Application and Standard Development of WSN

introduction

In recent years, the explosive development of wireless technology has spawned a variety of industrial, scientific and medical (ISM) band wireless standards. Thanks to these new standards, various wireless applications have penetrated into every aspect of our daily lives. Undoubtedly, wireless sensor networks (WSNs) are an important application that benefit the most from these standards.

Imagine a farmer in the Midwest of the United States who is faced with the challenge of monitoring the body temperature of thousands of cattle on a daily basis in order to prevent animal diseases such as foot-and-mouth disease that threaten the survival of his herd. Using wireless technology, these challenges can be easily overcome by installing a temperature sensor with a wireless transmitter on each cow and transmitting its temperature readings to a master terminal. This is a simple example of WSN, which shows that the use of wireless technology can save a lot of time and cost. This article will briefly introduce ISM bands and WSNs, as well as some of the wireless standards that support them.

1 Overview of Industrial Science and Medical Frequency Bands

The ISM band is a part of the spectrum that can be used by anyone without a license. The only requirement for developing products in the ISM band is to comply with certain regulations for this part of the spectrum. These regulations vary from country to country. In the United States, the Federal Communications Commission (FCC) is responsible for setting these regulations, while in Europe the European Telecommunications Standards Institute (ETSI) is the regulatory body. Part 15 of the FCC Rules and Regulations defines the frequency band requirements in the United States. Figure 1 illustrates the various frequencies and bands and lists the corresponding regulatory bodies.

The 2.4 GHz band and several sub-1 GHz bands are the most widely used ISM frequency space today. Because the 2.4 GHz band is so cluttered, some product development activities are moving to the 5 GHz band - but this trend is still very limited due to effective communication range issues. 2.4 GHz is a common band, while the sub-1 GHz bands allocated for low-power wireless applications vary from country to country. In the United States, the most common remaining bands are 902-928 MHz, while in Europe most wireless communication activities are concentrated in the 868 MHz frequency range.

The 2.4 GHz band is recommended when interoperability with other systems is required and when working in different geographical spaces is a key issue. The main disadvantages of using the 2.4 GHz band are its crowded space and limited communication distance due to the poor transmission characteristics of the 2.4 GHz frequency.

Choosing to design products in the sub-1 GHz band can help solve some of the issues faced in the 2.4 GHz band; however, the sub-1 GHz band also has some limitations of its own:

Limited Duty Cycle

No interoperability with other systems

Different geographical working restrictions (for example, wireless instruments designed for the 902-928 MHz band in the United States will not work properly in Europe)

Several standards have emerged to operate in the ISM frequency space, depending on the frequency, target data rate, distance, and desired level of interoperability. Figure 1 shows some of the standards most commonly used by wireless engineers in product development.

Figure 1 Different wireless standards operating in the ISM band

2 Overview of Wireless Sensor Networks

It is clear that "smart" environments represent the next evolutionary stage for buildings, utilities, industry, homes, transportation, and agriculture. As a result, interest in WSNs is steadily increasing. A WSN consists of many sensors distributed over a geographic area.

WSNs generally consist of a host or "gateway" that communicates with a large number of wireless sensors via a radio communication link. Data collection is done at the wireless sensor nodes, compressed, and transmitted directly to the gateway, or if required, other wireless sensor nodes can be used to pass the data to the gateway. The gateway then ensures that the data is input data to the system.

Each wireless sensor is considered a node, which has wireless communication capabilities and also has certain intelligence for signal processing and network data. Depending on the type of application, each node can have a specified address. Figure 2 shows a general block diagram of a node. It generally includes a sensor device, a data processing microcontroller, and a wireless connection RF module. Depending on the network definition, the RF module can play the role of a simple transmitter or a transceiver (TX/RX). When designing a node, it is very important to pay attention to current consumption and processing power. The memory of the microcontroller is very dependent on the software stack used.

Figure 2 General structure of a WSN node

Figure 3 shows a WSN applied in a home environment. In this network, we can observe different types of sensors, such as motion detectors, radiators, temperature monitoring, etc.

2.1 WSN targets four main goals

(1) Read some parameter values ​​at a given location and send them to the main processing center. In an agricultural application environment, for example, the herd of cattle mentioned earlier, reading the body temperature of each cow can help determine which cow needs closer monitoring.

(2) Monitoring the occurrence of certain events, for example, in medical applications, monitoring blood pressure and pulse and heart rate peaks.

(3) Tracking the movement of specific objects is widely used in the military field to track enemy vehicles.

(4) Helps classify detected objects, especially in traffic control applications.

2.2 Two main topologies used in WSN

A) Star Network: As shown in Figure 4, a star network consists of a point-to-multipoint wireless connection where a single host connects to several nodes in a bidirectional or unidirectional manner. This topology is very interesting if low power consumption and low software overhead are key parameters. The limitation is the effective communication range, as each node must be within the communication range of the host. Several standards can be used to implement this topology. Bluetooth, IEEE 802.15.4 or proprietary systems are some of the most widely used standards. Note that the Bluetooth platform has not gained widespread acceptance due to some limitations of the Bluetooth protocol.

Figure 4 Star network topology for WSN applications

B) Mesh Network: In a mesh network topology, as shown in Figure 5, nodes are connected together with many redundant interconnects. If a node fails, there are many other ways for two nodes to communicate. This topology has good reliability, but it comes at a high cost in terms of current consumption and software overhead. This topology can be implemented through proprietary or Zigbee standards.

Figure 5 Mesh network topology used in WSN applications

3 Conclusion

WSNs are evolving every day, and new standards are emerging more and more. However, it is important to note that most of these standards have not yet reached a mature level. Instead, they are still in the early stages of development. A serious WSN design engineer will study his network needs in depth in terms of architecture and the capabilities of specific standards in order to meet key requirements such as current consumption, maximum allowed number of nodes, battery life, data rate, and operating frequency.