## 1 Introduction  
The miniaturization of electronic components and the evolution of wireless communication technologies have stimulated the development and use of Wireless Sensor Networks (WSNs) in various applications, such as environmental monitoring, seismic detection, structural health monitoring, and smart spaces, among others. In general, WSNs consist of small sensors which use wireless short range communication. Furthermore, these networks have severe energy consumption, processing power, memory, and bandwidth constraints [1]. Monitoring is important for debugging and analyzing the operation of WSNs. By using a monitoring system, several pieces of information about the operation of the network can be obtained, such as topology discovery, reboot of nodes, isolated nodes, routing loops, packet loss, and network latency, among others [2]. In a WSN, network monitoring can be divided into active monitoring and passive monitoring. In active monitoring, code lines are inserted in the application running in sensor nodes to obtain information about the operation of the network. In this case, the monitoring packets are sent along with data packets of the network, modifying the behavior and operation of the monitored network and consuming the resources of this network. For example, in the active monitoring system Simpathy [3] about 30% of the bandwidth is used by monitoring traffic. Furthermore, faults in the network may affect the monitoring system and prevent the delivery of monitoring information when it is most needed. In passive monitoring, a monitoring network needs to be deployed in addition to the network to be monitored, named the target network. The monitoring network captures and analyzes packets transmitted by the target network, not consuming any resources of the target network. Furthermore, a fault in the target network does not affect the operation of the monitoring system. However, the deployment of an additional network with the purpose of monitoring the target network increases the overall cost of the WSN. Therefore, passive monitoring systems are more suitable when the goal is to reduce the use of target network resources and/or isolate the faults of the target network from those of the monitoring network. In this paper, we focus on passive monitoring. The lifetime of a WSN can be up to several years and not all problems arise during the first weeks after the deployment of the network [4]. Therefore, an energy-efficient monitoring system is necessary when there is a need for monitoring a WSN in a real scenario over long periods because the lifetime of the monitoring network is extended and, consequently, the target network benefits from the monitoring for a longer time. [PERSON] _et al_. [5] describe the use of a WSN for monitoring the oceans and emphasize the importance of monitoring this network using an energy-efficient passive monitoring system. All things considered, initially, we have identified, analyzed and compared the main passive monitoring systems proposed for WSN. During our research, we did not identify any passive monitoring system for WSN that aims to reduce the energy consumption of the monitoring network. Therefore, in this paper we propose an energy-efficient passive monitoring system for WSN named Energy-efficient **P**assive **MO**nitoring **S**ystem (EPMOSt). The main goal of this system is to reduce the energy consumption and, consequently, extend the lifetime of the monitoring network. Furthermore, EPMOSt provides monitoring information using an SNMP agent. In this way, any management tool that supports the SNMP protocol can be integrated with this monitoring system. The remainder of the paper is organized as follows: in Section 2, the main passive monitoring systems proposed for WSN are analyzed and compared. Section 3 presents the monitoring system proposed in this paper and describes the details of its implementation. Section 4 describes the experiments and metrics used to evaluate the proposed monitoring system.