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USB 3.1: What’s in a Name?
By Morten Christiansen, Technical Marketing Manager, USB IP
Clarity around names and nomenclature enable effective communication about a topic. Standards bodies define the nomenclature for their standards and specifications, and engineers and designers use what makes sense for them, often generating nicknames as well. Long term, the ecosystem around a standard will coalesce and use the most effective terminology. This article describes the USB 3.1 terminology as defined in the USB-IF specification to give designers and users a starting point for understanding the wide range of applicable USB terms.
The USB 3.0 specification is also known as the SuperSpeed USB specification. The SuperSpeed USB specification introduced a subtle change in spelling compared to the previous USB speeds. They are now officially named “low-speed,” “full-speed,” and “high-speed,” and are often abbreviated to LS, FS, and HS.
The inappropriate use of names and terminology can cause considerable confusion and grievance. For example, when USB designers use the term “BC,” you can assume the discussion relates to the USB-IF Battery Charging specification, version. 1.2. However “BC” could also refer to EN 62684:2010, IEC 62684:2011, or ITU TL.1000 which are all based on the USB-IF Battery Charging specification version 1.1. CCSA YD/T1591 2006, yet another BC specification, actually refers to a prerelease version (0.77) of the original USB-IF battery Charging specification that was officially released as version 1.0 in 2007.
The USB 3.1 specification supersedes the USB 3.0 specification and had to accurately describe the USB 3.0 or legacy SuperSpeed features, the new USB 3.1 features, and generic features. Of vital importance is backwards compatibility between devices, hubs and hosts built to the USB 3.0 specification and the new USB 3.1 specification. All combinations of old and new hosts, hubs and devices operating in any mode must be accurately described with the new nomenclature.
The new nomenclature has to be both accurate and efficient (Figure 1).
The term Enhanced SuperSpeed system is used to describe the any combination of a device/host, protocol layer, link layer, and physical layer that adheres to the USB 3.1 specification.
At the electrical or physical level, PHYs operate in Gen 1 or Gen 2 mode. Gen 1 uses the 5 Gbps signaling rate described in the USB 3.0 specification. Gen 2 uses a new 10 Gbps signaling rate described in the USB 3.1 specification.
SuperSpeed USB is used to describe the legacy link, protocol and host/device controllers at the architecture or system abstraction level for Gen 1 operation. The term SuperSpeedPlus USB is used to describe the new link, protocol and controller operation for Gen 2.
Figure 1: Enhanced SuperSpeed terminology as described in the USB 3.1 specification
“Enhanced SuperSpeed” vs “USB 3.1”
Describing a device that supports speeds up to 10 Gbps as well as backward compatibility for USB 3.0 systems can be tricky. To correctly and accurately describe a new USB 3.1 device, take the following statement as an example:
This SuperSpeedPlus device has an Enhanced SuperSpeed port and supports Enhanced SuperSpeed speeds.
The two last parts of the statement describe a device that can operate at either Gen 1 or Gen 2 speeds. It would have been sufficient to state “This device has an Enhanced SuperSpeed port.” Or “This device supports Enhances SuperSpeed speeds.”
The statements “This is an Enhanced SuperSpeed device” and “This is a USB 3.1 device” are useless as they do not describe if the device is capable of SuperSpeed only or either SuperSpeed or SuperSpeedPlus operation. The term “USB 3.1 SuperSpeed device” could be used to describe a device that is only capable of Gen 1 operation. However, “SuperSpeed only device” is a better description.
An Enhanced SuperSpeed device can be described differently at different times. A SuperSpeedPlus capable device operating in SuperSpeed or Gen 1 mode is demoted to being a SuperSpeed device. An accurate description is that an Enhanced SuperSpeed device is operating either in SuperSpeed or SuperSpeedPlus mode.
An Enhanced SuperSpeed hub or host must support both SuperSpeed and SuperSpeedPlus devices at the same time. An Enhanced SuperSpeed hub must be capable of operating in both SuperSpeed and SuperSpeedPlus mode at the same time. And one would describe a specific Enhanced SuperSpeed port on an Enhanced SuperSpeed hub or host as operating in either SuperSpeed or SuperSpeed Plus mode.
Some examples of correct usage of the new nomenclature are shown in Table 1:
Table 1: USB Terminology
Conclusion
Correctly describing all aspects of an Enhanced SuperSpeed system is not simple, even for the initiated. Many designers and architects closely involved with the USB 3.1 specification do not use this new terminology consistently or correctly. A common and understandable simplification is to use the term USB 3.0 for Gen 1 operation and USB 3.1 for Gen 2 operation, or 5G USB and 10G USB.
Designers may also choose to simplify the terminology by using LS, FS, HS, and SS to describe low-speed, full-speed, high-speed and SuperSpeed USB operation respectively, and introduce SSP for SuperSpeedPlus. This simple solution is accurate enough for everyday use.
Overall, designers and users must be careful about how they use the terminology around the USB specifications to ensure clarity and effective communication.
For more information on the USB 3.1 specification, download the white paper, USB 3.1: Evolution and Revolution.