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USB Technology: Multi-TT Hub Goes Head-to-Head With Single-TT

1:02 PM - September 9, 2003 by Patrick Schmid

1 – The Hub Hubbub: Intelligent Vs. Conventional USB 2.0 Hubs
2 – Universal Serial Bus: ABC
3 – Single TT Or Multi TT
4 – Standard Hub With One TT: DLink DUB-H4
5 – In The Other Corner, The Multi-TT: LinXcel USB 2.0 Hub

6 – Test Configuration
7 – Test Results
8 – USB 1.1 Performance
9 – Conclusion: Multi-TT Worthwhile For High Loads

USB has been a common standard for several years now. Unfortunately, many people mistakenly believe that USB 2.0 has made poor performance a thing of the past. The naked truth is that USB 2.0 devices can churn through data in no time, and USB 1.1 devices can be optimally interfaced with PCs only if they are not connected through a USB hub.

Time and time again, users have been stopped dead in their tracks because their USB 1.1 hubs only have a bandwidth of 12 mbit/s that has to be distributed among all their devices. You would think that the solution ought to be simple - just switch to USB 2.0, which offers 480 mbit/s - and there you have it: several slow devices running at an acceptable speed. Right?

Not really. Not all hubs use the same method to insert the 12 mbit/s signals into the 480 mbit/s data stream. The insertion is the responsibility of special units in the USB hubs called transaction translators, or TTs for short. The TTs can't work with several devices simultaneously, which creates the bottleneck you bump up against whenever you connect several USB 1.1 devices at once.

We received a hub with multiple TTs from Cypress Semiconductor, a major manufacturer of this type of component, and put it up against a conventional hub with a single TT in a test that simulates real-world conditions.