Section: New Results

Autonomous wireless networking

Participants : Isabelle Augé-Blum, Bernard Tourancheau, Fabrice Valois, Ibrahim Amadou, Cédric Chauvenet, Quentin Lampin, Alexandre Mouradian, Bilel Romdhani.

Designing protocols for large scale wireless sensors networks is a challenging issue, if realistic environments are considered. Finding a trade-off between energy consumption and delay, or capacity, is difficult. The most promizing ideas rely on zero-protocol approaches and on virtual coordinates use. the special case of VANETs is presented in the next section.

In [64] , we focus on Wireless Sensor Networks (WSNs) in a more realistic case than classical studies and previous works: we consider wireless sensor nodes having different transmission ranges according to the environment and/or to the wireless chipset. The main consequence of this heterogeneity is the existence of asymmetric links. Such links in a WSN degrade the performance of most protocols which have not been designed to support this heterogeneity and to deal with asymmetric links: so, mainly, these links are pruned. Under this assumption, we propose a routing protocol for data collection from sensors nodes to the sink node in heterogeneous WSNs. Our proposal detects and takes benefit from asymmetric links caused by this heterogeneity. Our proposal, denoted MURA, (1) provides a high delivery ratio, (2) reduces the number of duplicated packets and (3) reduces the number of hop counts by exploiting the asymmetric links.

Due to the efficiency and scalability of greedy routing in WSNs and the financial cost of GPS chips, Virtual Coordinate Systems (VCSs) for WSNs have been proposed. A category of VCSs is based on the hop-count from the sink, this scheme leads to many nodes having the same coordinate. The main advantage of this system is that the hops number of a packet from a source to the sink is known. Nevertheless, it does not allow to differentiate the nodes with the same hop-count. We propose in [87] a novel hop-count-based VCS which aims at classifying the nodes having the same hop-count depending on their connectivity and at differentiating nodes in a 2-hop neighborhood. Those properties make the coordinates, which also can be viewed as a local identifier, a very powerful metric which can be used in WSNs mechanisms.

Duty-cycled medium access protocols allow for long lasting autonomous networks by periodically putting nodes to sleep. However, this life expectancy improvement comes at the cost of a lesser network capacity and a poor adaptability to bursty traffic loads. Indeed, existing contention algorithms do not provide efficient algorithms to dynamically elect multiple senders per wake-up periods. In [84] , the medium is divided in several logical chan- nels (eg. obtained by a time/frequency division of the communication medium) and we propose to allocate them dynamically among senders. For this purpose, we propose a joint contention/scheduling algorithm, named Extended Slot Se- lection (ESS), that schedules multiple sender/receiver pairs to available logical channels.

Energy-efficient communication protocol is a primary design goal for Wireless Sensor Networks (WSNs). Many efforts have been done to save energy: MAC with duty cycle, energy-aware routing protocols, data aggregation schemes, etc. Recently, beacon-less strategies have emerged as new direction to improve considerably the WSN lifetime. However, the main contributions are not suitable to real radio environments be- cause of hole avoiding strategies based on either planarization or explicit neighbor solicitations. We propose in [34] PFMAC (Pizza- Forwarding Medium Access Control), which combines beacon- less geo-routing and energy efficient MAC protocol via a cross- layer design to save more energy with higher reliability. PFMAC supports radio interferences, asymmetric radio links, etc. PFMAC supports a greedy forwarding strategy and, a reactive and optimized neighborhood discovery at 2-hop to deal with holes. Intensive simulations are proposed to highlight the behavior and the performance of PFMAC compared to BOSS over BMAC.

To provide for reliability in Wireless Sensor Net- works (WSNs), Medium Access Control (MAC) protocols must be adapted by mechanisms taking cross-layer approaches into account. In [51] , [52] , we describe AreaCast which is designed for enhancing reliability in WSNs. AreaCast is a MAC layer mechanism inde- pendent of the routing layer, but uses only local topological and routing information to provide a communication by area instead of a traditional, node-to-node communication (i.e., unicast). In AreaCast, a source node addresses a set of nodes: an explicit relay node chosen as the next hop by a given routing protocol, and k other implicit relay nodes. The neighboring nodes select themselves as implicit relays according to their location from the explicit relay node. This mechanism uses overhearing to take advantage of the inherent broadcast nature of wireless commu- nications. Without changing the routing protocol, AreaCast is able to dynamically avoid a byzantine node or an unstable link, allowing to benefit from the inherent topological redundancy of densely deployed sensor networks. Simulation results show that AreaCast significantly improves the packet delivery rate while having a good reliability-energy consumption trade-off.

Improving the network lifetime is an important design criterion for wireless sensor networks especially if we want to use standard solution like IPv6. In [38] , we propose a novel approach which applies source-coding on addresses in heterogeneous IPv6 Cluster-based wireless sensor network. We formulate the problem of maximizing the network lifetime when Slepian-wolf coding is applied on addresses in network composed of line-powered and battery-powered sensors. The numerical results show that a significant network lifetime improvement can be achieved (about 25% in typical scenario). In [36] , we investigates the sinks mobility in IPv6-based wireless sensors networks and specially in the new IETF proposed protocol RPL (Routing Protocol for Low power and Lossy Networks). We also show that even the mobility of sinks is not an explicit design criteria, the use of mobile sinks improves the network lifetime.