USB Bandwidth

Tue Apr 04 20:55:44 CST 2023

0_USB-Bandwidth-Hornmic

USB Bandwidth

What is bandwidth?

Bandwidth, also known as bandwidth, refers to the amount of data that can be transmitted at a fixed time, i.e., the ability to transmit data in the transmission pipeline. In digital devices, bandwidth is usually expressed in bps, which is the number of bits per second that can be transmitted. In analog devices, bandwidth is usually expressed in cycles per second or hertz (Hz). 

The term "bandwidth" has two different meanings in computers:

1. indicates the band width

  The bandwidth of a signal is the range of frequencies occupied by the various frequency components contained in the signal. Bandwidth is especially important for Basic Output System (BIOS) devices, such as fast disk drives, which can be hindered by low bandwidth buses. 

2. Indicates the capacity of a communication line to transmit data  

The "maximum data rate" that can pass from one point in the network to another per unit of time. A good analogy for the concept of bandwidth is a highway. The amount of data that can be transmitted on a line per unit of time is commonly measured in bps (bits per second). The bandwidth of a computer network is the highest rate of data that can pass through the network, i.e., how many bits per second.

USB Bandwidth

For USB, there are two differential pairs per channel, one for transmit and one for receive.

1_USB_two_differential_pairs_Lane

The figure below shows the USB Type-C receptacle and plug, each with two channels, namely high-speed channel 0 and high-speed channel 1, which we call Lane 0 Adapter and Lane 1 Adapter. i.e. Channel 0 Adapter and Channel 1 Adapter. USB is full duplex communication, each channel has two differential pairs, one for transmitting and one for receiving. Similarly, differential pairs TX1+ and TX1- are used to send data, and differential pairs RX2+ and RX2- are used to receive data, and the two differential pairs form high-speed channel 0, i.e. Lane 0 Adapter. The channels connected to Lane 0 Adapter and Lane 0 Adapter are called Lane 0 and Lane 1.

2_USB-C-dual-Lane-channels

Single-channel mode means that only one of lane 0 and lane 1 is used to transfer USB data, and the other is used for other purposes. For example, USB 3.2 Gen 1 and USB 3.2 Gen 2 are both used to transfer data through a single channel. The USB 3.2 Gen 1 has a transfer speed of 5Gbps, because single channel is sending and receiving at the same time, so the transfer speed refers to the speed of one of the sending or receiving. Take lane 0, 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 the speed of RX2+, RX2- differential channel is 5Gbps, when using lane 1 to transmit data, the speed is the same. USB 3.2 Gen 2 The transfer 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 transfer speed of USB 3.2 Gen 2 is doubled to 10Gbps through the differential channels sent or received by lane 0. Similarly, when transmitting data with lane 1, the transfer speed of the differential channels sent or received is also 10Gbps.

3_USB-C-functional-Model

Dual-channel mode is where both lane 0 and lane 1 are used to transfer USB data. For example, USB 3.2 Gen 1x2 and USB 3.2 Gen 2x2 for USB 3.2 and USB4 Gen2x2 and USB4 Gen3x2 for USB4 are used to transfer data via dual channel. How to determine 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 "x2" or the word "x1" is written, it means single-channel mode. If there is no "x2" or it says "x1", it means single channel mode.

With the same USB version specification, the dual channel mode transfer speed is twice as fast as the single channel transfer speed. For example, the transfer speed of USB 3.2 Gen 1x2 is twice as fast as USB 3.2 Gen 1 because USB 3.2 Gen 1 uses only one 5Gps channel to transfer data, while USB 3.2 Gen 1x2 uses two 5Gps channels to transfer data, so the transfer speed becomes 10Gbps.

The transfer speed of USB 3.2 Gen 2 for single-channel transmission is doubled from USB 3.2 Gen 1 (5Gbps) for the same single-channel transmission to 10Gbps, and the transfer speed of USB 3.2 Gen 2x2 for dual-channel transmission naturally becomes twice that of USB 3.2 Gen 2, i.e. 20Gbps.

USB4 Gen2x2 is a dual-channel transfer that maintains the same transfer speed of 20Gbps as USB 3.2 Gen 2x2. USB4 Gen3x2 is also a dual-channel transfer that doubles the speed of USB4 Gen2x2 to 40Gbps.

4_USB-cable-compatibility-summary

From USB 3.2 onwards only support USB Type-C interface, and no longer support Type-A and Type-B interface. Why is this? Because from USB 3.2 defines the USB data transmission dual-channel mode, that is, two sets of differential pairs used to send, two sets of differential pairs used to receive, requiring four sets of differential pairs, only the USB Type-C interface can provide four sets of differential lines. Single-channel mode is one set of differential pairs for transmitting and one set of differential pairs for receiving. type-A and type-B interfaces can only provide two pairs of differential lines, which is only applicable to single-channel mode.



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By HornmicLink_Henry @230404 20:58