Nancy El Rachkidy

Performance analysis of the on-the-air activation in LoRaWAN

Long-range low-power communications are starting to replace short-range low-power communications in monitoring applications based on wireless sensor networks, both in rural and urban environments, due to the small deployment cost of long-range technologies. In this paper, we focus on the MAC layer of the recent LoRaWAN (long-range wide area network) standard, and specifically on the on-the-air activation procedure, which defines how nodes join an existing network. We propose a Markov chain model of the on-the-air activation, and we derive the expected delay and the expected energy required to join the network. We analyze the impact of several parameters on these metrics, including traffic conditions, duty cycles, and channel availability. We also discuss the impact of the regional settings of the standard. To the best of our knowledge, this is the first analysis of the MAC layer of LoRaWAN.

Improving QoS in Wireless Sensor Networks Using a Multi-Stack Architecture

Wireless sensor networks deployed nowadays are traditionally mono-stack: they are operating according to a single combination of one MAC protocol and one network (NWK) protocol. This work proposes a new multi-stack architecture in which several combinations of MAC and NWK protocols are used. This can be achieved by dividing time into time-intervals and activating different combinations during each period. Simulations of this approach prove that QoS is mitigated among the combinations. To alleviate the complexity of the time-intervals dimensioning, a formal description of a queue exchange algorithm that allows frames from a time-interval to be sent during another time-interval is proposed. This algorithm improves significantly the global performance of the network.

Pivot Routing Improves Wireless Sensor Networks Performance

Nowadays, wireless sensor networks (WSNs) are used in several applications such as environmental monitoring. When network size and data rate increase, congestion becomes as an important issue, especially when an emergency situation generates alarm messages in a specific area of the network. In this paper, we describe the pivot routing protocol named PiRAT, which avoids congested paths by using intermediate pivot nodes. Simulations show that PiRAT has better performance than previous protocols in terms of packet loss, end-to-end delay, congestion and node overload. Moreover, we show that the load-balancing ability of PiRAT allows it to benefit from nodes having independent low duty cycles.

Routing protocol for anycast communications in a wireless sensor network

In wireless sensor networks, there is usually a sink which gathers data from the battery-powered sensor nodes. As sensor nodes around the sink consume their energy faster than the other nodes, several sinks have to be deployed to increase the network lifetime. In this paper, we motivate the need of anycast communications in wireless networks, where all the sinks are identical and can gather data from any source. To reduce interference and congestion areas on the wireless medium, the path from a source to a sink has to be distant from the path connecting another source to another sink. We show that determining distant paths from sources to sinks is an NP-hard problem, and we propose a linear formulation in order to obtain optimal solutions. Then, we propose a sink selection and routing protocol called S4 and based on realistic assumptions and we evaluate it through simulations. Finally, we conclude that anycast routing protocols in wireless sensor networks should not compute paths independently for each source, but rather consider all the sources simultaneously.

PiRAT: Pivot Routing for Alarm Transmission in wireless sensor networks

Wireless sensor networks are increasingly used for remote monitoring, fire detection, emergency response. Such networks are equipped with small devices powered by batteries and designed to be operated for years. They are often based on the ZigBee standard which defines low power and low data rate protocols. As network size and data rates increase, congestion arises as a problem in these networks, especially when an emergency situation generates alarm messages in a specific area in the network. Indeed, congestion occurs as the alarms converge to a specific destination, which results into packet losses and higher delays. In this paper, we propose a solution for congested links, called the PiRAT (Pivot Routing for Alarm Transmission) protocol. It is based on multi-path routing in order to add some diversity in routing the alarms. PiRAT uses intermediate nodes as pivots to reach the destination. Simulation results show that PiRAT has better performance than previous protocols in terms of packet loss, end-to-end delay, congestion and node overload.

Congestion Reduction Using a MAC Scheduling Mechanism in a Wireless Sensor Network

In many synchronized MAC protocols for wireless sensor networks such as IEEE 802.15.4 in beacon-enabled mode, periods where all the nodes are active alternate with periods where all the nodes are inactive. This approach is used in order to save the energy of nodes as they are powered by small batteries. However, having all nodes active simultaneously can yield to congestion, which increases the packet loss rate and the delay. In this paper, we propose a new MAC scheduling mechanism that distributes the activities of nodes into several periods, thus reducing the number of active nodes during each period. The scheduling is based on the routing information provided by the network layer. We propose an heuristic to compute this schedule, and we derive a protocol with limited overhead. We compare their performance with the performance of IEEE 802.15.4 where all nodes are active simultaneously, as well as with the optimal solution computed using an integer linear program. The simulation results show that our heuristic can greatly improve both packet loss rate and delay in a large variety of scenarios without increasing the energy consumption.

Improving the AODV-based ZigBee routing protocol through pivots

Wireless sensor networks are often deployed for monitoring purposes: when nodes detect the occurrence of a significant event, they transmit an information to a control entity in a multi-hop fashion. When data rate increases, congestion becomes a fundamental issue, especially when an emergency situation generates alarm messages originating from a specific area of the network. Indeed, congestion increases delays and packet losses, and yields to an unfair use of the energy of nodes. In this paper, we propose to improve the ZigBee routing protocol, aiming at reducing traffic congestion. The proposed solution uses intermediate nodes, denoted as pivots, which are selected by the data sources in order to reduce congestion on paths. Simulation results highlight the significant improvement achieved in terms of packet losses and average delays, with respect to the ZigBee routing protocol, while the overhead generated in the network is maintained under control. A mathematical model to derive the average path length and the number of pivots is also provided.

Address assignment for wireless sensor networks in mines

Monitoring of gas leaks and toxic substances in mines is made possible using wireless sensor networks. However, wireless networks in mines form linear topologies that are unusual in other types of networks. In this paper, we show that the ZigBee standard is not suitable to mines because of the addressing mechanisms it proposes. Our contributions are three-fold. First, we show that the distributed address assignment mechanism induces a strong limitation on the depth of the network. Second, we show that the stochastic address assignment mechanism induces address conflicts that are costly to resolve. Third, we propose a new mechanism where nodes are able to request unused addresses. We show that our new mechanism is able to cope with large linear topologies such as those existing in mines.

Changing the routing protocol without transient loops

Computer networks generally operate using a single routing protocol. However, there are situations where the routing protocol has to be changed (e.g., because an update of the routing protocol is available, or because an external event has triggered a traffic with different quality of service requirements). In this paper, we show that an uncontrolled change of the routing protocol might yield to transient routing loops (even if the involved routing protocols are loop-free). We show that it is possible to achieve a loop-free change for multiple destinations using a strongly connected component approach producing successive steps, where each step contains nodes that can change the routing protocol in parallel. Our aim is to reduce the number of steps in order to reduce the time required for the network to change from one routing protocol to another. Simulation results show that our strongly connected component approach greatly reduces the number of steps compared to the state of the art, and thus it greatly reduces the time for the change.

MAC protocol for volcano monitoring using a wireless sensor network

Many existing MAC protocols for wireless sensor networks try to achieve simultaneously low latency and low power consumption. For many monitoring applications, however, there are two separated types of traffic. Most of the traffic is periodic, and is not subject to latency constraints: energy savings is the main objective for this traffic. A small proportion of the traffic has strict latency requirements, and the energy consumed for this priority traffic is not an issue. In this paper, we introduce a MAC protocol that can achieve this traffic differentiation in a WSN. We propose to combine the low power listening approach for periodic traffic with opportunistic encounters mechanisms for urgent traffic. We show that our approach provides a good ratio of consumed energy per delivered packet of the periodic traffic, while keeping a low loss rate for the priority traffic.