Who provide service

Hi my name is Marlon, I currently on 3d semester of ccna, I will have the oportunity to link 2 lans geographically separated,i understand that my router @ one end (dte) could be connected to a csu/dsu (dce)for connection via dial up network, if thisis correct can sombody tell me who provides the service for wan data transmission? and do i need a transiver to make the connection from the serial interface of router to csu/dsu, or how is it physically connected?
Thanks!