Features
- Create, modify, and release sessions
- Maintain the tunnel between UPF and RAN
- Selection and control of UPF (ARP/ND)
- Configure UPF traffic steering
- Collect UPF billing data
- SCC mode selection for the session
- I-SMF
- IMS P-CSCF discovery
- Policy related functions
Specific function explanation
Session management
Properties of PDU session
S-NSSAI and DNN
Network slice identification & data network name
S-NSSAI has a DNN list configuration, if it does not exist, the default value in
the slice will be used. Once all the parameters in the table are modified, it is equivalent to re-establishing a session
PDU session establishment
SMF checks the UE’s request and the UE’s subscription data in UDM.
UDM related SMF subscription data: DNN and S-NSSAI granularity
- Allowed PDU session type & default PDU session
- Allowed SSC mode and default SSC mode
- QoS information: signed Session-AMBR, default 5QI and default ARP
- Static IP address/prefix
- Signed user plane security strategy
- Charging attributes associated with the PDU session
- SMF supports the PDU session type sent by the UE
SMF supports the UE to establish multiple sessions of the same DN or different DNs.
The ID of the PDU that is registered to register the granular PDU session is unique in the UE to
establish a session request including:
- PDU session ID
- PDU session type
- Requested SSC mode
- 5G SM capabilities
- UE maximum data rate
- Always-on PDU session request
PDU session modification
Usually:
modify QoS, change PS data off state
At the same time, it supports UE modification & network side modification
SMF modification session trigger conditions : PCF policy change, SM subscription data change in UDM, according to local configuration changes, RAN resource changes
SMF can decide whether to send updates to PCF
PDU session release
Resources released by the session:
- IP address and prefix assigned to the PDU
- UPF resource of PDU session (N3 N9 N19)
- Access resources used by PDU sessions
Simultaneous support: UE-initiated release & network-side release.
Network-side SMF supports local configuration, or the entity that releases
SMF notification triggered by a message sent by PCF, DN, UDM, and AMF entities: PCF, DN
Secondary authentication/authorization during PDU session establishment
The DN-AAA server performs secondary authentication and authorization on the PDU session.
DN-AAA belongs to DN | 5GC When
DN-AAA belongs to DN, the DN-AAA server deployed inside the enterprise is responsible for authentication and authorization, and determines whether users can access the internal resources of the enterprise through the 5G network
. Time for secondary authentication and authorization: when the PDU session is established
Data source for authentication and authorization: DN specific ID authentication/authentication information carried when UE initiates session establishment; DN-related SMF local policy.
Activate or deactivate UP connection
RAN and N3 tunnel = UE-CN user plane connection = UP (user plane) connection of
PDU session This management occurs when the UE establishes multiple PDU sessions
- UE CM-IDLE state: UE or the network requests activation
- UE CM-CONNECTED state: UE requests the service by requesting activation
SMF supports the deactivation of the following scenarios:
- During the Handover handover process, all QoS flows are rejected by the RAN
- No data during UPF detection period
- AMF informs the UE to move out of the permitted service area
Deactivation is actually: notifying the RAN to release radio resources, and notifying the UPF to delete the RAN tunnel information of the N3 tunnel
Always-PDU session: from the idle state to the connected state are activated during each converted user plane resources
VoNR PDU will use always-on session
In this scenario SMF can not disable the PDU is connected UP
Session Continuity (SSC)
SSC mode 1: The user IP does not change during the session, and the UPF serving as the anchor point does not change
SSC mode 2: Both the user's IP and UPF need to be released and then re-created
SSC mode 3: between mode 1 and mode 2, at the anchor point Before the change, create a session with the new anchor point, and then release the old resource
SSC mode selection: make the user subscribe part of the data. Operators will provide UE with SSC selection policy (SSCMSP) through UE Route Selectin Policy, URSP.
Support different session types
The following types of sessions are supported, and only one of them can be selected for each PDU.
IPv4, IPv6, IPv46 : (Required)
Ethernet, Unstructured
Among them, Unstructured PDUs are generally used in application scenarios where data is transferred to the destination via the N6 interface through point-to-point tunneling technology.
Support location reporting and management
Area of Interest (Area of Interest): A geographic area within the designated 3GPP system.
For 3GPP access, the area of interest includes the following:
- A set of TA lists
- Cell ID
- RAN ID
- PRA ID(s)
- LADN DNN
describes the definition and composition of the interest area and then describes the SMF processing of the interest area:
SMF subscribes to the AMF " UE Mobility Time Notification " service
UE enters or leaves the interest area
SMF receives notification from AMF
SMF according to the location of the UE Information determines how to handle PDU sessions.
When the area of interest is LADN, SMF provides LADN DNN information to AMF and subscribes to the " UE Mobility Time Notification " service
Support I-SMF function
I-SMF stands for Intermediate SMF or Intermediate SMF.
Function: Add, delete and modify the SMF to the PDU session.
Location: AMF, I-SMF, SMF
Meaning of existence: If the original SMF cannot control the UPF of the service area for control, I-SMF will take over for control.
I-SMF is not applicable to SSC modes 2 and 3.