Section: New Results

Wireless Sensor Networks

Deployment of Wireless Sensor Networks

Participants : Ines Khoufi, Pascale Minet, Anis Laouiti.

In 2016, we studied two types of deployment for wireless sensor networks:

  • those ensuring full area coverage and network connectivity;

  • those covering some given Points of Interest (PoI) and ensuring network connectivity.

Deployment of sensor nodes to fully cover an area has caught the interest of many researchers. However, some simplifying assumptions are adopted such as the knowledge of obstacles, a centralized algorithm... To cope with these drawbacks, we propose OA-DVFA (Obstacles Avoidance Distributed Virtual Forces Algorithm), a self-deployment algorithm to ensure full area coverage and network connectivity. This fully distributed algorithm is based on virtual forces to move sensor nodes. We show how to avoid the problem of node oscillations and to detect the end of the deployment in a distributed way. We evaluate the impact of the number, shape and position of obstacles on the coverage rate, the distance traveled by all nodes and the number of active nodes. Simulation results show the very good behavior of OA-DVFA. This work done in collaboration with Anis Laouiti has been presented at the CCNC 2016 conference [35].

We also focus on wireless sensor networks deployed to cover some given Points of Interest (PoIs), achieve connectivity with the sink and be robust against link and node failures. The Relay Node Placement problem (RNP) consists in minimizing the number of relays needed and the maximum length of the paths connecting each PoI with the sink. We propose a solution that determines the positions of relay nodes based on the virtual grid computed by the optimal deployment for full area coverage. We compare our solution with two different solutions based respectively on (1) the straight line that builds the shortest path between each PoI and the sink, (2) the Steiner point that connects PoIs together. We then extend these algorithms to achieve k-connectivity. Our solution outperforms the Steiner points solution in terms of maximum path length on the one hand, and the straight line solution in terms of total number of relay nodes deployed on the other hand. We also apply our solution in an area containing obstacles and show that it provides very good performances. This study has been presented at the MASS 2016 conference [34].

Path Planning of Mobile Wireless Nodes Gathering Data

Participants : Ines Khoufi, Pascale Minet, Nadjib Achir.

Mobile wireless nodes in charge of collecting data from static wireless sensor nodes constitute a very attractive solution for wireless sensor networks, WSNs, where the application requirements in terms of node autonomy are very strong unlike the requirement in terms of latency. Mobile nodes allow wireless sensor nodes to save energy.

In 2016 we focused on the path planning problem of mobile wireless nodes gathering data according two different objectives:

  • to ensure the monitoring of a given area;

  • to visit some given Points of Interest (PoI) in a delay less than a given latency.

For the first objective, we are interested in area monitoring using Unmanned Aerial Vehicles (UAVs). Basically, we propose a path planning approach for area monitoring where UAVs are considered as mobile collectors. The area to be monitored is divided into cells. The goal is to determine the path of each UAV such that each cell is covered by exactly one UAV, fairness is ensured in terms of the number of cells visited by each UAV and the path of each UAV is minimized. To meet our goal, we proceed in two steps. In the first step, we assign to each UAV the cells to visit. In the second step, we optimize the path of each UAV visiting its cells. For the first step, we propose two solutions. The first solution is based on cluster formation, each cluster is made up of the set of cells monitored by a same UAV. The second solution is based on game theory and uses coalition formation to determine the cells to be monitored by each UAV. In the second step and for both solutions, we propose to apply optimization techniques to minimize the path of each UAV that visits all its cells. This study done in collaboration with Nadjib Achir was presented at the PEMWN 2016 conference [32].

For the second objective, we use game theory to model the problem. Game theory is often used to find equilibria where no player can unilaterally increase its own payoff by changing its strategy without changing the strategies of other players. In this paper, we propose to use coalition formation to compute the optimized tours of mobile sinks in charge of collecting data from static wireless sensor nodes. The associated coalition formation problem has a stable solution given by the final partition obtained. However, the order in which the players play has a major impact on the final result. We determine the best order to minimize the number of mobile sinks needed. We evaluate the complexity of this coalition game in terms of the number of rounds and the processing time needed to get convergence, as well as the impact of the number of collect points on the number of mobile sinks needed and on the maximum tour duration of these mobile sinks. In addition, we show how to extend the coalition game to support different latencies for different types of data. Finally, we formalize our problem as a multi-objective optimization problem. We compare the coalition game with a genetic algorithm: for 20 nodes to visit, the coalition game requires a processing time 327 times less than the genetic algorithm. The coalition game provides a scalable solution. These results have been presented at the IPCCC 2016 conference. This work was done in cooperation Mohamed-Amine Koulali and Abdellatif Kobbane [33].

Centralized Scheduling in TSCH-based Wireless Sensor Networks

Participants : Erwan Livolant, Pascale Minet, Thomas Watteyne.

Scheduling in an IEEE802.15.4e TSCH(Time Slotted Channel Hopping 6TiSCH) low-power wireless network can be done in a centralized or distributed way. When using centralized scheduling, a scheduler installs a communication schedule into the network. This can be done in a standards-based way using CoAP. In this study, we compute the number of packets and the latency this takes, on real-world examples. The result is that the cost is very high using today's standards, much higher than when using an ad-hoc solution such as OCARI. We conclude by making recommendations to drastically reduce the number of messages and improve the efficiency of the standardized approach.

Using an IEEE 802.15.4e TSCH network

Participants : Ines Khoufi, Pascale Minet, Erwan Livolant, Thomas Watteyne.

Most wireless sensor networks that are currently deployed use a technology based on the IEEE 802.15.4 standard. However, this standard does not meet all requirements of industrial applications in terms of latency, throughput and robustness. That is why the IEEE 802.15.4e amendment has been designed, including the Time Slotted Channel Hopping (TSCH) mode.

In 2016, we evaluated the time needed for a joining node to detect beacons advertising the TSCH network. This time may be long due to channel hopping in the TSCH network. The beacon advertising policy is left unspecified by the standard. We propose DBA, a Deterministic Beacon Advertising algorithm. DBA ensures a collision-free and regular transmission of beacons on all the frequencies used by the TSCH network. DBA outperforms two solutions already published that are Random Horizontal and Random Vertical. Some results have been presented as a poster at the IPCCC 2016 conference [48].

The medium access in a TSCH network is ruled by a schedule that determines for each pair (slot offset, channel offset) the transmitting node(s) and the receiving node(s). Each node in the TSCH network must have this schedule. The question is how to install it on all nodes. We proposed and evaluated different ways of installing a schedule in a TSCH network, comparing them in terms of the number of messages required. This study has been presented at the AdHocNow 2016 conference [36].

The OCARI Wireless Sensor Network

Participants : Erwan Livolant, Pascale Minet, Mohammed Tahar Hammi.

Wireless Sensor Networks and Industrial Internet of Things use smart, autonomous and usually limited capacity devices in order to sense and monitor industrial environments. The devices in a wireless sensor network are managed by a controller, which should authenticate them before they join the network. OCARI is a wireless sensor network technology providing optimized protocols in order to reduce the energy consumption.

To enhance OCARI security and ensure a robust authentication of devices, we propose a strong authentication method based on the One Time Password algorithm and deployed at the MAC layer. This method is specially designed to be implemented on devices with low storage and computing capacities. This work has been done in collaboration with Mohammed Tahar Hammi from Telecom ParisTech and presented at the PEMWN 2016 conference [30].

We also evaluated the performances of the building of an OCARI network. The goal was to identify the most time consuming steps among node association, neighborhood discovery, routing tree building, stabilization of the routing tree and node coloring.

Security in Wireless Sensor Networks

Participants : Selma Boumerdassi, Paul Muhlethaler.

Sensor networks are often used to collect data from the environment where they are located. These data can then be transmitted regularly to a special node called a sink, which can be fixed or mobile. For critical data (like military or medical data), it is important that sinks and simple sensors can mutually authenticate so as to avoid data to be collected and/or accessed by fake nodes. For some applications, the collection frequency can be very high. As a result, the authentication mechanism used between a node and a sink must be fast and efficient both in terms of calculation time and energy consumption. This is especially important for nodes which computing capabilities and battery lifetime are very low. Moreover, an extra effort has been done to develop alternative solutions to secure, authenticate, and ensure the confidentiality of sensors, and the distribution of keys in the sensor network. Specific researches have also been conducted for large-scale sensors. At present, we work on an exchange protocol between sensors and sinks based on low-cost shifts and xor operations. This study was published in [21]. After this publication, we have been working on the performance evaluation of the solution to determine the memory overhead together with both computing and communication latencies.

Massive MIMO Cooperative Communications for Wireless Sensor Networks

Participants : Nadjib Achir, Paul Muhlethaler.

This work is a collaboration with Mérouane Debbah (Supelec, France).

The objective of this work is to propose a framework for massive MIMO cooperative communications for Wireless Sensor Networks. Our main objective is to analyze the performances of the deployment of a large number of sensors. This deployment should cope with a high demand for real time monitoring and should also take into account energy consumption. We have assumed a communication protocol with two phases: an initial training period followed by a second transmit period. The first period allows the sensors to estimate the channel state and the objective of the second period is to transmit the data sensed. We start analyzing the impact of the time devoted to each period. We study the throughput obtained with respect to the number of sensors when there is one sink. We also compute the optimal number of sinks with respect to the energy spent for different values of sensors. This work is a first step to establish a complete framework to study energy efficient Wireless Sensor Networks where the sensors collaborate to send information to a sink. Currently, we are exploring the multi-hop case.