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A hub can be used as an active USB repeater to extend cable length for up to 5 metre (16 feet) lengths at a time. Active cables (specialized connector-embedded one-port hubs) perform the same function, but since they are strictly bus-powered, externally powered USB hubs would likely be required for some of the segments.
A USB device pulls one of the data lines high with a 1.5 kΩ resistor. This overpowers one of the 15 kΩ pull-down resistors in the host and leaves the data lines in an idle state called J . For USB 1.x, the choice of data line indicates what signal rates the device is capable of:
USB up to 2.0 allows a host or hub to provide up to 2.5 W to each device, in five discrete steps of 100 mA, and SuperSpeed devices (USB 3.x) allows a host or a hub to provide up to 4.5 W in six steps of 150 mA. USB-C allows for dual-lane operation of USB 3.x with larger unit load (250 mA; up to 7.5 W) [96]. USB-C also allows for Type-C Current ...
USB 2.0 provides for a maximum cable length of 5 metres (16 ft 5 in) for devices running at high speed (480 Mbit/s). The primary reason for this limit is the maximum allowed round-trip delay of about 1.5 μs. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost.
The physical phenomena on which the device relies (such as spinning platters in a hard drive) will also impose limits; for instance, no spinning platter shipping in 2009 saturates SATA revision 2.0 (3 Gbit/s), so moving from this 3 Gbit/s interface to USB 3.0 at 4.8 Gbit/s for one spinning drive will result in no increase in realized transfer rate.
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Generally run length is the number of bits for which signal remains unchanged. A run length of 3 for bit 1, represents a sequence 111. For instance, the pattern of magnetic polarizations on the disk might be +−−−−++−−−+++++, with runs of length 1, 4, 2, 3, and 6.
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