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Supports de communications ubiquitaires pour les réseaux à l'échelle de la ville

Pierre Brunisholz 1, 2 
2 Drakkar
LIG - Laboratoire d'Informatique de Grenoble
Abstract : Wi-Fi is everywhere in cities, whether through the growing number of publicaccess points, or the massive private access points deployment, in the formof set-top boxes for the major part.If we assume that all these access points are usable in order to allow anydevice to access the Internet, then we would potentially have network coveragethroughout the city.This assumption led us to wonder if Wi-Fi could be used as a city-wide network.This network could, more specifically, be used in a context of mobility.However, Wi-Fi was not designed to manage mobile users, and devices have tooften change their access points when they no longer have connectivity.This mechanism, called handover, can be long because devices must first detecttheir connectivity losses before they can start looking for the next accesspoint to associate with.It can be particularly long for devices such as smartphones because they areenergy constrained and therefore do not apply an aggressive handover policy.In this context we tried to characterize the possible Wi-FI applications for amoving user, considering the handover duration, the user speed and the accesspoints density in the city.We found that for slow-moving users, the impact of the handover is smallcompared to the their overall connectivity.This allows them to use bandwidth-intensive applications as long as they areto some extend delay-tolerant.However, when the user’s speed increases, the impact of handover’s durationgradually degrades the user’s connectivity, so that high speed users can nolonger expect to use different access points.Fast moving devices spend more time performing handovers with new access pointsthan transmitting application data.Retransmissions play an important role in the duration of handover.In order to study in detail the retransmissions in 802.11, we have set up atestbed allowing us to observe the sequences of retransmitted messages usingdifferent implementations of 802.11 when we suddenly make the access pointdisappear.We compared these sequences with the one described in the standard, and we wefound that the maximum number of retransmissions as well as the growth in thecontention window were not respected.In addition, these implementations spend a lot of time trying to retransmitbefore initiating their handover procedures.Retransmissions are also used in the rate control algorithms to determine ifthe link is deteriorating.However, during contention, the number of losses increases with the higherprobability of collisions.In order to observe the impact of retransmissions on the rate controlalgorithms during contention, we have set up a testbed composed of about thirtyidentical stations.We found that the rate control algorithm used underperforms compared to theuse of a single modulation.Finally, we proposed a geographical addressing scheme compliant with theInternet infrastructure.It allows to obtain both a hierarchical division of the world, and ahierarchical prefix for the addresses, similar to the one used in the CIDRformat.We show that this addressing scheme can be used in multicast addresses to sendmessages to specific geographical areas (minimum area of one square meter).
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Submitted on : Thursday, October 31, 2019 - 3:54:45 PM
Last modification on : Wednesday, July 6, 2022 - 4:16:22 AM
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  • HAL Id : tel-02341839, version 1


Pierre Brunisholz. Supports de communications ubiquitaires pour les réseaux à l'échelle de la ville. Systèmes embarqués. Université Grenoble Alpes, 2019. Français. ⟨NNT : 2019GREAM033⟩. ⟨tel-02341839⟩



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