GSM is a mobile communication modem. It is stands for global system for mobile communication (GSM). GSM system was developed as a digital system using time division multiple access (TDMA) technique for communication purpose.

GSM is a mobile communication modem. It is stands for global system for mobile communication (GSM). It is widely used mobile communication system in the world. GSM is an open and digital cellular technology used for transmitting mobile voice and data services operates at the 850MHz, 900MHz, 1800MHz and 1900MHz frequency bands.

GSM system was developed as a digital system using time division multiple access (TDMA) technique for communication purpose. A GSM digitizes and reduces the data, then sends it down through a channel with two different streams of client data, each in its own particular time slot. The digital system has an ability to carry 64 kbps to 120 Mbps of data rates.


The GSM module is communicate the microcontroller with mobile phones through UART. To communicate over UART or USART, we just need three basic signals which are namely, RXD (receive), TXD (transmit), GND (common ground).

GSM modem interfacing with microcontroller for SMS control of industrial equipments. The sending SMS through GSM modem when interfaced with microcontroller or PC is much simpler as compared with sending SMS through UART. In this scheme RTS and CTS signals of serial port interface of GSM Modem are connected with each other.

The transmit signal of serial port of microcontroller is connected with transmit signal (TxD) of the serial interface of GSM Modem while receive signal of microcontroller serial port is connected with receive signal (RxD) of serial interface of GSM Modem.

The SMS message in text mode can contain only 140 characters at the most. It depends upon the amount of information collected from GPS Engine that you need at the base station for tracking vehicle or person.

Figure: 1 Interfacing UART to Microcontroller

The Working Procedure of a GSM Modem

When the GSM modem is connected to the microcontroller, then it communicates with a mobile via a UART protocol and requires three basic signals such as TXD, RXD and GND. The GSM modem is connected to the microcontroller, it controls the industrial appliances through an SMS. When the modem receives the SMS from an operational phone, serially that data is sent to the microcontroller. This microcontroller compares this data with the stored data. If the compared data match with the stored data, then the microcontroller generates corresponding signals to control the load.

There are various cell sizes in a GSM system such as macro, micro, pico and umbrella cells. Each cell varies as per the implementation domain. The coverage area of each cell varies according to the implementation environment.

Time Division Multiple Access

TDMA technique relies on assigning different time slots to each user on the same frequency. It can easily adapt to data transmission and voice communication and can carry 64kbps to 120Mbps of data rate.

GSM Architecture

A GSM network consists of the following components:

1. A Mobile Station:  It is the mobile phone which consists of the transceiver, the display and the processor and is controlled by a SIM card operating over the network.

2. Base Station Subsystem: It acts as an interface between the mobile station and the network subsystem. It consists of the Base Transceiver Station which contains the radio transceivers and handles the protocols for communication with mobiles. It also consists of the Base Station Controller which controls the Base Transceiver station and acts as a interface between the mobile station and mobile switching centre.

3. Network Subsystem: It provides the basic network connection to the mobile stations. The basic part of the Network Subsystem is the Mobile Service Switching Centre which provides access to different networks like ISDN, PSTN etc. It also consists of the Home Location Register and the Visitor Location Register which provides the call routing and roaming capabilities of GSM. It also contains the Equipment Identity Register which maintains an account of all the mobile equipments wherein each mobile is identified by its own IMEI (International Mobile Equipment Identity)number.

4. The confidentiality of a call and secrecy of the GSM subscriber is just ensured on the radio channel, this is a major step in achieving end-to- end security.

Interfacing With AVR Microcontroller:

The Communication between AVR and modem takes place through USART protocol. GSM Modem SIM900 works on TTL level hence we can interface directly to RXD and TXD pins of AVR microcontroller. There is no need of using Voltage or level converter between them. The modem must be Initialized using the commands and then the process you are about to carry out must be selected.


These are the set of commands that are predefined in order to establish a communication between controller and modem. Also with the help of these commands a SMS or call has been made out.

Features of GSM modem:

>> High Quality Product- SIM900A

>> Frequency – 900/1800 MHz.

>> Modem interface- RS232 Serial interface

>> Configurable baud rate

>> Wire antenna (SMA connector  with GSM antenna  optional) with SIM card holder

>> Built in network status LED

>> Inbuilt powerful TCP/IP Protocol stack internet data transfer over GPRS

>> Power Management  : 4.5V -12V DC


>> SMS based Remote Control & Alerts

>> Security Applications

>> Sensor Monitoring

>> GPRS Mode Remote Data Logging

>> GSM controlled devices

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