Yannick Léo

Non-altering time scales for aggregation of dynamic networks into series of graphs

Many dynamic networks coming from real-world contexts are link streams, i.e. a finite collection of triplets (u,v,t) where u and v are two nodes having a link between them at time t. A great number of studies on these objects start by aggregating the data on disjoint time windows of length Δ in order to obtain a series of graphs on which are made all subsequent analyses. Here we are concerned with the impact of the chosen Δ on the obtained graph series. We address the fundamental question of knowing whether a series of graphs formed using a given Δ faithfully describes the original link stream. We answer the question by showing that such dynamic networks exhibit a threshold for Δ, which we call the saturation scale, beyond which the properties of propagation of the link stream are altered, while they are mostly preserved before. We design an automatic method to determine the saturation scale of any link stream, which we apply and validate on several real-world datasets.

Taking Benefit from the User Density in Large Cities for Delivering SMS

In this paper, from a measurement study and analysis of SMS based on traces coming from a nationwide cellular telecommunication operator during a two month period, we propose a DTN (Delay Tolerant Network) like network protocol for delivering SMS. More precisely, we perform a temporal and spatial analysis of the Mexico City cellular network considering geolocalized SMS. The temporal analysis allows us to detect events and to check for overloading periods, with abnormal or unexpected traffic, and to study the evolution of classical parameters such as activity or distance between source and destination. The spatial analysis is based on the Voronoï diagram of the base stations covering Mexico City. We explain how SMS traffic can be characterized. Such key characterization allows us to answer the question: is it possible to transmit SMS using phones as relay in a large city such as Mexico City? We defined a simple network protocol to transmit SMS from a source to a destination. This DTN like protocol does not need routing nor global knowledge. The protocol takes benefit from the locality of SMS, the density of phones in Mexico City and the mobility of phone users. We studied a mobile dataset including 8 millions users living in Mexico city. This gave use a precise estimation of the average transmission time and the global performance of our approach. After 30 minutes, half of the SMS were delivered successfully to destination.

Taking Benefit from the User Density in Large Cities for Delivering SMS Experimentations in Mexico City

In this paper, from a measurement study and analysis of SMS based on traces coming from a nationwide cellular telecommunication operator during a two month period, we propose a DTN (Delay Tolerant Network) like network protocol for delivering SMS. More precisely, we perform a temporal and spatial analysis of the Mexico City cellular network considering geolocalized SMS. The temporal analysis allows us to detect events and to check for overloading periods, with abnormal or unexpected traffic, and to study the evolution of classical parameters such as activity or distance between source and destination. The spatial analysis is based on the Voronoï diagram of the base stations covering Mexico City. We explain how SMS traffic can be characterized. Such key characterization allows us to answer the question: is it possible to transmit SMS using phones as relay in a large city such as Mexico City? We defined a simple network protocol to transmit SMS from a source to a destination. This DTN like protocol does not need routing nor global knowledge. The protocol takes benefit from the locality of SMS, the density of phones in Mexico City and the mobility of phone users. We studied a mobile dataset including 8 millions users living in Mexico city. This gave use a precise estimation of the average transmission time and the global performance of our approach. After 30 minutes, half of the SMS were delivered successfully to destination.