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

  Frames consist of 1 or more transactions.
  
  Transactions consist of 2 or 3 packets.
    Token packet - from host.
    Optional data packet (IN or OUT) - from either host or device. 
    Handshake packet (used to acknowledge transaction) - from either.

    check out : http://www.cypress.com/file/134171/download

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

  All frames are initiated by hub with 1st packet going 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. * On a particular bus.

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

Low/Full speed - packet size is 8,16,32,or 64 byte. High speed - 512 byte packet. Superspeed - 1024 byte packet. Additionally supespeed supports 16 packet bursts or 16K of data in a frame. * primarily applies to bulk transactions. Other types have restrictions.