However, many of the existing approaches lack adaptability or the framework necessary to rapidly build and sustain an accurate LQ prediction model. The primary contribution of this dissertation is the introduction of a novel way of blending machine learning with fuzzy logic so that an adaptable, yet intuitive LQ prediction model can be formed. Another significant contribution includes the evaluation of a unique active and incremental learning framework for quickly constructing and maintaining prediction models in robot networks with minimal sampling overhead
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