CSFB Failure Causes Scenarios and Counters.

First we have to check before the investigating of the CSFB problems –> is to understand CSFB performance counters and when do they count during the CSFB procedures:

Understanding of CSFB Counters.

L.CSFB.PrepAtt counter:

 is incremented by 1 each time the eNB receives the IE “CS Fallback Indicator” from the MME contained in an INITIAL CONTEXT SETUP REQUEST message or in a UE CONTEXT MODIFICATION REQUEST message.

L.CSFB.PrepSucc counter:

is incremented by 1 each time the eNB successfully sends an INITIAL CONTEXT SETUP RESPONSE message or a UE CONTEXT MODIFICATION RESPONSE to the MME.

L.CSFB.E2W / L.RRCRedirection.E2W.CSFB  counter:

The L.CSFB.E2W and L.RRCRedirection.E2W.CSFB counters are incremented by 1 each time the eNodeB sends an RRC Connection Release message to the UE and informs the UE to perform a CSFB procedure to WCDMA network. In this case, if the CSFB procedure is initiated by a UE in idle mode (when the UE initiates a CSFB, the initial state is determined by the eNodeB), the L.CSFB.E2W.Idle counter is incremented by 1.

L.CSFB.E2G/ L.RRCRedirection.E2G.CSFB 

L.CSFB.E2G and L.RRCRedirection.E2G.CSFB counters are incremented by 1 each time the eNodeB sends an RRC Connection Release message to the UE and informs the UE to perform a CSFB procedure to GERAN.

CSFB Failure Scenarios.

Below are the most common scenarios where CSFB failure scenarios can occur:

Scenario 1: 

CSFB Execution Failure (L.CSFB.E2W/ L.CSFB.E2G=0), i.e. CSFB is completed in the preparation phase but no redirection to UTRAN/GERAN.

CSFB Preparation: L.CSFB.PrepAtt = L.CSFB.PrepSucc

Redirection: L.CSFB.PrepSucc > L.CSFB.E2W+ L.CSFB.E2G

Possible Causes:

Ø  No UTRANNCELL configuration on LTE cell

Ø  No UTRANNFREQ configuration on LTE cell

Ø  No GERANNFREQGROUPARFCN configuration on LTE cell

Ø  No GERANNCELL configuration on LTE cell

Ø  Lacking/missing UTRAN/GERAN neighbor configuration

Ø  UTRANNCELL information with BlindHPriority = 0

Ø  GERANNCELL information with BlindHPriority = 0

Scenario 2: 

CSFB Execution Failure (L.CSFB.E2W/ L.CSFB.E2G >0), i.e. CSFB is completed in the preparation phase but low success redirection to UTRAN/GERAN (Low CSFB Execution success Rate)

Possible Causes:

Ø  Missing CO-Sector Utran/Geran Neighbors

Ø  Configure High BlindHOPriority to Non co-site Neighbors

Ø  Wrong External Configuration

Scenario 3: 

CSFB Preparation not triggered (L.CSFB.PrepAtt=0),

Possible Causes:

Ø  UtranCsfbSwitch not turned ON

Ø  GeranCsfbSwitch not turned ON

Ø  Missing CSFB License on LTE Site

*Scenario: CSFB triggered (Context request from MME to eNB) but not completed in the preparation phase due to interaction with another procedure

CSFB Preparation: L.CSFB.PrepAtt > L.CSFB.PrepSucc

Possible Causes:

Ø Interaction or conflict with simultaneous procedure such as S1, X2, or IRAT handover procedure.

Scenario 4:

CSFB triggered (Context request from MME to eNB) but not completed in the preparation phase due to RR procedure failure

CSFB Preparation: L.CSFB.PrepAtt > L.CSFB.PrepSucc

Possible Causes:

Radio resource procedure failed which could be due to RF related issues. The following RR procedures can be affected when CSFB is triggered (Context Setup Request from MME to eNB):

Ø  Security Mode

Ø  RRC Reconfiguration

Ø  UE Capability

Ø  Measurement

Scenario 5: 

CSFB Extended Service Request initiated but succeeding procedures not completed

CSFB Preparation: L.CSFB.PrepAtt > L.CSFB.PrepSucc

Possible Causes:

Ø  RF related reasons.

Ø  UE Initiates a procedure with one site and have succeeding procedures on another site.

*Scenario: CSFB triggered (Context request from MME to eNB) but not completed in the preparation phase due to other reasons

CSFB Preparation: L.CSFB.PrepAtt > L.CSFB.PrepSucc

Possible Causes:

Ø  UE doesn’t have IRAT capabilities.

Ø  Timer expiry that could be RF related reason if on the RR layer or other reasons if timer expiry on the S1 interface