Interface of USB has not been changed many times until now.
◎USB 3.0 was released in Jan of 2008 and its transmission speed can be 5Gbps, called SuperSpeed.
◎USB 3.1 was released in 2013 and its transmission speed can be upgraded to 10Gbps.
At that time, USB-IF would like to let hardware vendors know that two speed of 5Gbps and 10Gbps can be handled at the same time so that he rename USB 3.0 as USB 3.1. Transmission speed of USB 3.1 Gen 1 can be 5Gbps and one of USB 3.1 Gen 2 can be 10Gbps.
The USB3.2 dual channel announced by the USB Association in 2017 finally officially established the specification in this year.
The USB 3.2 standard will allow master-slave devices to operate in dual-channel 5 Gbps (10G) or dual-channel 10 Gbps (20G). Compared to current Type-C cables, the transmission bandwidth can be doubled to allow maximum transmission rate up to 20GB/s.
In addition to dual-channel support, USB 3.2 continues to use the SuperSpeed USB layer data rate and encoding technology, Update the hub specification to implement from single-channel to dual-channel operation. Besides, USB 3.2 is also support backward compatible with older versions device.
To avoid confusion on names, USB-IF redefines these names to USB 3.2 as follows:：
◎USB 3.1 Gen 1 changed to USB 3.2 Gen 1 rate 5Gbps.
◎USB 3.1 Gen2 changed to USB 3.2 Gen 2 rate 10Gbps.
◎USB3.2 changed to USB 3.2 Gen 2×2 rate 20Gbps.
Why is it marked as 2×2? Since the traditional USB only supports single-sided transmission, but the new USB 3.2 will support double-sided, and the 10Gbps transmission speed will up to 20Gbps. Since only Type-c can support double-sided transmission, USB3.2 is only standardized on Type-c.