I'm able to send AT commands to GSM modem,
but i am not able interface my 8051 to GSM model.
can i get sample inerfacing code for 8051 micro controller.
"null modem usart architecture"
Null-Modems and U(S)ARTs are completely differnt issues.
"moreover i can bid that more than 70% modems are ttl compactible" No. Again that is not correct
Again, you need to distinguish a true Modem from an embedded module: A Modem will use RS232 levels and, therefore will require and RS232 Transceiver; An embedded Module most probably will not use RS232 levels - and it may not even use standard logic levels.
Consider Wavecom, for example: Their Fastrack is a Modem and it most certainly does use RS232 levels; Their Integra is an embedded Module and it does not use RS232 levels
Remember also that an RS232 transceiver inverts the signal...
"i just want to help him out"
Giving him stuff that is blatantly wrong does not help him! In fact, connecting RS232 levels direct to an 8051's pins could well damage them!