links:
http://www.engineersgarage.com/microcontroller/8051projects/interfacing-gsm-8051-microcontroller-circuit-code
http://www.engineersgarage.com/microcontroller/8051projects/gsm-interface-8051-microcontroller-circuit-code
http://www.engineersgarage.com/microcontroller/8051projects/interface-gsm-module-8051-microcontroller-circuit-code