USB - universal serial cables.
  NRZI data encoding - no change 1, change 0.
    And bit stuffing (USB 1,2)
    - simpler than RLL, a zero is "stuffed" after 6 consecutive 1's.

    USB 3.0 uses RLL 8b/10b encoding.

    All use differential signaling.

   Signaling

  Cable connectors keyed (shaped) to go in one way.

  Styles
    Key aspect of cables/ports is that USB is tree topology, so cables have
      to connect in particular direction. And host to host or device to device
        connections invalid.

    A - flat - from PC or hub.

    B - to device which may be an extension hub. 
 
    Mini A/B - used to connect compact devices. 
      Same plug - additional pin allows devices to identify if connection
      is type A or B.

    Micro-USB also available.

    C - 24-pin symetrical plug and port. 
      Duplicate pair of USB 1/2 D+/D-, only one pair, either, used.
      Contains a chip for ID and configuration of other pins.

      4 differential pair (2 in each direction) for Superspeed+
      
      Supports USB On-The-Go - the ability of a device to the root hub or
        a device.

      Designed to be used in some non-USB connnections.

      Some cable versions available that are actually only USB 1/2

   
  Cable lengths limited to 5M (twisted pair) for high speed 
    and 3M (not twisted, keyboard, mouse) for low speed. 

  4 wire cable
    Pair of transmit/receive data lines, - half duplex.
    (USB 3 has 2 additional pair for parallel bidirectional).
 
    Power/Ground for low power devices (5 volt).
      .5 - 2.5 Watts.  10 mA to 500mA. (Active hub)
      > .5, use active hub. 

  Data lines used to transmit address, data, and control info.