Normally, the USB host will maintain activity on the USB bus at least every millisecond according to USB specification. A USB device will enter suspend when there is no activity on the USB bus (idle) for a given time. The USB device will resume operation when it receives any non idle signaling.
To signal that the USB device shall enter Low-power mode (suspend), the USB host stops activity on the USB bus, which becomes idle. Only the USB device pull-up and host pull-downs act on D+ and D-, and the USB bus is kept at a constant J state. It is up to the USB device to detect this lack of activity, and enter the Low-power mode (suspend) within a specified time.
The USB host can decide to suspend or resume USB activity at any time. If remote wake-up is enabled, the USB device may signal to the USB host to resume from suspend.