It collects all the information related to PDU session management from various network components (e.g., UPF, PCF, UDM) and controls / orchestrates those network components based on request from AMF [SMF:1].

SMF is a critical 5G core component that performs a fundamental role in the 5G Service-Based Architecture (SBA). The SMF is primarily responsible for interacting with the decoupled data plane, creating, updating, and removing Protocol Data Unit (PDU) sessions and managing session context with User Plane Function (UPF).

The most fundamental function of SMF is PDU session management and you can take all other functions as subsidiary functions to better manage PDU s ession.

Types of SMF:

• Centralized SMF: Manages sessions for multiple UPFs, providing a centralized point of control.

• Distributed SMF: A distributed SMF is co-located with the UPF, providing a decentralized point of control and reducing latency.

Key Functions:

• Session establishment
• Session modification
• Session release
• QoS Management
• Charging and Billing

SMF Procedures:

• Session Establishment Procedure
• Session Modification Procedure
• Session Release Procedure
• QoS Management Procedure

In summary SMF plays a vital role and provide high-quality user experience, efficient resource allocation, and accurate billing.

• PDU Session Establishment
• IP Address Management
• QoS Management
• Policy Control
• Charging
• Downlink Data Notification
• Subscription Verification
• Traffic Usage Reporting
• Packet Routing and Forwarding
• Traffic Steering

SMF interacts with the other functions to gather information about subscription, session, etc. At its core, the SMF is the orchestrator of user sessions. Think of it as the “brain” behind each user's connection to the 5G network. It doesn't just establish and terminate sessions; it dynamically manages them throughout their lifecycle. This includes mobility, QoS changes, and even applying policy updates in real-time. Here are the some interactions:

• AMF - Interface N11
• UDM - Interface N10
• UPF - Interface N4
• PCF - Interface N7

Let's deep dive each of them:

1. AMF (Access & Mobility Management Function):
   1. This is a critical interaction for session establishment and control. The AMF is responsible for managing the initial connection between the user equipment (UE) and the 5G Core Network. When a user wants to access data services, the AMF triggers the SMF via the N11 interface to:
      1. Create a new PDU session: The AMF provides the SMF with information about the user and the requested services, allowing the SMF to establish a dedicated PDU session for the user's data traffic.
      2. Modify an existing session: If the user's QoS requirements or network conditions change, the AMF can request the SMF to modify the existing session parameters, ensuring optimal performance.
      3. Terminate a session: When the user disconnects or their session expires, the AMF informs the SMF to release the allocated resources and terminate the PDU session.
      4. This dynamic interaction between the AMF and SMF ensures that user sessions are efficiently managed throughout their lifecycle, from initial attachment to final detachment.
2. UDM (Unified Data Management):
   1. This interaction is crucial for the SMF to understand the user it's dealing with. Imagine a user trying to connect to the network. The SMF needs to know:
      1. Is this user authorized? (Authentication credentials from UDM)
      2. What services are they subscribed to? (Subscription data from UDM)
      3. What are their specific data plan limitations? (Data volume, speed caps from UDM)
      4. Are there any roaming agreements in place? (Roaming information from UDM)
      5. This information allows the SMF to create a tailored session for the user, ensuring they get the service they expect.
3. UPF (User Plane Function):
   1. This is where the “rubber meets the road” in terms of data flow. The SMF instructs the UPF on how to handle the user's data packets. Think of the UPF as a sophisticated traffic manager on the highway (the data network). The SMF provides the UPF with instructions like:
      1. Route this user's traffic through a specific path. (e.g., for optimized gaming performance)
      2. Apply this QoS policy to prioritize their video streaming. (e.g., ensuring smooth video calls)
      3. Enforce data usage limits according to their plan.
      4. Filter certain types of traffic. (e.g., for parental controls)
      5. The SMF dynamically adjusts these instructions as needed, based on changing network conditions, user mobility, or policy updates.
4. PCF (Policy Control Function):
   1. The PCF is the policy decision-maker. It provides the rules that the SMF enforces. This interaction is essential for:
      1. Dynamic QoS adjustments: Imagine a user moving from a Wi-Fi network to a congested 5G cell. The PCF can detect this and instruct the SMF to adjust the QoS parameters, ensuring a seamless experience.
      2. Charging and billing: The PCF can track data usage and apply the appropriate charging rules based on the user's plan.
      3. Network slicing: The PCF can assign users to specific network slices with tailored performance characteristics. (e.g. low latency for critical IoT devices.)
      4. Security policy enforcement: The PCF can define security policies, and the SMF ensures they are applied to the user's session.

Bibliography:

1. https://www.sharetechnote.com/html/5G/5G_Core_SMF.html
2. https://techcommunity.microsoft.com/blog/telecommunications-industry-blog/what-is-the-5g-session-management-function-smf/3693852