Q. What are the distance limitations of USB?
A. The maximum range of USB is limited by the length of an individual USB cable and the number of cables that can be connected in series through USB hubs. The maximum length of a USB cable in 5 meters (16 ft.). The maximum number of USB hubs that can be daisychained (connected in series) is five. Thus, if a device is connected to a PC through five hubs, then the maximum distance from the device to the PC is 30 meters (98 ft.) (6 cables @ 5 meters [16 ft.] each).
Q. Why is USB cable length limited to 5 meters (16 ft.)?
A. This the maximum capability of the standard drivers that are provided in USB devices and hubs.
Q. Why is the number of hubs limited to five?
A. Each hub delays the USB signal by a certain 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 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 a 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 were 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 do USB extenders solve the delay problem?
A. A USB extender generates local responses that comply with the USB timing restrictions while the data is being retrieved from the remote source.
Q. Do USB extenders support all device speeds?
A. USB extenders support both 1.5-Mbps and 12-Mbps speeds for USB 1.1 and 480-Mbps speed for USB 2.0.
Have any more burning USB questions? Leave a comment, or go to USB.org.