Gérard Chalhoub

Medium access control for a tree-based wireless sensor network: Synchronization management

Energy efficiency is a primordial issue in the wireless sensor networks. This is achieved by deactivating nodes when possible. In this paper we describe the MAC protocol MaCARI that synchronizes nodes in order to schedule active and inactive periods. MaCARI divides time into three periods: a synchronization period, a scheduled activities period where communications are constrained by a tree and an unscheduled activities period where nodes can communicate whenever in range. With this synchronization, nodes are able to save energy during specific time intervals. Therefore, we focus on the synchronization period and apply an optimization to reduce its duration. We validate this approach by simulations under different tree topologies.

Cluster-tree based energy efficient protocol for wireless sensor networks

Wireless sensor networks are conceived to monitor a certain application or physical phenomena and are supposed to function for several years without any human intervention for maintenance. Thus, the main issue in sensor networks is often to extend the lifetime of the network by reducing energy consumption. When the network topology does not change very often, a clustering technique can be used to manage the network activity. Some applications have high priority traffic that needs to be transfered with bounded end-to-end delay. The IEEE 802.15.4 standard defines a MAC protocol that saves energy by putting nodes periodically in sleep mode. The ZigBee protocol defines a hierarchical addressing mechanism by creating cluster-tree based on regrouping the star topology of the IEEE 802.15.4 standard. In this paper, we show how these two standards fail to guarantee quality of service and energy saving at the same time. We propose a time segmentation approach that saves energy, enables quality of service in terms of guaranteed access to the medium and improves the overall performance of the network. This time segmentation is achieved by synchronizing nodes activity using a tree-based beacon diffusion. The tree-based topology is inspired from the cluster-tree proposed by ZigBee. Our solution guarantees end-to-end delay for high priority traffic. The efficiency of our solution is proven by simulation and implementation.

Cross-layering in an Industrial Wireless Sensor Network: Case Study of OCARI

Wireless sensor networks are adapted to monitoring applications. Specific solutions have to be developed for industrial environments in order to deal with the harsh radio conditions and the QoS (quality of service) requirements of industrial applications. In this paper, we present the main protocols used in the OCARI project, and we describe their use of cross-layering techniques. We show how it enables us to improve the performance of the network. For each protocol, we give a performance evaluation of its main characteristic.

Overview on Multi-Channel Communications in Wireless Sensor Networks

In this paper, we focus on multi-channel MAC protocols specifically designed for Wireless Sensor Networks (WSNs). The use of multiple channels helps overcome interferences and thus enhance the overall network performance. In order to establish channel and slot allocation to enable simultaneous transmissions, many protocols have been proposed in the literature. Some of them are based on a TDMA approach which offers a more deterministic behavior but requires a strict synchronization, while others are based on CSMA/CA which is an asynchronous medium access algorithm that suffers from collisions due to its probabilistic behavior. Different classifications have been proposed in previous survey papers depending on the periodicity of channel switching, on whether they are centralized or distributed, synchronized or asynchronous, etc. In this paper, we present a survey of the different techniques used in WSNs and that are applicable on the IEEE 802.15.4 standard. We detail some of the most popular protocols and discuss their advantages and weaknesses. We conclude the paper with an open discussion concerning future research directions.

Node Coloring and Color Conflict Detection in Wireless Sensor Networks

In wireless sensor networks, energy efficiency is mainly achieved by making nodes sleep. In this paper, we present the combination of SERENA, a new node activity scheduling algorithm based on node coloring, with TDMA/CA, a collision avoidance MAC protocol. We show that the combination of these two protocols enables substantial bandwidth and energy benefits for both general and data gathering applications. As a first contribution, we prove that the three-hop node coloring problem is NP-complete. As a second contribution, the overhead induced by SERENA during network coloring is reduced, making possible the use of these protocols even in dense networks with limited bandwidth. The third contribution of this paper is to show that applying any slot assignment algorithmwith spatial reuse based on node neighborhood without taking into account link quality can lead to poor performances because of collisions. The use of good quality links will prevent this phenomenon. The fourth contribution consists of optimizing end-to-end delays for data gathering applications, by means of cross-layering with the application. However, color conflicts resulting from topology changes, mobility and late node arrivals can give rise to collisions. As a fifth contribution, we show how the MAC layer can detect color conflicts, and cope with them at the cost of a slightly reduced throughput. Then, we discuss the tradeoffbetween requesting SERENA to solve the color conflicts and dealing with them at the MAC layer, our third contribution. The combination of SERENA and TDMA/CA is evaluated through simulations on realistic topologies.

Hybrid Multi-Channel MAC Protocol for Wireless Sensor Networks: Interference Rate Evaluation

This paper evaluates a channel allocation algorithm of a Hybrid Multi-Channel MAC protocol, HMC-MAC, that tries to improve communication performance in Wireless Sensor Networks (WSNs). We also analyze different techniques that are used in multi-channel MAC protocols proposed for WSNs. These protocols often use either a schedule-based or a contention based MAC protocol to communicate with each others. HMC-MAC takes advantage of the synchronous activity of TDMA to achieve collision free communications, of the flexibility offered by CSMA/CA to offer a scalable network, and of the parallel transmissions over different channels provided by FDMA to enhance the overall throughput. HMC-MAC assumes that sensor nodes are equipped with one half-duplex radio interface. We define a new method for channel allocation that enables nodes to choose the most convenient channel in its three-hop neighborhood in order to minimize interference as much possible. Simulation results demonstrate that HMC-MAC provides smaller interference rate and outperforms other mechanisms.

Evaluation of a Fast Symmetric Cryptographic Algorithm Based on the Chaos Theory for Wireless Sensor Networks

Wireless sensor networks are being more and more considered for critical applications where security issues are a priority. Security protocols are based on complicated algorithms that are very time consuming. The limited processing capabilities of sensor nodes make these protocols even more time consuming. Security algorithms based on the theory of chaos have been introduced in order to reduce the complexity of cryptographic operations. A novel chaos based algorithm aiming at enhancing the security robustness has been proposed for wireless sensor networks. On the other hand, a version of the well known AES algorithm has been adopted by various wireless sensor networks standards such as ZigBee, WirelessHART and ISA100.11a, for it is considered as a reliable and robust algorithm. In this paper, we evaluated both algorithms on TelosB motes and prove that the chaos based algorithm is much faster than the AES based algorithm and still achieve the same cryptography quality.

Channel allocation evaluation for a multi-channel MAC protocol

In recent years, the use of multi-channel MAC protocols in the Wireless Sensor Networks has attracted the attention of many researchers. In this paper, we analyze the different techniques that are used in multi-channel MAC protocols proposed for WSNs. These protocols often use either a centralized or a distributed manner to select and allocate channels. We present HMC-MAC, a Hybrid Multi-Channel MAC protocol that takes advantage of the synchronous activity of TDMA to achieve collision free communications, of the flexibility offered by CSMA/CA to offer a scalable network, and of the parallel transmissions over different channels provided by FDMA to enhance the overall throughput. It is based on a decentralized method for channel allocation that enables nodes to choose the most convenient channel in its 3-hop neighborhood. The method used to evaluate this channel allocation scheme is based on the number of conflicts generated by the use of the same channel in a three-hop neighborhood. We present simulation results using NS-2 that show the efficiency of the proposed channel allocation method.

Node Coloring in a Wireless Sensor Network with Unidirectional Links and Topology Changes

In wireless sensor networks, energy efficiency is achieved by making nodes sleep. In this paper, we propose SERENA, a node activity scheduling algorithm based on three-hop coloring. We study the performance of SERENA by simulating the conditions of a realistic environment including unidirectional links. The existence of unidirectional links causes color conflicts, which can be solved by exploiting a collaboration with the MAC layer. Such cross layering approach takes into account the real wireless environment and so enables substantial performance improvements (e.g. better reactivity to topology changes). Finally, we evaluate the benefits brought by this solution, namely the increase of network lifetime and spatial reuse.

Security architecture for wireless sensor networks using frequency hopping and public key management

The security aspect of wireless sensor networks has taken the attention of numerous researchers in the past several years. It has recently been proven that public keys are now feasible in wireless sensor networks but still consume a lot of processing time and memory. In this paper we propose the use of public keys based on ECC to exchange symmetric keys that will be used to encrypt communications. In addition, we propose a time segmentation approach that enables frequency hopping time slotted communications. Nodes secretly exchange frequency hopping sequences that enable them to fight against jamming and eavesdropping.