SMF
Session Management Function - manages PDU sessions in 5G networks.
Session Management Function
The Session Management Function (SMF) is a key network function in the 5G Core network responsible for session management, including PDU session establishment, modification, and release. It also handles UE IP address allocation and selection of UPFs.
SMF Overview
The SMF manages PDU sessions for UEs and controls the user plane through the UPF. It acts as the control plane function for session management and coordinates with other network functions to provide connectivity services.
The SMF combines functionality from multiple 4G network elements, including the MME (for session management), PGW-C (for gateway control), and PCEF (for policy enforcement control).
Key SMF Functions
The SMF performs the following key functions:
- Session Management: Establishes, modifies, and releases PDU sessions
- UE IP Address Allocation: Allocates and manages IP addresses for UEs
- UPF Selection and Control: Selects appropriate UPFs and controls them via N4 interface
- Traffic Steering: Controls traffic routing and steering policies
- QoS Control: Enforces QoS policies and manages QoS flows
- Charging Data Collection: Collects charging information for billing
- Lawful Intercept: Supports lawful intercept requirements
- Policy Enforcement: Enforces policies received from PCF
SMF Interfaces
The SMF connects to other network functions through various interfaces:
| Interface | Connected NF | Purpose |
|---|---|---|
| N4 | UPF | User plane control |
| N7 | PCF | Policy control |
| N10 | UDM | Subscriber data management |
| N11 | AMF | Session management signaling |
| N16 | SMF | Inter-SMF communication |
| N40 | CHF | Charging |
SMF Services
In the service-based architecture, the SMF provides the following services:
- Nsmf_PDUSession: PDU session management service
- Nsmf_EventExposure: Event exposure service for subscribing to SMF events
PDU Session Management
The SMF handles various PDU session procedures:
- PDU Session Establishment: Creates new PDU sessions for UEs
- PDU Session Modification: Modifies existing PDU sessions (QoS, traffic filters, etc.)
- PDU Session Release: Releases PDU sessions when no longer needed
- Service Request: Handles service requests for existing PDU sessions
UPF Control via N4
The SMF controls UPFs through the N4 interface using PFCP:
- PFCP Session Management: Establishes, modifies, and deletes PFCP sessions
- Packet Processing Rules: Configures PDRs, FARs, QERs, and URRs
- Traffic Steering: Controls how traffic is routed through the network
- Usage Reporting: Collects usage information for charging and monitoring
QoS Management
The SMF manages QoS for PDU sessions:
- QoS Flow Management: Creates, modifies, and deletes QoS flows
- QoS Rule Management: Manages QoS rules for UEs
- QoS Enforcement: Ensures QoS requirements are met
- Reflective QoS: Supports reflective QoS for UEs
Network Slicing Support
The SMF supports network slicing through:
- Slice-specific Session Management: Manages sessions for specific slices
- Slice-specific UPF Selection: Selects UPFs based on slice requirements
- Slice-specific QoS: Applies QoS policies based on slice requirements
- Slice-specific Charging: Applies charging policies based on slice
Related Specifications
The SMF is specified in the following 3GPP documents:
- TS 23.501: System Architecture for the 5G System
- TS 23.502: Procedures for the 5G System
- TS 29.502: 5G System; Session Management Services
- TS 29.244: Interface between the Control Plane and the User Plane nodes
- TS 24.501: Non-Access-Stratum (NAS) protocol for 5G System