School of Specs 23.501v20.2.0

3GPP 23.501 v20.2.0 — the document's own text

6.2.2 SMF

Taught in 4. What each network function does (The 5G system architecture, in depth), 18. Being served by somebody else's network (The 5G system architecture, in depth), 2. Who does what in the core (The 5G system architecture, overview), 2. The pieces a 5G core is made of (The 5G system architecture, quick start).

The Session Management function (SMF) includes the following functionality. Some or all of the SMF functionalities may be supported in a single instance of a SMF:

  • Session Management e.g. Session Establishment, modify and release, including tunnel maintain between UPF and AN node.
  • UE IP address allocation & management (including optional Authorization). The UE IP address may be received from a UPF or from an external data network.
  • DHCPv4 (server and client) and DHCPv6 (server and client) functions.
  • Functionality to respond to Address Resolution Protocol (ARP) requests and / or IPv6 Neighbour Solicitation requests based on local cache information for the Ethernet PDUs. The SMF responds to the ARP and / or the IPv6 Neighbour Solicitation Request by providing the MAC address corresponding to the IP address sent in the request.
  • Selection and control of UP function, including controlling the UPF to proxy ARP or IPv6 Neighbour Discovery, or to forward all ARP/IPv6 Neighbour Solicitation traffic to the SMF, for Ethernet PDU Sessions.
  • Configures traffic steering at UPF to route traffic to proper destination.
  • 5G VN group management, e.g. maintain the topology of the involved PSA UPFs, establish and release the N19 tunnels between PSA UPFs, configure traffic forwarding at UPF to apply local switching, N6-based forwarding or N19-based forwarding, manage traffic forwarding in the case that a SMF Set or multiple SMF Sets are serving a 5G VN.
  • Termination of interfaces towards Policy control functions.
  • Lawful intercept (for SM events and interface to LI System).
  • Support for charging.
  • Control and coordination of charging data collection at UPF.
  • Termination of SM parts of NAS messages.
  • Downlink Data Notification.
  • Initiator of AN specific SM information, sent via AMF over N2 to AN.
  • Determine SSC mode of a session.
  • Support for Control Plane CIoT 5GS Optimisation.
  • Support of header compression.
  • Act as I-SMF in deployments where I-SMF can be inserted, removed and relocated.
  • Provisioning of external parameters (Expected UE Behaviour parameters or Network Configuration parameters).
  • Support P-CSCF discovery for IMS services.
  • Act as V-SMF with following roaming functionalities:
    • Handle local enforcement to apply QoS SLAs (VPLMN).
    • Charging (VPLMN).
    • Lawful intercept (in VPLMN for SM events and interface to LI System).
  • Support for interaction with external DN for transport of signalling for PDU Session authentication/authorization by external DN.
  • Instructs UPF and NG-RAN to perform redundant transmission on N3/N9 interfaces.
  • Generation of the TSC Assistance Information based on the TSC Assistance Container received from the PCF.
  • Support for RAN feedback for BAT offset and adjusted periodicity as defined in clause 5.27.2.5.

NOTE 1: Not all of the functionalities are required to be supported in an instance of a Network Slice.

In addition to the functionalities of the SMF described above, the SMF may include policy related functionalities as described in clause 6.2.2 of TS 23.503 [45].

In addition to the functionality of the SMF described above, the SMF may include the following functionality to support monitoring in roaming scenarios:

  • Normalization of reports according to roaming agreements between VPLMN and HPLMN; and
  • Generation of charging information for Monitoring Event Reports that are sent to the HPLMN.

The SMF may also include following functionalities to support Edge Computing enhancements (further defined in TS 23.548 [130]):

  • Selection of EASDF, obtain and/or provision DNS security information of the EASDF and provision of its address to the UE as the DNS Server for the PDU session;
  • Usage of EASDF services as defined in TS 23.548 [130];
  • For supporting the Application Layer Architecture defined in TS 23.558 [134]: Provision and updates of ECS Address Configuration Information to the UE;
  • For supporting the HR-SBO as defined in clause 6.7 of TS 23.548 [130].
  • For supporting Local Offloading Management as defined in clause 6.10 of TS 23.548 [130].

NOTE 2: In case an I-SMF is inserted to locally manage edge computing related information and the traffic is locally offloaded, only the I-SMF performs the EASDF selection and uses the EASDF services for the locally offloaded PDU sessions.

  • For supporting the N6 delay measurement as described in clause 5.8.2.23.
  • Based on N6 delay measurements and user plane latency (between the 5G AN and candidate UPF(s)):
    • Supporting the (re)selection of local PSA UPF as defined in clause 6.2.3.2 of TS 23.548 [130].
    • Supporting to trigger (re)discovery of EAS(es) as defined in clause 6.2.3.2 of TS 23.548 [130].

The SMF and SMF+ PGW-C may also include following functionalities to support Network Slice Admission Control:

  • Support of NSAC for maximum number of PDU sessions as defined in clauses 5.15.11.2, 5.15.11.3 and 5.15.11.5.
  • Support of NSAC for maximum number of UEs as defined in clauses 5.15.11.3 and 5.15.11.5.

The SMF+ PGW-C may also include the support of NSSAA over EPS as specified in Annex L of TS 23.502 [3].

The SMF may also include following functionalities:

  • Providing per-QoS flow Non-3GPP QoS assistance information to the UE (e.g. PEGC) and formulation of the CN PDB based on non-3GPP delay budget from UE (e.g. PEGC) as described in clause 5.44.3.4.
  • Support of PDU Set based Handling as described in clause 5.37.5.

In addition to the functionalities of the SMF described above, the SMF may also include functionalities to support Network Slice Replacement as described in clause 5.15.19.

The SMF may also include functionalities to support indirect UPF event exposure service subscription on behalf of the consumer NF(s) as described in clause 4.15.4.5 of TS 23.502 [3].

In addition to the functionality of the SMF described above, the SMF may provide support for SMF overload control based on NWDAF analytics.

In addition to the functionality of the SMF described above, the SMF may provide support for energy related features as described in clause 5.51.

In addition to the functionality of the SMF described above, the SMF may include the following functionality to support communication via satellite access and/or satellite backhaul:

  • Support for reporting satellite backhaul category and its modification to PCF as described in clause 5.43.4.
  • Support for reporting to PCF the UE's serving satellite identifier and its change as defined in clause 5.4.14.