USB4: still the same USB?
At the end of 2020, the release of devices with support for the new generation USB4 / Thunderbolt 4 interfaces is expected. These interfaces are similar, but have a number of fundamental differences. Among these differences, the most significant can be distinguished: the specifications for USB4 are publicly available and anyone can study the basic principles of this interface, unlike Thunderbolt 4.
In this article we will look at the changes that the USB4 interface has undergone in comparison with the previous version (USB 3.2), analyze the USB4 architecture and list its main characteristics.
More information can be found in specifications for USB4.
What do we know about USB4?
Main characteristics of USB4:
Connector: similar to modern Intel interfaces (Thunderbolt 3/4), USB4 only supports USB-C.
- USB-C connector
Data transfer rate: here the situation is already a little more complicated and everything is not so simple, let’s try to figure it out: the minimum supported speed for a USB4-certified device is 20 Gb / s. But 40 Gbps can also be supported if the host, device and cable are capable of it. And this bandwidth is in no way inferior to its competitor from Intel – Thunderbolt 4.
- 20 Gbps minimum
- 40 Gbps support capability
Tunneling interfaces: One of the main tasks that had to be solved during the development stage of the USB4 interface was the unification of several different protocols operating through the USB-C connector into a single physical interface. Main interfaces operating in tunneling mode:
- Enhanced SuperSpeed USB (USB3) (previous generation);
- DisplayPort (DP);
- PCI Express (PCIe) (optional).
Bus configuration support: support for ad hoc connection between two hosts (host-to-host).
USB4 Power: For USB4 operation, power is installed and regulated according to USB Type-C and USB PowerDelivery (PD) specifications. The ability to transfer power up to 100 W.
Thunderbolt 3 Support: a host or USB4 device can also communicate with devices that support Thunderbolt 3 connectivity. However, this functionality is optional, so support for this feature is only up to the device developers.
Comparison with USB 3.2
After looking at the main characteristics of USB4, we can highlight a number of significant changes compared to the previous version of the standard:
Connector: previous versions of connectors are not supported. Due to the need to use an additional signal line of data (Sideband Channel), it is not possible to use connectors such as USB Type A / B.
- Rejection of USB Type A / B, miniUSB, microUSB connectors
Data transfer rate: for the latest version of USB 3.2 Gen2 x 2, the maximum data transfer rate is 20 Gb / s, but this speed was achieved only using two data lines simultaneously, that is, only for the USB-C connector. On previous versions of connectors, the speed was half that – 10 Gbps.
- Data transfer rate doubled from 20 to 40 Gbps
Nutrition: power distribution in the USB 3.2 standard is regulated similarly to the USB 2.0 standard, with an increase in the current consumption for devices operating on the SuperSpeed bus. The USB BC (Battery Charging) and USB PD standards are complementary to expand power options. For USB4, unlike USB 2.0 and USB 3.2, its own device power model is not defined and is regulated only by the USB PD and USB Type-C specifications.
- Unlike USB 3.2, USB4 does not have its own power model defined
Supports additional connectivity options: there are no additional features at all for the USB 3.2 interface. There are no tunneling modes for DP and PCIe interfaces, there is no way to establish a special connection between two hosts. Only when using the USB-C connector do several optional alternative modes (USB-C Alt Mode) appear, for example DisplayPort Alternate Mode, but this functionality refers specifically to the use of the USB-C connector and is regulated by the specifications of a particular vendor, and not by the USB 3.2 standard.
- For USB 3.2 there is no possibility of additional third-party connections at all
Difference between USB4 architecture and USB 3.2
The figure below shows the connection architecture of the USB 3.2 system. As we can see from the figure, the system has two parallel operating interfaces – Enhanced SuperSpeed and USB 2.0, which provides backward compatibility of the USB 3.2 interface with the earlier version of USB 2.0. Since both buses work in parallel, they can be active at the same time.
The USB4 system architecture has a number of differences. For joint work with the USB 2.0 interface, there is still a bus that functions independently of other interfaces. Since data exchange via the USB 3.2 interface is carried out over the same data lines that provide data exchange for other supported interfaces, it is necessary to use a tunnelled protocol. A detailed diagram of the USB4 system architecture is shown in the figure.
To tunnel interfaces such as USB 3.2 and PCIe, you need to use special Protocol Adapters. So, for tunneling the USB 3.2 interface, a special hub (Enhanced SuperSpeed Hub) is used. In turn, for PCIe, a special switch (PCIe Switch) is used, which is necessary to process packets associated with the routing protocol and provides data buffering. No intermediate logic is required for DP tunneling. The connection is established directly as an end-to-end connection.
Each router (Router) of the system has a unit responsible for synchronization and time distribution. In the diagram, it is designated as TMU (Time Management Unit).
The Router is the main building block required to build the USB4 architecture. It is responsible for mapping tunnel protocol traffic to USB4 packets, forms and routes packets through the USB4 structure. Through the internal TMU, the router synchronizes the time across the entire USB4 transmission structure. The Connection Manager located on the host side is responsible for configuring and locating the router on the line. There are only two types of routers: Device Router and Host Router.
Full USB4 support requires a USB4 port on both sides. It consists of transmit and receive lines (RX / TX) and a two-wire additional channel (Sideband (SB)) (SBTX / SBRX). The USB4 port can operate in two modes: single-channel or dual-channel. In single-lane mode, Lane 1 will be disabled. In dual channel mode, lines 0 and 1 are linked and provide a common data link. The figure below shows both modes of operation.
An additional SB channel is required to initialize the device on the line with the host and to control between ports.
Important understand that the USB 2.0 interface is not included in the USB4 router and works in parallel with it.
So, for a USB Type-C port with USB4 support, the full connection mode includes:
- USB4 port;
- USB 2.0 data bus (D + / D-);
- CC-line configuration channel required for USB PD protocol transmission;
- power bus (VBUS / VCONN / GND).
New levels in the USB4 functional model
The figure below shows the functional model of USB 3.2. There are three main levels: physical level (Physical Layer), channel level (Link Layer) and the highest level – level protocol (Protocol Layer).
We already see a different model in USB4, as significant changes have been made. The lowest of all remains physical level (Physical Layer), which in turn consists of two sublevels: logical (Logical Layer) and electric (Electrical Layer). One level above is located transport layer (Transport Layer). The highest equally spaced levels are: configuration layer and adapter protocol layer (Protocol Adapter Layer). The adapter protocol layer was added due to the tunneling implementation. It is used to process data from tunneled interfaces.
The figure below is a schematic diagram of the USB4 functional model.
Let’s consider each of them in more detail:
Electrical Layer defines the electrical characteristics for the USB4 connection: signal voltage levels, jitter, scrambling and signal encoding.
Logical Layer located above the electrical level and below the transport level. This layer establishes connections between two routers and is necessary for the transfer of byte streams between them.
Transport Layer determines the format of the transmitted packet, routing, synchronizes transmission in time and controls transmitted streams. At this level, tunnelled packets and Control Packets are transmitted over the bus.
Configuration Layer is required to process incoming management packets and provides router configuration. This layer also defines the addressing scheme for control packets in the domain and ensures that there is a reliable transport mechanism for control packets that provide access to the router’s configuration space.
Protocol Adapter Layer required to convert packets between the transport layer and the tunneling protocol. This layer defines the type of tunneling protocol.
New communication channel (Sideband Channel)
The main difference between the USB4 interface from the previous version and at the same time one of the reasons for the impossibility of using the new interface with previous versions of the connector is the appearance of a new additional communication channel located on the additional lines of the USB Type-C connector (SBU1 / SBU2).
This channel performs a number of functions:
- initialization of the data line;
- connecting and disconnecting devices from the USB4 port;
- enabling and disabling the data line;
- input and output from sleep state (Sleep state).
Additional channel transactions are sent on the SBTX line and are received on the SBRX line. The figure below shows several examples of connecting SBTX and SBRX between two routers using active, passive cables and built-in retimers on the devices.
There are three main types of transactions for a given channel. Each of them is responsible for its own functionality:
- Link Type (LT) – LT transaction designed to initialize a link. This transaction is also used to signal changes in the state of the connected adapter, such as a disconnected data line or a transition to a low power state;
- Administrative Type (AT) – AT transaction designed to read and write information to a special configuration area;
- Re-timer Type (RT) – RT transaction required for communication between routers and retymers.
Each USB4 port implements a set of channel configuration registers. Each line retimer also has its own configuration. A router uses AT transactions to access another router’s register space, or an RT transaction to access the retimer’s register space.
Promisethat Intel gave in 2017 was accomplished, making the USB4 standard incorporate much of Thunderbolt 3. As a result, we can say that USB4 remains the same standard interface, which acts as a data exchange between a host device and a wide range of simultaneously available peripherals. devices. At the same time, many changes appeared in it, which look extremely positive and promising at the moment: getting rid of different versions of the interface and merging them into one common name (USB4), abandoning various connectors towards one single one (USB Type-C), an attempt to make generally available the combination of various interfaces, such as DisplayPort, PCI Express, USB3, adding new additional features, for example, host-to-host connectivity – all these factors, as well as the openness of the standard (unlike Thunderbolt 4) indicate that USB4 has every chance of becoming a more “mainstream” interface than Thunderbolt 4.