Your question is about USB power provided to a peripheral. A
peripheral must power up with less than 100 ma. If a device is
acknowledged by the USB hub, then up to 500 ma may be delivered.
However, how accurate is the measuring circuit for each port? If
measuring 10% high, then a USB peripheral that should not create a
surge may do so anyway.
what is the difference between a usb port, hub and system!
A hub is the central point from which ports branch off. A hub may
support one port, four ports, eight ports, etc. Hubs can be daisy
chained. For example, your computer probably has a hub containing
four ports. From the root hub, one can daisy chain more hubs for up
to something like 127 USB ports / devices. Each port can power a
maximum of 500 ma at 5 volts. However, if a four port hub does not
have its own power source, then it can only distribute that 500 ma
from its 'root' hub meaning that each device may be limited to 100 ma.
USB ports exist in three forms - low speed, full speed, and high
speed. This could get complex. But the industry standard simplifies
which is one reason why USB obsoleted serial and parallel port
devices.
Which USB port is failing? Another diagnostic tool is a white LED
light powered by a USB port. Light will not define a port as good but
may discover a defective port. A port that should provide at least
100 ma must easily light a 20 ma LED. If not, well, problem
identified. But if the light does light, port is unknown (which is
completely different from 'good').