1GPRS Technology Introduction
GPRS (General Packet Radio Service) is a wireless packet switching technology based on the second-generation mobile communication system GSM. It is particularly suitable for intermittent, bursty, or frequent, small-volume data transmissions, and also for occasional large-volume transmissions. GPRS can achieve a maximum transmission rate of 171.2 kbps, with an average rate of 53.6 kbps in practical applications. GPRS provides connectivity between mobile users and data networks, offering mobile users a high-speed wireless interface and X.25 services. GPRS uses packet switching technology, allowing each user to simultaneously occupy multiple wireless channels, and multiple users can share the same wireless channel, thus effectively utilizing resources. Users are always online, and billing is based on data usage, reducing service costs.
Using GPRS for data transmission has the following advantages:
① Wide access range. GPRS is an upgrade of the existing GSM network, which can make full use of the nationwide telecommunications network and provide convenient, fast and low-cost deployment of remote network access for user data terminals.
② High transmission rate. The theoretical maximum speed can reach 171.2 kbps, which is more than ten times the speed of circuit-switched data services in the current GSM network. The speed of the next-generation GPRS service can even reach 384 kbps, which can fully meet the needs of user applications.
③ Quick login. GPRS access has a short waiting time and can quickly establish a connection, with an average time of 2 seconds.
④ Always online, providing real-time online functionality. "Real-time online" or "always online" means that users are always connected to the network. Even when there is no data transmission, the terminal remains connected to the network, making accessing services very simple and fast.
⑤ Pay-as-you-go billing. Users only use wireless resources while sending or receiving data, and are charged based on the number of data packets they send and receive. Users are not charged even if they are online when there is no data usage.
⑥ Seamless switching. Data transmission does not affect voice signal reception. There are two switching modes for data and voice services: automatic and manual, depending on the terminal.
2. System Overall Structure
Based on the design requirements and functions to be implemented of the street light remote monitoring and control system, the system is roughly divided into a three-layer network structure: a central control room, a centralized controller, and street light controllers. The overall structure of the system is shown in Figure 1.
The first-level central control room houses a PC server responsible for monitoring all streetlights in the city. The second-level centralized controller manages all streetlights on a single street. The third-level streetlight controller manages all lights on the same pole. A GPRS wireless communication network is used between the first and second levels; although the distance between these two levels is large, the communication cost is high due to the small number of communication participants. The second and third levels utilize narrowband power line carrier communication technology, transmitting signals using existing power lines without the need for additional cabling, resulting in virtually no operating costs. This is particularly suitable for situations with many communication targets.
In addition, this design also features power metering functions. Voltage and current transformers collect real-time power parameters of streetlights in each section, analyze and store the collected data, or transmit the operating parameters (including voltage, current, and switching quantities) of each section back to the monitoring center through inspections. The monitoring terminal can automatically detect emergencies such as tripping, circuit breaking, voltage abnormalities, power supply failures, and abnormal light control, and promptly upload alarm data to the monitoring center so that on-duty personnel can understand the situation and take timely action. The GPRS communication network serves as the data transmission channel between the monitoring center and the wireless data acquisition and monitoring terminal. A fixed IP address is used to actively and promptly upload the collected operating parameters to the monitoring center via the GPRS network.
3 System Hardware Design
3.1 GPRS Transmitter Module Circuit Design
The GPRS module primarily enables wireless internet access. Several mature products are available on the market, such as the Sony/Eircsson M47c and the Simmons MC35. Here, we choose the Cello CMS91, a dual-band GSM/GPRS Class 10 module. Its main advantages include low power consumption, simple interface, comprehensive AT command functionality, support for GPRS Class 10, multimedia application development, and lower price. It also provides SMS (Short Message Service) and voice functions. The GPRS module provides an RS232 interface, which can be used to control the module, such as dialing and switching modes. Once connected to the Internet, the collected data can be transmitted wirelessly to any host with a public IP address using TCP/IP, thus achieving wireless data transmission. Figure 2 shows the circuit diagram of the GPRS transmitting module composed of the CMS91.
In this design, the CMS91 module functions as a wireless modem user's application system. It needs to connect to the operator's Internet access server via PPP (LCP/PAP/IPCP) before communicating using TCP/IP/UDP or higher-level application programs (such as HTTP, FTP, etc.). This module integrates an antenna receiver module, which requires a SIM card slot for actual use. The GPRS terminal communicates with the device via an RS232 interface. The MAX232 level conversion chip is used to convert the microprocessor's TTL level to RS232 level. The MAX232 meets the requirements of TIA/EIA-232-F and 1TUv.28 standards, operates at 3–5.5V, has one driver and one receiver, and a maximum data rate of 250kbps. This chip features electrostatic discharge protection and automatic disconnection.
3.2 Power Line Carrier Module Design
The power line interface module consists of a line driver and a line interface. Its main functions are:
① In transmit mode, it is used to amplify and filter the transmission signal (AT0) sent by ST7537;
② In receive mode, the receive signal is provided to the ST7537's receiver port from the power line;
③ It has protection circuits to resist spike pulses and overloads.
The block diagram of the power line interface module is shown in Figure 3. The line driver amplifies the output signal (AT0) of the ST7537. A line interface is used to make the line driver compatible with power lines. A transformer is used in the line interface, and its function is as follows:
Isolate other circuits from the power lines;
Send the transmission signal to the power line;
Extract the received signal from the power line;
Filter out harmonics in the transmitted signal.
The circuit schematic of the power line interface module is shown in Figure 4.
The composite transistors Q1, Q2, Q3, and Q4 form a push-pull amplifier. Resistors R1 and R2 help the amplifier achieve optimal performance.
In (receive mode), the ST7537 outputs signals PABC=1 and
This turns off bipolar transistors Q1 and Q5, cutting off the power supply to the power amplifier, and the power amplifier stops working.
The transformer consists of one main winding and two auxiliary windings. The winding ratio is 4:1:1, with the following parameters: main winding 9.4 μH, auxiliary winding 140 μH, C1 = 2.2 nF. To prevent nonlinear distortion, C2 must have excellent linearity. C3 filters out 50/60 Hz signals from the power line and has short-circuit protection. When the phase is unknown, an additional capacitor C4 is added to C3 to form a discharge circuit, preventing the risk of electric shock.
To prevent damage to the circuit from spike signals, a bidirectional Zener diode is used. When the voltage value is greater than or equal to the Zener diode voltage, the Zener diode will be shorted to ground, protecting the grounding components of the interface circuit from being burned out.
In addition, the system uses the Dallas Semiconductor DS1302 trickle-charge clock chip. This chip is a programmable I2C serial interface clock chip and also provides 31 bytes of non-volatile SRAM for data storage. Its advantages include a simple circuit structure, the ability to use any I/O port of the microcontroller as SCL and SDA signal lines, easy programming, and low cost.
4 System Software Design
The system primarily uses a CMS91 wireless modem for remote transmission of historical data, real-time data, and report information. The CMS91 is configured for internet access and data transmission via AT commands from a microcontroller. Upon receiving a correct response from the CMS91, a physical channel is established between the CMS91 and the GPRS network. The microcontroller controls the modem's operation by sending different AT commands to it.
After the CMS91 is powered on, the application needs to operate the CMS91's ON/OFF control bit through the P0 port. The CMS91's formal startup process takes approximately 3-5 seconds. If a valid SIM card is connected to the CMS91, it will be attached to the GPRS network. Serial port read/write operations on the CMS91 are still implemented by the interrupt service routine. After power-on reset, the program first sets parameters such as the operating frequency, and then performs dialing and PPP negotiation. After successful PPP negotiation, the system obtains its local IP address. Once it obtains its own IP address, the system is effectively connected to the Internet. However, to communicate with another IP terminal connected to the Internet, an end-to-end TCP connection is required. After a successful TCP connection, the entire program will maintain this connection state. Once in the TCP connection state, it may receive data from another IP terminal connected via TCP. After layer-by-layer unpacking and processing, various application layer data above the TCP layer can be obtained. To send data to the other party, an interrupt request must be made first, and data can only be sent after the TCP connection is established. This part of the TCP/IP protocol processing is handled by the microcontroller embedded in the CMS91.
The flow chart of the GPRS module's sending and receiving subroutines is shown in Figure 5.
5. Conclusion
This paper presents a remote street light monitoring system based on GPRS and PLC. Compared to traditional clock and photoelectric control street lights, this system effectively monitors street light circuits, enabling remote control, telemetry, and remote signaling functions, and operates stably and reliably. The design utilizes GPRS and PLC for communication, eliminating the need for re-laying cables and building a new communication network, resulting in very low operating costs and significant application and promotion value.
