In the source code what i have to mention for "CNTR+Z"?
can u give me any example abt this
When you press cntr+z in some serial terminal software, it will simply send ascii char representing this cntr+z.
Here is some link http://www.cisco.com/en/US/docs/ios/.../papascii.html

So in fact you must send this char to gsm modem. It would be string with something like "\x1a".