hey guys, i had a question over here. can any1 give me a full explanation about whatsapp's charges?

currently i'm using prepaid, and i normally stay at home and use wifi to keep my whatsapp connected.

but the moment i when out, i wish to stay connected too, it will automatically turn into the data network usage, may i know that will it keep on charge for the usage of network or it charge at the moment some1 send msg to me through whatsapp?

hope to get a full and detailed reply on it...thanks guys...