5G • Services
RAT Fallback
Voice RAT Fallback Procedure
Fallback mechanism redirecting voice calls from 5G NR to 4G LTE while remaining connected to 5G Core.
Technical Explanation
In RAT Fallback (used in Option 7 NGEN-DC or Option 4 NE-DC), when a voice call is initiated, the UE is handed over from a 5G NR radio cell to an LTE ng-eNodeB cell, but remains connected to the 5G Core network, avoiding inter-core signaling with legacy EPC MMEs.
Key Functions
- Hands over voice call to LTE radio while remaining anchored in 5G Core
- Avoids inter-system core network handovers between 5GC and EPC
- Provides voice continuity using 5GC QoS framework over LTE radio
Specifications
3GPP TS 23.501, TS 23.502
Interfaces
UuN2
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