# Classification between Failed Nodes and Left Nodes in Mobile Asset Tracking Systems +
## 3 System Component  
### Description  
This section formalizes the system architecture of a medical asset tracking system developed in this work and the features of each component composing the system. The system architecture is shown in Figure 2. This system is composed of four-tuple \(S=(T,G,A,M)\), where \(T\) is the asset tracking application, \(G\) is the gateway, \(A=a_{1},a_{2},...,a_{m}\) is the set of anchor nodes and \(M=m_{1},m_{2},...,m_{n}\) is the set of mobile nodes [24].  
A wireless sensor network is developed based on the ZigBee specifications and on IEEE 802.15.4 [26]. The devices included in the network form a multiple-depth tree by themselves,  
Figure 2: System architecture [24].  
exchanging messages with one another. They operate under the beacon-enabled mode to achieve a low duty cycle. A personal area network coordinator (PANC) included in the gateway starts the network formation process, and each node maintains a neighbor list table. This table typically contains neighbors' addresses and received signal strength indication (RSSI) values for each connection. The anchor nodes and the mobile nodes are developed on the same hardware platform with the same software running on them. However, their functions are different with respect to their device type.  
A mobile node is attached to a medical asset and always becomes a leaf node of the network. This node is a battery-powered device with a battery-saving sleep mode. If an anchor node and a mobile node can exchange radio signals, then the anchor node becomes a neighbor of the mobile node. However, a mobile node does not include other mobile nodes as a neighbor. Initially, each mobile node selects a parent node from neighbors that send signals that exceed a threshold. If the signal strength from the current parent is less than the threshold, the mobile node selects another parent node. Each mobile node wakes up periodically and exchanges polling messages with its parent to check whether the wireless connection between them is valid. After confirming the connection, this mobile node gathers RSSI values from its neighboring anchor nodes and sends them to its parent. After transmitting the data, the node enters a sleeping phase.  
Anchor nodes are stationary nodes at a fixed location that form a tree network structure to transmit sensing data generated by each node to a gateway. They are installed in a monitoring region or on the boundaries of the target monitoring area and provide a high spatial resolution and a monitoring infrastructure for the mobile nodes. Every anchor node already knows its own position and acts as a router, which forwards messages for mobile nodes and other anchor nodes toward and from the gateway. Each anchor node uses heartbeat messages to detect disconnections of its neighbors.  
The gateway is considered as an interface between the application and the wireless sensor network. The gateway receives RSSI values reported by a mobile node through the anchor nodes. The trilateration algorithm in the gateway determines the location of the mobile node using RSSI values received from the node. This location algorithm also uses filters to eliminate invalid RSSI values and to increase the location accuracy. This filter is a simple threshold-based algorithm. This location algorithm will not be discussed in detail in this paper as the details are outside the scope of this study.  
The asset tracking application system gathers the location data of all individual nodes from the gateway. It takes the role of keeping track of the current locations of all mobile nodes, determining the enter/exit state of each mobile node, counting the number of assets within (and outside) the monitoring area and displaying all of the gathered data on its graphical user interface.