# Design and Performance of a XBee 900 MHz Acquisition System Aimed at Industrial Applications
## 3 Design of a Wireless Monitoring System
### Architecture of the Monitoring System  
This subsection describes the architecture proposed for the monitoring system (see Figure 1). It is implemented by means of a star topology, since it allows scheduling the communication traffic of the system in a more deterministic way. Two kinds of nodes are used: The _Coordinator Node_ (CN), at the core of the star topology, which centralizes all operations of the monitoring system. It holds the transaction clock, and it is responsible for storing the values of the remote sensors. This node manages the wireless communication with the _Smart Measuring Nodes_ (SMN), according to a schedule that fulfils the time requirements of the planned system. The Smart Measuring Nodes are responsible for acquiring the values of the process sensors and providing the network connectivity with the Coordinator Node. A prototype for the Smart Measuring Nodes was specifically designed for easing the connectivity of the sensors located remotely. The designed module allows the acquisition of up to four analog sensors per node.  
Figure 1: General architecture overview for the wireless monitoring system.  
Xbee wireless technology, which is based on the IEEE 802.15.4 standard, was chosen to connect the Coordinator Node to the Smart Measuring Nodes. This technology was chosen, for several reasons: (1) It is a well-proven, robust and low-cost technology; (2) it is a technology able to operate outside the 2.4 GHz band (namely, at the 900 MHz band), so that interferences with WiFi technology may be avoided; (3) it provides low jitter, so that the required sampling frequencies of the monitoring system may be achieved; (4) it is a very flexible technology that may be configured for different topologies and scenarios. Even though XBee technology is typically used at noncritical applications, it is highly configurable. For this work, a configuration was selected which fulfils the QoS requirements of the monitoring system. In particular, the XBee 900 HP antennas were chosen since they operate at the 900 MHz band, providing a communication channel without interferences with WiFi technology. In addition, they allow selecting the transmission signal power at different levels in order to adapt the quality of the signal to different scenarios.  
The Coordinator Node is the key component of the architecture, since it holds the transaction clock and manages the network messages with all Smart Measuring Nodes, which are responsible for acquiring the values of the sensors next to the process. Moreover, critical applications may require a combination of wired and wireless connections, acquiring specific sensors to be acquired by means of wired connections, whereas other sensors can be acquired by wireless means. In this scenario, it is necessary to select a platform for the Coordinator Node that fulfils all the following requirements:  
1. Industrial usage and acceptation, since it may be used in industrial scenarios and applications.
2. Real-time embedded platform, since the Coordinator Node requires a precise clock for centralizing wireless communications with Smart Measuring Nodes and a real-time programming capabilities.
3. Scalable platform that allows to implement hybrid (i.e., by means of wired and wireless links) connectivity with the sensor devices.