The USB cable has four wires. Two wires carry 5V power. Some devices may use this to power themselves, while others will be self-powered (i.e., battery or mains powered).

The other two USB wires carry the serial data. Data transfer is in one direction only at a time. Serial data is either transferred at full speed, 12 megabits per second (Mbs), or at low speed, 1.5 Mbs. All the serial data is at one of these speeds, so there is no rate conversion in hubs. A device is either a full-speed device or a low-speed device.

Note that there is no intention to uprate these communication speeds, unlike IEEE 1394. Move to IEEE 1394 if you need higher speed.

The USB protocol defines what appears on the serial bus. The host starts all transactions, but data transfer direction is switched, if necessary, during a transaction. The protocol allows devices to be plugged in at runtime and configured, and for devices to be unplugged at any time. Communication occurs only between the host and one device at a time.

Hubs repeat downstream data to all downstream ports (but not full-speed data to low-speed devices). For upstream data, hubs understand the protocol so they know which port (if any) to route upstream. Hubs do not buffer data.