1 Introduction Wireless sensor network is a brand-new information acquisition and processing technology, which has been increasingly widely used in real life. With the development of communication technology, embedded technology and sensor technology, sensors are gradually developing towards intelligence, miniaturization and wireless networking [1]. At present, the main research at home and abroad is on the design of low-power hardware platform topology control and network protocol, positioning technology, etc. This design takes the sensor that detects the intensity of ultrasonic waves as an example to realize a wireless sensor network. The parking space indicator light is turned on or off according to the intensity of the ultrasonic waves detected by the sensor, so as to determine whether a vehicle has entered the detection area. This sensor network integrates embedded technology, sensor technology and short-range wireless communication technology and has a wide range of applications. The system does not require any changes to the field structure, does not require any support from the original fixed network, can be quickly deployed, is easy to adjust, and has good maintainability and scalability. 2 IEEE 802.15.4 standard The IEEE 802.15.4 standard [2] is suitable for wireless personal area networks (WPANs) with low rate, low power consumption, low complexity and short distance data transmission. During wireless transmission within the network, a carrier sense multiple access mechanism (CSMA/CA) with collision avoidance is adopted, supporting superframe structure and time slot guarantee mechanism (GTS). The network topology can be a star network or a point-to-point peer network. The standard defines three data transmission frequencies, namely 868MHz, 915MHz, and 2.4GHz. The first two transmission frequencies adopt BPSK modulation, and the latter adopts 0-0PSK modulation. Each frequency supports wireless data transmission rates of 20 kbit/s, 40 kbit/s, and 250 kbit/s, respectively, with a transmission distance between 0m and 70m. The wireless transmission module with a frequency of 2.4GHz is used in this paper. 3 Implementation of Wireless Sensor Network 3.1 Network Platform Construction The wireless sensor network platform consists of three parts: an ultrasonic sensor module, a microprocessor module, and a wireless transmission module [3], as shown in Figure 1. The microprocessor module and the wireless transmission module are integrated on a single board, while the ultrasonic sensor module is connected to the microprocessor through an interface, so that different sensor modules can be used in various occasions. 3.1.1 Ultrasonic sensor module Because ultrasonic waves are highly directional and can travel a long distance in a medium, ultrasonic waves are often used for distance measurement, such as rangefinders and level measuring instruments, which can be realized by ultrasonic waves [4]. Ultrasonic detection is often fast, convenient, simple to calculate, easy to achieve real-time control, and can meet the requirements of industrial application in terms of measurement accuracy. In order for cars to automatically avoid obstacles, a ranging system must be equipped so that it can obtain distance information (distance and direction) to obstacles in a timely manner. The three-direction (front, left, right) ultrasonic ranging system introduced in this paper provides a motion distance information for the background staff to understand the environment in front, left and right [5]. Figure 1 Wireless sensor network node structure Figure 2 Wireless sensor network node communication topology The SL-SRF-25 ultrasonic sensor, when connected to a power source, can be used as an ultrasonic ranging device. The obstacle distance is displayed by a 3-digit LED digital tube. The 3-digit LED digital tube adopts a modular insertion method, which is convenient for debugging, inspection and use in different occasions. The measurement range is 10cm-250cm. When the distance is less than 100cm, the error is 1-2cm; when the distance is greater than 100cm, the error is 4-5cm. The SL-SRF-25 ultrasonic sensor can also be specified to output segmented distance detection signals from the microcontroller's I/O port. 3.1.2 Microprocessor Module The processor module uses the Mica2 model node from the University of California, Berkeley. The node board provides the following functions: a 433MHz center frequency wireless communication interface, which can be customized through programming to provide various communication powers from -20dB to 10dB; multiple transmission rates from 0.3kbps to 38.4kbps in Manchester encoding; and multiple communication frequencies can be set around 433MHz with a frequency interval of 76kHz. Its high speed and large-capacity RAM facilitate data packet processing. 3.1.3 Wireless Transmission Module The wireless transmission module uses the SRWF-501 low-power wireless module RF transceiver from Sunray Electronics Technology Co., Ltd. This chip requires very few external components, boasts stable performance, and extremely low power consumption. The transceiver provides three serial ports with three interface modes: COM1 is a TTL level UART interface, COM2 is a standard RS-232 interface, and a standard RS-485 interface. It features crystal frequency stabilization, a built-in digital phase-locked loop, and the frequency can be flexibly set within the range of 300-1000MHz according to user needs. Automatic noise filtering simplifies user interface programming, making it as convenient as wired connections. Automatic "receive" and "transmit" switching eliminates the need for dedicated transmit/receive control lines; it is in normal "receive" mode when not transmitting data and automatically switches to "transmit" mode when transmitting data, returning to "receive" mode after transmission is complete. It has a low transmit power: a maximum transmit power of 10mW. The SRWF-501's selectivity and sensitivity exceed the requirements of the IEEE 802.15.4 standard, ensuring the effectiveness and reliability of short-range communication. 3.2 The system software platform uses the TinyOS system development environment developed by the University of California, Berkeley. TinyDB is the query processing system of TinyOS, which can extract data and information from sensor nodes in wireless networks. TinyOS provides TinyDB with a visual JAVA API window for real-time querying. 3.3 Network Type In this paper, the wireless sensor network adopts a star topology (as shown in Figure 2), with a network coordinator as the central node, which can communicate with any ordinary node. Ordinary nodes contain ultrasonic sensors to measure and sample the ultrasonic signal intensity parameters in the surrounding environment, send the collected data to the central node, and can analyze and process the data and commands sent by the central node to complete the corresponding operations. If two ordinary nodes need to transmit data, they must go through the central node, which will then transmit the data to the corresponding node. 3.4 Network Process The wireless sensor network is a self-organizing network. If a fully functional node is activated, it may establish a network and set itself as the network coordinator. Other ordinary nodes can apply to join the network [6]. In this way, a wireless sensor network with a star topology can be built. The wireless sensor network in this paper supports a superframe structure. After energy scanning and active channel scanning, the network coordinator periodically sends beacon frames according to the set parameters. Ordinary nodes first undergo energy scanning and passive channel scanning to obtain parameters containing network characteristics in the beacon frame, such as beacon sequence number, superframe sequence number and network label. They synchronize with the network coordinator through a synchronization request and then associate with the network coordinator through a matching request. During the association process with the network coordinator, the network coordinator assigns a 16-bit short address to each ordinary node requesting association [7]. In this way, communication can be carried out using the short address in subsequent data transmission, improving communication efficiency, reducing energy consumption during transmission, and thus extending the lifespan of the network. 3.5 Data transmission mechanism 3.5.1 Data format The IEEE 802.15.4 standard defines four types of frames, namely beacon frames, data frames, command frames and acknowledgment frames [2]. (1) Beacon frame: used by the network coordinator to broadcast a beacon to its neighboring nodes in the first time slot supporting the superframe structure. When a nearby node receives the beacon frame, it can apply to join the network. Since the wireless sensor network system in this paper adopts a relatively simple star topology, the structure of the beacon frame is different from that of the IEEE 802.15.4 standard: the address field of the beacon frame only contains the network label and short address of the source node, and does not contain the destination node information (because it is sent by broadcast). (2) Data frame: used to transmit data containing ultrasonic intensity information. The address field contains the network label and short address of the source node and the destination node. Since there are two transmission directions for data frames: from ordinary nodes to the central node and from the central node to ordinary nodes. (3) Command frame: used to build wireless sensor networks, transmit synchronization data, etc. The format of the command frame is not much different from other types of frames. (4) Acknowledgment frame: used to confirm that the target node has successfully received the data frame or command frame. When the target node successfully receives the data frame or command frame, it sends an acknowledgment frame to the sender. The sender receiving this acknowledgment frame indicates that the transmission was successful. If no acknowledgment frame is received within the specified time, the data frame or command frame is retransmitted. The frame type is defined as acknowledgment frame in the frame control field. The sequence number of the acknowledgment frame must be the same as that of the acknowledgment frame, and the payload length must be zero. The acknowledgment frame is sent immediately after the acknowledgment frame, and there is no need to use the CSMA-CA mechanism to compete for the channel [8]. 3.5.2 Transmission process In the entire wireless sensor network, the ordinary nodes periodically read the ultrasonic data on their sensors and send the ultrasonic data to the central node. The central node processes the received data and then transmits it to the corresponding node to control the parking space position flag on it. First, the network coordinator checks the received data frame. The central node in Figure 2 determines whether it is sensor data of the designated node. If the received data is data on the designated node, the data is compared with an ultrasonic intensity threshold to set the control variable (used to control the opening and closing state of the parking space) [9]. Otherwise, no transmission operation is performed. Then, it is determined whether the idle node has joined the network. If an idle node is found in the network, the central node sends the control variable as the payload of the data frame to it. Otherwise, no data frame with control variable is sent. 4 Conclusion In the wireless sensor network parking control system we designed, ordinary nodes send the ultrasonic data they collect to the network coordinator. The network coordinator sends the data frame containing the control variables to the node with the parking space occupancy mark, and can also transmit the ultrasonic data to the computer through the serial port. The changes in the ultrasonic signal can be monitored through the background software on the computer. The occupancy status of the parking space can be determined from the ultrasonic sensor. This paper discusses the design and implementation of the wireless sensor network from several aspects such as the formulation of the wireless transmission protocol and the control of the transmission process. In practical applications, as long as the specific sensor is replaced, it can be applied to various sensor networks. Since the wireless sensor system is flexible in networking and adopts a modular design, it has good portability and scalability. With the improvement of people's living standards, this system has a broad application prospect in the future traffic monitoring field. It has a broad prospect in the future traffic monitoring field [10], smart home appliances, and intelligent adjustment of the home environment.