This chapter presents an overview of Universal Serial Bus 4 (USB4™) architecture and key
concepts. USB4 is similar to earlier versions of USB in that it is a cable bus supporting data
exchange between a host computer and a wide range of simultaneously accessible peripherals.
However, USB4 also allows a host computer to setup data exchange between compatible
peripherals. The attached peripherals share bandwidth as configured by the host computer. The
bus allows peripherals to be attached, configured, used, and detached while the host and other
peripherals are in operation.
When configured over a USB Type-C® connector interface, USB4 functionally replaces USB 3.2
while retaining USB 2.0 bus operating in parallel. Enhanced SuperSpeed USB, as defined in USB
3.2, remains the fundamental architecture for USB data transfer on a USB4 Fabric. The
difference with USB4 versus USB 3.2 is that USB4 is a connection -oriented, tunneling
architecture designed to combine multiple protocols onto a single physical interface, so that the
total speed and performance of the USB4 Fabric can be dynamically shared. USB4 allows for USB
data transfers to operate in parallel with other independent protocols specific to display,
load/store and host-to-host interfaces. Additionally, USB4 extends performance beyond the 20
Gbps (Gen 2 x 2) of USB 3.2 to 40 Gbps (Gen 3 x 2) over the same dual-lane, dual-simplex
architecture.
This specification introduces the concept of protocol tunneling to USB bus architecture. Besides
tunneling Enhanced SuperSpeed USB (USB3), display tunneling based on DisplayPort (DP)
protocol and load/store tunneling based on PCI Express (PCIe) are defined. These protocol
tunnels operate independently over the USB4 transport and physical layers. Additionally, USB4
allocates packets for bus configuration and management, and packe ts can be allocated
specifically for host-to-host data connections.
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