What is S-GW?
The S-GW (Serving Gateway) is a user plane element in the 4G LTE Evolved Packet Core (EPC) that routes and forwards user data packets between the eNodeB and the P-GW. It serves as the local mobility anchor for inter-eNB handovers — when a UE moves between cells, the S-GW maintains the user plane tunnel, avoiding P-GW re-establishment. In 5G, the S-GW’s functions are absorbed into the UPF (User Plane Function).
How Does S-GW Work?
The S-GW connects to eNBs via the S1-U interface (GTP-U tunnels) and to the P-GW via the S5/S8 interface (GTP or PMIP tunnels). When a UE attaches to the network, the MME selects an S-GW based on the UE’s location and establishes GTP tunnels from eNB to S-GW to P-GW. During X2 handover between eNBs, the S-GW acts as the anchor — the source eNB forwards data to the target eNB, and path switching updates the tunnel from S-GW to the new eNB. The S-GW also handles charging data collection, lawful intercept, and inter-operator traffic routing (via S8 for roaming). In idle mode, the S-GW buffers downlink data and triggers paging via the MME.
Use Cases
4G LTE user plane data routing, mobility anchoring during handovers, downlink data buffering in idle mode, charging data generation, and inter-operator roaming traffic forwarding.
3GPP / Standards Reference
3GPP TS 23.401 (EPC architecture — S-GW functions), 3GPP TS 29.274 (GTPv2-C — S11, S5/S8 interfaces)
Related Terms
EPC | PDN-GW or P-GW | UPF | eNB | Core Network
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This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
