Why are all USB hubs 4 ports?

First introduce a concept:

"Transaction Translators", there are in each Hub, it is responsible for transmitting data in the hub, we call it "TT".

In the Hub, there are two ways to set TT, one is called Single TT, and the other is called Multi TT.

1>Single TT means that there is only one TT in the Hub

2>Multi TT means that each port in the Hub has a TT

The maximum bandwidth that a TT can handle is limited, so 4 ports are already the maximum load for a Single TT. The hubs sold online and on the roadside for less than 100 yuan are basically Single TT. This kind of hub can only support up to 4 ports. If it is more, it will be connected in series, and the speed is very slow. The reason is the processing capacity of TT. There is an upper limit, and it can't handle too many devices when it comes to packets. In theory, Multi TT can be connected to a lot. I have seen 7-port and 8-port ones. The 7-port one is in the company, so I can't show it. Let me show you the one on my PC:

So as long as it is Multi-TT, theoretically you can connect as many as you want, but the speed is slow.

Because the address length contained in the USB request is an 8-bit number, and there are only 127 devices in the Root Hub, the USB can theoretically be connected to a Level 7 Hub (in reality, more than 3 signals are very poor, only the quality is good) can only be), so it is meaningless to design too many mouths, and the cost is too high.

Question:

A USB hub with an independent power supply, will the device still consume the power of the host when plugged into it?

Yes, will consume power.
The meaning of the USB HUB with power supply is to provide additional power supply. After all, the power supply of the USB port of the computer is limited, and it is impossible to complete the power supply of multiple devices after connecting the HUB, so now HUBs with more than four ports are generally equipped with independent power supply.

• [cf7ic]