Four general types of transfer frames.
  Control (message) - configure devices, give commands, check status.

  Isochronous (stream) - used with time sensitive devices - phones, sound. 
    Precise time intervals but no re-transmit on error.

  Bulk (stream) - large data transfers to/from non-time sensitive devices
    e.g printers,  Zip drives.

  Interrupt (stream) - for devices that need a quick response. Small packets
    of data (keyboard, mouse, etc.). 

    USB does not support device initiated interrupts. 

    Master hub polls devices for attention need (50 m-Sec (?)) in round robin 
      fashion.

    Device id itself as interrupt driven (possibly when 1st plugged in).  
      Keyboard/mouse - yes
      USB drive - no

    "Interrupt" driven devices may be polled more often than other devices.

    Size of packet sent by device limited in size.