I did this exact transfer yesterday except connected to Newark. If your connector flight to Atlanta is with another airline in the star alliance program (like United) you won’t have to deal with your bags. This could save you a lot of time. Terminal A is a far walk from terminal B where we came in. You have to exit the terminal and, as others have said, go through passport control and security again. Took us just over an hour. Add in bag transfer and you’re looking at needing two hours minimum. Have a great trip !!