Global System for Mobile communication (GSM) is digital cellular system used for mobile devices. It is international standard for mobile which is widely used for long distance communication. There are various GSM modules available in market like SIM900, SIM700, SIM800, SIM808, SIM5320 etc.SIM900A module allows users to send/receive data over GPRS, send/receive SMS and make/receive voice calls. It communicates serially with the devices like microcontroller, PC using AT commands. To interface SIM900A module with cellular network, it requires SIM card provided by a network operator.

It has been a particularly successful cellular phone technology for a variety of reasons including the ability to roam worldwide with the certainty of being able to be able to operate on GSM networks in exactly the same way - provided billing agreements are in place.

The GSM system was designed as a second generation (2G) cellular phone technology. One of the basic aims was to provide a system that would enable greater capacity to be achieved than the previous first generation analogue systems. GSM achieved this by using a digital TDMA (time division multiple access approach). By adopting this technique more users could be accommodated within the available bandwidth. In addition to this, ciphering of the digitally encoded speech was adopted to retain privacy. Using the earlier analogue cellular technologies it was possible for anyone with a scanner receiver to listen to calls and a number of famous personalities had been "eavesdropped" with embarrassing consequences.

The GSM cellular technology had a number of design aims when the development started:

• It should offer good subjective speech quality

• It should have a low phone or terminal cost

• Terminals should be able to be handheld

• The system should support international roaming

• It should offer good spectral efficiency

• The system should offer ISDN compatibility

GSM cellular technology uses 200 kHz RF channels. These are time division multiplexed to enable up to eight users to access each carrier. In this way it is a TDMA / FDMA system. The base transceiver stations (BTS) are organised into small groups, controlled by a base station controller (BSC) which is typically co-located with one of the BTSs. The BSC with its associated BTSs is termed the base station subsystem (BSS).

Further into the core network is the main switching area. This is known as the mobile switching centre (MSC). Associated with it is the location registers, namely the home location register (HLR) and the visitor location register (VLR) which track the location of mobiles and enable calls to be routed to them. Additionally there is the Authentication Centre (AuC), and the Equipment Identify Register (EIR) that are used in authenticating the mobile before it is allowed onto the network and for billing.

GSM supports voice calls and data transfer speeds of up to 9.6 kbps, together with the transmission of SMS (Short Message Service).GSM operates in the 900MHz and 1.8GHz bands in Europe and the 1.9GHz and 850MHz bands in the US.

GSM services are also transmitted via 850MHz spectrum in Australia, Canada and many Latin American countries. The use of harmonised spectrum across most of the globe, combined with GSM’s international roaming capability, allows travellers to access the same mobile services at home and abroad. GSM enables individuals to be reached via the same mobile number in up to 219 countries.

Terrestrial GSM networks now cover more than 90% of the world’s population. GSM satellite roaming has also extended service access to areas where terrestrial coverage is not available.

In GSM module interfacing with pic microcontroller article I have discussed RXD and TXD pins of GSM module and pic microcontroller should be connected with each other in reverse manner. Because transmitted data of pic microcontroller receive to GSM module and similarly transmitted data from GSM TXD go to RXD receive pin of pic microcontroller. This is a wired communication to send data from one device to another.

There are many methods of wired communication but pic and GSM interfaced through 2 wire serial communication. Serial communication mean to send data bit by bit. There is one important to consider while using serial communication that is baud rate. Baud rate is the number of bits transfer per second from one device to another. You should check baud rate compatibility between two devices. Usually SIM900D GSM module support 9600 baud rate with UART type serial communication. UART mean universal Asynchronous receiver and transmitter. Pic microcontroller have built hardware to implement UART serial communication process between pic microcontroller and GSM module. SIM900D GSM module also support UART serial communication.

• It provides very cost effective products and solutions.

• Advanced versions of GSM with higher number of antennas will provide high speed download and upload of data.

• SAIC and DAIC techniques provide very high transmission quality.

• The phone works based on SIM card and hence it is easy to change the different varieties of phones by users.

• The GSM signal does not have any deterioration inside the office and home premises.

• It is easy to integrate GSM with other wireless technology based devices such as CDMA, LTE etc.

• Improved spectrum efficiency

• International roaming

• Compatibility with integrated services digital network (ISDN)

• Support for new services.

• SIM phonebook management