What is a Hub

The simplest way is to think of a USB hub as a combination of a separator and a repeater. A hub provides an electronic interface between USB devices and the host. The hub is directly responsible for supporting many of the properties of USB, providing a user-friendly interface and avoiding complex usage issues. In addition to providing an interface for the host implementation, the hub can also provide more interfaces (fully utilizing the separator function), and can also transfer data on the upstream port (connected to the host) to the downstream port (connected to the target device), thereby playing the role of a repeater. The hub is responsible for detecting connection and disconnection events on the downstream port and reporting relevant information to the host. The hub must be able to support various USB peripherals of any speed specification connected to the downstream port (see the following content for details). The hub must also be able to detect and recover from any bus failure caused by an error condition of the target device.

In addition, the hub is responsible for managing the power supply of downstream ports, reporting power supply problems to the host, and notifying the user. The relevant specifications define two types of hub power supply: bus power supply and self power supply.

A bus-powered hub gets all its power from the upstream ports, but can only draw a maximum of 100mA from the upstream connection when powered up, and can be configured to draw no more than 500mA. When fully configured, power is divided between the hub, non-removable functional devices (composite devices), and external ports. A bus-powered hub needs to provide 100mA to each downstream port, so no more than four external (user-accessible) ports can be powered downstream on a bus-powered hub. With 100mA per port and four ports, the available current for the downstream connection is 400mA. If a fifth downstream port is added, it will also require 100mA, leaving no current available for the hub itself. A bus-powered hub can provide four external downstream ports and one or more internal ports, provided that the hub itself and all functional devices connected to the internal ports consume less than 100mA combined.

A self-powered hub gets its power from a local power source, such as a wall transformer or other local power source. It does not draw power from its upstream connection. A self-powered hub can draw up to 100mA from an upstream port, allowing that port to function even when the rest of the hub is powered down. A self-powered hub should provide 500mA to each downstream port. Although the specification does not limit the number of downstream ports that a self-powered hub can support, in practice this is typically limited to 7 ports, primarily for power considerations. A 7-port hub must draw more than 3.5A (500mA per port for 7 ports) to ensure proper operation of the hub.