Research and Discussion on rapid construction of GPRS data transmission products

1、 GPRS and its data transmission service

GPRS (General Packet Radio Service) is the mainstream technology and an important milestone in the evolution of global mobile communication network technology to 3G mobile communication. It is called 2.5 generation mobile communication. Compared with the traditional GSM circuit dial-up switching, GPRS has a qualitative leap in resource utilization efficiency, switching capacity and performance. Although GPRS has brought us a new wireless communication function, However, it only widens the road in the field of data communication. In recent years, with the increasing popularity of GPRS mobile phones and the promotion of this technology by operators, the use of GPRS mobile phones for wireless internet access, sending and receiving e-mail and short messages has been widely used, but the business development of wireless data transmission using GPRS was not ideal. As early as a few years ago, China Mobile has formulated the strategy of giving priority to data services and giving priority to voice services, but up to now, this strategy has not been successfully implemented, but implemented in the opposite direction, or formed an abnormal situation of giving priority to supporting the owner

In addition to the human factors such as the understanding of wireless data services, a major factor is the lack of relevant wireless data transmission products. Because these functions can not be completed only by GPRS mobile phones, they must be realized by corresponding GPRS communication equipment. Due to the large voice service market and fast economic benefits, major manufacturers at home and abroad focus on the research and development of mobile phones, The development of GPRS wireless data transmission products has the problems of long development cycle and slow economic benefits; At present, in addition to the products of well-known foreign manufacturers such as Siemens, Sony Ericsson, Nokia and Longco, dozens of domestic companies such as Huawei, ZTE, panda and digital China have successively launched corresponding digital transmission products, but generally speaking, Domestic companies mostly focus on agent sales and OEM sales, and there are few truly independent research and development. In view of the above situation, this paper puts forward the methods and means to quickly build GPRS wireless data transmission products based on wireless communication module, so as to make the products quickly go to the market and meet the needs of modern social life

2. Building wireless communication module using GPRS

2.1 online debugging of at command and module

Wireless communication module is the core part of some wireless communication manufacturers' general communication products, including MCU, power supply system, data storage, control and display, voice processing and other functions, The first mock exam is the integration of high frequency circuit and RF part in the wireless communication module. The development of different communication products is based on the same module, which adds a small amount of external circuits, mainly software updates and modifications, which makes the development difficulty and development risk of hardware greatly reduced.

At present, several major communication companies in the industry have launched their own wireless communication modules, such as Sony Ericsson GR47 / 48 series, Motorola g2o series, wavecom Q2400 series, etc. as a wireless communication module for building GPRS wireless modem, it should generally have the following functions:

*It has GSM / GPRS dual frequency and dual mode

*Built in TCP / IP protocol based on GPRS

*GSM circuit data / fax function, GPRS packet data function

*Point to point (MT / Mo) and short message area broadcasting,

Reply to call information, call or emergency call, maximum rate, upgrade maximum rate and half rate, echo cancellation and noise reduction, full duplex handsfree

*Single antenna interface, SIM 3V / 5V and sim detection use wireless communication module to develop communication products. First, be familiar with the function of the module and various AT commands. For those involved in the field of wireless communication, at command will not be a strange term. Use serial line to connect the computer serial port with the module, put SIM card in the SIM card slot, and connect the antenna, Finally, power on and let the module start working. At this time, you can use the "super terminal" or serial debugging program provided by windows to select the serial port number used and communicate with the module at a certain baud rate. The "syntax" exchanged between them is at instruction

For example, for Q2400 series modules, first of all, the relevant data inside the module can be obtained through a series of at instructions: a + cgmi can obtain the manufacturer's certificate from the module; At + CGMM can obtain the model of the module; At + cgmr can obtain the version number of the internal software of the module; At + CIMI can read the user serial number in the SIM card

If a valid SIM card has been inserted into the module, you can dial and hang up. In the "super terminal" environment, you can also use at instructions to realize the operation of phonebook, various settings of communication module, short message operation, fax service operation, etc. after mastering the basic instructions, call control instructions, network service instructions After some commonly used basic at instructions such as security instructions and phonebook operation instructions, the key point is to master how to transmit short messages and data. There are two main ways to realize SMS: at + cnma, i.e. text format and at + cnma = = n, < PDU length > i.e. PDU format. PDU format is actually a protocol for packaging short messages. The realization of Chinese short messages must be in PDU format, Using at command mode to be familiar with PDU format can deepen users' understanding of this problem Understanding and mastering of methods.

2.2 system design of GPRS wireless modem

As a module specially developed for mobile devices, wavecom's Q2400 series has strong interface functions. In the development of GPRS wireless modem, we mainly apply its interface functions such as communication, voice, SIM card and data storage. Its main structure is shown in Figure 1. In order to ensure that GPRS cordless mdoem can stably transmit data without frequent drop or loss of data, A stable power supply with certain power and reasonable power management are essential

GPRS wireless modem provides two working modes: transparent channel and non transparent channel. In the transparent channel mode, GPRS wireless modem does not process the user's data, directly sends the data, and the received data is also transmitted to the user intact. In this mode, the short message and voice functions can be realized by the user through at command or by calling API. In the non transparent channel mode, the user communicates with GPRS wireless modem in frame format through serial port, which can be realized by calling API

Figure 1 structure diagram of GPRS wireless modem

In order to facilitate users to develop application programs, we have compiled and developed API frames with simple operation and powerful functions. Users only need to operate these frames to provide Internet access services and realize application data transmission on the Internet. User API functions are divided into three categories: basic serial port transceiver API function at the lowest layer, frame API function, sock et API function, voice API and short message API

In GPRS wireless communication, there are two forms of IP address of data center computer as the destination of user data transmission on the Internet. One is to use fixed IP. Because the number of domestic fixed IP is limited, most of them use dynamic domain name as the IP address of data center computer. When GPRS wireless modem is configured, The fixed IP or dynamic domain name of the data center computer needs to be input into the modem. For the dynamic domain name, the dynamic domain name needs to be resolved into the IP address of the data center computer through the domain name server inside the modem. When the GPRS wireless modem is in data communication, The user data and the IP address of the computer in the data center are converted through TCP / IP protocol to generate a TCP / IP packet containing user data. The TCP / IP packet is transmitted to the mobile GPRS network by GPRS wireless modem.

2.3 Application of GPRS wireless modem

For the needs of GPRS wireless data transmission application, we have developed GPRS wireless modem with USB interface and RS-232 interface and PCMCIA card suitable for mobile computer. It can dial up the Internet in a specified way, realize point-to-point and point-to-multipoint data transmission, and send and receive short messages

The product supports TCP / IP version 4.0, PPP and UDP industrial standard protocols; It has a user configurable general socket, three wire serial ports (TXD, RXD, GND) with MCU interface and standard modem interface; Internet speed 300 BPS ~ 85 Kbps, client speed 300 BPS ~ ll5.2 Kbps, 8 data bits, 1 stop bit, odd check; It can realize voice and short message functions, and users can send and receive data directly through transparent channels. In addition, it also provides API interface functions of C language for users' application development.

The product developed by us has passed the technical test of the post and Telecommunications Department and the comparative test with similar foreign products, and its performance and stability have reached the level of similar foreign products. After more than one year of testing and use, the product has been used in the survey and investigation of forest resources

People's mobile Internet access, power remote monitoring and data acquisition have been widely used

3、 Concluding remarks

From the above development process, it can be seen that since the high-frequency circuit part and RF part with the greatest technical difficulty are processed in the wireless module, it undoubtedly greatly reduces the difficulty of hardware development. The wireless communication module can be used to build an external GPRS wireless modem, which is mainly used for wireless internet access and networking of desktop computers and industrial computers, It can be connected with desktop computer or industrial computer through RS-232 port; It can be built into a built-in PCMCIA card GPRS wireless modem, which is mainly used for PDA and laptop wireless internet access equipped with PCMCIA interface slot; Of course, it can also be built into an embedded GPRS wireless modem, which can be directly embedded into any product that needs wireless data transmission

Therefore, the application of wireless communication module can enable developers to turn their ideas into reality as soon as possible on the basis of small hardware development cost, so as to shorten the whole R & D cycle