In this work we present a measurement study of user mobility in Second Life. We first discuss different techniques to collect user traces and then focus on results obtained using a crawler that we built. Tempted by the question whether our methodology could provide similar results to those obtained in real-world experiments, we study the statistical distribution of user contacts and show that user mobility in Second Life presents similar traits to those of real humans. We further push our analysis to radio networks that emerge from user interaction and show that they are highly clustered. Lastly, we focus on the spatial properties of user movements and observe that users in Second Life revolve around several points of interest traveling in general short distances.Using maximum likelihood estimation, we show that our empirical data best fit to power-law with cutoff distributions, indicating that contact time distributions in a virtual environment has very similar characteristics to those observed in real-world experiments.
Performance and reliability of content access in mobile networks is conditioned by the number and location of content replicas deployed at the network nodes. In this work, we design a practical, distributed solution to content replication that is suitable for dynamic environments and achieves load balancing. Simulation results show that our mechanism, which uses local measurements only, approximates well an optimal solution while being robust against network and demand dynamics. Also, our scheme outperforms alternative approaches in terms of both content access delay and access congestion.
In this paper, we investigate schemes for energy-efficient multi-hop broadcasting in large-scale dense Wireless Sensor Networks. We begin with an initial simplified study of the schemes for relay selection. Our first finding is that MPR-based (Multipoint Relay) mechanisms work poorly in a dense network while the recently proposed Multicast Protocol for Low power and Lossy Networks (MPL) protocol based on Trickle performs better. However, Trickle requires to overhear packet retransmissions in the vicinity, while sensor nodes try to avoid overhearing by periodically waking up and going to sleep to save energy.We propose Beacon-based Forwarding Tree (BFT), a new scheme that achieves similar performance to MPL, although it fits better the case of nodes with low radio duty cycling MACs of the type of beacon-enabled IEEE 802.15.4. Our scheme also guarantees network coverage and its optimized version results in the shortest path distance to the broadcast source at a cost of lesser load mitigation. We compare and discuss the measured performance of MPL on top of Contiki-MAC and BFT over beacon-enabled 802.15.4 on a Contiki testbed. The experimental results of the comparisons show that BFT may achieve very good performance for a range of broadcast intensity, it has a predictable power consumption, a remarkable low power consumption for leaf nodes, and low loss rates. On the other hand, MPL over ContikiMAC can obtain very low duty cycles for low broadcast traffic.
Performance and reliability of content access in mobile networks is conditioned by the number and location of content replicas deployed at the network nodes. Facility location theory has been the traditional, centralized approach to study content replication: computing the number and placement of replicas in a network can be cast as an uncapacitated facility location problem.The endeavour of this work is to design a distributed, lightweight solution to the above joint optimization problem, while taking into account the network dynamics. In particular, we devise a mechanism that lets nodes share the burden of storing and providing content, so as to achieve load balancing, and decide whether to replicate or drop the information so as to adapt to a dynamic content demand and time-varying topology. We evaluate our mechanism through simulation, by exploring a wide range of settings and studying realistic content access mechanisms that go beyond the traditional assumption matching demand points to their closest content replica. Results show that our mechanism, which uses local measurements only, is: (i) extremely precise in approximating an optimal solution to content placement and replication; (ii) robust against network mobility; (iii) flexible in accommodating various content access patterns, including variation in time and space of the content demand.
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