Page 51 - ITU-T Focus Group IMT-2020 Deliverables

ITU-T Focus Group IMT-2020 Deliverables 2

6.2.3 Network function descriptions

6.2.3.1 Control plane network functions
NSSF (Network Slice Selection Function)

• Selects core network slice instance to accommodate the UE.
NOTE – It is assumed that UE can indicate required network slice type to the core network and then the core
network selects a particular network slice instance for the UE. Further clarifications on how UE and CN
interact to select a NSI are required.

MMCF (Mobility Management Control Function)
• Provides mobility support, including session continuity, for UEs connected to IMT-2020 core via
mobile and/or fixed access networks.

– When a UE is simultaneously connected to the same core network over heterogeneous access
networks, the UE is served by a single MMCF via interworking function (IWF).
• Performs access specific mobility control, e.g. UE reachability management, mobility restriction, etc.

• Performs access common mobility control, e.g. UE registration and location management, etc.
• Routes NAS (Non-Access Stratum) signalling messages (e.g., session management related) to
corresponding session management control function.

– A UE with multiple established PDU sessions may be served by different instances of SMCF. The
MMCF selects the SMCF functions for the PDU sessions. MMCF may select different SMCF
instances for different PDU sessions.
• Terminates RAN-CP interface and NAS (including ciphering and integrity protection)
• Access Authentication

SMCF (Session Management Control Function)
• Responsible for the setup of the IP or non-IP traffic connectivity (i.e. PUD session) for the UE as well
as controlling the user plane for that connectivity (e.g., selection/re-selection of user plane network
functions and user path, enforcement of policy and charging rules, etc.).

– Multiple SMCF can serve one UE in a network slice instance, e.g. to support different data
networks.
– Data plane network functions are selected/re-selected based on UE location, UE subscription
profile, session and service continuity mode selected for the PDU session, data network, user
plane network function load distribution, etc.
• Allocates UE IP address (including optional authorization).
• Provides session continuity with respect to UE mobility.
• Terminates session management related NAS messages
• Initiates access network specific session management information

– When SMCF needs to send NAS session management signalling to a UE, it provides information
allowing the MMCF to retrieve the corresponding UE NAS signalling context.
• Handles policy and charging rules

UACF (Unified Authentication Control Function)
• Responsible for the authentication of the identity (e.g., user identity) that is presented to the
network, when a UE requests to receive service(s) from the network and network slices.
SIDB (Subscriber Information DB function)
• Provides and manages user subscription data, policy data (e.g., on QoS and charging), session/user
related context and state.

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