Printer Friendly PDF Version

Pages <next> - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - <back>

Q. Why is the number of hubs limited to five?
A. Each hub delays the USB signal by a finite amount of time. When the Host Controller issues a request for data from a device, this request must pass through each hub in the chain, incurring incremental delay as it does so. A similar effect is experienced by the reply (data) from the device as it passes back through the chain of hubs to the Host Controller. The number of hubs is limited to five in order to place a limit on the round-trip delay of a signal from Host Controller to Device and back to Host Controller.

Q. Why does USB impose a limit on round-trip delay?
A. This is required to keep occupancy of the bus high. The Host Controller is not able to process commands for any other device while it is waiting for a reply.

Q. What would happen if the delay was too long?
A. The Host Controller would believe that the transaction had failed. Repeated failures might result in the device being taken out of service.

Q. How does ExtremeUSB solve the delay problem?
A. The ExtremeUSB protocol generates local responses that comply with the USB timing restrictions while the data is being retrieved from the remote source.

Q. Does ExtremeUSB support all device speeds?
A. ExtremeUSB supports both 1.5 Mb/s and 12 Mb/s speeds for USB 1.1 and 480 Mb/s for USB 2.0.

Q. Why are competitors' products limited to 50 - 60 meters (164 – 197 feet)?
A. They are limited to this distance because they are unable to compensate for the round-trip delay. In addition, these products are unable to support the full complement of USB hubs because they consume the delay budget that is allocated to each hub in a chain.

Pages <next> - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - <back>