2.4G wireless applications in industry and medical fields

With the continuous development of society, the advantages of wireless have gradually emerged. For example, wireless communication has a large coverage area and is almost not restricted by the geographical environment; wireless communication can be set up at any time, links can be added at any time, and installation and expansion are convenient; wireless communication can quickly (within tens of minutes) establish a communication link to achieve the purpose of temporary, emergency, and disaster relief communication; while wired communication has geographical restrictions and a longer response time. Wireless communication exceeds traditional wired communication methods in terms of reliability, availability, and anti-destruction, especially in some special geographical environments, which can better reflect its superiority. With the maturity of wireless technology, industries such as industry and medical care have begun to use 2.4G communication more and more, and 802.15.4, ZigBee and Wi-Fi have also been more widely used. ZigBee and Wi-Fi each have obvious characteristics, and many characteristics are complementary. Combining the two has a good application prospect. In order to help engineer friends answer more design and application problems at close range, this issue of Expert Sitting specially invited Liu Kai, technical director of Beijing Boxun Technology Co., Ltd., to share his design experience.

Netizen Yueyun: What is the difference between ZigBee and Wi-Fi?

The comparison between ZigBee, Wi-Fi, Bluetooth and several wireless technologies is shown in the following table:

Comparison of ZigBee, Wi-Fi, Bluetooth and several wireless technologies

Wi-Fi is currently being used in large quantities, mainly in the local area network of PCs and other devices in home and office environments. After the deployment of 3G, there will be some applications of 3G+Wi-Fi, and China Telecom's "Tianyi" includes this part. It is foreseeable that many embedded Wi-Fi devices will become more popular as the network becomes more convenient, such as Wi-Fi POS machines and Wi-Fi scales in supermarkets. There are also many applications in industrial environments, mainly in the Wi-Fi access of serial port devices, which are used in industrial wireless data acquisition systems.

ZigBee and IEEE802.15.4 devices are mainly concentrated in: wireless sensor detection and low-level control in industry; personal monitoring instruments, low-power wireless medical equipment; high-end toys; electrical appliance networking and control; wireless consumer equipment; HVAC and lighting control, etc. At present, batch applications are mainly in asset tracking, logistics management, intelligent lighting, remote control, medical care and remote meter reading systems.

Netizen UCanDoIt: How does 2.4G technology solve the problem of frequency band congestion?

802.15.4 uses DSSS, and 802.11 uses DSSS and OFDM. In actual use, we have tested in office buildings, factories and other environments. Communication is more affected by obstruction and distance, and congestion does not cause much impact.

Netizen jackywoo: 802.15.4 and Zigbee technologies are the most ideal choices for WSN networks, with the characteristics of low power consumption, but how to achieve low power consumption specifically and what factors need to be considered?

IEEE 802.15.4 defines an optional MAC layer superframe structure. The superframe includes two parts: active and inactive. In the inactive part, the device can enter low power mode (sleep state); the active part is divided into contention period and non-contention period. The contention period is provided for devices accessed in CSMA-CA mode. The non-contention period consists of several guaranteed time slots and is provided for some devices that need to reserve a certain data bandwidth. This superframe structure reflects a major feature of IEEE 802.15.4's low power consumption. The introduction of the inactive period limits the activation time of transceivers between devices and puts them in sleep state when there is no data transmission, thereby greatly saving power expenditure.

Zigbee devices are divided into full-function devices and reduced-function devices. Compared with full-function devices, reduced-function devices have a simpler protocol stack and smaller memory, and can only interact with a full-function device. Full-function devices have complete IEEE 802.15.4 protocol functions and can interact with any node within their transmission range. The two types of devices can be combined to form mesh networks, star networks, and tree networks.

In addition to the power consumption considerations in transmission, low-power systems also have power relations between the CPU and other parts of the system. For example, the JN5139 SoC itself can turn off the RF and run the CPU part alone. It can also turn off the SoC's on-chip ADC, serial port and other peripherals. For sleep, it can also provide sleep modes that retain RAM content for quick startup, sleep modes that use interrupts/comparators/timers to wake up, and deep sleep modes that can only be woken up by reset. In this way, the entire system can reasonably achieve a balance between functionality and power consumption.

Netizen Feng+Yun: What is the bandwidth of Zigbee signal?

ZigBee uses IEEE802.15.4 at the bottom layer, which means that the physical bandwidth is the bandwidth of IEEE802.15.4, which is 250kbps. However, there is a difference between physical bandwidth and effective data rate. For ZigBee, the data rate also needs to consider the network topology, data routing relationship, data volume in the network and other issues. These factors must be fully considered in actual applications. Because there are many factors involved, the actual calculation cannot be given here. In experience, routing has the greatest impact on data rate. Each additional level of routing increases the time by 100ms to 200ms. Therefore, ZigBee is not a real-time network. Secondly, the concurrent data of the network also has a great impact on the rate. In short, ZigBee is suitable for low-speed sensor applications, and the actual bandwidth must consider the actual situation of the network.