Design of wireless vehicle management and dispatching system based on GPRS/WLAN/GPS technology

    Preface

    With the rapid increase in the number of vehicles, efficient vehicle management and dispatching systems have become a tool urgently needed by various vehicle management and dispatching departments. Wireless communication technologies represented by  GPRS (General Packet Radio Service) and WLAN (Wireless Local Area Network), wireless positioning technologies represented by GPS (Global Positioning System), and increasingly mature embedded system hardware and software have provided a technical foundation for advanced wireless vehicle management and dispatching systems.

    In the vehicle management and dispatch system implemented in this paper, the vehicle terminal is an embedded system based on the Intel XScale processor, and the external WLAN and GPRS realize wireless communication with the management and dispatch center; the external GPS module performs real-time positioning. The management and dispatch center is a software system running on a PC with a friendly human-machine interface. The PC expands the WLAN and GPRS modules, so that the current location of the vehicle in the system can be obtained in real time and dispatched. At the same time, the system reserves a rich space for functional expansion, which can be used for secondary development.

    1 System overall scheme demonstration and selection

    In the overall scheme of this system, wireless communication technology, wireless positioning technology and embedded system hardware and software technology are the three key elements for the realization of the vehicle dispatching system. The following introduces the main alternative technologies in these three aspects and selects the most suitable technical solution through demonstration.

    In terms of wireless communication, both GPRS and WLAN have their own advantages and disadvantages. The main advantage of CPRS is that it has a wide coverage area and can support voice transmission; its disadvantage is mainly that the data transmission speed is slow, and its usage fee will be proportional to the data flow. The advantages and disadvantages of WLAN show complementary characteristics with GPRS. Its main advantage is that it can provide a higher data transmission rate and the usage fee is very low. Once the WLAN network is set up, except for a little maintenance work, there is almost no other cost involved; its disadvantage is that it does not support voice services yet, and the coverage area is generally small, which cannot be compared with GPRS at present. Based on the above analysis, from the perspective of optimizing the function of this system and facilitating use, it is decided to add both to the system. At the same time, considering the differences in data rate and usage fee between GPRS and WLAN, when the vehicle is in the WLAN coverage area, the management and dispatching center will give priority to communicating with the vehicle through the WLAN network; when the vehicle is outside the WLAN coverage area but in the GPRS coverage area, it will communicate with the vehicle through GPRS.

    In terms of wireless positioning systems, there are currently many options, including the US GPS, Russia's CLONASS, and my country's independently developed "Beidou Satellite Positioning and Navigation System". Among them, GPS has obvious advantages in performance, coverage, positioning accuracy, etc., and is currently the most widely used. Therefore, it was decided to use GPS as the wireless positioning module of this system.

    There are many options for embedded system hardware and software. The following discussion is divided into two parts: hardware and software.

    In terms of hardware, the choice of embedded CPU is very important. Currently, the most widely used embedded CPUs are: Intel's XScale series, mainly used in handheld terminals; SAMSUNG's S3C24XX series, mainly used in consumer electronics; FreeScale's PowerPc and ColdFire series, mainly used in POS machines and industrial control; Atmel's AT91 series, mainly used in industrial control, etc. According to the various peripheral modules such as WLAN that need to be expanded in this system and the application requirements that may need to process multimedia services, Intel's XScalePXA255@400 MHz is selected as the CPU of the vehicle-mounted equipment. This CPU has a main frequency of 400MHz and is widely used in personal handheld terminals produced by internationally renowned manufacturers such as HP and Dell. It has the characteristics of excellent performance and stable operation. Based on the selection of the CPU, 64 MB SDRAM, 16 MB NOR Flash memory and 128 MB NAND Flash memory are selected to form the core embedded system together with the CPU.

    In terms of embedded system software, the first thing is the choice of operating system (OS). The main alternatives are WinCE.Net and embedded Linux. Among them, WinCE.Net has a user interface and operation method that is more similar to Windows, which makes actual use more convenient. At the same time, since the vehicle terminal of this system needs to allow end users to customize and develop application software by themselves, it is also important to have a convenient software development platform. In this regard, WinCE.Net also has obvious advantages, so WinCE.Net is selected as the OS of the vehicle equipment.

    In addition to the above three aspects of demonstration, there is also the issue of the management and dispatch center. In order to facilitate the implementation, the management and dispatch center uses a high-performance PC as a server, accesses the GPRS network and WLAN network through expansion modules, and runs the management and dispatch software at the same time to complete the management and dispatch work.

    Based on the above scheme demonstration and selection, we can obtain the system scheme block diagram shown in Figure 1.

    2 System Hardware Design

    As mentioned above, the management and dispatching center plans to use PC as the hardware platform. Therefore, the hardware design work of this system is mainly focused on the vehicle-mounted terminal. According to the above scheme demonstration and selection, the vehicle-mounted terminal hardware block diagram shown in Figure 2 can be obtained.

    In the expansion module, the GPS module uses the JP7 module designed and produced by FALCOM. It is a 12-channel GPS receiver with an ultra-small volume of 25.4 mm×25.4 mm x3 mm. It supports 3D/2D/differential positioning, and the maximum positioning error in 3D positioning mode is 10 m. In this system, this module is connected to the core embedded system through a two-wire serial port, so that the vehicle-mounted equipment can obtain accurate position and time information in real time.

    The CPRS module is SIEMENS MC35i, which is a full-featured GSM/GPRS module that supports dual-band EGSM900 and GSM1800, supports GPRS Class 8, and has a data rate of up to 85.6 kbit/s. In this system, this module is connected to the core embedded system through an 8-wire serial port, and all operations are controlled by the core system through AT commands. It can be used as a cellular phone that can make calls and send and receive text messages, and can also be used as a wireless modem when it needs to access the GPRS network.

    The choice of WLAN module is mainly due to the fact that there are various WLAN network card products with stable performance and diverse interfaces on the market, and the PXA255 CPU we selected can easily expand the PCMCIA interface, so we decided to use a WLAN network card with a PCMCIA interface. The network card model selected in this system is Cisco LMC352, mainly because this network card has good performance and also supports external dual antennas, so that the antenna can be installed outside the vehicle terminal or even on the car shell, thereby reducing the shielding effect of the car shell on the WLAN signal.

    In addition to the above-mentioned expansion modules, a USBDevICe interface is also introduced from the CPU for software debugging; two RS-232 serial ports are expanded for connecting other serial devices in the future; one VGA interface and two PS-2 interfaces are expanded, so that a display, keyboard and mouse can be used on the vehicle terminal to realize visual operation of the vehicle terminal, which is also convenient for debugging and updating software.

    3 System software design

    3.1 Management and dispatch center software

    The software flow of the management and dispatching center is shown in Figure 3.

    The management and dispatch center software runs on a PC or small server with extended GPRS and WLAN modules, which can access the GPRS and WLAN networks at any time. The control and data transmission after accessing the network are controlled by the management and dispatch software. The software exit path is not marked in Figure 3, because in actual use, users can close the software and exit the management and dispatch software system at any time.

    The development environment of this software is Visual Studio.NET 2003, using C language. The software first initializes the serial port connected to the GPRS module, and also detects whether the local extended WLAN module has found and connected to the nearby AP. Under normal circumstances, the initialization of both can be completed in a short time, and then enter the idle state. When the user has a dispatch requirement, it is necessary to first determine the ID (identifier) ​​of the Japanese standard vehicle. This ID can be either the unique IP address or license plate number of each vehicle terminal, or an easier-to-remember ID can be assigned to each vehicle artificially. Then it is determined whether the vehicle corresponding to this ID is in the area covered by the WLAN network. This is achieved by sending a query message to the vehicle terminal on the WLAN network. If the vehicle is in the WLAN network, the vehicle terminal immediately sends a message to the dispatch center to confirm that it is in the WLAN network. At this time, the software will give priority to using WLAN to communicate with the vehicle; if the vehicle is not in the WLAN network, it cannot receive the query information from the dispatch center, and therefore cannot reply to the dispatch center for a confirmation message. At this time, the dispatch center will use GPRS to communicate with the vehicle. The use of GPRS involves a mobile IP problem: each time the vehicle-mounted terminal dials up to connect to the GPRS network, its IP address is generally different, so it is impossible to use the method of binding the IP address in the WLAN network to the vehicle. At this time, the common solution is for the dispatch center to use SMS to first require the vehicle-mounted terminal to report its IP address, and then communicate. When the management dispatch center issues an instruction, the software waits for the vehicle-mounted terminal to reply with a confirmation message. Through the vehicle confirmation method, it can be ensured that the vehicle accurately receives the instructions issued by the management dispatch center, minimizing the probability of misoperation of the system.

    3.2 Vehicle Terminal Software

    The vehicle terminal software runs on an embedded vehicle terminal based on XScale PXA255, using WinCE.Net 4.2 as the operating system. By stripping away the specific business contents that are very different in different vehicle management and dispatching systems, we can get the vehicle terminal software flow chart shown in Figure 4. The software exit path is not marked in the figure, because in actual use, users can close the software and exit the vehicle terminal software system at any time.

    The vehicle-mounted application software of this system is developed in the Smart Client engineering mode of Visual Studio.NET, using C language. As shown in Figure 4, this software first performs initialization work, opens the ports where GPRS/WLAN/GPS are located, and starts power supply. For the GPRS module, it will automatically search for the GSM network after power-on. If the location is covered by the GSM network, it will automatically connect and stabilize in an idle state where it can make/receive calls and send and receive text messages at any time; if there is no GSM network coverage, it will search the network again at a fixed time. For the WLAN module, after power-on, it will be controlled by the driver to start searching for the WLAN network. If a suitable AP is found, it will automatically connect and set encrypted authentication information to prevent attacks; if it is not found, it will search the network again at a fixed time. For the GPS module, after cold start, input the control command at the default baud rate of 4800 bit/s to adjust the baud rate to 9600 bit/s. At the same time, the command controls the GPS to output positioning information at a fixed time interval in the subsequent work. The time interval set by this software is 1 s. Considering that the general positioning error of civil GPS in non-differential mode is 1 m to 15 m, this time accuracy can meet the requirements of most applications.

    After initialization, the software starts two working threads, one for processing GPS positioning information and the other for processing wireless network information. The thread that processes GPS information must first determine whether it has been effectively positioned. According to actual measurements, if the GPS module is used for the first time or has been placed for a long time, the internal backup power of the module has been exhausted, so it cannot provide the most recent position information as a reference for positioning after cold start. At this time, effective positioning requires 5 min to 10 min. In other cases, it takes about 10 s from cold start to effective positioning. After effective positioning, this thread receives the current position, speed, time and other information output by the GPS module through the serial port once per second, and performs corresponding processing and recording. The thread that processes wireless network information is mainly responsible for coordinating GPRS and WLAN networks so that the vehicle terminal can communicate with the management and dispatch center effectively and reliably. This thread first scans whether there is query information on a port agreed with the dispatch center in advance on the WLAN network. If there is, it returns the query confirmation information and waits for the actual instruction; after receiving the instruction, it first returns a confirmation message to the dispatch center, and then takes corresponding actions according to the instruction content. If no suitable AP is found to connect during the query stage, it queries whether the GPRS module has received the text message sent by the dispatch center. If not, it will return to the state of scanning WLAN network; if yes, it means that the dispatch center needs to communicate with this terminal through GPRS, then the vehicle terminal should dial up to connect to the GPRS network immediately and send the obtained IP address back to the dispatch center in the form of SMS. The subsequent communication process is similar to the WLAN network, and the vehicle terminal and the management dispatch center also interact through command information and confirmation information.

    The above software flow description does not involve specific business such as instruction content. This is because the specific business of each user and unit of the vehicle management and dispatching system will be very different. In view of such differences, this system provides a wealth of space for customization and modification, which can be targeted for secondary development of different specific businesses, and can also expand many practical functions. For example, the destination of this trip can be entered on the vehicle terminal. Through the interaction between the terminal and the management and dispatching center, relying on the powerful database and electronic map system of the center background, suggestions can be given for this trip route and intuitively displayed in the form of a map on the display device of the vehicle terminal. If the location information during the trip is recorded in the form of a file and uploaded to the PC of the management and dispatching center when appropriate, the function of trajectory playback can be realized in combination with the electronic map, the actual route of the vehicle can be reproduced, and the vehicle can be better managed. If voice communication is required, the voice can be transmitted through the GSM module to realize the function of the vehicle phone.

    4 Conclusion

    This paper discusses the design of a wireless vehicle management and dispatching system based on GPRS/WLAN/GPS technology, gives the overall system block diagram and its functional modules, and implements the hardware and software of the system. The vehicle-mounted terminal part of this system fully utilizes the advantages of low usage fees and high data transmission rate of WLAN, the advantages of wide coverage of GPRS network, and the advantages of real-time positioning and easy system integration of GPS; it integrates various functional modules with high-performance embedded systems to achieve a vehicle-mounted terminal with excellent performance, rich functions and powerful functions. The management and dispatching center uses a PC as the server of the system, expands GPRS and WLAN modules, completes the sending and receiving of commands and data under the control of the management and dispatching software, and realizes the management and dispatching functions; at the same time, it reserves a rich space for functional expansion, which can be used for secondary development.

    The hardware and software technical indicators of this system have reached the technical requirements of industrialization. The equipment works stably and reliably. It has begun mass production and is used in the transportation vehicle management and dispatching system of Shenzhen Yantian Port. The number of vehicle-mounted terminals has reached about 1,000 and there are still continuous orders. It has a very broad market prospect!