5G Interview Questions Answers 27-31

Q27. What is R(AN)? What is the essential function of R(AN) in 5G NR?

Ans. An NG-RAN node is either:

a gNB, providing NR user plane and control plane protocol terminations towards the UE; or
an ng-eNB, providing E-UTRA user plane and control plane protocol terminations towards the UE.

The gNBs and ng-eNBs are interconnected with each other by Xn interface. The gNBs and ng-eNBs are also connected by NG interfaces to the 5GC, more specifically connected to the AMF by NG-C interface and to the UPF by NG-U interface (see 3GPP TS 23.501).
The gNB and ng-eNB have the following functions:

Functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);
IP and Ethernet header compression, encryption and integrity protection of data;
Selection of an AMF at UE attachment when no routing to an AMF can be determined from the information provided by the UE;
Routing of User Plane data towards UPF(s);
Routing of Control Plane information towards AMF;
Connection setup and release;
Scheduling and transmission of paging messages;
Scheduling and transmission of system broadcast information (originated from the AMF or OAM);
Measurement and measurement reporting configuration for mobility and scheduling;
Transport level packet marking in the uplink;
Session Management;
Support of Network Slicing;
QoS Flow management and mapping to data radio bearers;
Support of UEs in RRC_INACTIVE state;
Distribution function for NAS messages;
Radio access network sharing;
Dual Connectivity;
Tight interworking between NR and E-UTRA;
Maintain security and radio configuration for User Plane CIoT 5GS Optimisation, as defined in TS 23.501 [3] (ng-eNB only).

NOTE 1: NB-IoT UE is only supported by ng-eNB, see 3GPP TS 36.300

Q28. What is SMF? What is the essential function of SMF in 5G NR?

Ans. The Session Management function (SMF) includes the following functionality:

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.
Termination of interfaces towards Policy control functions.
Lawful intercept (for SM events and interface to LI System).
Charging data collection and support of charging interfaces.
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.
Roaming functionality:
Handle local enforcement to apply QoS SLAs (VPLMN).
Charging data collection and charging interface (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.

NOTE: 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 TS 23.503.
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/accounting information for Monitoring Event Reports that are sent to the HPLMN.

Q29. What is UPF? What is the essential function of UPF in 5G NR?

Ans. The User plane function (UPF) includes the following functionality:

Anchor point for Intra-/Inter-RAT mobility (when applicable).
Allocation of UE IP address/prefix (if supported) in response to SMF request.
External PDU Session point of interconnect to Data Network.
Packet routing & forwarding (e.g. support of Uplink classifier to route traffic flows to an instance of a data network, support of Branching point to support multi-homed PDU Session, support of traffic forwarding within a 5G VN group (UPF local switching, via N6, via N19)).
Packet inspection (e.g. Application detection based on service data flow template and the optional PFDs received from the SMF in addition).
User Plane part of policy rule enforcement, e.g. Gating, Redirection, Traffic steering).
Lawful intercept (UP collection).
Traffic usage reporting.
QoS handling for user plane, e.g. UL/DL rate enforcement, Reflective QoS marking in DL.
Uplink Traffic verification (SDF to QoS Flow mapping).
Transport level packet marking in the uplink and downlink.
Downlink packet buffering and downlink data notification triggering.
Sending and forwarding of one or more "end marker" to the source NG-RAN node.
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 UPF 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.
Packet duplication in downlink direction and elimination in uplink direction in GTP-U layer.
TSN Translator (NW-TT) functionality.
High latency communication.
ATSSS Steering functionality to steer the MA PDU Session traffic.

NOTE: Not all of the UPF functionalities are required to be supported in an instance of user plane function of a Network Slice.

Q30. What is PCF? What is the essential function of PCF in 5G NR?

Ans. The Policy Control Function (PCF) includes the following functionality:

Supports unified policy framework to govern network behaviour.
Provides policy rules to Control Plane function(s) to enforce them.
Accesses subscription information relevant for policy decisions in a Unified Data Repository (UDR).

NOTE: The PCF accesses the UDR located in the same PLMN as the PCF.
The details of the PCF functionality are defined in TS 23.503.

Q31. What is AF? What is the essential function of AF in 5G NR?

Ans. The Application Function (AF) interacts with the 3GPP Core Network in order to support the following services:

Application influence (effect) on traffic routing;
Accessing Network Exposure Function;
Interacting with the Policy framework for policy control;
IMS interactions with 5GC.

Based on operator deployment, Application Functions considered to be trusted by the operator can be allowed to interact directly with relevant Network Functions.

Application Functions not allowed by the operator to access directly the Network Functions shall use the external exposure framework via the NEF to interact with relevant Network Functions.