What is N26 Interface in 5G and what’s its importance

N26 interface is an inter-Core Network interface between the MME and 5GS AMF in order to enable interworking between EPC and the NG core. Inter-RAT Mobility procedures involving mobility between LTE-EPC and 5G-NR-5GS utilizes the N26 interface which enables the exchange of Mobility Management(MM) and Session Management (SM) states between the source and target RAT network.

1. The N26 Interface: Architecture and Purpose

The N26 interface is an inter-Core Network interface connecting the Mobility Management Entity (MME) in EPC to the Access and Mobility Management Function (AMF) in 5GC.

Its primary function is to enable the exchange of Mobility Management (MM) and Session Management (SM) states between the source and target Radio Access Technologies (RATs). While the N26 interface supports a subset of the functionalities found in the S10 interface (intra-EPC MME-to-MME), it is essential for tight interworking.

Why N26 Matters

• Latency Reduction: When N26 is present, the network can push context data directly between cores during Inter-RAT procedures, significantly reducing handover latency.
• Service Continuity: It is critical for seamless voice fallback. For instance, a Voice over NR (VoNR) call can be maintained as a VoLTE call during an Inter-RAT handover only if the session context is successfully transferred.
• IP Address Continuity: N26 support ensures IP address preservation for UEs operating in Single Registration Mode (where the UE is registered to only one CN at a time).

Note on Implementation: Support for N26 is optional. Operators can deploy “Interworking without N26,” but this often requires the UE to operate in Dual Registration mode to maintain session continuity, which impacts UE battery life and signaling overhead.

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2. Interworking Modes and Discovery

A 5G network generally operates in one of two interworking modes regarding legacy EPC. The network must inform the UE of its capabilities to ensure the device handles mobility correctly.

Interworking With N26

In this mode, the network supports seamless handover. The UE operates in Single Registration mode. The context transfer happens network-side, transparent to the application layer regarding IP preservation.

Interworking Without N26

If the N26 interface is absent, the network cannot transfer the session context directly.

• Requirement: The network must still support procedures to provide IP address continuity for UEs in both Single-Registration and Dual-Registration modes (e.g., via “handover attach” mechanisms).
• Signaling:
  • 5GC: During Initial Registration, the AMF provides an indication that “interworking without N26” is supported.
  • EPC: Optionally, the MME may provide the same indication during the Attach procedure (as defined in TS 23.401).

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3. Pre-Requisites: Subscription and Restriction Checks

Before any mobility procedure is attempted, the network must validate that the user is actually permitted to access the target core. This validation chain flows from the HSS/UDM down to the RAN.

1. HSS/UDM: Stores the subscription data, including Core Network Type Restrictions (e.g., restriction towards 5GC) and RAT Restrictions (e.g., restriction towards NR).
2. MME: Downloads these restrictions. If the UE is camped on EPC, the MME evaluates whether a move to 5GC is allowed.
3. eNB: The MME forwards these restrictions to the eNodeB.
4. Decision: The eNB and MME use this data to permit or deny mobility to 5GS or NR-connected-to-5GS.

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4. Mobility from LTE to NR: Technical Mechanics

When a UE is in the LTE Connected state and mobility to 5GS is triggered, the system faces the complexity of mapping LTE EPS Bearers to 5G PDU Sessions.

The Mapping Mechanism

For seamless mobility, the mapping between the two domains must be established before the handover occurs.

When bearers are activated in LTE, the network (specifically the PGW+SMF) maps the EPS Bearer ID (EBI) to a corresponding 5G PDU Session ID.

• PCO/ePCO Provisioning: If the network supports N26 and the UE supports both S1 and N1 modes, the network configures the UE with 5GS QoS rules.
• These rules are sent inside the Protocol Configuration Options (PCO) or Extended PCO (ePCO) IE within the ACTIVATE DEDICATED (or DEFAULT) EPS BEARER CONTEXT REQUEST.
• The UE stores these rules and applies them immediately upon inter-system change to N1 mode.

PGW+SMF Selection

The MME plays a crucial role in selecting the gateway. For a PDN connection to be movable to 5G, the MME must have selected a combo node (PGW+SMF) for that PDN.

During the mobility procedure, the PGW+SMF determines which PDN connections can be relocated based on:

1. Capabilities of the target 5GS deployment.
2. Subscription data.
3. Operator policies regarding Seamless Session Continuity (SSC).

Upon successful mobility, the 5GS informs the UE (via PDU Session modification signaling) specifically which EPS bearers were successfully migrated.

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5. Handling Multiple PDU Sessions and Partial Failures

In real-world scenarios, a UE often holds multiple PDU sessions (e.g., one for IMS/Voice, one for Internet). It is possible that some sessions can be moved to 5GC while others cannot.

Scenario: A UE has an IMS bearer (anchored on a combo PGW+SMF) and an Internet bearer (anchored on a legacy PGW).

1. Selection: The MME attempts to relocate connections. It identifies that the IMS bearer can be moved.
2. Rejection: The MME/AMF detects that the Internet PDN cannot be moved (e.g., the AMF cannot retrieve the SMF address, or the gateway lacks 5G capability).
3. Outcome: The PGW+SMF and NG-RAN reject the specific PDN connections that cannot be transferred.
4. UE Behavior: The handover proceeds for the supported sessions (IMS). For the dropped Internet session, the UE must trigger a new PDU Session establishment request in 5GS after the mobility procedure is complete.

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