question on OFDM preamble design.

In OFDM systems, we have preamble transmitted before data for various
purpose. In 802.11a preamble, we have short training sequence for AGC,
packet detection, and others; after short training sequence, we have
long training sequence for channel estimation and others.

In general, for the training sequence, it is a certain number of
repetition of a certain sequence. For example, in 802.11a, the short
training sequence = [ S S S S S S S S S S] while each S is a short
sequence with duration D for example. Basically, D is the period of
this periodic sequence, with total length 10*D.

My question is, what determines the period (D) of the sequence, and
what determines the repetition times (10 in 802.11a case) of the same
sequence? thanks a lot for your help.

Philgo