Description
Session Management Congestion Control (SMCC) is a sophisticated framework introduced in 5G to safeguard the core network's control plane, particularly the Session Management Function (SMF), from congestion and overload. Unlike user plane congestion, SMCC focuses on the signaling storm that can occur when a massive number of UEs simultaneously attempt to establish, modify, or release PDU sessions. This congestion can be triggered by legitimate events (e.g., a stadium filling up) or malicious attacks (e.g., a Distributed Denial-of-Service attack targeting the N11/N7 interfaces). The SMCC architecture involves coordinated actions between the SMF, the Access and Mobility Management Function (AMF), and the Policy Control Function (PCF).
SMCC operates through a multi-stage process of monitoring, decision, and action. The SMF continuously monitors key performance indicators (KPIs) such as the rate of incoming session management requests, processing latency, and CPU/memory utilization. When predefined thresholds are crossed, the SMF enters a congestion state. It then applies congestion control policies, which are often pre-configured or dynamically provided by the PCF. A primary action is for the SMF to signal congestion information to the AMF using the N11 interface. The AMF, acting as a signaling aggregator from many UEs, can then apply back-pressure. This involves rejecting new session management requests from UEs with a wait timer or redirecting them, effectively throttling the load at the source.
The mechanisms within SMCC are granular. They can differentiate between types of session management procedures (e.g., prioritizing session establishment for emergency services over regular browsing) and can apply controls based on UE subscription, network slice (S-NSSAI), or Data Network Name (DNN). For instance, during congestion, the SMF might reject requests for a non-critical IoT slice while continuing to accept requests from a slice supporting public safety. The SMF may also implement local queuing and prioritization algorithms. By proactively managing the signaling load, SMCC ensures that the SMF remains responsive for essential sessions and that congestion does not cascade to cause a wider network control plane failure, thereby maintaining overall network stability and service availability.
Purpose & Motivation
SMCC was created to address a critical vulnerability in 5G and future networks: control plane signaling overload. Previous generations had congestion control, but 5G's service-based architecture, network slicing, and support for massive IoT exponentially increase the scale and complexity of session management signaling. A single SMF instance could be responsible for millions of sessions. Without SMCC, a surge in signaling—whether from a legitimate mass event or a coordinated attack—could overwhelm the SMF, leading to service denial for all users, including those with high-priority services.
The technology solves the problem of ensuring service reliability and fairness in a hyper-connected environment. It allows the network to gracefully degrade under load rather than catastrophically fail. The motivation for its standardization in Release 17 stemmed from operator experiences and the foresight that as networks become more software-defined and sliced, a standardized, interoperable approach to session management congestion was necessary. SMCC provides the tools to implement "smart" congestion control that can distinguish between different services and slices, which is a fundamental requirement for delivering on 5G's promises of guaranteed service level agreements (SLAs) for diverse use cases, from enhanced mobile broadband to ultra-reliable low-latency communications.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (38 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the SMCC (Session Management Congestion Control) function was newly introduced as part of the Session Management Event Exposure Service. This allowed the SMF to expose events related to PDU Sessions, including mechanisms for congestion control, such as notifications about buffered or discarded downlink packets at the UPF. The introduction also included corrections to the AF influence traffic steering control within this framework.
- Correction to the AF influence traffic steering control TS 29.508CR0012
In Release 16, the SMCC function was enhanced by introducing new event exposure capabilities for user data congestion. Specifically, the Nnwdaf_EventsSubscription and Nnwdaf_AnalyticsInfo services were extended to support user data congestion information, allowing for the subscription and notification of such events. Furthermore, the concept of a confidence level for user data congestion information was formally introduced.
- Support PDU session establishment event TS 29.508CR0068
- Enhance the Nnwdaf_EventsSubscription service to support User Data Congestion TS 29.520CR0056
- Enhance the Nnwdaf_AnalyticsInfo service to support user data congestion TS 29.520CR0057
- Confidence for User Data Congestion Information. TS 29.520CR0150
In Release 17, the SMCC (Session Management Congestion Control) function introduced new analytics capabilities, specifically a "New event for SM congestion control experience" for exposure via the Nsmf_EventExposure service. This release also extended support for User Data Congestion Analytics within the NWDAF framework, enhancing the Nnwdaf_EventsSubscription and Nnwdaf_AnalyticsInfo APIs to incorporate these new congestion-related events and data. Furthermore, updates were made to provide PDU session information to support UE communication analytics and to correct and clarify various data attributes and API definitions related to congestion reporting.
- New event for SM congestion control experience TS 29.508CR0146
- Provide PDU session information for supporting the UE communication analytics TS 29.508CR0170
- Extension to User Data Congestion Analytics with GPSI TS 29.520CR0301
- Extension to User Data Congestion Analytics in Nnwdaf_EventsSubscription API TS 29.520CR0314
- Extension to User Data Congestion Analytics in Nnwdaf_AnalyticsInfo API TS 29.520CR0315
- Updates to User Data Congestion Extension in Nnwdaf_EventsSubscription API TS 29.520CR0325
+ 9 more changes
In Release 18, the SMCC function was enhanced with new capabilities for PDU Session Traffic Analytics, including support for analytics subsets and data collection for traffic analytics, exposed via the Nsmf_EventExposure service. It also introduced enhancements for user data congestion reporting, such as support for ordering criteria and immediate reports for NWDAF subscriptions. Furthermore, the release added support for monitoring congestion information and collecting the list of access types used for a PDU session from the SMF.
- Support of the Congestion Information Monitoring TS 29.508CR0233
- Supporting data collection for PDU Session Traffic Analytics TS 29.508CR0232
- Collect list of Access Types used for the PDU session from SMF TS 29.508CR0235
- Access type change for MA PDU session update TS 29.508CR0261
- User consent enhancements for NWDAF data management TS 29.520CR0620
- Enhancement of user data congestion for AnalyticsInfo Service TS 29.520CR0666
+ 10 more changes
In Release 19, the SMCC function was enhanced to support new RAT Type information for PDU Sessions within the event exposure framework. Furthermore, the release introduced procedures for handling subscription termination specifically triggered by a PDU session release event. These updates expanded the granularity of network congestion control by incorporating more detailed session context and lifecycle events.
Explore further
Broader topics and technologies where SMCC plays a role.
Defining Specifications
3GPP specifications that define or reference SMCC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 29.508 vj40 | 5G Session Management Event Exposure Service | Rel-19 |
| TS 29.520 vj40 | 5G Network Data Analytics Services Stage 3 | Rel-19 |