Why are USB hubs mostly 4, 7, and 10 ports?

USB hubs are undoubtedly one of the most popular peripherals out there, but while we're enjoying their convenience, have we ever wondered why USB hubs are 4, 7, and 10 ports the most common?

USB hub uses a star topology to extend multiple USB ports. The core of a USB hub is a master control chip (also known as a bridge chip), which allocates resources from a USB port (upstream port) on a PC to the USB port (downstream port) extended by the USB hub through software and hardware algorithms.

You may have noticed that USB hubs on the market almost always have 4, 7, or 10 USB ports. In theory, a single USB hub bridging chip can be extended to 4 USB ports.Why isn't there a hub with 8 or 12 USB ports?

The reason goes back to the USB hub topology. Since a hub bridge chip can only expand 4 USB ports, to expand more USB, you have to use more bridge chips at the same time. Take the 7-ports USB hub as an example, it’s topology is shown in the figure below. As we can see, "Master chip 2" splits 1 USB port under "Master chip 1" into 4 ports again, with a total of 7 ports. In the same way, 3 bridging chips can be extended to 10 USB ports, and so on. 

So how do I buy a USB hub?

If you need a USB hub of more than 4 ports, or usually need to use the hub to connect the mobile hard disk, it's much better to choose the product with external power supply. As we all know, the maximum current that USB2.0 and USB3.0 can transmit is 500mA and 900mA respectively, while a mobile hard disk (HDD) needs 450mA to 800mA (depending on the model) to drive stably. In other words, when you connect a USB2.0 hub to a portable hard disk, the portable hard disk will perform very reluctantly. If the USB3.0 hub is connected to a keyboard and mouse when it is connected to a portable power supply, the portable hard disk also faces the risk of insufficient power supply. The problem comes, the unstable power supply of the mobile hard disk easily causes hardware failure, resulting in the damage of the hard disk and internal important data.

So whenever it's possible to connect a removable hard disk with a USB hub in the future, whether it's a USB2.0/3.0 hub, you should choose a model with an external power supply to make up for the lack of power from the USB port itself. In fact, there's a lot more to consider when choosing a USB Hub, and if you're interested, check out the blog before to learn more about USB.