Slot synchronized topology-transparent scheduling for sensor networks W. Chu, C.J. Colbourn, V.R. Syrotiuk * Department of Computer Science and Engineering, Arizona State University, P.O. Box 878809, Tempe, AZ 85287-8809, USA Available online 16 March 2005 We propose a slot synchronized topology-transparent medium access control (MAC) protocol for sensor networks. Most of the earlier scheduled approaches require a stronger form of synchronization where nodes are synchronized on frame boundaries. Synchronizing on slot boundaries is simpler to achieve since it does not require global time, making the protocol practical for sensor networks. The benefit of topology-transparency is that it ensures a bounded delay to each neighbour as long as the number of actively transmitting nodes in a neighbourhood is at most D max . The node schedules are independent of which nodes are active. Such a topology-transparent solution is particularly suitable for sensor networks, where nodes may not have unique identifiers, eliminating costly neighbour discovery. We formulate the problem as a combinatorial design problem on cyclic superimposed codes and give a constructive proof of existence along with a delay bound. q 2005 Elsevier B.V. All rights reserved. Keywords: Topology-transparent scheduling; Synchronization; Superimposed codes 1. Introduction Advances in wireless communications, micro-electro- mechanical systems (MEMS) technology, and digital electronics have contributed to the development of small, low-power sensor nodes. A sensor node consists of a sensor with data processing and communications capabilities. A sensor network is collection of sensor nodes that may be used to communicate what is sensed continuously, or to detect specific events. Since the position of the sensor nodes may not be known in advance, there is a need for the network to coordinate in a distributed manner, similar to the self-organizing capabilities of a mobile ad hoc network (MANET). Sensors come in many types (e.g. thermal, infrared, acoustic) and can monitor a wide variety of conditions (e.g. temperature, humidity, pressure). The variety in types of sensors and in their usage has precipitated applications for sensor networks that span a range of personal, corporate, and national interests [1]. As a result, research on sensor networks has grown rapidly in order to support the implementation of these emerging applications. There are several differences between sensor networks and MANETs [2]. In particular, the number of sensor nodes deployed in a sensor network is expected to be several orders of magnitude higher than the number of nodes in a typical MANET. While sensor nodes may be more densely deployed, they are also limited in power, computational capability, and memory capacity. Since sensor nodes are prone to failure, destruction, and energy depletion, the topology of the network changes frequently for these reasons rather than from node mobility. Sensor nodes may not have global identifiers because of the amount of overhead assigning such identifiers for a large numbers of sensors. Furthermore, the network tends to operate as a collective structure, addressed by attribute, rather than supporting many independent point-to-point flows. Traffic tends to be variable and highly correlated. The protocols designed for MANETs are not directly usable in sensor networks due to these differences. In this paper, our focus is on the medium access control (MAC) protocol. The MAC protocol is fundamental in any network whose basis is a broadcast channel, since it determines usage of the communication resources. Not only is it important that the channel be shared efficiently, it should also be shared fairly and conserve energy. Computer Communications 29 (2006) 421–428 www.elsevier.com/locate/comcom 0140-3664/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2004.12.026 * Corresponding author. Tel.: C1 480 965 7034; fax: C1 480 965 2751. E-mail address: syrotiuk@asu.edu (V.R. Syrotiuk).