Techlte Blog

LTE CSFB (Circuit Switch Fall Back)


To make a voice call there are mainly two solutions:

  • CSFB
  • IMS based Volte Call

There are pre conditions for CSFB:

  • When UE is in LTE RAT it should be registered with MSC .
  • UE is not directly connected to MSC .
  • MME is connected to MSC via SGs interface.

SGs Interface:

  • SGs interface is between MME and MSC.
  • LA Update is transferred by MME to MSC over SGs

There must be MME-MSC/VLR SGs association which is created at below occasion:

  1. During Combined EPS and IMSI attach.
  2. During Combined TA and LA Update.

Combined EPS and IMSI attach procedure:

In Attach request message UE send IE Type : combined EPS and IMSI attach.

Note : EPS –PS Domain.

          IMSI-CS Domain.

 Pre Conditions:

Message in UE Log:

CSFB Call flow

Type of CSFB implementation done in the network.

1.Blind CSFB
2.Measurement Based CSFB
3.RIM or Flash CSFB.

Note : When in legacy network (3G/2G) the UE can have PS data sessions. PS data is supported in 2G network only when DTM (Dual Transfer Mode) is supported. When UE supports DTM it can have both voice and data simultaneously.

What happens after VOICE Call get over:

If RAT is 2G

  • UE will remain in 2G until data transfer completed or resume data flow after call completion.
  • UE go to LTE without resuming data flow.

If RAT is 3G

  • UE can wait for data transfer to complete before cell reselection to LTE.
  • UE go to LTE cell during ongoing data flow.