## 4 Evaluation and Experiments
### Experiments
#### 4.2.1 Construction of Testing Platform  
The online lake water quality monitoring process is shown in Figure 16.  
We constructed a testing platform in a laboratory environment, debugged and tested the function of each module, and assessed the reliability of the system. Sensors, monitoring nodes, gateway nodes, and other components must be properly combined together. The test platform for this experiment is shown in Figure 17. It consists of three monitoring nodes, gateway nodes, sensors, and a PC.  
The monitoring nodes are located in the bottom layer of the system. The monitoring nodes read and process sensor data, and communicate with the gateway node. A monitoring node contains power, data acquisition, wireless communication, and microprocessor control modules. It uses a storage battery as a power supply. The power module provides power to the other modules. The data acquisition module is a RS485 bus interface circuit and sensor module. It records the real-time water data, which is transmitted to the control chip for processing. The processor module is used to coordinate the operation of each module, to control the sensor data readings, and for receiving, processing, and transmission. The wireless communication module performs the ZigBee communication and networking functions. ZigBee technology is a kind of short distance, low complexity, low power consumption, low rate and low-cost two-way wireless communication technology [30]. The selected ZigBee communication  
Figure 16: Online lake-water quality monitoring process.  
Figure 17: Testing platform for a lake water quality monitoring system.  
module is developed by Mechatronics and the information laboratory of East China University of Science and Technology.  
The gateway node controls Zigbee's internal network, and joins the external and internal networks. It is the key point of the communication and data transmission process. The gateway node focuses on either the communication between the coordinator and each Zigbee node, or the communication between a GPRS module and the remote monitoring server. The gateway node is mainly composed of a power supply module, a microprocessor control module, an Electrically Erasable Programmable Read-Only Memory (EEPROM) module, a GPS information acquisition module, a Zigbee communication module, and a GPRS communication module. The microprocessor control module is used for data processing and coordinates the work of the other modules to ensure an orderly data communication process. The EEPROM memory module is used to store the data during processing, so that no data are lost. The GPS information acquisition module collects a real-time geographic position. The Zigbee communication module receives the data communicated by the monitoring nodes, and the GPRS communication module remotely transmits all kinds of received information to the monitoring center using a mobile network.  
In this system, real-time data and water quality analysis results for monitoring the lake were presented to users in the form of webpages. We also included a management function for an administrator. Figure 18 shows the visualization interface, which presents information about the received data and the system management. The design of the interface helps the administrator check the data and management options, and control the state of the system.