The GSM system is a relatively mature, complete, and most widely used system in the current mobile communication system based on time division multiple access technology. The GSM short message system has a wide range of users for its fast, convenient, and low-cost features. It is also a remote monitoring system. Provides a new technical means. The use of GSM short message (SMS) for remote monitoring has the characteristics of low investment, low cost, high reliability, etc., and has a high cost performance in the case of low real-time requirements for operation and monitoring.
Wireless GSM communication module TC35T
TC35T is a special GSM modem launched by SIEMENS. It is mainly composed of six parts: GSM baseband processor, GSM radio frequency module, power supply module, flash memory, ZIF connector and antenna interface. It can complete the transmission of voice, data, short message and fax. TC35T has a standard industrial interface and a complete SIM card reader, so it is very simple to use, and its structure is shown in Figure 1.
Principle and implementation of GSM remote monitoring
The GSM remote monitoring system is composed of a control terminal and a controlled terminal. As shown in FIG. 2, the control terminal can be a mobile phone or a TC35T module and a PC.
In this design, the controlled end is composed of a single chip microcomputer, a GSM module, a detection part, and a control part. The PC is used for monitoring and control, and the main control program sends a short message to the controlled device through the GSM module TC35T. After receiving the short message, the GSM module TC35T at the controlled end transmits it to the microcontroller through the serial port R232. The microcontroller processes the received short message, extracts control commands from it, and then controls the controlled device accordingly. The status information of the controlled device is also sent to the short message service center (SMSC) through the GSM module in the form of short message, and then sent to the PC by the short message service center. After receiving the short message, the PC can send back the short message to control according to the content of the short message.
AT command introduction
The AT instruction set was jointly developed by Nokia, Ericsson, Motorola, HP and other manufacturers for the GSM system, which includes the control of short messages. There are three ways to control SMS: Block mode, Text mode and PDU mode. The Text mode is relatively simple. Many Nokia mobile phones support this mode, but the disadvantage is that they cannot send and receive Chinese text messages. Sending and receiving short messages in Chinese or mixed Chinese / English must use PDU mode, and the short message body is transmitted after hex encoding. In this design, we need to implement the transmission and reception of Chinese short messages, so the PDU mode is used to send and receive short messages.
When the mobile terminal initiates (ie sends) the PDU format is:
The PDU format for mobile terminal is:
Among them, SMSC is the short message service center address, DA / OA is the source / destination address, PID is the protocol identification, DCS is the data encoding, UDL is the length of user data, UD is the length of user data, VP is the effective time, and MR indicates that the information is sent , SCTS indicates the time when the short message arrives at the service center.
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