The top layer of the USB4 function stack layer is the protocol adapter layer and the configuration layer, which do not interfere with each other. The next-level is the transport layer, and the last level is the physical layer. The physical layer is divided into logical layer and electrical layer from top to bottom. The USB4 function stack relies on USB4 ports, protocol adapters and control adapters. There are channel adapters in USB4 ports. Protocol adapters, control adapters and channel adapters are collectively referred to as adapters.
From the figure above, we can see that the protocol adapter implements the functions of the protocol adapter layer and the transport layer, the control adapter implements the functions of the configuration layer and the transport layer, and the channel adapter of the USB4 port implements the functions of the entire physical layer. The physical layer includes the logical layer and electrical layer. Below, we will introduce all the layers of the function stack in order from the bottom to the top.
1. Electrical layer
The electrical layer defines the electrical signaling characteristics of the USB4 link, including scrambling, encoding, jitter and voltage. A low bit error rate should be met to achieve reliable communication.
Scrambling: Multiply the original signal with the scrambling code (pseudo-random code) to obtain a new signal. The opposite process is called descrambling.
Encoding: Convert information and data into specified electrical pulse signals.
Jitter: The short-term deviation of a specific moment in a signal from its ideal time position, which needs to be within a specified range.
Voltage: The voltage of the electrical pulse signal needs to be within the specified range.
Bit error rate: the proportion of erroneous codes to the total codes.
2. Logic layer
The logical layer establishes a USB4 link between two routers and provides services for transmitting and receiving byte streams between the two routers.
The logical layer is located above the electrical layer and below the transport layer. It treats data traffic (data transferred per unit time) to and from the transport layer as a stream of bytes.
The services provided by the logic layer are:
Establish and maintain USB4 links with link partners
Performance scalability through different speeds and widths
Error detection and recovery mechanism
Operate using different media such as passive cables, active cables and retimers
Support mechanisms such as clock compensation, data scrambling and forward error correction codes
Power management
USB4 links are assisted and managed by an accompanying sideband data channel that enables the following functions:
Configure USB4 link parameters
Interact with the retimer (if present) and perform USB4 link feedforward equalization transmitter handshake
Ensure correct power-down/wake-up sequence for USB4 link transceivers and retimers
3. Protocol adapter layer
The protocol adapter layer performs the mapping between tunneling protocol traffic and USB4 transport layer packets. The protocol adapter layer is defined by the types of tunneling protocol traffic that is sent and received.
4. Configuration layer
The configuration layer performs router configuration tasks and processes incoming control packets. The configuration layer provides an addressing scheme for control packets within the domain, processes control packets, and provides a reliable transport mechanism for control packets. Control packets give the connection manager access to the router's configuration space.
5. Transport layer
The transport layer forwards tunnel data packets and control data packets through the bus. It defines packet format, routing, Quality of Service (QoS) support, flow control and time synchronization. The transport layer is where protocol multiplexing is performed.
In short, the USB4 system structure relies on the adapter to transmit data. The protocol adapter layer and transport layer form the protocol adapter, the configuration layer and transport layer form the control adapter, and the logical layer and electrical layer form the channel adapter.
The protocol adapter layer implements the mapping between USB4 data and the transport layer. The configuration layer is responsible for configuring the router. The transport layer forwards the control data packets of the configuration layer and the USB4 tunnel data packets of the protocol adapter layer. The logic layer implements data transmission and communication between the two routers. For receiving services, the electrical layer ensures the reliability of communication data transmission and reduces the bit error rate. The division of tasks between each layer jointly realizes the stable and reliable transmission of USB4 data on the USB4 system structure.
