RRC Connection Release Message Causes

Often while doing a drive test the RRC collection released gets logged by our data collection tools and later, while debugging our drive tests we see, RRC connection Release. Many times we wonder why we received that message. Is it because we finished our 500 GB file download? Did we unintentionally press a button and we ended the call? What happened?

There are many cases where and when the UE receives an RRC connection release:

a)     Going to idle mode: In this case, the UE will receive an RRC connection release from the eNodeB due to the expiration of the inactivity timer (in most networks configured to approximately 10 seconds).

Release Cause: User Inactivity

b)    Drop Call: RLC Failure: When the number of retransmissions at the RLC layer in the Downlink direction reaches its maximum value given by the parameter MaxRetxThreshold, the eNodeB releases the context and sends an RRC connection release to the UE.

Release Cause: Other.

c)     Drop Call: RRC Connection Reestablishment Reject: Either because the feature is not adopted or because a race condition occurred in which the case just presented happened first, the eNodeB responds with a RRC connection reestablishment reject to the UE.

Release Cause: Other.

d)    Tracking Area Update: During a successful tracking area update, the eNodeB will send an RRC connection release to the UE after sending a tracking area update message (from the MME) when no new GUTI is allocated or after the tracking area update complete message is received from the UE, if it received a new GUTI.

Release Cause: Other.

e) Redirection or CSFB to UTRAN:

If the UE is performing CSFB, it will fall back to UTRAN after receiving RRC Connection Release message that includes the target frequency of UTRAN.

Release Cause: Other

e)     During Detach: Either during normal detach or abnormal detach, both by an UE initiated detach or network initiated detach, the UE receives an RRC connection Release from the network. Elements in the Network that may cause a detach message sent from the MME to the UE are:

1. Expiration of timers at the P-GW for the last bearer the UE had, capacity issues or errors.
2. Errors or Capacity issues at the S-GW
3. Expiration of timers at the MME (t3412) without TAU, errors at the MME, configuration problems, etc.

Release Cause: Other or Normal.

Given the above, the RRC connection release message is caused by many reasons. Before arriving to a conclusion just by analyzing a simple UE logfile, a cell trace or MME trace analysis is required to arrive to sounded conclusions.