Who Triggers the USB suspend?

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Gone through the USB spec but I am too confused. I have few questions.
Hope I get reply to them here:-

1. USB Device goes into the suspend mode when there is not bus-
activity for 3 ms. Does this means that the USB device is monitoring
the bus? I believe that the host will monitor the bus not device, when
there is no activity host will SET Suspend on the port. Am I correct?

2. There is SOF frame/Keep Alive coming to the device to avoid device
going to suspend mode. Who triggers this SOF Frame, Host Controller
hardware or Host Controller driver or our device's driver?

3. Apart from no bus activity what are the other conditions in which
host can send suspend signal to the device?

4. How does the USB device hardware behave on recieving the suspend?
Will it Pull down the D+ resistor. If yes how will it recover back to
the resume state?

5. If my device hardware doesnt support suspend/resume, how will the
host react. Is there any way to tell host that device hardware doesnt
support suspend/resume?

Regards
Neeti.

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