LTE: What does mean by cause "loadBalancingTAUrequired" of RRC Connection Release ?

Question

Can someone please explain about the cause "loadBalancingTAUrequired" ?

Answer 1

The RRC CONNECTION RELEASE procedure can be used for MME load balancing. If an MME feels overloaded, it can simply move the calls to other MME in the MME pool. The source MME releases the S1 connections of the UE towards the eNodeB asking the UE to perform a “load balancing TAU”. Then the eNodeB sends RRC CONNECTION RELEASE message to the UE with cause ‘load balancing TAU required'. After receiving this message, the UE initiates Tracking Area Updating procedure which is redirected to another MME

Answer 2

Adding to the answer above, the trigger for rrcConnectionRelease with cause "Load Balancing TAU" occurs at the time of MME selection for sending Initial UE Message after rrcConnectionSetupComplete (+ NAS Attach Req) is received. eNB applies the NNSF (network node selection functionality) to select MME and if MME selection procedure fails, rrcConnRel is triggered due to this cause.

Comments

I don't think RRC connection release is triggered with this cause while doing MME selection to send the Initial UE Message.

Answer 3

When load re-balancing between MMEs is required at the core network then a MME initiates S1-Release procedure with the cause "load balancing TAU required" for the ECM-CONNECTED mode users. eNodeB receives S1-AP UE Context Release Command with the same cause and perform RRC release procedure for the same UE.