USB transaction translator

Any USB hub that supports a higher standardthan USB 1.1 (12 Mbit/s) will translate between the lower standard and thehigher standard using what is called a transaction translator (TT). Forexample, if a USB 1.1 device is connected to a port on a USB 2.0 hub, then theTT would automatically recognize and translate the USB 1.1 signals to USB 2.0on the uplink. However, the default design is that all lower standard devicesshare the same transaction translator and thus create a bottleneck, aconfiguration known as the single transaction translator. Consequently, multitransaction translators (Multi-TT) were created, which provide moretransaction translators such that bottlenecks are avoided.

A USB hub consists of a Hub Repeater section, aHub Controller section, and a Transaction Translator (TT) section.

The Hub Repeater is responsible for managingconnectivity between upstream and downstream facing ports which are operatingat the same speed. The Hub Repeater supports full-/low-speed connectivity andhigh-speed connectivity.

The Hub Controller provides status and controland permits host access to the hub.

The Transaction Translator (TT) takes high-speedsplit transactions and translates them to full-/low-speed transactions when thehub is operating at high-speed and has full-/low-speed devices attached. If alow or full speed device is connected to the hub operating at high speed, thedata transfer route includes the Transaction Translator (TT). If a high speeddevice is connected to this high speed hub the route only includes the HubRepeater and no Transaction Translator (TT).

For more information, please review the chapter11.14 Transaction Translator of USB 2.0 specification.