The proposed standard will include 12MBit/Sec operation as an initial/fall back operating mode and will allow 12MBit/Sec hardware and software to continue to operate on individual lines to higher speed ports and hubs. The major planned limitation is that a lower speed device in the signal path will limit all devices further out to its speed. It is intended that USB2.0 will be able to utilize current USB1.1 cabling and connectors with 40mv signals delivered into terminated lines whose terminations will be switched in when high speed modes are invoked.
The USB2.0 proposal is somewhat controversial in that many observers feel that speeds have been dictated by marketing desire to challenge IEEE-1394 devices rather than by engineering judgment. It has been suggested that the 30-40MBit/sec required for full frame video might have been a more realistic target that would also resolve most current problems with insufficient USB1.1 bandwidth to support multiple devices. It has also been pointed out that meeting emission requirements with existing cabling may be difficult at 10 to 40 times the transmission frequency.
USB2.0 will be like USB1.1 in that it will be time division multiplexed and will not, in general, be able to deliver its full bandwidth since each device on a controller must be allocated enough bandwidth to satisfy its peak needs. It is possible that it will use a shorter frame than USB1's 1ms and therefore will be less subject to start up delays and wasted bandwidth for small packets. There is no capability in USB1.1 for devices to communicate to other devices using USB, and this will be true of USB2.0 as well.
Return To Index Copyright 1994-2008 by Donald Kenney.