[Discuss-gnuradio] OFDM preamble

Hi.
I wanted to know about the preambles that are inserted in the ofdm packet. I know they are inserted for the synchronization and frequency offset purposes.
But I want to know where in the OFDM packet they are inserted. I have following questions.

1. How can I vary the length of preambles? Can I do that ?

2. On which parameters length of preamble depends?

3. What is the length of preamble sequence if I am using fft-length=512, occupied carriers=200 cp-length=128, size of ofdm packet=400 bytes, modulation='bpsk'.

4. Where are they inserted in the OFDM packet? I mean before Payload?

       --------------------------------------------
      preamble| payload|
       --------------------------------------------
5.  Both the preambles are inserted at the start ?

6. Which modulation is used for preambles?

Need your help

Regards