The difference between Gen1 and Gen2, Gen3 -ChangZhou Sun-Rise Electronic Co.,Ltd.

After the USB interface has experienced USB1.0, USB1.1, USB2.0, USB3.0, USB3.1 and USB3.2 (the past and present of USB - USB2.0, USB3.0, USB3.1, USB3.2 , What is the difference between USB4.0), ushered in the current USB4, under the alternation of these standards, the transmission rate of the USB interface has been increasing, following the increase in the transmission rate and the complexity of the naming of the interface standard, take USB3. 1, it is actually divided into USB3.1 Gen1 and USB3.1 Gen2, which makes many consumers confused and confused. Everyone probably has this question: They are all USB standard interfaces, why should they be divided in this way? What is the difference between USB3.1 Gen1 and Gen2? Now USB4 has a Gen3 cable, today we will come to popular science, the difference between USB4 standard and Gen1, Gen2, Gen3.

Before introducing USB4, I have to mention the Thunderbolt protocol jointly launched by Intel and Apple. The Thunderbolt protocol is a combined protocol that integrates the DisplayPort protocol (DP for short) and the PCI-Express protocol (PCIe for short). The Thunderbolt 1 and Thunderbolt 2 protocols were launched successively, but the responses were mediocre. Later, the Thunderbolt 3 protocol (TBT3 for short) was launched, and the transmission speed reached 40Gbps. Later, Intel chose to hand over the protocol to the USB-IF Association. In 2019, USB-IF integrated the Thunderbolt 3 protocol (TBT3 for short) on the basis of USB 3.2, and launched USB4 Gen 2x2 and USB4 Gen 3x2. The difference between this renaming and the previous one is that it still retains the name of USB 3.2. USB4 doubles the transmission speed of the original USB 3.2 to 40Gbps, which is the speed of USB4 Gen 3x2. USB4 supports both USB 3.2 and TBT3 transmission. In short, USB4=USB 3.2+TBT3. Like USB 3.2, USB4 also has single-channel mode and dual-channel mode. The USB4 interface, like the USB 3.2 interface, supports the USB PD fast charging protocol.

One sentence summary: Gen refers to the transmission rate, the theoretical transmission rate of Gen2 is 10Gbps, while Gen1 is 5Gbps, USB3.2 Gen2x2 is 20Gbps, USB4 Gen3x2 is 40Gbps, as shown in the figure below

USB is a protocol specification for transmitting data, and it is also an interface technical specification. In 1996, USB Standardization Organization (USB Implement Forum, referred to as USB-IF), a non-profit organization composed of seven industry giants including Intel, Microsoft, Compaq, DEC and IBM in the United States, NEC in Japan, and Northern Telecom in Canada, launched USB 1.0, followed by faster USB 1.1 and USB 2.0. In 2008, the USB 3.0 Promoter Group Industry Technology Alliance composed of Intel, Apple, HP, Microsoft, Renesas Electronics, STMicroelectronics and Texas Instruments released the USB 3.0, then transferred to USB-IF to manage and maintain. From 2013 to 2019, the USB 3.0 Promoter Group jointly launched by Intel and other large companies successively launched the USB 3.1, USB 3.2 and USB4 protocol specifications, which were released and updated on the USB-IF official website.

It can be seen from the evolution diagram of USB that USB has gone through 24 years from USB 1.0 to USB4. The most obvious improvement is the increase in bandwidth. USB 2.0 and before mainly support low speed (1.5Mbps), high speed (12Mbps) and full speed (480Mbps). When USB 3.0 starts to enter the super speed mode, and every time it is upgraded, the bandwidth doubles, from the initial 5Gbps to the current 40Gbps.

The USB interface is divided into USB1.0, USB 1.1, USB2.0, and USB 3.0 according to the specification. The USB2.0 interface and USB 3.0 interface we often talk about are classified according to the USB specification. In 2013, USB 3.0 was renamed as USB 3.1 Gen 1, and USB 3.1 Gen 2 with 10Gbps bandwidth was launched at the same time, both of which are collectively referred to as USB 3.1. In 2017, USB 3.1 Gen 1 and USB 3.1 Gen 2 were renamed USB 3.2 Gen 1 and USB 3.2 Gen 2, respectively. At the same time, USB 3.2 Gen 1x2 with a bandwidth of 10Gbps and USB 3.2 Gen 2x2 with a bandwidth of 20Gbps have been added. These four are collectively referred to as USB 3.2. At this point, it has entered the era of USB 3.2, and the name of USB 3.0 has become history. In short, USB 2.0 is still retained, and USB 3.0 has now been renamed USB 3.2 Gen 1 by the USB-IF Association, and there are more USB 3.2 Gen 2, USB 3.2 Gen 1x2 and USB 3.2 Gen 2x2. Among them, USB 3.2 Gen 1x2 and USB 3.2 Gen 2x2 indicate the dual-channel mode of USB 3.2 Gen 1 and USB 3.2 Gen 2, while USB 3.2 Gen 1 and USB 3.2 Gen 2 are single-channel modes.

Both USB 3.1 and USB 3.2 were introduced to increase bandwidth, and the design goals of USB4 remain the same. However, the release of this specification is also to integrate the USB Type-C ecosystem and reduce confusion for end users. Although the new USB4 standard introduces a new underlying protocol, it is still compatible with the existing USB3.2, USB2.0 and Thunderbolt 3. USB4 will adopt a two-way dual-simplex architecture, which doubles the transmission bandwidth of Type-C. The transmission rate must reach at least 20Gbps, and 40Gbps is optional. The maximum transmission rate is twice that of the previous generation of USB 3.2. The certified interface and cable Two different Logos will also be provided (Intel announced the USB4 interface connector, which is divided into two versions). For 40Gbps USB4, the encoding method is still 128b/132b used by USB 3.2, and 20Gbps will use 64b/66b, that is, 40Gbps USB4 needs to send 130 bits for every 128 bits transmitted. Then, each Lane of the USB4 protocol Support 8 * 128 / 130 = 7.877 Gbps = 984.6 MB/s rate, a USB4x2 channel (USB4 Gen 3 x 2 is full-duplex, dual-channel data transmission, each channel sends and receives data at the same time), x2 The available bandwidth is 7.877*2 = 15.754 Gbps = 15.754 GB/s, and the two-way bandwidth is as high as 31.5GB/s. The encoding method determines the bandwidth and transmission speed.

For USB, each channel has two differential pairs, one for transmit and one for receive. PCIe is also full-duplex communication, which will send and receive data at the same time, and the definition of the channel also conforms to this. The figure below shows the USB Type-C female socket, which has two channels, high-speed channel 0 and high-speed channel 1, which we call Lane 0 Adapter and Lane 1 Adapter. That is, channel 0 adapter and channel 1 adapter. The adapter here can be understood as a signal converter. USB is full-duplex communication, each channel has two differential pairs, one for sending and one for receiving. That is, the differential pair TX1+ and TX1- is used to send data, the differential pair RX2+ and RX2- is used to receive data, and the two differential pairs form high-speed lane 0, that is, Lane 0 Adapter. Similarly, the differential pair RX2+ and RX2- is used to receive data, the differential pair TX2+ and TX2- is used to transmit data, and the two differential pairs form high-speed lane 1, that is, Lane 1 Adapter. Lanes connected to Lane 0 Adapter and Lane 0 Adapter are called Lane 0 and Lane 1. Single-lane mode means that only one of lane 0 and lane 1 is used to transmit USB data, and the other is used for other purposes. For example, USB 3.2 Gen 1 and USB 3.2 Gen 2 both transmit data through a single channel. Among them, the transmission speed of USB 3.2 Gen 1 is 5Gbps, because a single channel is sending and receiving at the same time, so the transmission speed refers to the speed of one of sending or receiving. Take lane 0 as an example, there are two differential pairs TX1+, TX1- and RX2+, RX2-, the transmission speed of lane 0 refers to the speed of TX1+, TX1- differential channel is 5Gbps, or RX2+, RX2- differential The speed of the channel is 5Gbps. When using lane 1 to transmit data, the speed is the same. The transmission speed of USB 3.2 Gen 2 is 10Gbps, which is doubled on the basis of USB 3.2 Gen 1. In fact, it means that the transmission speed of USB 3.2 Gen 2 is doubled to 10Gbps through the sending or receiving differential channel of lane 0. Similarly, use lane 1 When transmitting data, the transmission speed of the differential channel for sending or receiving is also 10Gbps.

Dual-channel mode means that both lane 0 and lane 1 are used to transmit USB data. For example, USB 3.2 Gen 1x2 and USB 3.2 Gen 2x2 of USB 3.2, USB4 Gen2x2 and USB4 Gen3x2 of USB4, all transmit data through dual channels. How to judge whether it is single-channel mode or dual-channel mode? If the number after "Gen" has the word "x2", it means dual-channel mode. If there is no word "x2" or the word "x1" is written, it means single-channel mode. With the same USB version specification, the transmission speed of the dual-channel mode is twice that of the single-channel transmission speed. For example, the transmission speed of USB 3.2 Gen 1x2 is twice that of USB 3.2 Gen 1, because USB 3.2 Gen 1 uses only one 5Gps channel to transmit data, while USB 3.2 Gen 1x2 uses two 5Gps channels to transmit data, so The transmission speed becomes 10Gbps. The transmission speed of USB 3.2 Gen 2 with single-channel transmission is doubled on the basis of USB 3.2 Gen 1 (5Gbps) with the same single-channel transmission, and becomes 10Gbps. Similarly, the transmission speed of USB 3.2 Gen 2x2 with dual-channel transmission Naturally, it becomes twice that of USB 3.2 Gen 2, that is, 20Gbps. USB4 Gen2x2 of USB4 is a dual-channel transmission, which maintains the same transmission speed of 20Gbps as USB 3.2 Gen 2x2. USB4 Gen3x2 of USB4 is also a dual-channel transmission, which is doubled on the basis of USB4 Gen2x2 and becomes 40Gbps. As mentioned above, starting from USB 3.2, only USB Type-C interface is supported, and Type-A and Type-B interfaces are no longer supported. Why is this? Because the dual-channel mode of USB data transmission has been defined since USB 3.2, that is, two sets of differential pairs are used for sending, and two sets of differential pairs are used for receiving. Four sets of differential pairs are required, and only the USB Type-C interface can provide four sets of differential lines. . In single channel mode, one set of differential pairs is used for transmission and one set of differential pairs is used for reception. Type-A and Type-B interfaces can only provide 2 pairs of differential lines, and are only applicable to single-channel mode. In short, because of the introduction of the dual-channel USB data transmission mode, USB 3.2 and later USB4 can only support the USB Type-C interface.

According to the model, the USB interface is divided into three types: Type-A, Type-B and Type-C. Standard-A belongs to Type-A, Micro-B belongs to Type-B, USB 3.1 interface and before, in addition to Type-C, there are also Type-A and Type-Micro B; Type-MINI B and other models, and to USB The 3.2 interface only supports Type-C. It is because the past connectors such as USB Type-A or Micro-B only support single-channel transmission and cannot support USB3.2 and USB4. The fastest USB transmission speed supports 40G (20Gbps x2), and can be transmitted at the same time.

ChangZhou Elec IMP&EXP Co.,Ltd.

Jackee Loo

April 22, 2023