CSFB to GSM/UMTS and related events

CSFB to GSM/UMTS is a reasonably simple procedure on paper, but can lead to quite some confusion when implementing in a live network. In the following article, we will explain CSFB and its related events.

For details on CSFB strategy read our article: CSFB Strategy: HO based or Redirection based

Question: Which events are triggered during CS Fallback to GSM/UMTS from LTE?

Answer:

CS fallback to UMTS can be performed using different scenarios:

  • RRC Connection Release with Redirection without Sys Info
  • RRC Connection Release with Redirection with Sys Info
  • PS handover with DRB.

CS fallback to GERAN can be performed using different scenarios:

  • RRC Connection Release with Redirection without Sys Info
  • RRC Connection Release with Redirection with Sys Info
  • PS handover
  • Cell Change order without NACC
  • Cell change order with NACC

RwR (Release with Redirect): Does not use events for CSFB. RRC Connection Release with redirection info – in this case RRC Connection release message contains the target UTRAN frequency information. Then the UE moves to the target RAT and searches for an acceptable cell by using the frequency information within the RRC Connection Release.

PS HO: This feature minimises the interruption on PS data session, when the eNodeB transfers the UE to UTRAN. It supports blind handover and handover to a WCDMA frequency based on measurements. These measurements use B1 and B2 events (B1 Inter RAT neighbour becomes better than threshold, B2 PCell becomes worse than threshold1 and inter RAT neighbour becomes better than threshold2)

  • Event B1: Neighbour becomes better than absolute threshold.

Inequality B1-1 (Entering condition)


Inequality B1-2 (Leaving condition)


  • Event B2: PCell becomes worse than absolute threshold1 AND Neighbour becomes better than another absolute threshold2.

Inequality B2-1 (Entering condition 1)


Inequality B2-2 (Entering condition 2)


Inequality B2-3 (Leaving condition 1)


Inequality B2-4 (Leaving condition 2)


Ref: 3GPP TS 36.331

For more information on measurements, counters or any Radio Network related query don’t hesitate to contact 3DB CONSULT at: toomas.sarv@3dbconsult.com

Additional info


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