Hi Experts.
For LTE, how eNodeB detects user inactivity for S1 release?
Does it through MAC CE or other procedure in LTE?
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When there’s no more scheduling in UL and DL then it will trigger user inactivity and S1 context release.
I read in netmanias only DL part for S1 is released:
S1 release may be triggered by either eNB or MME. eNB-triggered release can be caused by:
- user inactivity
- repeated RRC signaling integrity check failure
- release due to UE generated signaling connection release
- unspecified failure
- O&M intervention
However, after S1 release, the DRB and downlink S1 bearer are released in the user plane, and the ECM connection (RRC + S1 signaling connections) is lost in the control plane, releasing E-UTRAN resources. It should be noted that at this time only the resources for the downlink S1 bearer are released, and those for the uplink are kept in the network.
How eNodeB come to for user inactivity?
Is it through Scheduling grants sent by UE?
If it stopped then does it indicates user inactivity?
I honestly do not understand how can only DL part of S1 is released and UL is kept. 
S1 is either kept or released it cannot be half kept half released.
It says resources for Uplink are not released, not that S1 is not released in Uplink.
There will be inactivity timers running from RRC point of view so once expired bearer will be released.
If I am not wrong.
Parameter is there in eNodeB but not sure does it given to UE in any message or not value seen 5 s most of time.
Yes it will be sent via RRC reconfiguration message.
You can check in UE logs in RRC layer.
I think in MAC config there is IE like inactivitytimer.
In MAC cfg is DRX inactivity timer not UE inactivity timer.
About mac, I am not sure but long back I have seen from RRC messages and similar scenario observed.
And yes this could be also the reason.
Inactivity timer is not sent to UE.
It is the eNodeB MAC that store it and monitors the UL activity for UE.
Yes, I don’t think UE is informed about inactivity timer.
It is the eNodeB that has to send RRC connection release, so UE does not need to know UE inactivity timer.
Not sure but in RRC layer sub module level I have seen the messages there after release observed.
For example if there no Uplink data for say 30 sec UE is considered as inactive and RRC release is sent to UE and UE context release request is sent to MME.
In s1ap log the cause of context release request you can see UE inactivity.
Do you mean no SR request sent by UE to get UL grants for 30 s it mean it’s inactivity timer expired running at eNodeB?
If there is no transmission in both DL and UL. Not just UL.
It’s implementation specific.
Some implement SR some implement PHR.
Yes right. Inactivity timer is never given to UE till rel-13.
In rel-14 new inactivity timer is added called “UE inactivity timer”.
It’s recommended value is always higher than normal inactivity timer which eNB monitors.
This timer is introduced to avoid UE RRC state mismatch.
If sometime UE did not receive RRC release messgage, then after expiry of UE inactivity timer, UE explicitly releases all the resources and goes in to idle mode.
Why PHR for UL grants? It’s SR always.
I am saying for UE inactivity measurement.