USB3 Gen T Tunneling Over USB4

USB Promoter Group recently released USB4 Version 2.0 and this updated specification extends USB4 speed and data protocol performance, enabling manufacturers to develop products that can deliver up to 80 Gbps of data performance over the USB Type-C® cable and connector.

Protocol updates and optimizations are also being made to enable higher performance for USB 3.2 (USB3 Gen T Tunneling), DisplayPort, and PCI Express® (PCIe®) data tunneling to best use the higher available bandwidth.

USB3 Gen T is a type of USB3 Tunneling architecture where the Enhanced SuperSpeed protocol is extended to allow operation at the maximum bandwidth available on the USB4 link. GenT supports transmit and receive data bandwidth of up to 80 Gbps with a symmetric link and up to 120/40 Gbps transmit/receive or 40/120 Gbps transmit/receive data bandwidth with an asymmetric link.

The USB3 Protocol stack for a USB3 Gen T Protocol Adapter is the same as for a USB3 Gen X Protocol Adapter (USB3 Protocol Adapter compliant with USB4 Version 1.0). The Link and Protocol Layers in the internal Enhanced SuperSpeed Peripheral implement a subset of the Link Layer defined in the USB 3.2 Specification.

Key changes and optimizations are done for USB3 Gen T compared to USB3 Gen X tunneling in order to minimize tunnel overhead and maximize USB3 throughput:

• Encapsulation and transport of native USB3 LFPS, LBPM, Ordered Sets, Link management packets and a few link commands (LRTY, LBAD, LDN, LUP) are deprecated.
• Simplified the Link Training and Status State Machine. Eliminates LTSSM states which are irrelevant for USB3 tunnel operation over USB4.
• No link level retries are required, taking advantage of lower USB4 bit error rates to simplify USB3 link architecture.
• Link Credits increased to 64 for both Type 1 and Type 2, to facilitate a higher data rate.
• Out of band message (OOBM) tunneled packets are introduced for event indications such as Link ready, Link error, Port reset, Port disabled, and U3 resume.
• Link Command Aggregation and Coalescing feature is introduced for better link performance.

In the above system view diagram, we can see that USB3 GenT link is a point-to-point connection between a Downstream USB3 Gen T Port at the Host Controller and an Upstream USB3 Gen T Port at the Peripheral Device. A USB4 Hub supports USB3 Gen T Tunneling as a “pass through”, meaning the USB3 Gen T tunneled packets does not pass through any additional Protocol Adapters in USB4 Hub component.

Cadence has a mature Verification IP solution, for the verification of various aspects of USB4 Version 2.0 and Version 1.0 design, with verification capabilities provided to do a comprehensive verification of these. You may refer to the Cadence Simulation VIP for USB4 page for further details.