S5 and S8 Interfaces in LTE Explained

S5/S8 Interface Diagram

Quick facts

Contents

1. S5/S8 Interface Diagram
2. S5/S8 in the LTE Architecture
3. What S5 and S8 Are Used For
4. Difference Between S5 and S8
5. S5/S8 in the Bearer Model
6. User Plane on S5/S8
7. Control Plane on S5/S8
8. S5/S8 and the S-GW
9. S5/S8 and the P-GW
10. S5/S8 and Roaming
11. S5/S8 and Default / Dedicated Bearers
12. S5/S8 and Policy / Charging
13. S5/S8 and GTP Tunnel Management
14. S5/S8 and Mobility Continuity
15. Common Troubleshooting Angles for S5/S8
16. Related Pages
17. Key takeaways
18. FAQ
19. References

S5/S8 in the LTE Architecture

S5/S8 is the EPC gateway-chain interface between the access-side gateway and the PDN-facing gateway. It sits between the S-GW, which anchors the access-side user plane, and the P-GW, which provides connectivity to external packet data networks and operator IP services.

A simplified user-data path is UE → eNB → S1-U → S-GW → S5/S8 → P-GW → SGi → external network. S5/S8 is therefore one of the most important links in the LTE user-plane path.

What S5 and S8 Are Used For

S5/S8 extends LTE bearer connectivity from the access-side gateway deeper into the EPC. It carries the gateway-chain bearer path and the related control signaling needed to create, update, and maintain that path.

• Transporting the core bearer path between S-GW and P-GW
• Carrying both user-plane traffic and related control-plane signaling
• Supporting packet connectivity from LTE access into external PDNs
• Supporting roaming-aware separation between visited and home network gateways

Difference Between S5 and S8

S5 and S8 are closely related, but they are used in different architecture situations. In both cases the gateway chain is preserved, but S8 explicitly reflects the home/visited split in roaming deployments.

S5/S8 in the Bearer Model

The LTE bearer model chains together several bearer segments. A radio bearer carries traffic between UE and eNB, an S1 bearer carries traffic between eNB and S-GW, and an S5/S8 bearer carries traffic between S-GW and P-GW.

S5/S8 is where EPC bearer continuity is extended deeper into the core. Even if the radio bearer and S1 bearer are healthy, the EPS bearer path is not complete unless the S5/S8 segment is correctly established and maintained.

User Plane on S5/S8

The user plane on S5/S8 uses GTP-U in the EPS GTP-based model. User packets are encapsulated in GTP-U as they move between the S-GW and P-GW.

This is critical because S5/S8 is where the access-side user-plane anchor in the S-GW connects to the IP-anchor and PDN-facing role of the P-GW.

Control Plane on S5/S8

The control plane on S5/S8 uses GTPv2-C in the GTP-based model. This means S5/S8 carries gateway-to-gateway control signaling as well as user traffic.

That is why S5/S8 is not a pure transport link. It is also part of LTE bearer-control logic for session and bearer management between the S-GW and P-GW.

S5/S8 and the S-GW

The S-GW is the access-side gateway on the S5/S8 path. It anchors the user plane toward E-UTRAN and connects onward to the P-GW over S5/S8.

Without S5/S8, the S-GW could anchor access-side traffic, but it could not complete the packet path toward the PDN-facing side of the EPC.

S5/S8 and the P-GW

The P-GW is the PDN-facing gateway on the S5/S8 path. It connects the EPS bearer path to external packet data networks and operator IP services through SGi.

S5/S8 links the S-GW access-side mobility anchor role to the P-GW IP-anchor and service-connectivity role. This is why S5/S8 is so important to LTE service continuity.

• The S-GW keeps the access-side anchor stable.
• The P-GW keeps the IP/session side stable.
• S5/S8 bridges those two gateway roles.

S5/S8 and Roaming

S8 is especially important in LTE roaming. It is the interface between the visited-network S-GW and the home-network P-GW.

This allows the subscriber service and IP-anchor behavior to remain tied to the home network while the access-side mobility anchor can reside in the visited network.

• The visited side can support access and local gateway functions.
• The home side can still control packet-data connectivity and service anchoring.
• S8 reflects the gateway ownership and bearer-path split in roaming.

S5/S8 and Default / Dedicated Bearers

Both default bearers and dedicated bearers rely on S5/S8 for their gateway-to-gateway path. The default bearer cannot provide full packet connectivity unless S5/S8 is active, and dedicated bearers also depend on S5/S8 to extend service-specific treatment into the gateway chain.

This is why failures on S5/S8 can show up as attach succeeds but no real packet service, dedicated bearer policy looks correct but traffic never reaches the PDN side, or QoS/service continuity degrades deeper in the EPC.

S5/S8 and Policy / Charging

Policy and charging decisions are made elsewhere, but S5/S8 is still part of the actual gateway-chain path over which policy-controlled traffic must travel. PCRF/PCEF behavior can decide how traffic should be treated, but that treatment still depends on the bearer path existing correctly across S1-U, S5/S8, and SGi.

This is why S5/S8 problems can sometimes look like QoS or service-policy failures from the subscriber point of view, even when the root cause is gateway-chain transport or control failure.

S5/S8 and GTP Tunnel Management

Because S5/S8 uses both GTP-U and GTPv2-C, it is one of the clearest examples of LTE control/user-plane separation.

Control-plane failure may prevent bearer creation or update, while user-plane failure may prevent actual traffic delivery even when signaling succeeded. That distinction is central to EPC troubleshooting.

S5/S8 and Mobility Continuity

S5/S8 also matters during mobility because the bearer path must stay continuous as the UE moves. The S-GW serves as the access-side mobility anchor, while the P-GW remains the IP-facing anchor toward external networks.

If the S5/S8 gateway path is not aligned with current bearer state, user-plane continuity can break even when radio and S1 signaling look correct.

Common Troubleshooting Angles for S5/S8

S5/S8 troubleshooting usually means separating gateway control state from user-plane delivery. Gateway signaling may succeed while GTP-U traffic is blackholed, or user-plane tunnels may exist while control-plane bearer state is stale or inconsistent.

• Attach succeeds but there is no end-to-end data path
• S-GW and P-GW bearer state mismatch
• GTPv2-C control issues preventing session or bearer updates
• GTP-U user-plane issues causing traffic blackholing
• Roaming path failure on S8
• Dedicated bearer behavior fails between gateways
• Default bearer exists but PDN-side reachability fails
• Incorrect tunnel endpoint or TEID handling between S-GW and P-GW

Related Pages

Key takeaways

• S5/S8 is the interface between the S-GW and P-GW in LTE EPC.
• It carries the S5/S8 bearer, the gateway-chain segment of the EPS bearer path.
• S5 is typically used in non-roaming architecture, while S8 is used in roaming.
• The user plane on S5/S8 uses GTP-U, and the control plane uses GTPv2-C.
• Understanding S5/S8 is essential for diagnosing gateway-chain data problems, bearer issues, and roaming path failures in LTE.

FAQ

References

• 3GPP TS 23.401 EPS architecture, bearer model, roaming reference models, and S-GW/P-GW roles.
• 3GPP TS 29.281 GTP-U user-plane protocol used on S5/S8 and related EPS user-plane interfaces.
• 3GPP TS 29.274 GTPv2-C control-plane protocol for S5/S8 and other EPC control interfaces.