Is FaceTime over wifi or 4G?

I have a question about FaceTime. I am thinking about communicating with my sister through Facetime on an ipod and with friends through iPhone 4 but I am not sure if you must use the 3g connection, calling charges, or just wifi. I am in Taiwan and if I get the phone, my package will include unlimited 3g access, unlimited internet wifi access, but only a limited amount of calling per month. So what I am wondering is if I use facetime, is that using 3g connection or wifi or my 'call time.' Here it is separated in three different types of connections and charges.