The Evolution of USB (Universal Serial Bus) Standards

9 min readJul 4, 2023

In the 1990s, office desktops were a tangled mess of serial, parallel, and proprietary cables. Universal Serial Bus (USB) changed all of that, simplifying computer connectivity through a small, inexpensive interface. It is a plug-and-play interface that allows a computer to communicate with peripheral devices. USB-connected devices cover a wide range of electronics like keyboards, mice, game controllers, printers, scanners, digital cameras, and removable media drives, just to name a few. The USB interface can also be used to provide power to low-power peripherals such as smartphones. With billions of USB devices now in daily use, USB is the most widely used wired interface for laptops, tablets, phones, and other devices.

How Does USB Work?

When a peripheral device is attached via USB, the host computer will detect what kind of device it is and automatically load a driver that allows the device to function. Data is transferred between the two devices in small amounts known as ‘packets’. A set number of bytes (a unit of digital information) is transmitted with each packet. Other information is also sent, including- the source of the data, the destination of the data, the length of the data, and details of any errors that have been detected.

Four types of data transfer can occur :

Interrupt Transfer — Peripheral devices such as keyboards and mice use this type of data message to send smaller amounts of data. Such transfers are often used for less frequent but important requests. The devices generate the requests, though they must wait for the host to inquire about the specific data the remote device needs. Such requests are guaranteed to be reattempted if the first transfer fails. These transfers will also let you know about any changes to the status of the device.
Bulk Transfer — Used by printers and digital scanners for large amounts of data, this type of transfer is low-priority and not time-critical. The transfer will slow down if the host computer has several USB devices connected.
Isochronous Transfer — Audio, video, and other real-time data use isochronous transfer. Errors can occur during the transfer, though the transfer will not be interrupted to resend the packets. However, such transfers usually involve situations where the accuracy of the data is not critical, such as audio elements that may not be picked up by the listener. Missing these elements is preferable to retrying data, which could result in glitching audio.
Control Transfer — This type of data transfer is used to configure and control a USB device. The host sends a request to the device and the data transfer follows. Control transfers are also used to check status. Only one control request is handled at any one time.

USB Protocol

USB data is transferred in so-called transactions. Normally, they consist of three packets:

The token packet is the header defining the transaction type and direction, the device address, and the endpoint.
Data is transferred in a data packet.
The final status of the transaction is acknowledged in the handshake packet.

IN Transaction — IN Transaction is initiated with IN Token from host followed by data packet by the device. If the device is unable to send data, it uses STALL/NAK handshake. The host can return only one type of handshake ACK. If the host received a corrupted data packet it discards data and issues no response. No handshake packet for isochronous transfer.

OUT Transaction — OUT Transaction is initiated with OUT Token from host followed by data packet by the host. The device may return any one of three handshake types. If the device received a corrupted data packet it discards data and issues no response. No handshake packet for isochronous transfer.

Setup Transaction — Setup Transaction is similar to OUT, but the data payload is exactly 8 bytes (DATA0 only). Upon receiving the setup token, the device must accept the data. It is not permitted for a device to respond to a SETUP token with a NAK or a STALL.

Split Transaction — It is used to initiate a full-/low-speed transaction via the hub and some full-/low-speed device endpoints. The split transaction has two parts: Start Split transaction (SSPLIT) and Complete Split transaction (CSPLIT).

USB Standards

Released in 1996, the USB standard is maintained by the USB Implementers Forum (USB-IF). There have been four generations of USB specifications: USB 1.x, USB 2.0, USB 3.x, and USB4. There is also the less known Wireless USB standard which was not widely adopted. A group of seven companies began the development of USB in 1994: DEC, Nortel, NEC, Intel, Microsoft, IBM, and Compaq.

1. USB 1.x — It was released in January 1996, USB 1.0 specified data rates of 1.5 Mbit/s and 12 Mbit/s. Few USB devices made it to the market until USB 1.1 was released in August 1998. USB 1.1 was the earliest revision that was widely adopted and led to what Microsoft designated the “Legacy-free PC”. It provided a Master / Slave interface and a tiered star topology which was capable of supporting up to 127 devices and a maximum of six tiers or hubs. The master or “Host” device was normally a PC with the slaves or “Devices” linked via the cable. The cable length for USB 1.1 was limited to 5 meters, and the power consumption specification allowed each device to take up to 500mA, although that was limited to 100mA during start-up.

2. USB 2.0 — The USB 2.0 standard is a development of USB 1.1 which was released in April 2000. The main difference, when compared to USB 1.1, was the data transfer speed increase up to a “High Speed” rate of 480 Mbps. However, it should be noted that even though devices are labeled USB 2.0, they may not be able to meet the full transfer speed. The data encoding method for this version of USB is Unicode. In addition to the improvements in data capability USB 2 also saw an increase in the power provision to 1.8A. This enabled USB to provide charge for smartphones that were increasingly charging faster and also for more power-hungry peripherals such as external drives, etc. When compared to USB 1, this provided a much-needed improvement in current capability.

3. USB 3.x — The USB 3.0 specification was released on 12 November 2008. USB 3.0 adds a SuperSpeed transfer mode, with associated backward compatible plugs, receptacles, and cables. SuperSpeed plugs and receptacles are identified with a distinct logo and blue inserts in standard format receptacles. USB 3.0 also introduced the UASP protocol, which provides generally faster transfer speeds than the BOT (Bulk-Only-Transfer) protocol.

USB 3.1, released in July 2013 has two variants. The first one preserves USB 3.0’s SuperSpeed transfer mode and is labeled USB 3.1 Gen 1, and the second version introduces a new SuperSpeed+ transfer mode under the label of USB 3.1 Gen 2. SuperSpeed+ doubles the maximum data signaling rate to 10 Gbit/s while reducing line encoding overhead to just 3% by changing the encoding scheme to 128b/132b.

USB 3.2, released in September 2017, preserves existing USB 3.1 SuperSpeed and SuperSpeed+ data modes but introduces two new SuperSpeed+ transfer modes over the USB-C connector with data rates of 10 and 20 Gbit/s (1.25 and 2.5 GB/s). The increase in bandwidth is a result of multi-lane operation over existing wires that were intended for the flip-flop capabilities of the USB-C connector.

4. Wireless USB — The concept for wireless USB is that, as the name suggests, it provides a wireless connection over which data can be transferred. This USB standard has not been as widely adopted even though the standard was released in September 2010. Sometimes the abbreviation WUSB is used, although the USB-IF does not encourage the use of this abbreviation. Wireless USB uses frequencies in the band 3.1–10.6 GHz and provides a data bandwidth of 53–480 Mbps over a distance of up to 3 to ten meters.

5. USB 4 — The USB4 specification was released on 29 August 2019 by USB Implementers Forum. USB4 is based on the Thunderbolt 3 protocol specification. It supports 40 Gbit/s throughputs, is compatible with Thunderbolt 3, and backward compatible with USB 3.2 and USB 2.0. The architecture defines a method to share a single high-speed link with multiple end device types dynamically that best serves the transfer of data by type and application.

USB Standards and Patent Pool

Intel Corporation was granted a patent (US5694555A) on USB technology in 1997, but the protocol system was created in conjunction with six other companies. This technology has become a standard in the industry. However, from both a legal and a business perspective, patents, and industry standards can be problematic. Companies need to fully disclose all relevant patent information before trying to implement proposed industry standards. It might also be expensive to protect and maintain these types of patents and their proprietary nature might hinder wide-spread use. One solution to these problems is to create a “patent pool” — something used by companies associated with the USB standard. All holders of essential patents relating to USB technology — either the first or subsequent USB versions — have combined their patents into one pool called the USB Implementers Forum.

This organization allows other participating companies to license USB patents under RAND (Reasonable and Non-Discriminatory) terms without having to pay royalties. Implementers, who use the USB standard, are free to create products that support the standard without fear of lawsuits from the patent holders.

Top Players

The graph shows the top 10 players who have been assigned the most number of patent families related to USB technology. These patent families come under the CPC classification of G06F13/409 (Mechanical coupling) or H01R27/00 (Coupling parts adapted for co-operation with two or more dissimilar counterparts). There are a total of 1,681 patent families in this category out of which 1015 are still alive. The top 2 assignees, Hon Hai Precision Industry and Apple own 39 and 27 patent families respectively related to USB technology. HP, Intel, and Samsung are ranked 4th, 5th, and 6th positions with an equal number of patent families, i.e. 21, to their name.

Conclusion

With the ever-increasing requirements for faster and larger levels of data transfer as well as increased levels of convenience and capability, the concept for USB has evolved. The levels of functionality that are available today are enormous when compared to the first standard release in the late 1990s. Since then several new versions of the USB standard have been introduced, each providing a greater level of performance. However, as new standards bring more speed, power, and versatility to market, they also bring a complex assortment of features and capabilities to consider when deciding which cable or peripheral is right for your application.

References

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The Evolution of USB (Universal Serial Bus) Standards