During registration with 5G system, a 5G NR device can send S1 mode capability to the AMF using “5GMM capability” information element within “REGISTRATION REQUEST” message. This capability assists AMF whether the device support s1 interface (interface between LTE BS (eNB) and EPC) or not?
The 5G NR UE supporting S1 mode should operate in the mode of single registration or dual-registration for inter-system interworking with EPS as follows:
a) Single-registration mode
- If the IWK N26 bit in the 5GS network feature support IE (within e.g. “Registration Accept” message) is set to “interworking without N26 interface (interface between AMF and MME) is not supported”, the UE should operate in single-registration mode;
- If the IWK N26 bit in the 5GS network feature support IE is set to “interworking without N26 interface supported” and the UE only supports single-registration mode, the UE should operate in single-registration mode.
b) Dual-registration mode
- If the IWK N26 bit in the 5GS network feature support IE is set to “interworking without N26 interface supported” and the UE supports dual-registration mode, the UE may operate in dual-registration mode.
Here I try to depict how inter system change between 5G system and 4G system and vice versa can be done in RRC_Idle mode and in case of single registration mode.
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