Section: New Results

Scalable protocols for capillary networks.

Participants: Ibrahim Amadou, Quentin Lampin, Bilel Romdhani, Alexandre Mouradian, Isabelle Augé-Blum, Fabrice Valois

Beacon-less and opportunistic routing.

During the thesis of Ibrahim Amadou [1] , we were focused on the issues of energy in WSNs through energy-efficient routing and medium access control protocols. The contributions of research work can be summarized as follows. First, we were interested on the energy issues at the routing layer for multi-hop wireless sensor networks (WSNs). We proposed a mathematical framework to model and analyze the energy consumption of routing protocols in multi-hop WSNs by taking into account the protocol parameters, the traffic pattern and the network characteristics defined by the medium channel properties, the dynamic topology behavior, the network diameter and the node density. We showed that Beacon-less routing protocol is a good candidate for energy saving in WSNs.

We investigated the performance of some existing relay selection schemes which are used by Beacon-less routing protocols. Extensive simulations were realized in order to evaluate their performance locally in terms of packet delivery ratio, duplicated packet and delay. Then, we extended the work in multi-hop wireless networks and developed an optimal solution, Enhanced Nearest Forwarding within Radius, which tries to minimize the per-hop expected number of retransmissions in order to save energy.

We presented a new Beacon-less routing protocol called Pizza-Forwarding (PF) without any assumption on the radio environment: neither the radio range nor symmetric radio links nor radio properties (shadowing, etc.) are assumed or restricted. A classical greedy mode is proposed. To overcome the hole problem, packets are forwarded to an optimal node in the two hop neighbor following a reactive and optimized neighborhood discovery.

In order to save energy due to idle listening and overhearing, we proposed to combine PF's main concepts with an energy-efficient MAC protocol to provide a joint MAC/routing protocol suitable for a real radio environment. Performance results lead to conclude to the powerful behavior of PF-MAC.

In collaboration with Orange Labs, we designed QOR, an opportunistic routing protocol for wireless sensor networks [16] . QOR first builds a stable directed acyclic logical routing structure and a prefix-based addressing plan stemming from data sinks. This addressing plan is then used to define the potential forwarders set for each source and allows a strict scheduling and an unique selection of the forwarder for each transmission thanks to a cascading acknowledgment scheme. QOR is particularly suited for sensor networks that require high delivery ratio under severe energy constraints. Extensive simulations show the benefits of QOR over an implementation of the IETF routing protocol for Lossy and Low Power networks, RPL, tailored to provide high delivery ratios. Our case studies shows that QOR saves up to 50% energy and reduces the end-to-end delay of a factor of 4 times while maintaining similar delivery ratios.

Most existing routing protocols designed for WSNs assume that links are symmetric, which is in contradiction with what is observed in the field. Indeed, many links in real-world WSNs are asymmetric. Asymmetric links can dramatically decrease the performance of routing algorithms not designed to cope with them. Quite naturally, most existing routing protocol implementations prune the asymmetric links to only use the symmetric ones. In our experience, asymmetric links are a valuable asset to improve network connectivity, capacity and overall performance [20] ,[2] . We therefore introduced AsymRP (Asymmetric Convergecast Routing Protocol) [21] , a new routing protocol for collecting data in WSNs. AsymRP assumes 2-hop neighborhood knowledge and uses implicit and explicit acknowledgment. It takes advantage of asymmetric links to increase delivery ratio while lowering hop count and packet replication.

MAC and cross-layer mechanisms for QoS.

Protocols developed during the last years for Wireless Sensor Networks (WSNs) are mainly focused on energy-consumption optimization and autonomous mechanisms (e.g. self-organization, self-configuration, etc). Nevertheless, with new WSN applications appear new QoS requirements such as time constraints. Real-time applications require the packets to be delivered before a known time bound which depends on the application requirements. We particularly focused on applications which consist in alarms that are sent to the sink node (e.g. air pollution monitoring). We proposed the Real-Time X-layer Protocol (RTXP) [27] , a real-time communication protocol that integrates mechanisms for both MAC and routing layers. Our proposal aims at guaranteeing an end-to-end constraint delay, while keeping good performances on other parameters, such as energy consumption. For this purpose the protocol relies on a hop-count-based Virtual Coordinate System (VCS) which classifies nodes having the same hop-count from the sink, allows forwarder selection, and gives to the nodes an unique identifier in a 2-hop neighborhood allowing deterministic medium access. This protocol has better performances than state-of-the-art protocols, in terms of time constraints and reliability, even with unreliable radio links.

In the ARESA2 project, but also in a joint collaboration with Orange Labs, we studied receiver initiated MAC protocol to compare their performance to the more classical receiver-based MAC one [17] . We proposed the Self Adapting Receiver Initiated MAC protocol (SARI-MAC), a novel asynchronous MAC protocol for energy constrained Wireless Sensor Networks. SARI-MAC self-adapts to the traffic load to meet specified Quality of Service requirements at the lowest energy cost possible. To do so, SARI-MAC relies on traffic estimation, duty-cycle adaptation and acknowledgment mechanisms. Our performance evaluation assesses that SARI-MAC meets given QoS requirements in a energy efficient manner and outperforms the state of the art protocol RI-MAC in a broad range of traffic scenarios.

For energy constrained wireless sensor networks, lifetime is a critical issue. Several medium access control protocols have been proposed to address this issue, often at the cost of poor network capacity. To address both capacity and energy issues, we proposed a novel medium sharing protocol for Wireless Sensor Networks named Cascading Tournament (CT-MAC) [15] . CT-MAC is a synchronous, localized, dynamic, joint contention/allocation protocol. Relying on cascading iterations of tournaments, CT-MAC allocates multiple time slots to nodes that compete for accessing the medium. CT-MAC offers an unprecedented trade-off between traffic delay, network capacity and energy efficiency and stands out as a solid candidate for energy constrained sensor networks that must support heterogeneous traffic loads. Our simulations show that CT-MAC significantly outperforms the state-of-the-art SCP- MAC protocol.