5G/NR - Network Architecture - SMF  





SMF stands for Session Management Function. Simply put, it collects all the informations related to PDU session management from various network components (e.g, UPF, PCF, UDM) and controls / orchastrates those network components based on request from AMF.

Overall Architecture

The SMF and interactions with other network components within 5G Core network is illustrated below.

Followings are the name of each network component.

    AMF     Access and Mobility Management Function ==> Equivalent to MME in 4G

    AUSF    Authentication Server Function

    DN       Data Network

    NEF      Network Exposure Function

    NRF      Network Repository Function

    NSSF    Network Slice Selection Function

    PCF      Policy Control Function ==> Equivalent to PCRF in 5G

    (R)AN   (Radio) Access Network

    SMF     Session Management Function

    UDM     Unified Data Management ==> Equivalent to HSS in 4G

    UPF      User Plane Function ==> Equivalent to PGW in 4G

    SMSF   SMS Function

    SEAF    SEcurity Anchor Function ==> part of AMF function

    ARPF    Authentication credential Repository and Processing Function

    SIDF    Subscription Identifier De-concealing Function

SMF Functionality

Based on 23.501-6.2.2, the functionality of AMF is descrbed as follows. The most fundamental function of SMF is PDU session management and you can take all other functions as subsidiary functions to better manage PDU session.

  • PDU Session Establishement (23.502-4.3.2)
    • a UE initiated PDU Session Establishment procedure.
    • a UE initiated PDU Session handover between 3GPP and non-3GPP.
    • a UE initiated PDU Session handover from EPS to 5GS.
    • a Network triggered PDU Session Establishment procedure.
  • enforcement of the policy decisions related to service data flow detection
  • authorized QoS
  • charging
  • gating
  • traffic usage reporting
  • packet routing and forwarding
  • traffic steering

SMF and interaction with other components

Let me put high level overview on SMF and interactions with other components around SMF. Basically this is overall description on the illustrations shown above.

SMF (Session Management Function):

    It is responsible for establishing, maintaining, and terminating user sessions in the 5G core network. The SMF manages user plane resources and interacts with the UPF to ensure that data packets are correctly routed and forwarded. It's a central entity in the 5G core network when it comes to user sessions.

UDM (Unified Data Management):

    Manages user data, including subscription details and credentials.

    Interaction with SMF via Interface 10: The SMF might need to retrieve subscription details, session information, or other relevant data from the UDM. This ensures that the SMF has the necessary information to manage sessions effectively.

UPF (User Plane Function):

    Responsible for data packet routing and forwarding.

    Interaction with SMF via interface 4: The SMF interacts with the UPF to manage user plane resources and ensure that data is routed and forwarded correctly based on the session context.

PCF (Policy Control Function):

    The PCF determines and controls policy rules for user services. It provides the decision-making mechanism when it comes to policies like Quality of Service (QoS) rules, charging, and other service-specific behaviors.

    Interaction with SMF via Interface 7: The SMF interacts with the PCF to obtain the necessary policy rules for a user's session. Based on the policies provided by the PCF, the SMF can make decisions about how the session should be managed, including aspects like routing decisions, allocation of resources, and setting up QoS parameters.

UE-Initiated PDU Sesstion Establishment Process

The UE-Initiated PDU Session Establishment process in the 5G Core (5GC) network is a fundamental procedure that allows a User Equipment (UE) to establish a Packet Data Unit (PDU) session with the network. This process is essential for the UE to gain access to data services in the 5G network.

The UE-Initiated PDU Session Establishment process in the 5G Core network is a multi-step procedure that involves several network functions working in tandem to set up a data session for the UE. This process ensures that the UE gets the necessary resources and policy treatment for its data traffic.

The high level description on the interaction between SMF and other network components for the PDU session establishment can be described as follows :

  • PDU Session Establishment Request: The process begins when the UE sends a PDU Session Establishment Request to the Access and Mobility Management Function (AMF). This request contains necessary information such as the desired Slice/Service Type (SST), Session and Service Continuity (SSC) mode, and other relevant parameters.
  • AMF to SMF Request: The AMF forwards the request to the Session Management Function (SMF). The AMF selects an SMF based on the information provided by the UE and other network policies.
  • SMF Interaction with UPF: The SMF interacts with the User Plane Function (UPF) to set up the necessary user plane resources. The UPF is responsible for handling user data traffic and routing it appropriately.
  • Policy Control: The SMF may also interact with the Policy Control Function (PCF) to obtain policy rules for the new PDU session. These rules dictate how the user's data should be treated (e.g., Quality of Service, charging, etc.).
  • PDU Session Establishment Accept: Once the necessary resources are allocated and policy rules are set, the SMF informs the AMF, which in turn sends a PDU Session Establishment Accept message to the UE.
  • Data Transfer: With the PDU session established, the UE can now transfer data with the 5G network. The UPF handles the routing of this data to the appropriate destination, be it the internet, another UE, or a service within the 5G network.
  • Session Context: Throughout the session, the SMF maintains the session context, which contains all the necessary information about the PDU session, such as the allocated resources, policy rules, and other session parameters.

The detailed signaling procedure for PDU session Establishement is depicted in 3GPP as follows :


< 23.502 - Figure UE-requested PDU Session Establishment for non-roaming and roaming with local breakout >


1. PDU Session Establishment Request: The User Equipment (UE) initiates a request to establish a PDU session by sending the message to AMF. This is the initial communication signaling the UE's intent to establish a data session.

2. SMF selection: The AMF (Access and Mobility Management Function) selects an appropriate SMF (Session Management Function) for managing this PDU session.

3. Nsmf_PDUSession_CreateSMContext Request: AMF sends a request to the selected SMF to create a session management context. This context holds information about the session.

4. Subscription retrieval/ Subscription for updates: SMF interacts with the UDM (Unified Data Management) to retrieve subscription details or subscribe for updates. This ensures SMF has the latest subscriber information.

5. Nsmf_PDUSession_CreateSMContext Response: The SMF responds to the AMF indicating the result of the session management context creation, either successful or with reasons for failure.

6. PDU Session authentication/authorization: Before proceeding, there's an authentication and authorization process ensuring the UE has the right permissions to establish the session.

7a. PCF selection: The SMF selects a PCF (Policy Control Function) for policy decisions related to this session.

7b. SM Policy Association Establishment or SMF initiated SM Policy Association Modification: This step involves either establishing a new policy association or modifying an existing one based on the session requirements.

8. UPF selection: The SMF selects a UPF (User Plane Function) which will handle the user plane traffic for this session.

9. SMF initiated SM Policy Association Modification: The SMF might initiate a modification to the established policy association based on certain criteria or network conditions.

10a. N4 Session Establishment/Modification Request: SMF make a request to UPF to establish or modify a session on the N4 interface, which is between the SMF and UPF.

10b. N4 Session Establishment/Modification Response: The UPF responds to the SMF's request indicating the success or failure of the session establishment or modification on the N4 interface.

11. Namf_Communication_N1N2MessageTransfer: SMF sends a message to AMF via the Namf_Communication_N1N2MessageTransfer interface. This message contains a wealth of information regarding the PDU session:

12. N2 PDU Session Request (NAS msg): This step involves the AMF sending a PDU session request to RAN over the N2 interface, which is an NAS (Non-Access Stratum) message.

13. AN-specific resource setup (PDU Session Establishment Accept): The Access Network (AN) sets up specific resources for the session, and then a PDU Session Establishment Accept message is sent.

14. N2 PDU Session Response: Finally, a response for the N2 PDU session request is sent from RAN to AMF, indicating the result of the session establishment attempt.

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