Analytical Model of the LPL with Wake up after Transmissions MAC protocol for WSNs Cristina Cano 1 , Boris Bellalta, Jaume Barcel´ o, Miquel Oliver, Anna Sfairopoulou Departament de Tecnologies de la Informaci´ o i les Comunicacions (DTIC), Universitat Pompeu Fabra C/ Tanger, 122-140, 08018, Barcelona, Spain 1 cristina.cano@upf.edu Abstract—The LWT-MAC protocol extends the normal opera- tion of B-MAC by taking advantage of the local synchronization that can be achieved after each packet transmission. Sensor nodes wake up at the end of each successful transmission to send or receive messages eliminating the need for the long preamble transmission. Current LPL analytical models are extremely simple as they do not consider the energy waste due to collisions. In this work, a detailed analytical model of the LWT-MAC protocol, which considers the energy waste of collisions and overhearing for both saturated and unsaturated conditions, is presented. Results show a reduction of the energy consumption as the traffic increases and an improvement, in terms of throughput and delay, compared to B-MAC. Additionally, a performance evaluation is presented in which some key network parameters (check interval, traffic load and the number of sensor nodes) are modified in order to analyze the behavior of the studied LWT- MAC protocol. I. I NTRODUCTION Wireless Sensor Networks (WSNs) consist of small devices that sense environmental data and send it to a central device. Normally, sensor nodes are low-cost devices with reduced processing, memory and battery resources deployed in remote and large areas. The battery replacement in these networks is, therefore, too costly or even impossible making energy consumption the most important constraint. For this reason, the Medium Access Control (MAC) protocol is crucial as it directly influences the transceiver operation that is the most consuming component of a sensor node. The common approach to reduce energy consumption in WSNs is to peri- odically put the transceiver into sleep mode, working in a low duty cycle operation instead of continuously listen the channel as in traditional wired and wireless networks without power constraints. A well-known MAC protocol is B-MAC [1] in which each node periodically and independently of the others samples the radio channel to detect activity, what is known as Low Power Listening (LPL) operation. Then, when a node wants to send a message, it first sends a preamble long enough to overlap with the listen time (active part of the duty cycle) of the receiver. Note that, at very low loads, the energy consumption of the sensor nodes is extremely reduced. However, as the load increases (for instance, due to events occurrence) the collisions of preambles become a significant energy waste, even more important in large scale WSNs with hidden terminal problems. The LPL with Wake up after Transmissions (LWT-MAC) MAC protocol [2] was designed to maintain a low energy consumption at low loads while at the same time being able to react to instantaneous increases of the network load. A local synchronization after transmissions is adopted that ensures that all nodes that have overheard the last transmission will be awake to receive a new message, hence without requiring the long preamble transmission. Analytical models of WSNs MAC protocols allow to derive performance optimizations of the different parameters involved (duty cycle, contention window or packet size among others) depending on different network scenarios. However, existing analytical models of WSNs MAC protocols (for instance, the ones used in [1] and [3]) are extremely simple as they only consider the time to transmit a packet without taking into account the time and energy wasted in collisions. Usually, WSNs work under low load conditions but it is important to consider the case of high traffic, specially in event-based WSNs. More detailed analytical models of scheduled MAC protocols such as the S-MAC [4] and nanoMAC [5] have been done, however, the LPL has not been studied so exhaustively. In this work, the LWT-MAC protocol is studied from an analytical point of view. The presented LPL analytical model considers the energy waste due to collisions and overhearing in a single-hop network under saturated and unsaturated con- ditions. It models the behavior of the LWT-MAC protocol, although it can be easily adapted to model other LPL MAC protocols such as B-MAC. The rest of the paper is organized as follows: Section II describes the LWT-MAC protocol, then in Section III the analytical model is presented. Results are discussed in Section IV and finally some conclusions are given in Section V. II. THE LWT-MAC PROTOCOL The L PL with scheduled W ake up after T ransmissions (LWT-MAC) extends the operation of B-MAC by taking ad- vantage of the local synchronization of all nodes that overhear a transmission [2]. The LWT-MAC defines that all overhearing nodes wake up simultaneously at the end of each successful transmission in order to send or receive packets. Since all nodes that have overheard the last transmission will be awake the long preamble is no longer necessary (Fig. 1). In this 978-1-4244-3584-5/09/$25.00  2009 IEEE ISWCS 2009 146