## 2 Passive Monitoring in WSNs
Then the global trace is analyzed in order to detect faults and to assess the performance of the target network. The information obtained from this analysis is displayed in a tool developed by the authors. In PMSW, only data and ACK packets are captured, while control packets, such as routing packets and packets of cluster election, are not captured or analyzed. Table 1 shows the comparative analysis of the monitoring systems described in this section. We analyze the following characteristics:  
* Energy-efficient--indicates whether the system is concerned with minimizing the energy consumption of nodes of the monitoring network;
* Analysis mode--indicates whether the analysis of captured packets is performed online or offline;
* types of packets (data and/or control) captured by the monitoring network;
* Event analysis--indicates whether the monitoring system analyzes the captured packets to obtain information about the operation of the target network;
* Redundant packets--verifies whether the monitoring system captures redundant packets. Redundant packets are captured when two or more sniffers capture packets sent by the same node of the target network;
* Aggregation--verifies whether the monitoring system uses any mechanism to aggregate information of packets captured from the target network in order to reduce the number of packets transmitted by the monitoring network and, consequently, reduce the energy consumption of this network; and
* Visualization tool--describes the computational tool used to display information obtained from passive monitoring. The characteristic _analysis mode_ and _event analysis_ were defined in [11], while the remaining characteristics were identified from the study of passive monitoring systems discussed in this section. The capture of these characteristics is an additional contribution of this work. The main conclusions obtained by observing Table 1 are:  
1. None of these systems is energy-efficient; in other words, the proposed solutions are not concerned with minimizing the energy consumption of nodes of the monitoring network. Energy efficiency is important when it is necessary to monitor sensor networks deployed in remote locations, where it is impractical to replace the batteries of sniffers;
2. In SNIF, Pimoto and PMSW the analysis of captured packets is online. In SNTS and LiveNet captured packets are stored in non-volatile memory, which must be collected manually for future analysis (offline);
3. SNTS, SNIF and PMSW analyze the captured data in order to detect fault events and/or assess the performance of the target network, while Pimoto and LiveNet only display the traces of captured packets;
4. Only Pimoto does not capture redundant packets, because the target network is subdivided into monitoring islands and each island has only one sniffer. The capture of redundant packets increases the energy consumption of the monitoring network because sniffers transmit redundant packets through this network;
5. None of these systems implements an aggregation mechanism of packets captured from a target network. Aggregation reduces the number of bytes transmitted by the monitoring network and hence reduces the energy consumption of this network; and
6. Only Pimoto displays the monitoring information in a network management tool used by the community (_i.e._, Wireshark), and none of these systems provides the monitoring information using an SNMP agent.