USBHubs

A USB hub is a device that expands a single USB port into multiple downstream ports, allowing several USB peripherals to connect to a host computer or another hub. It has an upstream port that attaches to the host or to another hub, and multiple downstream ports that provide connections to devices. Hubs can be bus-powered, drawing power from the upstream connection, or self-powered, using an external power adapter to supply higher current to connected devices.

Hubs are built around a hub controller that handles port enumeration, power distribution, and data routing between the host and connected devices. Some hubs include features such as per-port overcurrent protection, LEDs, wake functionality, and charging negotiation. Downstream ports share the same USB bus, so total available bandwidth is divided among connected devices. Performance can vary depending on the USB version of the hub and the devices in use; USB 2.0 hubs operate up to 480 Mbps, while USB 3.x hubs support higher speeds and are backward compatible with USB 2.0 devices.

Standards and topology: a USB hub can be integrated into a tree structure with multiple hubs cascading from the root hub of the host. The USB specification limits the total number of devices in a tree to 127, including hubs. Cascading hubs is common, but deeper trees can introduce latency and bandwidth constraints.

Power considerations: unpowered hubs rely on the host’s power budget and are suitable for low-power devices. Powered hubs can supply more current per port and are better for external drives and charging. Some hubs offer charging-capable ports, though charging performance depends on the hub and device compliance with charging standards.

Use and limitations: hubs increase port count but do not increase total bandwidth; performance may degrade with multiple high-bandwidth devices. Proper power supply and compatible drivers help ensure stable operation.