CSFB (Circuit Switched Fallback) is an solution in 3GPP Release 8.
In this approach, the LTE provides data services, and when a voice call is to be initiated or received, it will fall back to the CS domain.
When using this solution, operators need to upgrade the MSC instead of deploying the IMS, and therefore, can provide services quickly. However, the disadvantage is longer call setup delay.
When using this solution, operators need to upgrade the MSC instead of deploying the IMS, and therefore, can provide services quickly. However, the disadvantage is longer call setup delay.
Network Requirements for CSFB
- 2G/3G available in the same coverage area.
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SGs interface between MME and MSC-S is defined.
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The UE is dual radio capable.
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UE is registered in CS domain.
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The MSC-S is updated and allow the CSFB to perfom paging via LTE.
In CSFB option the user connected to LTE will be redirected by the network to cell-reselection, from 4G to 2G or 3G access network to connect to the CS domain, when there is a call request.
CSFB performs following steps
1. Subscriber is registered in MSC but roam in LTE.
2. Incoming call to subscriber in LTE.
3. MSC Paging over SGs, S1 and eUu-Interface.
4. UE notifies MME with an Extended Service Request.
5. MME orders eNodeb to release and inform UE that CSFB is ok.
6. UE and RAN triggers an enhanced release with redirect.
7. UE sends a location update and call setup over 2G/3G radio.