Analysis of Three IoT-Based Wireless Sensors for Environmental Monitoring
The recent changes in climate have increased the importance of environmental monitoring, making it a topical and highly active research area. This field is based on remote sensing and on wireless sensor networks for gathering data about the environment. Recent advancements, such as the vision of the Internet of Things (IoT), the cloud computing model, and cyber-physical systems, provide support for the transmission and management of huge amounts of data regarding the trends observed in environmental parameters. In this context, the current work presents three different IoT-based wireless sensors for environmental and ambient monitoring: one employing User Datagram Protocol (UDP)-based Wi-Fi communication, one communicating through Wi-Fi and Hypertext Transfer Protocol (HTTP), and a third one using Bluetooth Smart. All of the presented systems provide the possibility of recording data at remote locations and of visualizing them from every device with an Internet connection, enabling the monitoring of geographically large areas. The development details of these systems are described, along with the major differences and similarities between them. The feasibility of the three developed systems for implementing monitoring applications, taking into account their energy autonomy, ease of use, solution complexity, and Internet connectivity facility, was analyzed, and revealed that they make good candidates for IoT-based solutions.
It presents an open-source wireless mesh networking module for environmental monitoring, with the aim of promoting this kind of routing in such applications. This is based on a low-cost RF transceiver, with more compact and less complex code than the one of a ZigBee design, and works in connection with a sensor node. The system was compared to an off-the-shelf product, XBee, with the conclusion that it can offer comparable or even better performance than commercial items. Other systems for monitoring applications belonging to different fields, all based on ZigBee, are reported in , , and . The major drawback of these consists in the requirement for a gateway in case data has to be sent over the Internet, a basic requirement for IoT scenarios. Various monitoring solutions based on BLE technology have appeared and are gaining ground especially in home automation, after its introduction in 2010 . The work in  consists in the development of a novel energy management approach for smart homes based on BLE enabled wireless networks. By offering low power, low cost, and reduced device dimensions, the authors believe that this technology has a high potential of becoming important for both the IoT and for smart homes. This trend will also be sustained by the availability of native support offered by current mobile devices, compared to IEEE 802.15.4, which will also reduce the cost of BLE devices. The simulation results show that this approach contributes to the reduction of peak load demand and electricity consumption charges, ultimately leading to monetary savings. Furthermore, it has been shown that the performance of the proposed BLE network is better than the one obtained in the case of IEEE 802.15.4 in terms of packet delivery ratio, delay, and jitter. With the continuous improvements brought to the protocol, such as the support for mesh networking, and the extension of the range offered, it is believed that this technology will be taken into consideration for implementing environmental monitoring applications. In , we reported the development of Wi-Fi sensors sending temperature and relative humidity measurements to a base station using UDP. A battery lifetime of 2 years with a 20 min measurement cycle was achieved. This encouraged the development of a device using HTTP, for investigating the power efficiency of this more reliable solution, from the communication point of view.
The two sensors that communicate using Wi-Fi technology are based on the same hardware, the difference between the two consisting in the protocol that was used, namely UDP or HTTP. The generic architecture of the devices based on Wi-Fi technology is presented in Fig. 1. It consists of the application processor, a CY8C3246PVI-147  programmable system on chip microcontroller (PSoC 3) produced by Cypress Semiconductor, a wireless local area network (LAN) module, RN-131C/G , and the temperature and relative humidity sensor (SHT21 ), all powered by a 3 V CR123A 1500 mAh battery. The choice of using a separate application processor removes the possibility of interfering with the communication stack on the WLAN module. Therefore, the processor in the developed devices is in charge with performing all the actions for the proper operation of the device, namely, power management, acquisition of data from the sensing unit, and communication. For transmitting the data to a base station, a serial link between the PSoC 3 device and the communication module, and an API (application programming interface), called WiFly, are used. The motivation for selecting the RN-131C/G wireless module consists in its low-power operation, providing 4 μA during sleep and short 210 mA pulses during transmission. The development of other cheaper wireless modules based on the IEEE 802.11 set of standards, such as the ESP8266 from Expressif, will multiply the range of possible solutions.
Three different wireless sensors for implementing IoT-based solutions for environmental monitoring were designed, developed, and analyzed. All of them are fabricated using commercial off-the-shelf discrete components and provide facile access to the Internet using a minimum of additional hardware and software resources. The analysis of the three implementations revealed the fact that Wi-Fi and BLE are two technologies suited for monitoring applications that can successfully compete with the widely used ZigBee protocol. As expected, Wi-Fi consumes more energy but enables the development of solutions with reduced total cost of ownership through the use of the existing infrastructure. The third solution, employing Bluetooth Smart communication, has also been proven to be efficient, and the promising results encourage the development of systems based on this technology. Furthermore, the refinement of the protocol and the addition of mesh networking capabilities to BLE devices will make it even more attractive for implementing such solutions. The analysis of the developed systems also indicate the fact that, with the availability of increasingly energy efficient transmission modules, power harvesting is applicable also to Wi-Fi technology, until recently considered to be too power hungry for designing wireless sensors. The analysis presented in this paper represents a starting point for the selection of a direction in the implementation of IoT-based environmental monitoring applications, providing an overview of the potential and challenges of each one of the three developed wireless sensors.
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