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Which interface is required for CSFB in LTE?

Circuit-Switched FallBack (CSFB) is a mechanism in LTE (Long-Term Evolution) networks that enables the transition from the LTE network to a legacy circuit-switched network (2G or 3G) to facilitate voice calls. CSFB is crucial for LTE networks that initially focused on providing high-speed data services but needed a solution for voice calls, which were traditionally handled by circuit-switched networks. The primary interface required for CSFB in LTE is the SGs interface.

SGs Interface:


  • The SGs interface is a logical interface connecting the LTE Evolved Packet System (EPS) and the Circuit Switched (CS) domain. It is used specifically for the exchange of signaling information related to the CSFB procedure.


  • The SGs interface facilitates communication between the LTE core network, represented by the Evolved Packet Core (EPC), and the CS domain, which may include legacy networks such as GSM (2G) or UMTS (3G).

Protocols Used:

  • The signaling on the SGs interface utilizes various protocols, including the Mobile Application Part (MAP) protocol. MAP is commonly used for signaling between different elements in the CS domain.

CSFB Procedure:

  • When a VoLTE-capable device initiates or receives a voice call, and the LTE network needs to hand over the call to a legacy CS network, the CSFB procedure is triggered. The SGs interface is involved in signaling to coordinate this transition.

Key Steps in CSFB Procedure:

1. Call Setup:

  • The process begins with the setup of the voice call within the LTE network.

2. Detection of Need for CSFB:

  • The network determines the need for CSFB based on the call requirements and the availability of LTE coverage for voice services.

3. SGs Signaling:

  • SGs interface comes into play for signaling between the LTE EPC and the CS network. This involves the exchange of information regarding the upcoming handover for voice service continuity.

4. Handover to CS Network:

  • Following the signaling on SGs, the handover of the voice call is initiated, and the device transitions to the legacy CS network to continue the call.

5. Completion of CS Call:

  • The CS network handles the voice call until completion, after which the device may return to the LTE network for data services.

Interworking with MSC and MME:

  • The SGs interface connects the Mobile Switching Center (MSC) in the CS domain with the LTE MME (Mobility Management Entity) in the EPC. This interworking ensures proper coordination for CSFB.

Fallback for Voice Services:

  • CSFB is specifically implemented for fallback scenarios where LTE coverage for voice services might be limited, ensuring that users can seamlessly continue voice calls using legacy CS networks.


The SGs interface plays a pivotal role in the CSFB mechanism within LTE networks. It facilitates the signaling and coordination required for a smooth handover of voice calls from the LTE EPC to legacy circuit-switched networks. This interworking is essential to ensure the continuity of voice services, especially in scenarios where LTE coverage for voice may be limited. The CSFB procedure, enabled by the SGs interface, demonstrates the interoperability between the evolved LTE network and traditional circuit-switched networks for voice call continuity.

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