Wireless sensor networks and ZigBee are popular targets for current research. This paper briefly introduces the main advantages of IEEE 802.15.4/ZigBee-based wireless sensor networks, focusing on the IEEE 802.15.4/ZigBee-based sensor node model and several alternative transmission module chips, and gives a The actual node design.
Wireless applications such as sensors and self-organizing networks do not require high transmission bandwidth, but require lower transmission delays and extremely low power consumption, enabling users to have longer battery life and more Device array. The IEEE802.15.4/ZigBee standard has low power consumption and low cost as the main targets, providing a platform for interconnecting and interconnecting sensor networks. At present, the research and development of wireless sensor networks based on this technology has received more and more attention.
IntroductionThe IEEE 802.15.4 specification is an economical, efficient, low data rate (250 kbps) wireless technology operating at 2.4 GHz and 868/928 MHz. The network layer and above protocols are developed by the ZigBee Alliance, and IEEE 802.15.4 is responsible for the physical layer and link layer standards. . The complete ZigBee protocol suite consists of a high-level application specification, an application convergence layer, a network layer, and a data link layer and a physical layer. The structure of the protocol stack is shown in Figure 1.
Figure 1 ZigBee protocol stack structure
Physical layerThe physical layer uses DSSS (Direct Sequence Spread Spectrum) technology to provide 27 channels for data transmission and reception. IEEE 802.15.4 defines two physical layer standards for the 2.4 GHz band and the 868/915 MHz band. The main functions of the physical layer include: active and dormant RF transceivers, channel energy detection, link quality indication of channel received data packets, idle channel evaluation, and data transmission and reception.
data link layerThe IEEE 802 series of standards divides the data link layer into a medium access layer MAC and a logical link control layer LLC. The MAC sublayer of IEEE 802.15.4 supports multiple LLC standards. The MAC sublayer uses the services provided by the physical layer to implement data frame transmission between devices; and the LLC sublayer provides connection-oriented and connectionless services to the device based on the MAC sublayer. The MAC sublayer function specifically includes: the coordinator generates and transmits a beacon frame, the common device synchronizes with the coordinator according to the coordinator beacon frame; supports association and disassociation of the PAN network; supports communication security of the wireless channel; uses the CSMA-CA mechanism Support for Protected Time Slot (GTS) mechanisms; supports reliable transmission between MAC layers of different devices. The LLC sublayer functions include: transmission reliability guarantee and control; segmentation and reassembly of data packets; and sequential transmission of data packets.
Significant advantages of the sensorBased on the IEEE 802.15.4 standard, coordinated communication between thousands of tiny sensors is possible. In addition, relaying data from one sensor to another by radio waves can make communication very efficient. In general, as the communication distance increases, the complexity, power consumption, and system cost of the device increase. Compared to existing wireless communication technologies, ZigBee technology's low power consumption and low speed are the most suitable standards for sensor networks. ZigBee technology is suitable for carrying services with small data traffic, especially sensor networks.
Low power consumption and low cost
In a ZigBee-based sensor network, a full-featured device can be used as a sink node. The terminal node generally uses a reduced function device to reduce system cost and power consumption and improve battery life.
Large capacity, short delay
Higher density nodes can be accommodated in a single network. A ZigBee network can accommodate up to 254 slaves and 1 master device, and an area can have 100 ZigBee networks simultaneously, specifically meeting the requirements of large-scale sensor arrays.
Simple protocol and high security
The ZigBee protocol stack averages only 1/4 of Bluetooth or other IEEE 802.11 lengths. This simplification is important for low cost, interactivity, and maintainability. ZigBee technology provides data integrity checking and authentication, provides a three-level security mode, flexible security attributes, and network security.
Design based on IEEE/ZigBee sensor nodes
Hardware reference model for sensor nodes
The wireless sensor network micro node generally consists of a sensor module, a data processing module, a data transmission module and a power management module. The sensor module is responsible for collecting information of the monitoring area and completing data conversion. The collected information may include temperature, humidity, light intensity, acceleration and atmospheric pressure; the data processing module is responsible for controlling processing operations, routing protocols, synchronous positioning, and power consumption of the entire node. Management and task management; the data communication module is responsible for wireless communication with other nodes, exchanging control messages and transmitting and receiving data; the sensor used by the power management module strobe, the node power is composed of two 1.5V alkaline batteries, which will be adopted in the future. Miniature button battery to further reduce the volume.
The sensor node implementation mechanism designed in this paper replaces the traditional serial communication module with IEEE/ZigBee transmission module, and sends the collected information data wirelessly. The node also includes an IEEE/ZigBee wireless communication module, a microcontroller module, a sensor module and interface, a DC power module, and an external memory.
Sensor module selection for each nodeWith the release of the IEEE/ZigBee standard, the world's major wireless chip manufacturers have successively launched wireless transceiver chips that support this standard. Most of these chips integrate the physical layer functions of the standard and can be used as communication modules for sensor nodes. The MAC layer function is implemented by using a microcontroller as a processing module.
·Wireless transceiver chip selection
The choice of wireless transceiver chip mainly considers the following factors:
1 Band: IEEE 802.14.5 defines two operating frequencies. In general, high frequencies provide high data rates, but high antenna requirements, which also means more energy is required. All countries have strict management and supervision of radio products. According to the regulations on domestic wireless spectrum management, only devices operating in the 2.4 GHz band can be selected.
2 Modulation: The large size, high density and narrow bandwidth of the wireless sensor network make it have serious internal communication interference. Therefore, WSN needs to implement a modulation and spread spectrum mechanism that is simple, has strong anti-interference ability, low power consumption, and low cost. Currently widely used include FSK and OQPSK. Among them, FSK has the advantages of simple equipment, convenient modulation and demodulation, and good anti-multipath delay performance.
3 Sleep current and wake-up time: The sensor is usually in sleep state, sleep wake-up time and sleep current are all indicators that must be considered. Table 1 lists the main indicators of several common transceiver chips. Considering the above factors, the RF chip suitable for domestic use is CC2420 and CC2430 operating in the 2.4GHz band.
Processor selection
The processor is the core of the sensor node. When selecting, it must meet several requirements such as small size, high integration, low power consumption, support for sleep mode, fast enough speed, and low cost. AVR microcontrollers have achieved an optimal balance between software/hardware overhead, speed, performance and cost, and are cost-effective microcontrollers. High-end ATMega series AVR microcontrollers, including ATMega8/16/32/64/128 models, integrated large-capacity memory (storage capacity is 8/16/32/64/128 KB respectively) and rich hardware Interface circuit with advanced RISC reduced instruction set structure.
·Sensor and power supply
The sensor should be selected according to actual needs, such as temperature, humidity, strength, acceleration, vibration and other sensors. The power supply uses a 5th battery.
Node reference design schematic
The circuit schematic of the sensor node reference design is shown in Figure 2. The CC2420 wireless transceiver chip is used as the transmission module, and the AVR Mega128 is used as the processor. Specific sensor devices are not included in the figure and can be added for specific applications. The physical layer protocol of IEEE 802.15.4 can be implemented by Mega128 and CC2420.
Figure 2 IEEE/ZigBee-based wireless sensor network node reference design circuit diagram
The circuit design mainly includes three key parts, namely the RF interface circuit, the processor interface circuit and the upper application interface circuit. The RF interface is the circuit between the RF pin of the CC2420 chip and the antenna. The CC2420's RF signal is differential. The optimal differential load is 115+j180Ω. The impedance matching circuit needs to be adjusted according to this value. This design uses a 50 ohm monopole antenna and the impedance matching circuit uses a BALUN. The balun circuit consists of low-cost inductors and capacitors (see Figure 2), including inductors L1, L2, and L3 and capacitors C3, C4, C5, and C6. The inductors L1 and L2 also provide a DC bias for the low noise amplifier and power amplifier inside the chip.
ConclusionThis paper focuses on the advantages of WSN based on IEEE 802.15.4/ZigBee standard and its node design. The low cost, low power consumption and simple application protocol have provided the interconnection of wireless sensor networks and a large number of micro-control applications. International standards, based on unified standards between micro-sensors of different manufacturers can achieve interconnection networking. The competition between open products will eventually lead to mass production of sensors and reduce costs, thus providing a powerful opportunity to promote the application of wireless sensor networks and the development of related industries.
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