A Hub expands a single USB port into several ports that are accessible by the Host. Hubs have one upstream port facing the Host, and multiple downstream ports facing the attached devices. Common USB 2.0 Hubs have four downstream ports.
To the Host a Hub will look like a Device with multiple endpoints. Each downstream port is represented by an endpoint in the Hub. In addition to the downstream-port endpoints, each Hub reserves Endpoint 1 IN to inform the Host that a down stream device has either been inserted or removed from the bus. (See USB enumeration for details on this process.)
Only one Device can be connected to a Host. If more than one Device exists in a system it can only be connected to the Host through a Hub. The Hub directly connected to the Host is called the Root-Hub. There can be up to five additional Hubs connected to the root-hub in series to create separate tiers. The maximum numbers of tiers in a USB system is seven.
USBs Hosts are required to make available a certain amount of power for enumeration and operation. (The amount of power required depends upon the specified USB Speed) A Hub can either draw power from the USB bus (bus-powered ) or have its own external power supply (self-powered).
- Bus-Powered Hubs will draw power from the Host and cannot add any additional power to Devices.
- Self-Powered Hubs are capable of supplying power to the Device to augment the power delivery of the Host. Self Powered Hubs report to the Host the amount of power available to each port.
During the enumeration process a Host will consider the power characteristics of a Hub when deciding which downstream Device configurations to activate.
Enhanced Hub Capabilities
With the newer specification (since USB 2.0) additional capabilities have been added to Hub. These include:
- Speed detection of the Device
- Power distribution and reporting
. See the Specifcations on USB Speeds for more information on the enhanced Hub capabilities.