Connect Siemens TC35

Hello everyone, I bought the Siemens GSM module (TC35). I would like to connect it to Arduino via serial port.

I searched the internet and I found that I use to connect 3 pins: TxD, RxD and DTR.

TXD should be pin 1, pin RXD should be 0. I can not understand what could be the DTR pin.

How can I do understand what it is?

Who can help me?