Each generation of the USB is backward compatible and a USB 3.1 topology may include devices operating at five speeds. USB 2.0 signals handle three of the data rates: 1.5 Mb/s Low Speed (LS), 12 Mb/s Full speed (FS), and 480 Mb/s High Speed (HS). USB 3.0 added an additional set of signals to USB cables and connectors to support a fourth speed, 5 Gb/s SuperSpeed (SS). USB 3.1 brings the 5th speed option, 10 Gb/s SuperSpeedPlus (SSP). The USB 2.0 and SS/SSP signals are physically separate and USB 3.1 refers to devices operating at LS/FS/HS as being in the USB 2.0 topology; devices operating at SS and SSP are in the Enhanced SuperSpeed (ESS) topology and are also referred to as ESS Gen1 and Gen2. In addition to the faster data rates, the ESS topology employs protocol optimizations including unicast packets, device asynchronous messages, as well as link level flow control, error handling, and power management.

All generations of USB rely on platform host controllers to manage devices attached to each bus instance. USB 2.0 employed UHCI/OHCI and EHCI compliant host controllers to handle low, full, and high speed devices. The advanced capabilities of USB 3.1 require a new generation of host controller. Course topics include Intel’s eXtensible Host Controller Interface (xHCI). A single host controller based on xHCI can manage both USB 2.0 and Enhanced SuperSpeed topologies as well as attached devices of any USB speed. The xHCI operational model, software interface (registers, memory data structures), doorbell-based work notification, and hardware transaction scheduling are all described.