NEF services and slicing impacts

The Network Exposure Function (NEF) in 5G inherits from the SCEF (Service Capability Exposure Function) in 4G and encompasses many functionnalities among which :

• exposure at NFs, in particular AFs, of capabilities or events occurring at other NFs;
• exposure at NFs, in particular AFs, of capabilities or events occurring at other NFs;
• provisioning of information coming from AFs to NFs in the 5GC e.g. Expected UE Behaviour, 5GLAN group information and service specific information;

• support for analytics exposure i.e. NWDAF analytics possibly exposed by NEF for external parties AFs.

These functionalities are expressed in terms of NEF services.

NEF services and slicing impacts :

• The majority of NEF services are not impacted by slicing aspects introduced in 5G typically because they relate to the UE itself independently of the slice (S-NSSAI) the UE is using e.g. Nnef_Location.
• Some NEF services indirectly relate to slicing information because they relate to PDU session(s) and a PDU session is activated for a (S-NSSAI, DNN) combination. This is the case for NEF services related to Non-IP Data Delivery (NIDD), i.e. NIDD Nnef_SMContext, Nnef_NIDD and Nnef_NIDDConfiguration.
• Some NEF services also indirectly relate to slicing information because they relate to application(s) and an application can be identified by a (S-NSSAI, DNN) combination in 5G (although other means are also possible) This is the case for Nnef_ServiceParameter, Nnef_TrafficInfluence and Nnef_ServiceParameter;
• NEF_Parameter_Provision allows an AF to configure parameters in the UDM for a (S-NSSAI, DNN) combination for managing 5G VN groups;
• Some NEF services relate to slicing e.g. Nnef_AnalyticsExposure because it’s possible for an AF to obtain network analytics information related to a specific slice type, i.e. a filter is possible in the subscription request sent to the NEF. For event exposure and analytics exposure, when the filter on S-NSSAI is not possible, the deployment of a NEF dedicated to a slice allows the AF to obtain notifications of the events in the slice, which is applicablewhen a slice is dedicated to an NPN (Non-Public Network), i.e. PNI-NPN.

NEF deployment and slicing impacts :

• 3GPP specifications make no assumption on NEF deployment in the PLMN and the operator can deploy the NEF per type of NEF service and/or per slice type.
• NEF centralization allows saving OPEX/CAPEX with a limited number of NEF(s) in the PLMN and also saving signalling for subscriptions at the NEF and from the NEF towards the other NFs in the PLMN.
• NEF distribution avoids tromboning effects in the control plane for services such as edge computing or URLLC services where AFs are supposed to be distributed: NEF may be deployed close to AFs.
• NEF distribution is also a solution to provide event exposure and network analytics exposure to an AF on a dedicated slice when the subscription to such events per slice type is not possible in the standard.
• Implementing one or a few centralized NEF(s) and more distributed NEFs in the PLMN is the most realistic situation to manage the above cases.