1 Medium Access Control for a Tree-Based Wireless Sensor Network: Synchronization Management G´ erard CHALHOUB, Alexandre GUITTON, Fr´ ed´ erique JACQUET, Antonio FREITAS, Michel MISSON Clermont Universit´ e / LIMOS CNRS Complexe scientifique des C´ ezeaux, 63177 Aubi` ere cedex, France Emails: {chalhoub,guitton,jacquet,freitas,misson}@sancy.univ-bpclermont.fr Abstract—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 synchroniza- tion 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. Keywords: energy-efficient MAC protocol, wireless sensor network, synchronization. I. I NTRODUCTION Wireless sensor networks might be the most cost-effective solution today to monitoring buildings, water dams or wide areas. Such networks have to run several applications (mon- itoring, data collection, etc.). The most distinctive feature of such networks is their ability to run for long periods of time. However, having many small range wireless nodes collaborating while applying energy-efficient mechanisms is an important challenge. This work is part of the OCARI project [1] that focuses on optimizing communications in an industrial network. Our ob- jective is to develop a MAC (medium access control) protocol that is compliant to the IEEE 802.15.4 standard [2]. Therefore, we assume the existence of two communicating entities: end devices and coordinator nodes. End devices are associated to sensors or actuators. These communicating entities have no routing features. They are also called reduced function devices in the 802.15.4 standard. Coordinator nodes are organized as a tree, whose root is the PAN coordinator. Each coordinator node is also the core of a star whose extremities are the end devices linked to it. A coordinator node has the ability to manage its end devices, to communicate with its parent (except for the root of the tree) and its potential children, and to relay or route packets. This organization is depicted on Fig. 1. In order to spare energy, MAC protocols define inactive periods during which nodes switch their radio devices off, which is the main source of energy consumption. They also save energy by scheduling transmissions to avoid collisions, which is a complicated task of the MAC protocols. In an industrial environment, the propagation conditions are specific A G H C E F D B A G H C E F D B coordinator node end device link tree links Figure 1. Example of a tree-based network: physical structure and tree. due to the presence of metallic machinery and mobile obsta- cles that perturb wireless communications. In order to avoid collisions caused by interferences, we schedule the activity periods of stars sequentially. However, this requires a precise synchronization of senders and receivers. In this paper, we propose a tree-based MAC protocol for a wide deployment of sensors. In our approach, we use the tree to synchronize all the nodes and split the activity into time slots. Within each time slot, a statistical approach such as CSMA/CA can be used. Moreover, by observing that the energy used to retrieve information increases as it is relayed by more wireless nodes, we decided to prioritize the communications between a node and its children in the tree by allocating small periods where father and children can communicate together. The tree is thus used as a default route by the routing protocol. The paper is organized as follows. Section II presents the state of the art of energy-efficient wireless MAC protocols. Section III describes our MaCARI protocol. In the description, we focus on the synchronization part of MaCARI, which is a fundamental mechanism to save energy. Section IV describes an optimization of the synchronization period and shows how the topology impacts on the synchronization delay of this mechanism. Finally, we conclude our work in Sect. V. II. STATE OF THE ART Many research papers have proposed MAC protocols in order to reduce the energy usage. We start this section with a brief review of several such protocols, and we describe two important wireless standards. Then, we describe two protocols for wireless sensor networks that synchronize nodes.