In general, there is a 32khz sleep clock which is used to run the module. This allows the main clock (usually a 19.2MHz or 26MHz TCXO) to shut off and a lot of the internal baseband chip Phase-Lock Loops to shut off as well.
Data sessions are not active during low power mode, while SMS is still received which can then be acted upon to restart a data connection. You could also program the device to wake the module at regular intervals, or based on an event.
When a device is in sleep mode it’s typically only consuming about 3mA or less of battery current.
If power optimization is critical – you should make sure that any other peripherals have similar functionality and can be put to sleep at the same time or even turned off all together to preserve system power.
In addition, you should evaluate the needs of the application to ensure that you wake it up as little as possible. Questions to ask might be:
- How long can I wait for the next update from the device?
- Can my update fit into an SMS message, limiting the need to wake up for a data connection?
- Are there times I can turn off network scanning or registration attempts to further limit power consumption?