## 3 System Description  
This study takes place as part of a whole system which includes a communication system from the sensor platform to a server application in a remote place. The communication system includes an installation application to configure the nodes in the slab installation phase. In both the slab installation and maintenance monitoring phases, the communication module is Digi's Xbee PRO S5 with a frequency of 868 MHz. Symmetric encryption is used by this module to provide secure communications. It helps to improve the security of the system, but as future work it would be interesting to apply the model proposed in [19] to analyze the security, privacy and dependability of the whole system. Nodes are deployed inside a custom case. This custom case is inserted in the slab track inside a special hole carved into the concrete. This case is sealed and protected from unauthorized attacks and also from external weather conditions. In general terms, this casing approach allows operators to easily swap the existing case with a new one (with a node with similar configuration and a new battery inside). The sensors deployed in the system are used to obtain acceleration, inclination and distance information. They are divided into groups controlled by a cluster head (coordinator) that reports the information to base stations located in the passing trains. As described in Figure 1, the coordinator receives the information from the nodes of its group and establishes the communication with the train. The information obtained from the system can be used to assist operators in the installation phase of the slab tracks and also to assess the structural health of the railway infrastructure. By analyzing vibration performance (in the frequency domain), inclination and distance, abnormal situations can be detected and the evolution of potential defects controlled. Based on this railway application, a description of the node requirements is given in the next subsection. In the wireless sensor network field it is well known that wireless transmission is one of the aspects that affect the life of the batteries the most, for this reason, it is worth remarking that the system is designed to retransmit as little information as possible in order to extend the life of the devices in terms of energy consumption. To address this issue nodes are organized in clusters which are controlled by a cluster-head. Furthermore, nodes are placed so that all nodes within the same cluster can communicate with their cluster-head in one-hop. Thanks to this design it is not necessary to carry out data fusion or aggregation techniques. Basically, the cluster-head receives information from its neighbors and retransmits it to one of the trains when it passes by. In the case the cluster-head is unable to retransmit all the information it will use the trains behind to complete its task. The cluster-head will always be able to transmit all the information received from their neighbors to the trains as they only generate data once a day (around 24 bytes) and furthermore, many trains pass by in a day, thus, a cluster-head has several chances to retransmit all the information. This architecture allows the complexity of the applications to be reduced as nodes don't need to transmit packets in a multi-hop way. The application of our system can be described in four simple actions:  
1. Nodes read data from their sensors once a day. 2. Nodes send the information obtained from the sensor to their cluster-head. 3. Cluster-head sends the information received from the sensors to the train passing by. Figure 1: Sensor4 PRI architecture. In order to clarify the design of the on-board components and the communication mechanisms of the proposed platform, the set of requirements are detailed in this section. The integrated sensors have two main functions:  
* Provide assistance in the installation phase of the slabs. The platform should provide an accurate estimation of the slab tilt and the distance between contiguous slabs.