Serial-Hardware Handshaking Mode

I’m using the lwIP Embedded WEB Server Demo using GCC on an AT91SAM7X256. I added the serial.c and serialISR.c task and it has been working successfully. Also I have removed the Web Server task and USB task as it will not be required for my application. I am now trying to make the communicating between my Serial port with my GSM modem (AT command) in Normal mode (no handshaking) and it is just not very reliable. Has any one has any idea or example on how to set up the serial and serialISR in Hardware Handshaking Mode (HWHSH) or modem mode? Please give me some Hint. Thanks in advance! Kinds Regards Hudson