Security researcher Fabien Sanglard has published a technical 'cheat sheet' to resolve widespread confusion surrounding USB naming conventions and performance specifications.
Writing on his website, fabiensanglard.net, on May 05, 2022, Sanglard explained that the guide was prompted by a personal error. "I spend time investigating a non-existing bug today because I misunderstood a USB term," Sanglard said.
The guide provides a breakdown of how marketing names like 'SuperSpeed' and 'SuperSpeedPlus' correspond to actual signal speeds and hardware requirements. According to the documentation, USB 1.1 operates at 12 Mbps with a 4-meter cable limit, while the latest USB4 40Gbps standards can reach 40,000 Mbps using 12-wire configurations.
Technical confusion often stems from the overlapping nomenclature used across different USB generations. The documentation clarifies that 'USB 3.0,' 'USB 3.1,' 'USB 3.2,' 'USB 3.1 Gen 1,' and 'USB 3.2 Gen 1' all refer to the same 5Gbps specification.
Sanglard’s breakdown details how the number of wires in a cable impacts performance and signal speed. A 4-wire setup is used for USB 1.1 and USB 2.0, while an 8-wire configuration is used for SuperSpeed USB 5Gbps and SuperSpeedPlus USB 10Gbps. The 12-wire configuration is utilized for SuperSpeedPlus USB 20Gbps, USB4 20Gbps, and USB4 40Gbps.
The guide also highlights the discrepancy between advertised signal speeds and effective data rates due to encoding overhead. For example, while USB 3.2 Gen 1x1 advertises a 5,000 Mbps signal, the effective rate after 8b/10b encoding drops to 4,000 Mbps, with a real-life rate of approximately 400 MiB/s.
Under the Gen naming convention, 'A' represents the generation and 'B' represents the number of lanes used. For instance, USB 3.2 Gen 2x2 utilizes 128b/132b encoding to achieve a signal total of 20,000 Mbps, resulting in an effective 19,392 Mbps and a real-life rate of 1,600 MiB/s.
Cable length limits also vary significantly by standard. USB 1.1 and USB 2.0 are limited to 4-meter cables, whereas SuperSpeedPlus USB 20Gbps and USB4 20Gbps require much shorter 1-meter cables. The highest-performing USB4 40Gbps standard is restricted to a 0.8-meter cable length.
