What is Universal Serial Bus (USB)?

2026 Executive Summary

As of January 2026, USB technology has evolved into the USB4 Version 2.0 standard, offering speeds up to 80 Gbps (symmetric) and power delivery up to 240 Watts via USB Type-C. While legacy interfaces (Type-A, Micro-USB) remain in use for older peripherals, the modern ecosystem is standardized on the versatile Type-C connector for data, video, and power. This guide covers the complete architecture from legacy USB 1.1 to the cutting-edge USB4 generation.

Catalog

Ⅰ How is the USB Software Structure Defined?

The USB software architecture relies on a single host managing a tiered system of clients, hubs, and function drivers. It includes the following specific layers:

1. Bus interface

The USB bus interface handles the critical interconnection between the electrical layer and the protocol layer. From the perspective of interconnection, similar bus interfaces are presented by the device and the host simultaneously, such as the Serial Interface Engine (SIE). The USB bus interface is implemented physically by the main controller.

The USB system uses a host controller to manage data transfer between the host and USB devices. The interface between it and the main controller depends on the hardware definition. The USB system manages resources such as bandwidth allocation and bus energy (power management), which is critical for modern PD 3.1 standards. The USB system comprises three basic components:

• Host Controller Driver (HCD): Maps different host controller devices to the USB system. The interface between HCD and USB is called HCDI.

• Universal Host Controller Driver (UHCD): Located at the bottom of the software structure, it manages and controls the main controller, handling communication while remaining hidden from other system software parts.

• USB Driver (USBD): Sitting on top of the UHCD, it provides a driver-level interface (USBDI) to meet existing device driver design requirements. USBD transmits data via I/O request packets (IRPs) through specific "Pipes."

2. Host software

In some operating systems, specific USB system software is not provided natively. In these legacy scenarios, the software was used to provide configuration information and loading structure to the device driver. In modern operating systems (Windows 11, macOS, Linux), the device driver uses the provided interface instead of directly accessing the USBDI structure.

3. USB client software

This is located at the highest level of the software structure and handles specific USB device drivers. The client program layer describes all software entrances directly acting on the device. When a device is detected by the system, these client programs act on the peripheral hardware. This shared feature places the USB system software between the client and its device, which is processed by the client program according to the device image formed by the USBD.

Ⅱ What Comprises the USB Hardware Architecture?

USB hardware utilizes a cascaded star topology connecting a host controller to up to 127 devices via hubs and cables. Modern USB 3.x and USB4 cables use multiple differential pairs for data and dedicated lines for power negotiation (CC lines). USB is a token-based bus where the host controller broadcasts tokens, and devices respond if the address matches. Crucially for 2026, the hardware supports advanced power states (suspend/resume) and Power Delivery (PD) up to 240W.

USB Topology: Host, Hubs, and Functions

The Host (Root): Built onto the motherboard or installed as an adapter card. It contains the main controller and a root hub, controlling data flow. Each USB system has one root hub.

The Hub: A specific component that provides "ports" to connect devices to the bus. It detects connections, manages power for attached devices, and handles bus failure recovery. Hubs can be self-powered or bus-powered.

The Function: The end peripheral (e.g., mouse, SSD, camera) connected to the bus.

Ⅲ The Evolution of USB Standards (1996–2026)

Legacy Standards (USB 1.1 & 2.0)

USB 1.0 appeared in 1996 with a speed of just 1.5Mb/s. By 1998, USB 1.1 increased this to 12Mb/s (Full Speed). USB 2.0, released in 2000, brought "High Speed" data at 480Mbps (60MB/s). While slow by 2026 standards, USB 2.0 is still widely used for keyboards, mice, and audio interfaces due to its reliability and low cost.

USB 3.x Generations (5Gbps - 20Gbps)

The USB 3.0 era introduced "SuperSpeed." It has undergone several renaming phases by the USB-IF. Here is the modern naming convention used in 2026:

• USB 3.2 Gen 1 (formerly USB 3.0): Transfer rate of 5Gbps.

• USB 3.2 Gen 2 (formerly USB 3.1): Transfer rate of 10Gbps.

• USB 3.2 Gen 2x2: Uses two lanes to achieve 20Gbps (Requires USB-C).

USB4 & USB4 Version 2.0 (The 2026 Standard)

USB4, introduced in 2019, is based on the Thunderbolt 3 protocol. It dynamically shares bandwidth between data, video, and power. As of 2026, the ecosystem is dominated by USB4 Version 2.0.

Key Specs for 2026:

• Max Speed: 80 Gbps (symmetric) or 120 Gbps (asymmetric for 8K displays).

• Tunneling: Encapsulates USB 3.2, DisplayPort 2.1, and PCIe traffic.

• Connector: Exclusively uses USB Type-C.

Ⅳ Guide to USB Connector Types: Legacy to Type-C

USB Type-A (Legacy Host Connector)

While modern devices have standardized on Type-C, legacy connectors remain in use for industrial and older consumer electronics. Below is a breakdown of these interface types.

Mini-B (Legacy)

The Mini-B 5-pin was once the standard for MP3 players, older digital cameras, and portable hard drives. It provided excellent anti-misplug performance but was relatively bulky compared to modern standards.

Micro-USB (Legacy)

Micro-USB became the global standard for mobile phones before Type-C. It supports data rates up to 480Mbps (USB 2.0). It is physically fragile compared to Type-C and supports lower charging wattages.

USB Type-C (The 2026 Standard)

Type-C is the universal interface for 2026, mandated by regulations (such as the EU Common Charger Directive) for mobile devices. It solves the "reverse insertion" problem and supports massive bandwidth.

Key Type-C Capabilities in 2026:

• Reversibility: Symmetrical design allows insertion in either direction.

• Power Delivery (EPR): Supports "Extended Power Range" up to 240 Watts (48V/5A), capable of powering high-end gaming laptops.

• Alt Modes: Can transmit DisplayPort, HDMI, and Thunderbolt signals simultaneously with data.

Frequently Asked Questions (2026 Update)

What is the difference between USB4 and Thunderbolt 4/5?

USB4 is built on the Thunderbolt protocol. While Thunderbolt 4 guarantees minimum specs (like 40Gbps and dual 4K display support), USB4 can vary. However, the new Thunderbolt 5 and USB4 Version 2.0 both support speeds up to 80Gbps, effectively converging the high-end performance capabilities in 2026.

Can I charge my laptop with any USB-C cable?

Not necessarily. While the plug fits, cables are rated for different power levels. Standard cables support 60W, while high-performance cables support 100W. To charge a high-powered 2026 laptop, you need a cable marked "EPR" (Extended Power Range), which supports up to 240W.

Is USB Type-A obsolete in 2026?

Partially. While most new smartphones and laptops feature exclusively USB-C ports, USB Type-A is still common on desktop motherboards, gaming consoles, and industrial equipment for backward compatibility with older mice, keyboards, and flash drives.