A USB hub is a device that distributes the USB signal to multiple USB ports. USB hubs can draw their power from the bus itself (bus-powered) or have their own power supply, usually in the form of an external plug-in power supply (self-powered). Self-powered hubs have the advantage that each device connected to them can draw up to 500 mA of power. In the case of bus-powered hubs, the hub and all the devices connected to it can draw a maximum of 500 mA together. The USB standard stipulates that connected devices first start in low power mode (100 mA) and, if more power is required, request it from the host before switching to normal mode. With USB 2.0 this can be up to another 4x 100 mA. If this request fails, the device must switch off. If a host (PC) is not connected, the USB ports are not enabled according to USB specification. However, many USB consumers use the USB port as a power source only, without being asked, and violate the USB standard by trying to draw more than 100 mA of current without permission from the host. This is only possible with USB hubs that do not comply with the USB specification. Here the upstream (host) bus power supply is internally shorted to the external power supply out of specification. The same 5 V supply is applied to the downstream ports without any protection elements. In extreme cases, this could damage the USB port of the host (PC) or mess up the power management of the computer, resulting in unstable behavior. Brennenstuhl products with USB hub, on the other hand, adhere to the USB specifications to avoid such damage.