The computer host, hub (or docking station) and external devices together form a USB4 system. That is, all the interdependent parts of USB4 are combined into a whole called USB4 system. The USB4 system structure (English name USB4 Fabric) includes one or more combinations of USB hosts, USB4 hubs and USB4 external devices.
USB4 system structure
The transmission of USB 3.2 data, DisplayPort data and PCIe data between USB4 hosts, USB4 hubs and USB4 external devices can abstract the following model.
1. Tunnel protocol
The way in which data from different protocols will be integrated in the future is called a tunnel protocol. What is transmitted is tunnel data. USB4 tunnel refers to the path for transmitting USB4 tunnel data. USB4 tunnel data refers to a data form that integrates USB 3.2 data, DisplayPort data and PCIe data. There are also commonly mentioned USB 3.2 tunnels, DisplayPort tunnels and PCIe tunnels respectively referring to the respective transmission paths of USB 3.2 data, DisplayPort data and PCIe data.
2. Router
The router mentioned in this article is a different concept from the router that is often used to separate wireless networks in our daily life. The router mentioned in this article refers to a basic module of the USB4 system structure. It has two functions:
Choose the best transmission route between two points
Transmitting USB4 data packets through the USB4 system structure
Routers are divided into two types: host routers and device routers. For example, a laptop with USB4 function must have a host router, a hub that extends a USB4 interface to multiple USB4 interfaces, a DisplayPort monitor that receives signal video, and a solid-state drive and a mobile hard drive. Such external devices must have device routers.
USB 3.2, DisplayPort and PCIe are equivalent to 3 passengers, and the router is equivalent to a taxi driver. After determining the starting point and destination, it chooses the best driving route to take the passengers to the designated location.
3. Adapter
Adapters provide interfaces for connections between routers and external entities. Each router contains up to 64 adapters. There are three types of adapters: protocol adapters, channel adapters and control adapters.
3.1. Protocol adapter
Use a protocol adapter to convert between supported native protocols and USB4 tunneling.
There are four types of protocol adapters: USB3 adapter, DisplayPort (DP) adapter, PCIe adapter and Host Interface (HI) adapter. The function of USB3 adapter, DisplayPort adapter and PCIe adapter is to convert USB 3.2 data, DisplayPort data and PCIe data into USB4 tunnel data, or to convert USB4 tunnel data into USB 3.2 data, DisplayPort data and PCIe data respectively.
3.2. Channel adapter
Channel adapters provide interfaces for channels. There is one channel adapter for each channel of the USB4 port.
3.3. Control adapter
The router contains a control adapter. The control adapter is an abstract concept and has no physical entity. The control adapter is the end consumer of control packets directed to the router. The control adapter also generates control packets, which are sent to the connection manager.
4. Time management unit
Time Management Unit (TMU, Time Management Unit) is the functional module of each router. The router allocates and synchronizes time throughout the entire USB4 system structure through its time management unit. There is a certain time delay in data transmission, and the time for data transmission needs to be allocated and synchronized timing is also required.
The synchronization management unit is to ensure that data transmission between different routers has a unified timing method. Just like in life, when two people get together, they all agree on the time according to Beijing time. If one party is far away from the meeting point and one party is close to the meeting point, then the closer party can start at a later time.
5. Retimer
A USB4 product can contain up to two onboard retimers per USB4 port. The function of the retimer is to reorganize the signal of the previous stage and pass it to the next stage. It has the function of recovering data and reduces the possibility of data transmission errors.
6. Connection Manager
A connection manager is a software entity responsible for enumerating, configuring, and managing routers. The connection manager performs tasks such as path setup and teardown, hot plugging and unplugging, and bandwidth management. The connection manager is part of the USB4 host system.
7. USB4 link
A link refers to a real line from one point to another point, called a physical line. There are no other nodes in between. A USB4 link refers to the connection between USB4 ports of interconnected routers. The figure below illustrates the connection between a USB4 host and a USB4 device.
It can be seen intui-tively that the USB4 link is the part circled by the red box. That is, the blue connection part of the USB4 port of the USB4 host and the USB4 device is connected to the USB Type-C female socket, and the green connection part is connected to the two USB Type-C female sockets. Together they form a USB4 link. Among them, the green connection represents a cable with USB Type-C male connectors on both ends. The blue connection represents the line on the circuit board between the USB4 port of the USB4 host and USB4 device to the USB Type-C female socket.