PSM TAU and Active timers

The PSM timers are negotiated on a network attach.
The active timer is the time that the modem will stay IDLE before moving to PSM.

For battery consumption this should be as short as possible.

However, it may be necessary to periodically receive an ACK when the device transmits data to the server.

In this instance the Active timer should be increased.

Does anyone know if the PSM timers can be re-negotiated with the network when already attached?


I’m not sure if PSM is enabled by default. However, you can request your own timers using the AT+CPSMS command. See section 10.4 here:

To enable PSM or to modify the timers call something like:

The timer values are specified as 8 binary digits in quotes. Where the upper 3 bits are the coefficient, and the lower 5 bits are the numerator.

For the Requested_Periodic_TAU timer the coefficients are:

For the Requested_Active_Time the coefficients are:

The network has to agree to the requested timers and may not, in which case the network will provide other timers. You can query the final timers using the AT+CPSMS? command. Requesting too short (especially for the Requested_Periodic_TAU) or too long timers will likely cause the network to reject/override the requested values.

Also note, there are potentially several timer value combinations (coefficient x numerator) which results in the same amount of time. Sometimes at first glance the values seem very different to what you requested but the overall time is the same. e.g 10m x 3 or 1m x 30.

Hi Gabriel,

Thanks for the detailed response.

I am already using the command “AT+CPSMS” to set the timers and they are correctly negotiated with the network during network registration after which the device moves to the ATTACH state.

The TAU is 24 hours and the Active timer is 6 seconds.

I am led to believe that the timers can only be re-negotiated during network registration, BEFORE the device is ATTACHED.

My question is, is it possible to re-negotiate the PSM timers with the network AFTER the device has registered and is already ATTACHED?


I believe you can renegotiate the timers at any time by making another call to AT+CPSMS.

I believe this results in an immediate TAU being sent with the request for those new parameters.

Similarly, it is possible to initiate a connection to a server (from the device) at any time during the PSM cycle. Doing so will bring the device out of PSM, and I believe an new PSM cycle will start aftwards.

OK thanks. I’ll give it a try.