SIGNAL INTEGRITY
The USB protocol is fairly demanding in terms of signal quality. An extender should be able to
transmit the USB signal without major distortion from the host to the device and back.
SIGNAL DELAY
In terms of USB extension, the most challenging limitation of the USB standard is the “time
budget.” This is the time required by the specification for communication back and forth
between the host and the USB device. The host (master) sends messages to every device and
expects the answers to come back within the allotted time; if this does not occur, the host will
time-out the transaction.
The specification requires a time budget no greater than 380 nanoseconds (ns) in each direction.
A separate time budget is designated for the device to formulate an answer, as follows:
▪ The signal must travel from host to device within 380 ns,
▪ The device must formulate an answer, and
▪ The answer must travel back from the device to the host within 380 ns.