Description
The Serving Gateway User plane function (SGW-U) is the packet-forwarding engine resulting from the disaggregation of the traditional Serving Gateway. Introduced with Control and User Plane Separation (CUPS), the SGW-U handles all the data plane processing for user traffic in the Evolved Packet Core (EPC). Its primary role is to serve as the mobility anchor for the user plane during inter-eNodeB handovers within LTE and to route and forward traffic between the Radio Access Network (RAN) and the Packet Data Network Gateway (PGW). It operates under the direct command of its associated control plane function, the SGW-C, using the Packet Forwarding Control Protocol (PFCP) over the Sxa reference point.
Functionally, the SGW-U performs packet inspection, classification, and enforcement actions based on rules installed by the SGW-C. These rules are conveyed through PFCP messages and include Packet Detection Rules (PDRs), Forwarding Action Rules (FARs), and QoS Enforcement Rules (QERs). A PDR defines how to identify a packet flow (using parameters like source/destination IP, port, and GTP tunnel endpoint identifiers). Once a packet matches a PDR, the associated FAR dictates the action, such as forwarding the packet to a specific next-hop tunnel (e.g., towards an eNodeB or a PGW-U), buffering, or dropping it. QERs allow the SGW-U to apply rate policing, marking, and scheduling to ensure the agreed-upon Quality of Service. The SGW-U is also responsible for encapsulating and decapsulating user IP packets within GPRS Tunneling Protocol (GTP-U) tunnels that connect it to the eNodeB (S1-U) and the PGW-U (S5/S8-U).
Architecturally, the separation allows the SGW-U to be implemented as a high-performance, streamlined packet processing node, often using hardware acceleration. It can be deployed in a distributed manner, much closer to the RAN than a centralized data center. This distribution is critical for reducing latency, which benefits applications like mobile gaming, autonomous vehicles, and augmented reality. In 5G interworking scenarios, the SGW-U can be collocated with or interconnected to a 5G User Plane Function (UPF) to facilitate seamless user plane handovers between 4G and 5G access. Its specifications span architecture (23.214), management (28.708, 32.867), the PFCP protocol (29.244), and security requirements (33.127).
Purpose & Motivation
The SGW-U was developed to overcome the inflexibility of the integrated SGW appliance. In traditional EPC deployments, the SGW's user plane was tied to its control plane, preventing independent optimization and scaling. This coupling was ill-suited for emerging trends like network function virtualization (NFV) and the need for low-latency edge computing. Scaling user plane throughput required provisioning entire new SGW instances, including redundant control plane resources, leading to inefficient capital expenditure.
The creation of the SGW-U, finalized in 3GPP Release 14 as part of CUPS, was driven by the desire for a more agile, cloud-native network architecture. By extracting the user plane into a separate function, operators can deploy lightweight, stateless SGW-U instances using commercial off-the-shelf hardware or virtualized platforms at optimal locations in the network topology. This enables traffic breakout at the edge for local services and reduces backhaul costs. The SGW-U's programmability via PFCP also opens the door for software-defined networking (SDN) principles in the mobile core, allowing for more dynamic traffic steering and service chaining. This separation was a direct precursor to the native control/user plane separation seen in the 5G Core's SMF and UPF, proving the concept for next-generation networks.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (87 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-14, normative work from Rel-15.
In Release 15, the SGW-U was formally introduced as part of the Control and User Plane Separation (CUPS) architecture for the EPC, enabling its flexible deployment separate from the SGW-C. The new Sxa reference point was defined between the SGW-C and SGW-U, and the S11-U interface between the MME and SGW-U was specified to support CP CIoT EPS Optimisation. Furthermore, the SGW-C was given the functionality to select an appropriate SGW-U based on parameters like the UE's location and the UP function's capabilities.
- Enable SGW-C & PGW-C selection of UPF to take UE's NR capabilities into account TS 23.214CR0047
- Correcting the condition for selection of SGW-U for NR as secondary RAT TS 23.214CR0055
- Interface between MME and SGW-U for IoT data transmission TS 23.214CR0050
- User plane reporting TS 29.244CR0041
- Reporting User Plane Inactivity on N4 TS 29.244CR0060
- Condition correction for SGW-U/PGW-U selection based on DCNR TS 29.244CR0069
+ 8 more changes
In Release 16, the SGW-U saw enhancements primarily to its PFCP protocol procedures and interfaces. Key additions included support for PFCP sessions to be successively controlled by different SMFs within a set, PFCP message bundling for efficiency, and mechanisms for PFCP association and session re-establishment. Furthermore, the release introduced explicit procedures for user plane path recovery reporting and GTP-U path QoS monitoring.
- Clarification of TEID allocation by gateway user plane TS 23.214CR0074
- Enhancement to the PFCP Association Release Procedure TS 29.244CR0240
- User Plane Forwarding with Control Plane CIoT 5GS Optimisation TS 29.244CR0247
- Update the PFCP association setup to support UE IP address Allocation by AAA/DHCP TS 29.244CR0252
- PFCP sessions successively controlled by different SMFs of a same SMF set TS 29.244CR0261
- PFCP messages bundling TS 29.244CR0285
+ 24 more changes
In Release 17, the SGW-U saw enhancements primarily through the extension of PFCP procedures over the N4mb interface, including support for user plane inactivity detection and reporting. The release also introduced mechanisms for reporting the restart of a GTP-U entity and for the control of start and stop usage measurement for a PFCP session. Furthermore, procedures for PFCP session set modification to restore sessions and the inclusion of PDRs and Traffic Endpoints in session modification messages were specified.
- PFCP Node related messages supported over N4mb TS 29.244CR0606
- User Plane (In)Activity Detection and Reporting over N4mb TS 29.244CR0608
- Reporting of the restart of a GTP-U entity TS 29.244CR0600
- Transport Level Marking information for PFCP sessions over N4mb TS 29.244CR0622
- 5GS User Plane Node TS 29.244CR0558
- Updates for Bridge/User plane Node ID configuration TS 29.244CR0585
+ 13 more changes
In Release 18, the SGW-U saw enhancements primarily in its PFCP-based control interface (Sxa), including extensions for new procedures like HR-SBO PDU sessions and the handling of TL-Containers in session modification. User plane inactivity detection and timer management were updated, and corrections were made to PFCP error handling and GTP-U extension header procedures. Additionally, new mechanisms were introduced for PFCP session deletion during SMF handovers and for the creation of Buffering Action Rules within session modification requests.
- User plane inactivity detection update TS 29.244CR0731
- PFCP extensions for HR-SBO PDU sessions TS 29.244CR0750
- TL-Containers in PFCP Session Modification/Deletion Request/Response TS 29.244CR0767
- New GTP-U PDU Set Information Container extension header TS 29.244CR0787
- LI of 5G Media Streaming (5GMS) (Control plane) TS 33.127CR0186
- PFCP Error Handling Corrections TS 29.244CR0740
+ 11 more changes
In Release 19, key enhancements for the SGW-U function included improved PFCP session resilience, such as the ability to restore sessions at an alternative SMF and exclude specific sessions from restoration upon an SMF failure. The release also introduced new user plane capabilities for GTP-U path QoS control information and the identification and marking of Data Burst Size in downlink GTP-U packets. Additionally, clarifications were provided for the User Plane Inactivity Timer and PFCP procedures for UE level measurements.
- PFCP sessions excluded from the restoration upon a SMF failure with SMF set being deployed TS 29.244CR0895
- Identification and marking of Data Burst Size in DL GTP-U packets TS 29.244CR0892
- Providing alternative SMF(s) per PFCP Session TS 29.244CR0911
- GTP-U Path QoS Control Information TS 29.244CR0887
- Restore PFCP Sessions at an alternative SMF TS 29.244CR0914
- User Plane Inactivity Timer clarification TS 29.244CR0971
+ 1 more changes
Explore further
Broader topics and technologies where SGW-U plays a role.
Defining Specifications
3GPP specifications that define or reference SGW-U, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.214 vj00 | Control and User Plane Separation for EPC | Rel-19 |
| TS 28.708 vj00 | EPC NRM Integration Reference Point Information Service | Rel-19 |
| TS 29.244 vj40 | PFCP Specification for Control/User Plane Separation | Rel-19 |
| TS 29.844 ve00 | Control and User Plane Separation for EPC Nodes | Rel-14 |
| TS 32.867 vf10 | Management Impacts of EPC CUPS | Rel-15 |
| TS 33.127 vj50 | Lawful Interception Architecture and Functions | Rel-19 |