Fabrice Theoleyre

Using multiparent routing in RPL to increase the stability and the lifetime of the network

Energy is a very scarce resource in Wireless Sensor Networks. While most of the current proposals focus on minimizing the global energy consumption, we aim here at designing an energy-balancing routing protocol that maximizes the lifetime of the most constraint nodes. To improve the network lifetime, each node should consume the same (minimal) quantity of energy. We propose the Expected Lifetime metric, denoting the residual time of a node (time until the node will run out of energy). We design mechanisms to detect energy-bottleneck nodes and to spread the traffic load uniformly among them. Moreover, we apply this metric to RPL, the de facto routing standard in low-power and lossy networks. In order to avoid instabilities in the network and problems of convergence, we propose here a multipath approach. We exploit the Directed Acyclic Graph (DAG) structure of the routing topology to probabilistically forward the traffic to several parents. Simulations highlight that we improve both the routing reliability and the network lifetime, while reducing the number of DAG reconfigurations.

Stability and efficiency of RPL under realistic conditions in Wireless Sensor Networks

The IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) is one of the emerging routing standards for multihop Wireless Sensor Networks (WSN). RPL is based on the construction of a Destination-Oriented Directed Acyclic Graph (DODAG), which offers a loop-free topology to route data packets. While several routing metrics have been proposed in the literature, it is unclear how they perform with RPL. In this paper, we analyze the impact of different PHY and MAC metrics on the stability and efficiency of RPL. We highlight the fact that realistic conditions lead to instabilities and oscillations in the routing structure. While minimizing the hop length leads to a stable but poor routing structure, more sophisticated link metrics such as ETX reflect more clearly the radio link quality but increase the number of DODAG reconfigurations. We also provided a detailed methodology to measure the DODAG stability and to implement efficiently each routing metric with RPL.

Virtual structure routing in ad hoc networks

Routing protocols are the main issue of ad hoc networks. Because flat propositions (reactive, proactive) are not sufficient and suffer from a lack of performance, new solutions should he investigated and proposed. On the other hand, virtual topologies propose to structure the network and to give a hierarchy between the strongest and the weakest nodes. We propose a new routing protocol, virtual structure routing (VSR), based on a virtual topology including both a backbone and clusters. The backbone is used to collect control traffic and to reduce overhead for route discovery. VSR uses clusters to define a route as a list of cluster IDs. This cluster topology is more stable than the physical topology. Hence, routes are more robust. VSR combines the assets of both flat approaches; intra-cluster routing is proactive while inter-cluster routing is reactive. Finally, routes are computed dynamically and a mechanism for route repair is proposed.

Multi-Channel Cluster Tree for 802.15.4 Wireless Sensor Networks

We propose MCCT (Multi-Channel Cluster Tree), a cluster-tree construction protocol for nodes in IEEE 802.15.4 beacon-enabled mode. By multiplexing transmissions across orthogonal channels, we reduce collisions between control and data frames, which leads to better packet delivery rate and fairness. We propose a method for constructing a cluster-tree suitable for minimizing beacon collisions. The protocol builds on a neighbor discovery procedure that uses a dedicated control channel while still sticking to the superframe structure of IEEE 802.15.4. We also specify a channel assignment and superframe scheduling method that takes into account channel diversity. We evaluate the proposed protocol through simulation and compare with other proposals: standard 802.15.4 and a representative of distributed solutions to the superframe scheduling problem - MeshMAC. The simulation results show that MCCT significantly improves packet delivery ratio, delay, and fairness. It also results in very good packet delivery ratio for increased network density.

Adaptive IEEE 802.15.4 MAC for Throughput and Energy Optimization

IEEE 802.15.4 defines a popular MAC standard for wireless sensor and actuator networks. With the default parameters, under medium to high load, 802.15.4 generates excessive collisions and packet losses. Low duty cycles even exacerbate the problem, because more nodes become active after long periods of sleep and contend for channel access. In this paper, we have applied the models that led to the Idle Sense access method for 802.11 to the 802.15.4 slotted CSMA/CA, taking into account the central role of the coordinator and also the bursty nature of the traffic. Surprisingly, the approach perfectly applies to 802.15.4 even if the principles of the two access methods fundamentally differ. Based on the model, we propose ABE, an adaptation method that adjusts the contention window to optimal values so that the network obtains high throughput along with low duty cycles leading to low energy consumption. The method converges to near-optimal backoff values even under bursty traffic and for any number of contending nodes.

Efficient topology construction for RPL over IEEE 802.15.4 in wireless sensor networks

IEEE 802.15.4-2006 represents a widely used standard for multihop wireless sensor networks. However, the standard exploits a tree structure in the MAC layer, which may lead to network partitions even after a single link or node failure, i.e. the well known single point of failure problem. Besides, the single path approach avoids the routing protocol to select by itself a next hop based on its own criteria. Moreover, transmissions must be appropriately scheduled in the IEEE 802.15.4 cluster-tree to avoid collisions. In this paper, we propose to modify the cluster-tree structure into a Cluster-Directed Acyclic Graph (DAG) to improve the robustness and the topology redundancy at the MAC layer. We also present a simple greedy scheduling algorithm integrated with the IEEE 802.15.4 MAC mechanisms. Simulations show that the proposed mechanisms optimize the MAC layer for multihop topologies. In particular, the routing protocol (e.g. RPL) is able to exploit efficiently the cluster-DAG and to reduce the number of packet losses and the end-to-end delay. Last but not least, the cluster-DAG structure leads globally to energy savings by reducing the number of transmissions at the MAC layer.

Assignment of Roles and Channels for a Multichannel MAC in Wireless Mesh Networks

A multichannel MAC improves throughput in wireless mesh networks by multiplexing transmissions over orthogonal channels. In this paper, we propose an efficient way for constructing the wireless mesh structure associated with Molecular MAC, a multichannel MAC layer designed for efficient packet forwarding. Molecular MAC outperforms other classical approaches, but requires a specific structure for efficient operation. First, we propose a centralized protocol that provides an upper bound for constructing such a molecular structure through a MILP (Mixed Integer Linear Programming) formulation that maximizes network capacity. Then, we present two distributed self-stabilizing heuristic protocols derived from the protocols for constructing respectively a Maximum Independent Set and a Spanning Tree. We compare the performance of the proposed protocols in terms of network capacity and route length.

Experimental Validation of a Distributed Self-Configured 6TiSCH with Traffic Isolation in Low Power Lossy Networks

Time Slotted Channel Hopping (TSCH) is among the proposed Medium Access Control (MAC) layer protocols of the IEEE 802.15.4-2015 standard for low-power wireless communications in Internet of Things (IoT). TSCH aims to guarantee high network reliability by exploiting channel hopping and keeping the nodes time-synchronized at the MAC layer. In this paper, we focus on the traffic isolation issue, where several clients and applications may cohabit under the same wireless infrastructure without impacting each other. To this end, we present an autonomous version of 6TiSCH where each device uses only local information to select their timeslots. Moreover, we exploit 6TiSCH tracks to guarantee flow isolation, defining the concept of shared (best-effort) and dedicated (isolated) tracks. Our thorough experimental performance evaluation campaign, conducted over the open and large scale FIT IoT-LAB testbed (by employing the OpenWSN), highlight the interest of this solution to provide reliability and low delay while not relying on any centralized component.

Improving the network lifetime with energy-balancing routing: Application to RPL

The devices composing Wireless Sensor Networks (WSN) are very limited in terms of memory, processing power and battery. We need efficient routing algorithms to ensure a long lifetime of the WSN. However, many solutions focus on minimizing the average energy consumption of all the nodes. Since the lifetime is rather given by the death of the first node or by the disconnection of the network, these solutions are currently insufficient. Here, we have rather chosen to focus on identifying the energy bottlenecks (i.e., the nodes that are more likely to be the first ones to run out of energy). Minimizing their energy consumption will surely improve the network lifetime. We define here the Expected LifeTime (ELT) routing metric to estimate the lifetime of these bottlenecks. We take into account both the amount of traffic and the link reliability to estimate how much energy such a bottleneck consumes on average. Moreover, we apply this metric to the RPL architecture, the de facto routing standard in the Internet of Things. We construct the Directed Acyclic Graph structure based on this ELT criteria while avoiding the creation of loops.

Neighbor Discovery with Activity Monitoring in Multichannel Wireless Mesh Networks

One way of improving performance of wireless mesh networks is to use multiple non-overlapping channels. At the same time, the mesh network must continuously self-adapt to varying radio conditions and topology changes. Thus, we propose a neighbor discovery protocol that fits the requirements of multichannel networks while dealing at the same time with the deafness problem. The proposed scheme also takes advantage of neighbor discovery to continually monitor channel activity. We provide a theoretical analysis of the average discovery time. Our simulations show that the solution integrated within Molecular MAC, a multichannel MAC, results in efficient discovery and channel assignment.