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Mixed Full- and High-Speed Bus Topologies

There are currently two types of USB buses in common use: USB version 1.1 buses which operate at Low/Full-Speed and USB version 2.0 buses which operate only at High-Speed. It is important to note that a High-Speed bus does not carry any Low- or Full-Speed transactions. On the other hand, the USB 2.0 specification requires all USB 1.1 devices (i.e., Low/Full-Speed) to be supported by USB 2.0. This is handled by allowing a USB 2.0 hub to connect Low/Full-Speed buses on its downstream ports to a High-Speed bus on its upstream port.

Connecting a USB 1.1 Device to a USB 2.0 Root Hub

The Host Controller will detect that the newly attached device can't be operated at High-Speed. As a consequence, it converts the corresponding port to a USB 1.1 port by handing it over to a companion USB 1.1 controller. The root hub's port is now the root of a USB 1.1 bus.

Connecting a USB 1.1 Device to a USB 2.0 External Hub

The hub will detect that the newly attached device can't be operated at High-Speed. As a consequence, it converts the corresponding downstream port to a USB 1.1 port. The hub will buffer and translate all transactions (so called split transactions) between the host and all devices below this port such that they are High-Speed towards the host and Low/Full-Speed towards the device.

Note that this scenario also applies to USB 1.1 hubs. A USB 1.1 hub connected to a USB 2.0 bus forms a complete Low/Full-Speed subtree and can host many Low/Full-Speed devices.

Connecting a High-Speed Device to a USB 1.1 Port

The USB 2.0 specification requires that all High-Speed devices must also be able to operate at Full-Speed. The device will detect that the bus it has been attached to does not support High-Speed. It will then identify itself as a Full-Speed device and can be operated as such (possibly with degraded performance).

Note that this scenario also applies to a High-Speed device connected to a USB 1.1 hub which in turn is connected to a USB 2.0 port. No device can be operated at High-Speed if it resides under a USB 1.1 port.

The Universal Serial Bus

Transfer Speeds

Transfer Types