What is the typical financial arraignment between major television networks and their affiliates?

I know that networks pay production companies for the right to broadcast their shows, and I know that networks ultimately make their money via the sale of advertising. My question is, do networks sell advertising and then pay individual stations to broadcast their shows (with their ads), or do individual stations sell advertisements and then pay networks for their editorial content? Or do networks and affiliates have some other financial arrangement?|||Individual stations sell advertizments and then pay networks