5G Stuff: N26, N1, S1 Interfaces

3GPP TS 23501 specifies this N26 interface in 4.3 for both Roaming and non-roaming cases. To provide flexibility to the networks, N26 is optional.  

Let us have a look at the N26 Interface which inter connects EPC (MME) and 5GS (AMF). 

In Idle mobility case
            - from 5GS to MME, AMF tansfers the MM context (UE Usage type) to MME
            - from MME to 5GS, MME tansfers the S-NSSAIs associated with the PDN connections and mapping info to AMF. 

When N26 is not supported, UE reports S-NSSAI received from PGW-C/SMF for MME to 5GS case using PDU Session Establishment Req.

In Connected mobility case
            - from 5GS to MME, AMF selects the target MME (based on Source AMF region ID, AMF set ID and target location) and transfers the UE context over N26 interface. After the HO successful, TAU is performed.
            - from MME to 5GS, MME selects the target AMF based on the target location (TAI or any other local info) and transfers the UE context over N26 interface. The PGW-C+SMF sends PDU Session IDs and related S-NSSAIs to AMF. After the HO successful, Registration is performed to secure the allowed N-SSAI.

N1 mode: A mode of a UE allowing access to the 5G core network via the 5G access network.

S1 mode:

If the UE is capable of S1 mode, there is a single UE’s usage setting at the UE which applies to both 5GS and EPS.