Transfer Types

All transfers to USB devices fall into one of four types, namely: control, isochronous, bulk and interrupt. The names of each of the types are indicative of the purpose of each of the transfers. Control and interrupt transfers are used for sending small,fixed-format messages, for purposes such as setting a configuration of a device or sending small sets of data across. Bulk and isochronous transfers are used for transferring larger sets of data. While isochronous (as the name suggests) gives some time guarantees with regard to the transfers, bulk transfers are highly flexible in their transfer mechanisms. Every endpoint that a device may posses has a specific transfer type associated with itself. Hence a device which wants to make transfers of multiple types, must have one corresponding endpoint for each of the types.