Inactivity timer in LTE

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Inactivity timer in LTE

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Hello Team,

 I have a query related to the Inactivity timer in LTE.

My DUT is registered on LTE with Default bearer assigned to it. I am able to do traffic initiation both sides.

But, after say a period of inactivity, the DUT because of the inactivity timer -> will break the session .

Does it enter Idle mode with no default bearer ? or it will still be in CONNECTED mode .

And after resuming data , will the UE goes back from Idle to CONNECTED mode again.

The requirement is data resumption has to be successful.


Can you please sahre your thoughts on this.


All Replies
  • User Inactivity means the network discovers that for a certain time period (defined by the value of the “user inactivity” timer) no user plane packets are exchanged between the UE and network.

    On expiry of User Inactivity timer, network releases default EPS bearer to save resources and hence UE is forced to enter Idle mode.  Once the user is put in idle mode, the default radio bearer is torn down, i.e., there is no RRC connection once the user enters idle mode.  When the user comes out of idle mode (due to traffic , paging, expiry of timers etc.), the UE has to reestablish the RRC connection before the bearers can get reactivated.

  • To add on, data resumption is possible when UE goes from idle to connected if the IP allocated to the DUT is not changed.

    There are 2 inactivity timers: one on the eRAN side and another on the EPC side. Usually, EPC inactivity timer >> eRAN inactivity timer.

    The one that Prakesh has mentioned refers to the eRAN inactivity timer.

  • Hi I have a doubt, regarding the EPC/eRAN Inactivity timer. I would like to know where the timer value will be communicated to the UE or to the network. In which message the timer value will be send?

  • These timers are internal to the network and I assume are also implementation -dependent and configurable. The UE does not see (nor is signalled about) these timers explicitly, the only thing the UE sees is that the RRC connection is released.

    There is an "inactivity timer" that is relevant for (and signalled to) the UE called "drx-InactivityTimer", but this is related to connected mode DRX functionality which is an entirely different topic altogether.

  • Can u give refrence to LTE Specs in which info is given regarding the EPC/ eRAN In activity timers.


  • Since the timer values are not exchanged across any interface, the specs do not describe them. These are a vendor-specific implementation item.

  • To save battery life and resources UE goes to idle mode after some time. but it does not release the default bearer, it just release the air interface. when required it can be re-connected.

    You should check the MO call flow for LTE. you will understand everything

  • Does "inactivity timer is implementation dependent" mean network does not need to implement the S1 release procedure by inactivity timer in eNB which is described in TS23.401 s5.3.5?