The Fidonet is a dialup-based system, where the nodes connect to each other and exchange files according to a set protocol. Since the connections are using the same phonelines that are used for the normal users of the BBS, the fidonet transfers only happen at special times of the day.
The transfers are done according to a hierarchy, where most nodes send messages up to the node above them.
There were also methods developed to transfer non-mail files by sending a mail to a special adress and as a reply receiving the file you were requesting.
 a web page about fidonet.