Data Flow Types

The communication on the USB is done in two directions and uses 3 different transfer types. Data directed from the host to a device is called downstream or OUT transfer. The other direction is called upstream or IN transfer. Depending on the device type different transfer variants are used:

• Control transfers are used to request and send reliable short data packets. It is used to configure devices and every one is required to support a minimum set of control commands. Here is a list of standard commands:
  • GET_STATUS
  • CLEAR_FEATURE
  • SET_FEATURE
  • SET_ADDRESS
  • GET_DESCRIPTOR
  • SET_DESCRIPTOR
  • GET_CONFIGURATION
  • SET_CONFIGURATION
  • GET_INTERFACE
  • SET_INTERFACE
  • SYNCH_FRAME

  Further control commands can be used to transfer vendor specific data.

• Bulk transfers are used to request or send reliable data packets up to the full bus bandwidth. Devices like scanners or scsi adapters use this transfer type.

• Interrupt transfers are similar to bulk transfers which are polled periodically. If an interrupt transfer was submitted the host controller driver will automatically repeat this request in a specified interval (1ms - 255ms).

• Isochronous transfers send or receive data streams in realtime with guaranteed bus bandwidth but without any reliability. In general these transfer types are used for audio and video devices.