RRC Connection Release in LTE

• The RRC CONNECTION RELEASE message is used to command the release of an RRC connection. E-UTRAN initiates the RRC connection release procedure to an UE in RRC_CONNECTED state
• The RRC CONNECTION RELEASE procedure with redirection information can also be used for CS-fallback to GERAN or UTRAN
• The RRC CONNECTION RELEASE procedure can also 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
• The eNodeB may provide a cell reselection priority for each frequency, by means of separate lists for each RAT including E-UTRA) in the RRC CONNECTION RELEASE message
• There is no RRC CONNECTION RELEASE COMPLETE message defined in LTE. So, UE leaves RRC_CONNECTED state without transmitting RRC CONNECTION RELEASE COMPLETE message

Some of the IEs in RRC CONNECTION RELEASE message :

Release Cause: 

The IE releaseCause is used to indicates the reason for releasing the 

RRC Connection(Ex:- loadBalancingTAUrequired, cs-FallbackHighPriority, or other)

RedirectedCarrierInfo: The redirectedCarrierInfo indicates a carrier frequency which is used to redirect the UE to an E-UTRA or an inter-RAT carrier frequency

IdleModeMobilityControlInfo: This IE provides dedicated cell reselection priorities.

FreqPriorityListX: Provides a cell reselection priority for each frequency, by means of separate lists for each RAT (including E-UTRA)

SystemInformation: Container for system information of the GERAN cell. Each OCTET STRING in SystemInfoListGERAN contains one complete System Information (SI) message as defined in TS 44.018

cellInfoList: Used to provide system information of one or more cells on the redirected inter-RAT carrier frequency. The system information can be used if, upon redirection, the UE selects an inter-RAT cell indicated by the physCellId and carrierFreq (GERAN) or by the physCellId (other RATs). utra-BCCH-Container: Contains System Information Container message as defined in TS 25.331.