Interworking &
Non-3GPP.
The module about how 5G reaches the world it does not own — the 4G it is replacing, the wifi in every building, the fibre to your home. The 5G core does not care how you arrive: give it N2 and N3, and 4G, wifi and fibre all become doors into the same 5G. This lesson builds it all — combined nodes and the N26 bridge, the N3IWF and the IPsec/EAP-5G tunnel over untrusted wifi, trusted and wireline access, and ATSSS, where your phone uses 5G and wifi at once — with real tunnel and protocol-stack diagrams.
Reaching every network — 4G, wifi, and fibre
The player screen is a live animation stage with real diagrams — IPsec tunnels carry packets through hostile wifi, the N26 bridge lights up on a 4G handover, and one connection splits across two accesses, exactly as the narration reaches it. Karaoke subtitles in English, two subtitle modes, fullscreen.
Every interworking & non-3GPP structure
Each structure the video opens — its interface, direction, purpose and clause. Click any row to open its full breakdown. Filter to find any of them.
| Structure | Interface | Dir | What it does | Clause |
|---|
Interworking & Non-3GPP, at protocol depth
Everything the video animates, as reference you can scan: how 5G lives beside 4G (combined nodes, N26, single vs dual registration), how the core accepts untrusted / trusted / wireline access, the EAP-5G registration over wifi, and the whole of ATSSS. Cited to TS 23.501 V19.6.0 §4.2.8, §5.17, §5.32.
Combined nodes
- PGW-C + SMF (control) · UPF + PGW-U (user plane).
- HSS + UDM (subscriber) · PCRF + PCF (policy).
- The same node holds your session on both sides.
- Mapping: 5G QoS flows ↔ 4G EPS bearers.
N26 — the context bridge
- Directly connects the AMF and the MME.
- Carries identity, security, and session context.
- Connected-mode → seamless handover; idle-mode → context fetch.
- Optional — its absence forces dual-registration.
| Single-registration (with N26) | Dual-registration (no N26) | |
|---|---|---|
| registration | one active at a time (5G or 4G) | independent in both cores at once |
| mobility | seamless handover, context over N26 | UE re-establishes sessions itself |
| security | 5G keys mapped to 4G keys — no re-auth | UE manages its own two contexts |
| who drives | the network converts the context | the UE decides when/how to move |
| UE capability | a UE that supports both N1 mode and S1 mode | |
Untrusted → N3IWF
- Any WLAN the operator can’t trust.
- UE builds an IPsec tunnel to the N3IWF (NWu).
- N3IWF terminates N2 + N3 to the core.
- Signalling SA + a child SA per PDU session.
Trusted → TNGF
- Operator-controlled access (TNAN, TNAP).
- EAP-5G at the access point — cleaner join.
- TNGF terminates N2 + N3.
- UE picks PLMN first, then a trusted access.
Wireline → W-AGF
- Fibre / cable into the home (FMC).
- 5G-RG (speaks 5G) or FN-RG (older).
- W-AGF terminates N2 + N3.
- One core for home broadband + mobile.
The universal rule: every access — gNB, N3IWF, TNGF, W-AGF — terminates N2 (control) and N3 (user) to the 5G core. A UE over 3GPP + non-3GPP to the same PLMN is served by a single AMF (two N1 instances). N1 NAS is protected the same over every access.
| Step | What happens |
|---|---|
| 1 | UE joins the WLAN and gets a local IP address |
| 2 | UE starts IKE_SA_INIT toward the selected N3IWF (agree on encryption) |
| 3 | IKE_AUTH + EAP-5G carries the 5G NAS Registration Request up to the AMF |
| 4 | the AMF runs primary authentication (5G-AKA, Module 6) inside EAP-5G |
| 5 | on success the AMF delivers an N3IWF key over N2 |
| 6 | IKE completes with that key → the signalling IPsec SA stands up |
| 7 | NAS now flows securely to the AMF; registration completes (a child SA per PDU session follows) |
The three powers
- Steer — choose which access carries a flow.
- Switch — move a flow between accesses, live.
- Split — spread one flow across both at once.
- Built on a Multi-Access PDU session (two paths, one UPF).
Steering functionalities
- MPTCP — for TCP; MPTCP Proxy in the UPF.
- MPQUIC — for UDP / IP / Ethernet.
- ATSSS-LL — whole flows at the IP layer, any traffic.
- Several at once; each flow uses exactly one.
| Steering mode | Strategy | Good for |
|---|---|---|
| Active-Standby | one access live, the other a hot backup | never-drop devices |
| Smallest-Delay | send over whichever path is faster right now | gaming, video calls |
| Load-Balancing | split by a ratio (e.g. 70/30) | big downloads |
| Priority-based | fill the preferred access, overflow to the second | cheap-first (wifi then 5G) |
The brain: the PCF provisions ATSSS rules in the UE and matching N4 rules in the UPF; the PMF probes both paths for real-time delay so smallest-delay steering always knows the faster road.
Filter by name or clause — INTERWORK · NON-3GPP · FUNCTION · ATSSS · PROCEDURE.
| Element | Family | Where | Clause | What it is |
|---|
The rule to remember: the 5G core does not care how you arrive — it only cares that your NAS reaches the AMF and your data reaches the UPF. So 4G, wifi, and fibre all become doors into the same 5G, and ATSSS lets a single connection use two of those doors at once.
Built from ETSI TS 123 501 V19.6.0 §4.2.8 · §5.17 · §5.32Interworking & Non-3GPP mastery check
Questions and answers reshuffle every load. 70%+ to consider Module 9 done.