A hub is an intelligent device that provides attachment points for devices and
manages each device’s connection to the bus. Devices that plug directly into the host computer connect to the system’s root hub. Other devices can connect to external hubs downstream from the root hub.

A hub manages power use, helps initiate communications with newly attached devices, and passes traffic up and down the bus. To manage power, a hub provides current to attached devices and limits current on detecting an over-current condition. To help initiate communications with devices, the hub detects and informs the host of newly attached devices and carries out requests that apply to the devices’ ports.

The hub’s role in passing traffic up and down the bus varies with the speeds of the host, device, and hubs between them. This chapter presents essentials about hub communications. You don’t need to know every detail about hubs in order to design a USB peripheral. But some understanding of what the hub does can help in understanding how devices are detected and communicate on the bus.