port of a hub changes status, this change is dealt with in an interaction between the host and this hub; with any hubs between the host and "changed hub" acting as transparent. To this aim, each hub has a single interrupt endpoint "1 IN" (endpoint address 1, hub-to-host direction) used to signal changes in the status of the downstream ports. When someone plugs in a device, the hub detects voltage on either D+ or D- and signals the insertion to the host via this interrupt endpoint. When the host polls this interrupt endpoint, it learns that the new device is present. It then instructs the hub (via the default control pipe) to reset the port where the new device was plugged in. This reset makes the new device assume address 0, and the host can then interact with it directly; this interaction will result in the host assigning a new (non-zero) address to the device. Transaction translator Any USB 2.0 hub that supports a higher standard than USB 1.1 (12 Mbit/s) will translate between the lower standard and the higher standard using what is called a transaction translator (TT). For example, if a USB 1.1 device is connected to a port on a USB 2.0 hub, then the TT would automatically recognize and translate the USB 1.1 signals to USB 2.0 on the uplink. However, the default design is that all lower-standard devices share the same transaction translator and thus create a bottleneck, a configuration known as the single transaction translator. Consequently, multi transaction translators (Multi-TT) were created, which provide more transaction translators such that bottlenecks are avoided. Note that USB 3.0 hubs do not currently perform transaction translation to super-speed for USB 2.0 devices. Electronic design Most USB hubs use one or more integrated controllers (ICs), of which several designs are available from various manufacturers. Most support a four-port hub system, but hubs using 16-port hub controllers are also available in the industry. The USB bus allows seven cascading tiers of ports. The root hub is the first tier, and the last devices are on the seventh tier, allowing five tiers worth of hubs between them. The maximum number of user devices is reduced by the number of hubs. With 50 hubs attached, the maximum number is . See also USB On-The-Go References External links Hub de:Universal Serial Bus#USB-Hubs