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Calculating Active timer, periodic TAU timer value during PSM(Power saving Mode) at EPC/MME

+2 votes

PSM: It is a specially kind of UE status that can minimize the energy consumption that is supposed to be even lower than normal idle mode energy consumption.

It was the network/MME decision whether UE can be allowed to enter into PSM mode. On agrees on network UE will enter into PSM state which is similar to switch-off.


  • If UE wants to enable PSM, it will include active timer value(T3324) in attach request/TAU request.
  • If operator policy is not allowed to enter into PSM then MME can ignore T3324 timer value and it will send attach/TAU accept without T3324 timer value. But, MME will send periodic TAU timer which can be used by UE to update current TA to MME in Idle Mode.
  • If MME  accepts PSM, it will re-assign T3324 timer value based on operator policy.


            T1 = T3324 value received from MME

            T2 = locally configured active time by MME O&M

            T3 = “Active Timer value” received from SCEF/CSE. (CSE will set this value based on at what time it was having IOT MT data to send UE)

           May be T3324 timer value = max{T1, T2, T3};

  • Also MME will re-calculate T3412 timer value as mentioned below.

               T1 = current T3412 timer value

               T2 = Maximum latency received from SCEF/CSE. (CSE will set this value at what interval , CSE may having MT data to send UE)

             May be T3412 timer value = min{T1, T2}

  • On receiving of attach/TAU accept with T3324 timer value, UE will start active timer value. Once expiration of this timer value, UE will release radio connection enter into PSM mode and state will change it to EMM_REGISTERED_NO_CELL_AVAILABLE.
  • In PSM mode if UE having some MO data, it can send it before active timer expiry. Other it should wait till next active timer starts. Means, after T3412 expiry.
  • In PSM mode if network having some MT data it should buffer it till next periodic TAU request received.
  • Once TAU request received in PSM mode and MME/SGW having some MT data to send, then MME will trigger paging.

In the same lines MME will re-calculate eDRX cycle length and paging occasion period in case of eDRX. I will explain eDRX case in other article.



Latest versions of TS 23.401, TS 24.301, TS 24.008, TS 29.122, TS 29.366, TS 29.026 and TS 23.682
posted Jul 12, 2018 by Althi Rajeswararao

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Good one....