Description
The Serving Gateway Control plane function (SGW-C) is a critical network function introduced as part of the Control and User Plane Separation (CUPS) architecture for the Evolved Packet Core (EPC) and its evolution into 5G. It represents the disaggregation of the monolithic Serving Gateway (SGW) into separate control and user plane entities. The SGW-C is responsible for all the control logic associated with managing user sessions. This includes processing signaling from the Mobility Management Entity (MME) or Access and Mobility Management Function (AMF), such as session establishment, modification, and termination requests. It manages the mobility anchoring point for inter-eNodeB handovers within LTE and acts as the local mobility anchor for 3GPP access when the user equipment (UE) moves between eNodeBs. The SGW-C also interfaces with the Policy and Charging Rules Function (PCRF) or Policy Control Function (PCF) to enforce QoS policies and charging rules for each data session.
Architecturally, the SGW-C communicates with its corresponding user plane function, the SGW-U, using the Packet Forwarding Control Protocol (PFCP) as defined in 3GPP TS 29.244. This protocol allows the SGW-C to instruct the SGW-U on how to handle user plane packets. For example, the SGW-C sends PFCP Session Establishment/Modification/Deletion requests to the SGW-U to create, update, or remove packet detection rules (PDRs), forwarding action rules (FARs), QoS enforcement rules (QERs), and usage reporting rules (URR). This separation allows the SGW-C to be deployed centrally for efficient control, while SGW-U instances can be distributed to the network edge to reduce latency for user traffic.
In a 5G context, the SGW-C is part of the interworking function between the 5G Core (5GC) and the EPC, particularly in non-standalone (NSA) deployment scenarios. It works in conjunction with the User Plane Function (UPF) and the Session Management Function (SMF). The SGW-C's role is to maintain continuity for 4G sessions as networks transition to 5G, ensuring seamless mobility and service. Its operation is detailed across multiple specifications, including architecture (23.214), management (28.708, 32.867), protocol (29.244), and security (33.127).
Purpose & Motivation
The SGW-C was created to address the limitations of the traditional, integrated Serving Gateway in 4G networks. The monolithic SGW combined both control and user plane functions in a single network element, which led to scaling inefficiencies. Scaling for increased control signaling (e.g., during mass device attach events) required scaling the entire box, including expensive user plane packet processing resources. Conversely, scaling for user plane throughput (e.g., during a video streaming surge) necessitated scaling the control plane capacity as well, leading to cost-ineffective and rigid deployments.
The introduction of the SGW-C, as part of the CUPS framework standardized in 3GPP Release 14, was motivated by the need for greater network flexibility, scalability, and innovation. By separating the control plane, operators can centralize and pool SGW-C resources in large data centers for efficient management and signaling processing. This separation enables independent scaling of control and user plane resources based on actual network demands. It also facilitates the deployment of user plane functions (SGW-U) at distributed locations closer to the radio access network, which is a key requirement for low-latency applications and edge computing. This architectural shift was a foundational step towards the fully cloud-native, service-based architecture of the 5G Core network.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (92 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-C was introduced as a new, separated control plane function that manages the SGW-U via the new Sxa reference point. Its newly specified responsibilities include session management, UE mobility support, and controlling procedures like user plane path failure reporting and load/overload control on the N4 interface. The release also defined the SGW-C's role in selecting the SGW-U based on parameters like UE location and UP function 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
- Load-overload control on N4 TS 29.244CR0053
- Reporting User Plane Inactivity on N4 TS 29.244CR0060
+ 11 more changes
In Release 16, the SGW-C saw enhancements primarily to its Packet Forwarding Control Protocol (PFCP) procedures with the user plane. These included new capabilities for PFCP session reestablishment after a UP function restart, PFCP message bundling, and support for PFCP sessions to be successively controlled by different SMFs within a set. Furthermore, the release introduced procedures for user plane path recovery reporting and integrated support for Control Plane CIoT 5GS Optimisation with user plane forwarding.
- 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
+ 27 more changes
In Release 17, the SGW-C saw enhancements primarily through the extension of PFCP procedures over the N4mb interface, including new capabilities for user plane inactivity detection and reporting. The release also introduced mechanisms for PFCP session restoration after a UPF restart and added support for controlling the start and stop of usage measurements per PFCP session. Furthermore, specific updates were made for PFCP node and session-related messages, including the handling of node IDs and session modification requests.
- PFCP Node related messages supported over N4mb TS 29.244CR0606
- User Plane (In)Activity Detection and Reporting over N4mb TS 29.244CR0608
- 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
- Correcting PFCP Associations setup description TS 29.244CR0596
+ 13 more changes
In Release 18, the SGW-C saw enhancements primarily to the PFCP interface for improved session management procedures. These included extensions for handling HR-SBO PDU sessions, the introduction of TL-Containers in PFCP Session Modification/Deletion messages, and refinements to the PFCP association release procedure for the MPAS feature. Additionally, updates were made to the control of the User Plane Inactivity Timer and its reporting mechanism from the user plane.
- Generalization of QoS monitoring control description TS 29.244CR0723
- 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
- LI of 5G Media Streaming (5GMS) (Control plane) TS 33.127CR0186
- Removal of UPF exposure of QoS Notification Control TS 29.244CR0738
+ 10 more changes
In Release 19, key enhancements for the SGW-C were introduced to improve PFCP session management resilience and QoS control. Specifically, new procedures were defined to restore PFCP sessions at an alternative SMF and to exclude certain sessions from restoration during an SMF failure, enhancing reliability in deployments with an SMF set. Additionally, the release added capabilities for GTP-U Path QoS Control Information and provided clarifications on the User Plane Inactivity Timer.
- PFCP sessions excluded from the restoration upon a SMF failure with SMF set being deployed TS 29.244CR0895
- Modification of Header Handling Control Rules TS 29.244CR0935
- 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-C plays a role.
Defining Specifications
3GPP specifications that define or reference SGW-C, 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 |