Thesis defense — Pierre Brunisholz — July 10

Ubiquitous communications for smart city networks #

The defence will take place on Wednesday 10 July 2019 at 14:00 in the auditorium of the IMAG building, 700 avenue Centrale, Domaine universitaire, 38400 Saint-Martin-d’Hères.

Abstract #

Wi-Fi is everywhere in cities, whether through the growing number of public access points, or the massive private access points deployment, in the form of set-top boxes for the major part. If we assume that all these access points are usable in order to allow any device to access the Internet, then we would potentially have network coverage throughout 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 too often change their access points when they no longer have connectivity. This mechanism, called handover, can be long because devices must first detect their connectivity losses before they can start looking for the next access point to associate with. It can be particularly long for devices such as smartphones because they are energy constrained and therefore do not apply an aggressive handover policy. In this context we tried to characterize the possible Wi-Fi applications for a moving user, considering the handover duration, the user speed and the access points density in the city. We found that for slow-moving users, the impact of the handover is small compared to the their overall connectivity. This allows them to use bandwidth-intensive applications as long as they are to some extent delay-tolerant. However, when the user’s speed increases, the impact of handover’s duration gradually degrades the user’s connectivity, so that high speed users can no longer expect to use different access points. Fast moving devices spend more time performing handovers with new access points than 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 a testbed allowing us to observe the sequences of retransmitted messages using different implementations of 802.11 when we suddenly make the access point disappear. We compared these sequences with the one described in the standard, and we found that the maximum number of retransmissions as well as the growth in the contention window were not respected. In addition, these implementations spend a lot of time trying to retransmit before initiating their handover procedures. Retransmissions are also used in the rate control algorithms to determine if the link is deteriorating. However, during contention, the number of losses increases with the higher probability of collisions. In order to observe the impact of retransmissions on the rate control algorithms during contention, we have set up a testbed composed of about thirty identical stations. We found that the rate control algorithm used underperforms compared to the use of a single modulation. Finally, we proposed a geographical addressing scheme compliant with the Internet infrastructure. It allows to obtain both a hierarchical division of the world, and a hierarchical prefix for the addresses, similar to the one used in the CIDR format. We show that this addressing scheme can be used in multicast addresses to send messages to specific geographical areas (minimum area of one square meter).

The full text is available on HAL: tel-02341839.