introduction
In recent years, wireless communication technology has been widely used. GPRS (General Packet Radio System) general wireless packet network is a relatively popular wireless communication network.
GPRS network has the advantages of wide coverage, fast data transmission speed, high communication quality, always-on and chargeable by flow rate.
This wireless transmission solution uses the Siemens GPRS wireless communication module MC35i, which supports the standard AT instruction set, but does not embed the TCP / IP protocol, but the system uses the Linux operating system with a complete TCP / IP protocol stack and PPP protocol not only reduces the complexity of the software system, but also increases the stability of the system. In this way, this solution can be used in different occasions, as long as the corresponding upper-layer software processing system is designed.
1 Overall system design
The system is mainly composed of GPRS communication module and control module, and uses embedded system to design GPRS terminal. The embedded system is composed of hardware and application software, and its architecture can be divided into five parts: microprocessor, memory, input and output, operating system and application software. The hardware of the system is composed of microprocessor, GPRS module, SIM card holder, external interface and extended data storage. The microprocessor can control the information reception and transmission of the GPRS module to carry out data communication with the external controller through the standard RS232 serial port. Its system principle structure is shown in Figure 1.
2 System hardware design
The hardware of this system is mainly based on ARM as the development platform, and the MC39i module of Siemens is used as the GPRS transmission module, so that the system can quickly complete the transmission work.
2.1 Embedded microprocessor unit
The CPU of this hardware platform uses Samsung ’s S3C2410 microprocessor based on the ARM920T core. The S3C2410 is a 32-bit microcontroller manufactured using a 0.18μm process. Can support TFT LCD controller, NAND flash memory controller, 3 UART, 4 DMA, 4 PWM timer, I / O port, RTC, 8 10-bit ADC, Touch Screen interface, IIC-BUS interface , IIS-BUS interface, 2 USB hosts, 1 USB device, SD host and MMC interface and 2 SPI. The S3C2410 processor can run up to 203MHz. In addition, by providing a complete series of peripheral devices, the S3C2410X can greatly reduce the cost of the entire system, eliminating the need to configure additional devices for the system.
2.2 GPRS module MC35i
MC35i is a lead-free product of the new-generation dual-band GSM / GPRS communication module recently launched by Siemens. Its compact package is an ideal solution for wireless high-speed data transmission in many application systems. It can perform all aspects of data, voice, SMS and FAX. Application, and low power consumption. It provides users with always-on, high-speed, and simpler means of accessing mobile data communications. MC35i has rich AT commands, powerful functions, and flexible and convenient operation. It is another very important terminal equipment of GPRS mobile communication system following GPRS mobile phones. Its appearance has injected new vitality into the development of GPRS.
MC35i has the characteristics of small size, light weight and low power consumption. The working voltage of MC35i is 3.3 ～ 4.8 V, the typical voltage is 4.2V, and the maximum working current is 2 A. The module can work in two frequency bands of EGSM900 and GSMl800. The power consumption when working in EGSM900 is 2 W, and the power consumption when working in GSM1800 is 1 W. It can be controlled by AT commands, and supports text and PDU mode short messages, group i fax. Common working modes of MC35i module include power saving mode, IDLE, TALK, data and other modes. The unique 40-pin connector (ZIF) can realize the bidirectional transmission of power connection, command, data, voice signal and control signal. MC35i module is mainly composed of GSM baseband controller, radio frequency module, power supply module, flash memory, ZIF connector, radio frequency power amplifier, antenna interface and other parts.
3 System software design
According to the system requirements, before the operating system Linux starts, the internal and external hardware of the system including the microprocessor and GPRS module should be initialized first. After the hardware initialization is ready, the system tasks can be divided and created according to the system requirements, and finally Linux is started. Scheduling of various tasks. The application program is divided into five tasks: initialization task, configuration task, data sending task, data processing task and user serial interrupt service. This article mainly completes the system initialization and dial-up connection to the Internet.
3.1 System initialization
In order to use TCP / PI protocol for data transmission. The hardware and software of the system must be initialized during design. Figure 2 shows the software flowchart of the system initialization part.
At the beginning of this system software, it is necessary to determine whether the MC35i module is normally powered on by checking whether the input level of the 0x90 port is high. If it fails to power on normally, the software will power on the MC35i module through the output of the 0x90 port.
When the MC35i module is powered on, the software will create a buffer area in the memory as a data buffer area including PPP, IP, TCP, UDP and other protocols. The storage method of the data in the cache area adopts a doubly linked list.
In order to enable timeout detection in the implementation of the TCP protocol, a 20ms clock interrupt should be installed in the software using the internal timer of the R8822CPU module.
The ETR186 can be connected to the MC35i module through its serial port COM2, and the software can be used to perform data transmission with the MC35i module by installing a serial port interrupt.
After completing the above operations, the system can use AT commands to log in to the GPRS network. If it is unsuccessful, it will retry. After exceeding the number of retries, an error report will be sent. The last step is to connect the PPP link and send data at the same time.
3.2 Dialing principle
The so-called initialization module is mainly to set APN: CMNET,
at + cgdcont = 1, ip, cmnet,
Then use the chat program to dial * 99 *** 1, #
After the dial-up connection is successful, the communication link can be established by pppd. The script is changed as follows: in ppp-on, the phone number is changed to * 99 *** 1 #, the account number and password are cleared, then the path of DIALER_SCRIPT is modified, and the following device is changed to / dev / ttyS0, and the rate is changed 115200, and then remove the crtscts parameter. At the same time, remove the account password in ppp-on-dialer. The modified script is as follows:
After the modification is complete, execute the script PPP-on / ppp-on to dial-up Internet access.
3.3 Wireless data transmission
When the GPRS dial-up is successfully connected to the Internet, wireless data transmission is possible, which can be achieved through network programming under Linux. There are two main ways to choose network programming based on TCP / IP protocol: TCP and UDP. When using UDP for communication, both parties must know the other party's IP address and port number. However, the current GPRS network does not provide fixed IP services, and the IP address of the GPRS terminal in the communication link is dynamically allocated by the gateway support node (GSN). Therefore, the GPRS terminal performs each data transmission. Both its IP address and port may change. The network data communication in this scheme adopts TCP mode. That is, stream sockets are used for network programming.
3.4 PC software design
The central server-side software should provide a visual monitoring interface to intuitively, conveniently, and quickly understand the status of the remote data collection terminal, find and handle abnormalities and faults in a timely manner, and communicate with the GPRS terminal. The upper computer part adopts the Windows platform, and the compiler adopts VC6.0. In order to facilitate network programming, in the early 1990s, Microsoft and several other companies jointly developed a set of network programming interfaces under WINDOWS, that is, Windows Sockets specification. This is actually not a network protocol, but a set of open, Can support multiple protocols, network programming interface under Windows, now Winsock has basically achieved nothing to do with the protocol, you can use Winsock to call the functions of multiple protocols, but the more commonly used is the TCP / IP protocol. Socket actually provides a communication port in the computer, and can communicate with any computer with a Socket interface through this port. When the application is transmitted on the network, the received information can be achieved through this Socket interface.
When developing WINSOCK API programming in VC, the following three files need to be used in the project, otherwise a compilation error will occur:
(1) WINSOCK.H: This is the WINSOCKAPI header file, which needs to be included in the project;
(2) WSOCK32.LIB: WINSOCKAPI connection library file. In use, it must be included in the project file as a non-default connection library for the project;
(3) WINSOCK.DLL: WINSOCK's dynamic link library, located in the installation directory of WINDOWS.
4 Conclusion
The system organically integrates wireless communication technology and embedded technology, which enables user terminals to connect to the GPRS network and the Internet in a cheap, fast, and reliable manner, and enables user terminal data information to be transmitted in a concise and real-time manner. This provides a reliable guarantee for the safe and stable operation of user terminals. Experimental results show that the system can receive data on the host computer very well, especially in low- and medium-speed services and low-rate data transmission, it can reflect good cost performance and meet real-time requirements.
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