原创 Hub TT

TT ：transaction translator. 事务转换器。USB通用串行总线是新一代标准接口总线，与USB1.0版本不同，USB 2.0不仅支持低速（1.5Mbps）和全速（12Mbps）外部设备，还支持高速（480Mbps）外部设备。为了将全/低速设备对高速设备可用带宽的影响减到最小，usb2.0提供了一种机制，该机制支持在Hub连接的是全/低速设备的情况下，主控制器与Hub之间以高速传输所以设备的数据。TT（transaction translator）is the main traffic control engine in an USB 2.0hub to handle the unbalanced traffic speed between the upstream port and the downstream ports.

All USB 2.0 hubs have at least one built-in transaction translator (TT) so that they can be downwards compatible. The TT automatically recognizes which USB category is connected on a particular port and translates any USB 1.1 signals to USB 2.0. There is a catch: you only have one TT for all your available ports, which keeps the bandwidth to the system down to no more than 12 mbit/s.

Overall performance suffers whenever all the devices on the hub have to share one transaction translator.

You can get around this problem by using higher-quality USB hubs that have as many TTs as they do ports. No longer do the devices on the hub have to wait their turn for some free bandwidth; the TTs optimally embed all the separate data streams in the USB 2.0 data stream.

Each port in this hub has its own transaction translator. Only three are in use, though: a USB 2.0 device is connected to the fourth port, and its data doesn't need to be translated before being transferred.