GSM – AN OVERVIEW
GSM, Global System for Mobile communications, is today the most successful digital mobile telecommunication system. This second-generation (2G) system provides voice and limited data services and uses digital modulation with improved audio quality.
The different versions of GSM are:
GSM Mobile communication system can be intelligently used by electronic devices that can collect some data and send it to the central place using SMS or GSM data call. It’s required In-Vehicle Tracking Systems because GPS (Global Positioning System) can normally only receive location information from the satellites but cannot communicate back with them. Hence we need some other communication system like GSM to send this location information to the central control room. Other technologies can also be used but they are more costly.
GSM Network Architecture
GSM Network consists of three main parts:
Mobile Station consists of:
Base Station Subsystem consists of:
Figure 1. Layout of generic GSM network
Network and Switching Subsystems consists of:
Network Databases – Home Location Register and Visitor Location Register together with MSC provides the call routing and roaming capabilities of GSM.
SMS is one of the unique features of GSM compared to older analog systems. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is sent to the sender. SMS also can be used in Cell Broadcast mode to send messages such as traffic or news updates. Messages can be stored on the SIM card for later retrieval. SMS is effective because it can transmit short messages within 3 to 5 s via the GSM network and doesn’t occupy a telephony channel. Moreover, the cost savings makes it a worthwhile choice. With SMS transmitting, gathering position data is easy and convenient.
THE GSM UNIT:
The GSM unit contains a GSM module along with a GSM transmitter antenna. The module functions according to its built and the antenna transmits the information to the Base Station wherein this is exposed to further processing. GPS is not a two-way system. It can either receive or transmit but not both. Due to its inability in doing so, GSM systems are used.
The GSM module that we are using in this unit is the SIMCOM SIM300 module. Designed for global market, SIM300 is a Tri-band GSM/GPRS engine that works on frequencies EGSM 900 MHz, DCS 1800 MHz and PCS1900 MHz. SIM300 provide RF antenna interface with two alternatives: antenna connector and antenna pad. The antenna connector is MURATA MM9329-2700. And customer’s antenna can be soldered to the antenna pad.
SMS is one of the unique features of GSM compared to older analog systems. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is sent to the sender. SMS also can be used in Cell Broadcast mode to send messages such as traffic or news updates. Messages can be stored on the SIM card for later retrieval. SMS is effective because it can transmit short messages within 3 to 5s via the GSM network and doesn’t occupy a telephony channel. Moreover, the cost savings makes it a worthwhile choice. With SMS transmitting, gathering position data is easy and convenient.
We use AT commands to control and program the SIMCOM SIM300 module. The data and control commands are exchanged between the PIC microcontroller and GSM module through the serial interface. There are many groups of AT commands, including: Call Control, Data Card Control, Phone Control, Computer Data Card Control, Reporting Operation, Network Communication Parameter, Miscellaneous, and Short Message Service. We use some of the SMS commands to communicate with the control center. The main AT commands for using SMS are listed below.
Let’s review an example of how to make a GSM module send and read a sample SMS in Text mode. First, initialize the GSM module with AT commands AT+CSCA and AT+CMGF. Using the former sets the SMS center number to be used with outgoing SMS messages. Remember, the number will be saved on the SIM card just like in normal mobile phones. There are two different modes—Text mode and Protocol Data Unit (PDU) mode—for handling short messages. The system default is PDU mode; however, Text mode is easier to understand. So, use the AT+CMGF=1 command to set the module to the GSM 07.05 standard SMS Text mode. The AT+CMGS command is used to send a short message. The GSM module can receive incoming short messages and save them on the SIM card automatically. You can use the AT+CMGR command to read an incoming short message from the SIM card storage, and then use the AT+CMGD command to delete it when you’re finished. If you want to read an SMS message, then send a AT+CMGR=x command to tell the GSM module which short message you want to read. Next, check the serial port to receive the message from the GSM module.
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