Research on Urban Traffic Control Data Communication System Based on GPRS

The data communication system is an important component of the urban traffic control system. Through data collection, transmission and management technology, it enables the monitoring center to accurately obtain the traffic flow and the operating status of traffic signals at each intersection of the road network, thereby creating conditions for ensuring the control effect of the road network. Therefore, how to quickly, accurately and in real time upload the traffic flow data and the operating status of traffic signals at each intersection to the monitoring center and how to transmit control instructions from the monitoring center to the signal has become an important link in the efficient operation of the entire control system. At present, the existing urban traffic control system network in my country mainly uses wired methods such as telephone or dedicated cables [1] for data communication, which can no longer meet the real-time and mobility requirements of data communication of intelligent transportation systems: ITS [2] and various intelligent vehicle-mounted mobile terminals. General Packet Radio Service (GPRS) is a high-speed, efficient and economical wireless system with the advantages of wide network coverage, fast access, traffic-based billing, real-time online and no wiring [3]. It is particularly suitable for intermittent, sudden or frequent, small-volume data transmission, as well as short-term sudden large-volume communication, and fully meets the needs of distributed urban traffic real-time, multi-point traffic signal data collection and monitoring for two-way data communication. This paper adopts the 32-bit ARM7TDMI core ARM microprocessor S3C44BOX and GPRS module GR47 to design an intelligent traffic signal with GPRS function, and realizes the urban traffic control data communication system based on GRPS network, which basically overcomes the shortcomings of traditional wired communication methods such as high cost, small coverage and poor scalability.

1 System composition and working principle

The urban traffic control data transmission system based on GPRS network is a distributed, centralized, networked, and fully open monitoring system, which consists of a monitoring center, a GPRS/Internet communication network, and intelligent traffic signals at intersections with GPRS functions, as shown in Figure 1.

The entire system operates in a client/server mode. The communication server, database server, and monitoring workstation are interconnected through Ethernet in the monitoring center. Under the control of the software system, the monitoring center receives and processes various data such as traffic volume information, current working status, and signal timing scheme from the intelligent traffic signal machines at each intersection in real time [4], and displays the signal machine location, operating status, and related information on the LED multimedia display screen and the central monitoring terminal of the monitoring center. At the same time, the communication server of the monitoring center can also transmit various control instructions and optimized signal timing parameters to the signal machines at each intersection, realizing the monitoring and control of the entire distributed intelligent traffic signal machine status. The communication server can be a computer connected to the Internet to complete the TCP/UDP link establishment, maintenance, data reception, storage, and image monitoring.

The traffic signal at the intersection is a fully automatic intelligent device. It can start the GPRS dial-up program to access the Internet through text messages, data detection, ringing, preset time and other methods, actively establish a TCP or UDP connection with the communication server of the monitoring center, and transmit data or images to the communication server to realize data transmission of urban traffic signals.

2 Hardware composition of intelligent traffic signal

The intelligent traffic signal with GPRS function is mainly composed of ARM microprocessor module, GRPS wireless communication module, storage module, power crystal oscillator module, traffic signal and JTAG debugging interface modules. Its structural block diagram is shown in Figure 2.

2.1 ARM Microprocessor Module

The ARM microprocessor module is the control core of the intelligent traffic signal, responsible for data collection, data storage and processing, GPRS communication, etc. According to the low power consumption requirement of the intelligent traffic signal, Samsung's S3C44BOX[5] was selected. It is a 32-bit high-speed processor based on the ARM7TDMI core reduced instruction set, operating at 66MHz, supporting Thumb (16-bit)/ARM (32-bit) dual instruction set, and can be well compatible with 8-bit/16-bit devices. It has the characteristics of high cost performance, low power consumption, small size, and high reliability. S3C44BOX provides direct support for two serial ports Uart0 and Uart1. In this system, one serial port is used to communicate with the GR47 module. At the same time, when debugging the application, another serial port needs to be connected to the PC to ensure the normal operation of the program.

2.2 GPRS wireless communication module

The GPRS wireless communication module is the communication basis of the entire system. It uses the programmable wireless communication module GR47[6] from Sony-Ericsson. It provides an RS232 port and can be controlled by AT commands. GR47 has two wireless data transmission channels: GPRS and SMS. It can quickly, safely and reliably realize data transmission, voice transmission and short message service. It can work in two frequency bands: 900MHz and 1800MHz. When the GPRS network is in good condition, the location, traffic flow and operation status of the intelligent traffic signal are transmitted to the monitoring center through GPRS communication and the instructions from the monitoring center are received. At the same time, GR47 continuously checks the GPRS network status. When the network is congested, GPRS is not covered or the TCP/IP connection is disconnected due to the erroneous operation of the center staff, it will immediately switch to SMS mode until TCP/IP is reconnected.

2.3 Storage Module

FLASH memory stores system startup code, embedded operating system, file system, application code or other user data that needs to be protected after the system loses power. SST39VF160 with 16-bit data bandwidth from SST company is selected. It has a storage capacity of 2MB and an operating voltage of 2.7V to 3.6V. Its address lines A0 to A20 are connected to the address lines ADDR1 to ADDR2 of S3C44BOX respectively. SDRAM memory is the main area when the system is running. System and user data and stack are all located in SDRAM memory. Its storage speed is much higher than FLASH memory and it has read/write properties.

2.4 Power supply and crystal oscillator module

The power supply circuit consists of three DC-DC converters (9V~5V, 5V~3.3V, 5V~2.5V), which respectively supply power to the peripheral circuits, the CPU I/O and the S3C44BOX core. The 10MHz crystal oscillator provides the working clock for the system, which is multiplied to 66MHz by the on-chip PLL circuit as the working clock of the microprocessor.

2.5 Traffic Signals

Traffic signal is an important part of traffic control system. Under the coordination of ARM microprocessor S3C44BOX, it executes control instructions such as turning off lights, flashing, all red, manual stepping, etc. transmitted from the monitoring center. It can also perform single-point autonomous time period control of the signal without central control. When the signal is running, it can continuously and accurately collect a large amount of on-site traffic data, and transmit the on-site data to the monitoring center in real time. The monitoring center can not only directly use the traffic data for traffic control through the collection, storage and processing of traffic data, but also provide accurate and scientific quantitative basis for traffic command and dispatch and urban traffic planning. Intelligent traffic signal uses chip 16C550 to realize serial/parallel expansion of data. Its 8 data lines D0~D7 are connected to the corresponding D0~D7 of microprocessor S3C44BOX. Traffic signal realizes two-way high-speed data exchange with microprocessor through MAX232 standard serial port. [page]

3 Software Implementation

3.1 Intelligent traffic signal application software design

The intelligent traffic signal application software is mainly composed of the main program and GR47 wireless communication and other functional modules. The main program mainly completes system initialization (including timer initialization, serial port initialization, port initialization, etc.), traffic signal data reading and wireless communication with GR47. The GR47 wireless communication module mainly completes communication module initialization, packet data protocol PDP (Packet Data Protocol) context activation, TCP/IP connection establishment, GPRS data reading/sending, SMS reading/sending, GRPS network status detection and other processing. In the development process of the application software, C language and assembly language are used for mixed programming, and ARM ADS (ARM Developer Suite) integrated development tools are selected. After the source program is edited, cross-compiled, and cross-linked on the PC to generate the target image in ELF format, the target image is finally downloaded to the FLASH on the target development board through the JTAG interface. The software flow is shown in Figure 3.

GPRS data transmission is the core function of the GR47 module. Before making a TCP/IP connection through the GPRS network, you must first establish a data account inside the GR47 module and indicate the correct APN name of the access service provider. The intelligent traffic signal uses the GRPS communication server IP address solidified in the FLASH memory to access the communication server of the monitoring center, which can be achieved through the AT+CGDCONT command, for example: AT+CGDCONT=1, "IP", "CMNET".

3.2 GPRS Communication Server Software Design

The main function of the communication server of the monitoring center is to realize the communication between the intelligent traffic signal and the monitoring center (database server and monitoring workstation). The communication server provides the internal Ethernet interface for the monitoring center and monitors the communication status of the network, supporting TCP/IP protocol and PPP protocol. The communication server software adopts WinSocket programming, mainly creating three kinds of Socket classes, namely listening Socket (CListenSocket), data receiving and processing Socket class (CServerSocket) and LAN communication Socket class (CUDPSocket). CListSocket regularly monitors the corresponding port, receives the connection request of the intelligent traffic signal, creates a Socket thread and completes the establishment and maintenance of the TCP link, cooperates with ADO database technology to realize data reception, storage and monitoring functions, and then puts the original socket back to the listening state to continue listening. For each intelligent traffic signal, a receiving thread is required to handle the reception of data. Therefore, it is necessary to establish a dynamic table in the memory to maintain the corresponding relationship between the data receiving thread handle and the intelligent traffic signal. The generated new thread contains CServer-Socket, which is responsible for data reception, processing and sending. The entire communication server software structure is shown in Figure 4.

By installing the intelligent traffic signal on the experimental vehicle, the real-time communication and control between the GPRS network and the communication server are realized. The system runs stably and the system response time is ≤2s. This system adopts GPRS wireless access mode, designs intelligent traffic signal, realizes urban traffic control data communication, solves the problems of traditional wired communication mode in network coverage, scalability, real-time and reliability, reduces operating costs, and will have broad market application prospects.

References:

[1]. ARM7TDMI datasheet http://www.dzsc.com/datasheet/ARM7TDMI_139812.html.
[2]. GPRS datasheet http://www.dzsc.com/datasheet/GPRS_1594650.html.
[3]. GR47 datasheet http://www.dzsc.com/datasheet/GR47_1135738.html.
[4]. RS232 datasheet http://www.dzsc.com/datasheet/RS232_585128.html.
[5]. SST datasheet http://www.dzsc.com/datasheet/SST_1180824.html.
[6]. SST39VF160 datasheet http://www.dzsc.com/datasheet/SST39VF160_1045221.html.
[7]. A20 datasheet http://www.dzsc.com/datasheet/A20_1063905.html.
[8]. MAX232 datasheet http://www.dzsc.com/datasheet/MAX232_1074207.html.