how to keep session alive??

hi. i wana knw how can i keep user session alive on ma page ? i have a page where user watch movies or do other stuuff..
what i want is that user's session will alive till he is on that page, regardless of any activity frm him on this page as video can b watch without interaction.

currenty what m facing is that i have 20 mint session time out other pages on ma web site,, ant at video play back i want that user  session vill live till he is on page, nw what is happening is that if user watch movies till 21 mints and then tries to rate  it show tht u must sign in to rate..

can i keep session alive through ajax as m also ua=sing ajax on ma page..
any sample or example
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