By PURPLELEC | 29 April 2025 | 0 Comments
USB4.0 Technology Analysis
USB stands for Universal Serial Bus Generation. The USB interface released the USB 1.0 specification in 1996, supporting low-speed 1.5 Mbps and full-speed 12 Mbps. Later, it released specifications supporting 480 Mbps, 5Gbps, 10Gbps, 20Gbps, etc., and released the latest USB4 specification in September 2019, supporting 20 Gbps and 40 Gbps.
USB4.0 adds 3 key points
1. USB4 only uses USB Type-C connector. USB4 signals use dual-channel transmission. Previous connectors such as USB Type-A or Micro-B only support single-channel transmission and cannot support USB4.
2. USB transmission speed supports up to 40G (20Gbps x2) and can transmit DisplayPort audio and video at the same time. Designed to combine multiple protocols into a single physical interface, the overall speed and performance of the USB4 architecture can be dynamically shared.
3. Compatible with USB 2.0 and USB 3.2 and supports Thunderbolt 3.
Three key points of USB4.0 connectors and cables
1. USB4 only uses USB Type-C connectors.
2. The supported adapter cable length of USB4 Cable adapter is reduced from 1 meter of USB 3.2 Gen2 to 0.8 meters of USB4 Gen3.
3. If you need to connect to a large-size screen or VR application, you can use an active cable. USB4 active cables are cables containing Repeater components (such as Re-timer, Re-driver and other active components), and conduits. The supported active cable length is 5 meters, while the optical fiber cable length can support 50 meters.
Three key points of USB4™ architecture
The main components of USB4 are Router, Adapter, and TMU (Time Management Unit).
1. Router is a main module of USB4. Router converts tunnel protocol into USB4 packets for transmission and performs time synchronization through TMU. It is mainly detected and managed by the Connection Manager built into the USB Host.
2. Adapter is built into the Router. Its main function is to perform protocol conversion for the internal communication between the Router and external components. For example, when the USB4 Host transmits USB3 data, the internal USB3 Host encapsulates it into USB4 Tunneled Packet through the USB3 Adapter. A Router can support up to 64 Adapters.
3. TMU is built into the Router and uses multiple Time Management Units (TMUs) to synchronize time between Routers.
USB4.0 is divided into 5 types according to the functional system
1. Protocol Adapter Layer: Responsible for the correspondence between USB4 and different protocols. And encapsulate different protocols into Tunneled Packet and transmit them in the USB4 interface.
2. Configuration layer: responsible for processing the control packet sent by the connection manager, and attaching the corresponding address in the path to ensure its reliable transmission mechanism.
3. Transport layer: defines the packet format, path, flow control and mutual control, and generates link management packets to provide time synchronization packets, flow control packets, etc.
4. Logical layer: responsible for establishing a USB4 connection between two devices, providing data transmission and reception, encoding and decoding, power management, error detection and repetition mechanisms, and communication using sideband channels for channel initialization, including speed and dual-channel communication.
5. Electrical layer: defines the characteristics of USB4 electrical signals such as voltage, reading, encoding, etc.
There are 4 types of USB4.0 products
1. USB4 Host: The product has more than one DFP. There is no UFP.
2. USB4 Hub: The product has one UFP and one or more DFPs.
3. USB4-Based Dock: The product has one UFP and one or more DFPs, and the product also has other device functions, such as storage or network functions.
4. USB4 device: The product has one UFP and no DFPs.
Tunnel protocols supported by USB4.0
According to the specifications, there are different requirements for the tunnel protocols that Host/ Hub/ Dock/ Device must support, as shown in the figure below. Those marked with "V" must be supported, and the rest are optional.
For example, the USB Host must support USB3, DisplayPort and Host-to-Host Tunneling, but may not support PCI Express and TBT3 Tunneling.
Transmission rates supported by USB4.0
USB4 supports 20Gbps for USB4 Gen2 and 40Gbps for USB4 Gen3. Does it mean that these two speeds must be supported if USB4 is announced?
• For USB4 Hub and USB4-Based Dock, both 20Gbps and 40Gbps must be supported.
• For USB4 Host and USB4 Device, only 20Gbps can be supported. (40Gbps can be supported additionally, but is not necessary).
Conclusion
The USB4 transmission rate is increased to 40 Gbps, and the bandwidth can be shared dynamically. When using a USB Type-C cable, it can be compatible with Thunderbolt 3 and Display Port products, which is a more convenient interface for consumers. However, for product developers, USB4 is a relatively big challenge. In addition to the difference in product design and previous USB3 product architecture, coupled with the high-frequency signal on the PCB and the attenuation on the connector, more attention should be paid to high-frequency impedance matching to ensure the quality of the transmitted signal during the development stage. GRL has accumulated considerable experience in high-speed interfaces such as Thunderbolt 3, USB3, DisplayPort, PCIe, etc., and can assist customers in rapid testing and verification services.
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