Abstract: This paper proposes to construct a cluster-tree wireless sensor network using Zigbee technology. First, the characteristics of wireless sensor networks are summarized; second, the design concept of the protocol for this type of wireless sensor network is elaborated; finally, the designed protocol is verified in a real hardware environment. Keywords: Wireless Sensor Networks; Zigbee; Self-health 1 Introduction Wireless sensor networks consist of multiple wireless network sensors. These sensors integrate sensor execution, controllers, and communication devices, forming resource-constrained embedded devices that combine sensing and drive control capabilities, computing power, and communication capabilities. Wireless sensor networks composed of these miniature sensors can monitor, sense, and collect information about various monitored objects within the network distribution area in real time, process this information, and transmit it to users who need it. Wireless sensor networks (WSNs) are characterized by self-organization, self-healing, and multi-hop capabilities, with nodes typically placed in fixed locations. Since some wireless sensors operate in harsh environments, the design of WSN protocols must fully consider energy efficiency and real-time data transmission. ZigBee is a short-range wireless communication technology with a unified technical standard, using the IEEE 802.15.4 protocol for its PHY and MAC layers. The WSN proposed in this paper operates on the globally available ISM (Industrial, Scientific and Medica) free band at 2.4 GHz, with a data transmission rate of 250 Kb/s, divided into 16 channels. Compared to other short-range wireless communication technologies such as Bluetooth or 802.11b, ZigBee has inherent advantages. ZigBee devices are low-power devices with energy detection and link quality indication capabilities. Furthermore, the use of a collision avoidance mechanism (CSMA-CA) prevents data transmission conflicts. In terms of network security, a 128-bit encryption algorithm is used to encrypt the transmitted data, ensuring high reliability and security during data transmission. Wireless sensor networks built using ZigBee technology are simple in structure, small in size, cost-effective, flexible in placement, easy to expand, low in cost, low in power consumption, and secure and reliable. This emerging wireless sensor network is bound to have broad application prospects. 2 Zigbee Wireless Sensor Networks Currently, Zigbee technology has already seen some applications in home networks, control networks, and mobile terminals abroad. However, existing Zigbee networks are limited to Zigbee Wireless Personal Area Network (WPN) topologies. The number of sensor nodes that each access point can accommodate is far less than the 255 nodes specified in the protocol. To achieve dense sensor network coverage, complex networking is necessary, which not only increases network complexity but also increases overall network power consumption and significantly reduces the lifespan of sensor nodes. This paper proposes a wireless sensor network with a clustered tree topology. The network diagram is shown in Figure 1: [align=center] Figure 1 Schematic diagram of a clustered sensor network[/align] In this network, nodes in adjacent areas form a cluster, each cluster has one and only one cluster head, and adjacent cluster heads cyclically form another cluster, thus repeating in this way to form a tree-structured sensor network. In this structure, the root node acts as the coordinator of the entire network and can be connected to a PC to receive data collected by the sensors and display and process the data. 3 Network Protocol Design 3.1 Self-organizing Wireless Sensor Networks are initially initiated and established by nodes of Full-Function Devices (FFDs). After the wireless sensor network is established, this initiating device acts as the coordinator of the entire network. This coordinator can be connected to a PC through a serial interface to process various received data and can also exchange data with other heterogeneous networks. The process of a node spontaneously establishing a network is as follows: First, the FFD node performs channel energy detection (ED) and selects the channel with the lowest detected energy peak as the data transmission channel for the wireless sensor network to be established. Then, it sends a cross-network beacon request frame on this channel to obtain information parameters of other wireless sensor networks within its operating range. After receiving the beacon frame, it selects an unused network label. Finally, based on the determined network channel number, network label, and other relevant parameters, it sets the values ​​of the relevant registers in the hardware. Thus, the network coordinator in the wireless sensor network is formed. Figure 2 is a schematic diagram of the device's spontaneous network establishment. [align=center] Figure 2 Schematic diagram of the device's spontaneous network establishment[/align] When a node wants to join an established wireless sensor network, it first presets the network label and the channel to be used, then sends an intra-network beacon request broadcast frame. After receiving multiple beacon frames with link quality signal parameters, it selects a node with better link quality and more remaining energy to connect to, and sends an entry request command frame to the corresponding coordinator. After the coordinator allows the connection, it assigns a short intra-network address to the node. Each node has a neighbor table, which is dynamically maintained. This neighbor table contains the address of a parent node (except the root node) and multiple child node addresses (except leaf nodes). By repeating this process, all nodes can self-organize into a clustered tree-like wireless sensor network. Figure 3 shows a diagram of a node joining the network handshake: [align=center] Figure 3 Node Joining Handshake Figure 4 Node Leaving Handshake[/align] Similarly, if a node wants to leave the network, it only needs to send a request command frame to its parent node. Upon receiving the request, the parent node will perform the corresponding operation and send a response frame. Figure 4 shows a diagram of a node leaving the network handshake. 3.2 Self-Healing and Self-Energy Saving of the Network In addition to the self-organizing capability of nodes, wireless sensor networks also possess self-healing and self-energy saving characteristics. When a node becomes detached from the sensor network due to environmental factors or changes in the original sensor network parameters, it can send an orphan request frame to the coordinator. Upon receiving the request frame, the coordinator determines if the node is one of its original child nodes. After making this determination, it sends a response frame to the node to determine whether to accept it as a child node again. Figure 5 illustrates the handshake process for an orphan request. [align=center]Figure 5 Handshake diagram of a node orphan request[/align] Because the coordinator in a wireless sensor network has multi-hop functionality, the node acting as the coordinator incurs additional energy overhead for forwarding received data. Therefore, we set a minimum energy limit and have the node periodically check its remaining energy. When the node's remaining energy falls below this limit, the coordinator sends an out-of-network command frame to all its child nodes. Subsequently, each child node performs the relevant in-network operations, detaches from its original parent node, and attaches itself to the new coordinator node. At this point, the original coordinator node becomes a leaf node, no longer responsible for data forwarding, thus reducing energy consumption, increasing the node's lifespan, and consequently extending the overall lifespan of the wireless sensor network. 3.3 Frame Formation and Forwarding Data acquired by each node through the sensor is processed into frames, which are then sent to their parent node. This process repeats until the network coordinator receives the data and hands it over to the PC for processing. The Zigbee protocol defines four types of frames: command frames, data frames, beacon frames, and acknowledgment frames. The format of a common frame is shown in Table 1: Table 1 Common Frame Format The frame control field mainly includes the frame type and source and destination address modes. 4. Conclusion In the test, we used three wireless sensor nodes to build a peer-to-peer network. One node was connected to the PC via a serial port, acting as the network coordinator, through which it could send the collected data to the PC. The wireless sensor nodes are primarily based on the Philips P89LPC932 microcontroller, with the Ubec UZ2400 wireless data transceiver chip based on the Zigbee protocol. The node hardware overview is shown below. Normally, the nodes are in a sleep state, activating to receive data upon interrupt request. [align=center]Figure 6 Wireless Sensor Node Hardware Overview[/align] Preliminary experimental results show that after the PC sends a self-organizing network command to the network coordinator, the other two wireless sensor nodes can successfully join the network, and data can be sent and received normally between the nodes. Simultaneously, the network coordinator can process its own collected data or data transmitted from other sensors through the PC. 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