USB Frames
  Frame - communication session - frame is a logical construct.

  Frames consist of 1 or more packets.

  Frames can be viewed as a bi-directional session.
    And may use multiple pipes.

  All frames are initiated by hub with 1st packet going upstream to
    devices.

  Subsequent packets within the frame can move between hub and device in 
    either direction depending on specific activity.

  For low and full speed devices, a frame (session) must be completed before
    a different one is started.

  For high speed (USB 2), protocol exists to split a frame up.