0 Introduction
Prisons have high requirements for the security, stability, and real-time performance of their security systems. Currently, the video surveillance, alarm, and access control systems used in prisons are all based on wired communication. This creates blind spots in areas where wiring is inconvenient, and the unchanging monitoring areas provide opportunities for criminals. To prevent inmate fights, self-harm, suicide, escapes, and attacks on police, and to promptly and accurately control and access information on prison violence and escape attempts, a three-pronged security system integrating human, physical, and technological defenses is proposed. This paper proposes a prison security system design based on wireless sensor networks. Using wireless instead of wired systems offers many advantages, such as reduced deployment difficulty, lower maintenance costs, and increased flexibility, especially in areas where wiring is impossible or inconvenient. As Helmut Macht, Chief Technology Officer of Siemens Building Technologies, stated, "The innovation of wireless communication will increasingly replace wired communication." However, due to the unique environment of prisons, adopting wireless technology within prisons faces many challenges. First, adopting wireless technology faces challenges such as multipath interference, transmission collisions, and obstacle reflections. These issues affect the reliability and scalability of wireless networks, as well as the data throughput of broadband communications. Second, since most existing monitoring systems are designed based on wired networks, their original high-level applications and protocols must be modified for use on wireless networks. Furthermore, cost is a significant factor for the widespread application of wireless technology in prison security systems; wireless communication modules must be suitable for the low-cost requirements of the monitoring system. The prison security system design based on wireless sensor networks studied in this paper does not simply replace wired with wireless. Instead, it builds upon existing wired prison security systems, retaining existing wired monitoring methods with high data transmission volumes, such as video and threshold monitoring, while incorporating static temperature and vibration-sensitive wireless sensor networks and dynamic real-time positioning wireless sensor networks. This improves the security system, eliminates monitoring blind spots, and ensures that the functions of each subsystem complement each other.
1. Wireless Network Technology for Prison Security Systems
A wireless sensor network (SNN) is a wired or wireless network composed of a group of sensors arranged in an ad hoc manner. Its purpose is to collaboratively sense, collect, and process information about objects within the geographic area covered by the sensor network and transmit it to observers. This type of sensor network integrates sensor technology, embedded computer technology, and wireless communication technology. It can collaboratively sense, monitor, and collect information about objects in various environments. Through collaborative information processing, accurate information about the sensed objects is obtained, and then transmitted to users who need this information via ad hoc methods. Sensors used in sensor networks differ from sensors in the general sense. Besides sensing changes in the measured physical quantity and outputting corresponding change information, they must also have remote communication capabilities; they must be intelligent sensors. Furthermore, to complete the communication function, in addition to sensing elements, they must also have a power supply, an embedded microprocessor, memory, and data transmission and communication components. Figure 1 shows a typical sensor network structure, which includes sensing nodes (sensors), observation nodes (sinks), and base stations.
In sensor networks, nodes are deployed in large numbers inside or near the objects being sensed, through methods such as aerial deployment and manual placement. These nodes form a wireless network through self-organization, collaboratively sensing, collecting, and processing specific information within the network's coverage area. This enables the collection, processing, and analysis of information from any location at any time. This self-organizing network transmits data back to the sink node (relay node) via multi-hop relays. Finally, the sink link transmits data from the entire area to a remote control center for centralized processing.
The basic building block of a wireless sensor network is the node. While sensor node designs vary across different applications, their basic structure remains the same. A typical node hardware structure, as shown in Figure 2, mainly includes the following units: a sensing unit (composed of a sensor and an analog-to-digital converter module), a processing unit (consisting of an embedded system, including a microprocessor, memory, signal conditioning circuitry, etc.), a communication unit (composed of a wireless radio frequency module), and a power supply section. In addition, other optional functional units include: a positioning system, a mobility system, and a self-powered system.
2. On-site execution layer design scheme
2.1 Static Monitoring System Design Scheme
Wireless sensor network nodes are deployed in prison cells, production areas, high points, solitary confinement cells, teaching areas, and medical rooms to achieve comprehensive coverage of the prison. The introduction of wireless sensor network technology into prison security systems is primarily due to the large number of sensor nodes in prisons, the extensive wiring work, and the fact that many areas are unsuitable for wired connections. Wireless access greatly facilitates sensor deployment, allowing for easy addition or relocation of sensor nodes. By periodically or irregularly changing the positions of monitoring sensor nodes, blind spots in existing wired monitoring systems can be eliminated, enabling target localization through sensor arrays. Furthermore, because the monitoring node positions frequently change, inmates cannot find monitoring loopholes, thus further enhancing the security system.
2.2 Design Scheme of Dynamic Real-time Monitoring System
By attaching nodes to prisoners, the system uses a positioning mechanism to monitor their activities in real time. However, due to the large number of prisoners, only two types of data need to be transmitted: first, when a prisoner's activity range exceeds the prescribed area; and second, when a node fails due to external force or energy depletion, the remote center must detect the missing node. Data acquired by the sensor nodes can be processed locally, then aggregated and transmitted to a base station (PC or PDA). The prison's remote server and monitoring system analyze the indicators of each monitored prisoner in real time, and an expert system assesses the prisoner's real-time condition. If any abnormality or danger is detected, an alarm is triggered and rapid action is taken. Simultaneously, the system can provide other auxiliary functions, such as prisoner location tracking. While the sensor network provides real-time monitoring of necessary vibration and position indicators, it also allows prisoners to move freely within a certain range, which helps maintain a positive mood and is beneficial for their rehabilitation. Since each cell is equipped with corresponding sensors, prison guards can also comprehensively monitor the prisoners' rest periods. When prisoners are in their cells or outside the cells (and sensor networks are required to be deployed outside the wards), prison guards can still locate and track them and obtain their vibration and location parameters in a timely manner.
The nodes are worn by prison guards. Using vibration sensors and positioning systems mounted on the nodes, the work area of the guards can be monitored in real time, ensuring their personal safety. On the one hand, in the event of an emergency, such as an inmate attacking a guard or a fight between inmates, the monitoring system and remote service center can react quickly, notifying police forces to arrive at the scene for support. On the other hand, by tracking the location of prison guards, their work status can be monitored, which helps improve work efficiency.
3. System hardware and software design scheme
3.1 Design of Sensor Nodes
Each sensor node consists of one or more sensor elements, a motor, and a battery pack. The sensors primarily detect vibration and temperature data, while the motor processes this data and transmits it to the base station via the network.
The radio frequency (RF) uses the nRF9E5, a system-on-a-chip (SoC) from the Norwegian company Nordic. It employs GFSK modulation, providing strong anti-interference capabilities; supports multi-point communication with a data transmission rate up to 0.1 MB/s; and features a unique Shock Burst signal transmission mode and transmit signal carrier monitoring function, effectively reducing power consumption and avoiding data collisions. Internal registers provide users with basic communication protocols, facilitating expansion and making it well-suited for wireless data transmission system designs. The nRF9E5 transceiver has three operating modes: Shock Burst receive (RX) mode, Shock Burst transmit (TX) mode, and idle mode. The operating mode of the nRF9E5 transceiver is determined by the special function registers TRX_CE and TX_EN.
The DS181320 digital vibration temperature sensor, developed by Dallas Semiconductor, is a miniaturized, low-power programmable single-bus digital vibration temperature sensor that directly converts measured temperature values into digital signal output. The DS181320 operates according to a strict single-bus protocol: initialization, followed by sending ROM commands, and finally, sending function commands. Initialization involves the host sending a reset pulse (achieved by pulling the bus low for at least 480μs), and the host waiting for a presence pulse from the DS181320. The DS181320 waits 15–16μs after detecting the rising edge of the reset pulse, and then sends the presence pulse by pulling the single-bus low for 60–240μs. Read and write operations on the DS181320 are implemented through read/write timing sequences. Therefore, using the DS181320 simplifies the circuit, improves node integration, and enhances the accuracy of vibration temperature measurement.
3.2 Software Design
This system uses a one-to-many communication mode, so the communication protocol is divided into three layers: the first layer is the physical layer, which is implemented by the nRF9E5 module hardware; the second layer is the network layer; and the third layer is the application layer.
The network layer employs the TEEN (threshold sensitive energy efficient sensor network protocol). This protocol is a responsive sensor network protocol, transmitting data only when the observed variable undergoes a sudden change. Since sensor nodes have limited energy and are difficult to replenish, using this protocol effectively conserves energy and extends network lifetime. Furthermore, in prison security systems, data transmission is only necessary when vibration and temperature data undergo sudden changes; continuously sending monitoring data to the sink node at a constant rate is unnecessary for prison security systems. Figures 3 and 4 are the software flowcharts for the host module and node module, respectively.
In practical applications, TEEN defines two threshold values, hard and soft, to determine whether monitoring data needs to be sent. The hard threshold sets the threshold so that when temperature and vibration exceed it, the node sends monitoring data to the sink node; the soft threshold sets the threshold so that when vibration and temperature changes exceed it, data is sent.
4. Simulation and Result Analysis
To verify the feasibility and effectiveness of the algorithm, we will temporarily disregard the impact of factors such as wind speed, wind direction, temperature, humidity, and noise on positioning accuracy, and assume that the speed of sound in the air is 340 m/s.
As shown in Figure 5, assume that three sensor nodes are distributed in a 100 m × 100 m area. In the figure, o represents the actual position of the target, * represents the position of the sensor node, and + represents the calculated position of the target.
Target location can be achieved using a statically distributed sensor array. Simulation results show that the error between the target location calculated by the target location system and the actual target location is within an acceptable range. In the event of an emergency, police forces can quickly and effectively take action based on the calculation results.
In a 100m × 100m square simulation area, a certain number of nodes (10, 20, ..., 100) are randomly and uniformly distributed, each node having the same communication distance of 10m. There are 5 cluster head nodes. Figure 6 shows the positioning error of the DV-Hop positioning algorithm with randomly selected anchor nodes. Simulation results show that the positioning error decreases as the number of sensor nodes increases. Therefore, in practical applications, the number of sensor nodes in the dynamic monitoring system can be appropriately increased, taking into account cost considerations.
5. Conclusion
The fundamental purpose of prison security systems is to bring greater reliability to management. The development of wireless network technology has brought broader possibilities for the integration of prison security systems, not only overcoming the inconvenience caused by cable limitations during the integration process, but also enabling many new applications. The wireless sensor networking method and the integration framework of wireless sensors and control networks presented in this paper enable the effective application of wireless network technology in prison security systems.
