Possible Duplicate:
length between nested lists

Possible duplicate:

Length between nested lists in beamer

In order to better distinguish groups of notions, I'd like to be able to control the vertical space between items of itemize when subitems are also used as follows:

\item level 1 a
\item level 2 a 
\item level 2 b 
\item level 1 b

How to automatically set-up a 2mm vertical space when subitems are used and nothing otherwise?