Question

Discuss in detail at least one of the key network functions.

Answer 1

Defination :

A key Network Function (NF) of SBA is the Network Repository Function (NRF), which provides NF service registration and discovery, enabling NFs to identify appropriate services in one another.

5G architecture is an evolution of current 4G architectures but based on a Service-Based Architecture (SBA). The 3GPP defines the SBA for a 5G core network as delivered by a set of interconnected Network Functions (NFs), with authorization to access each other’s services.

Some of the key differences / focus areas:

• In contrast to a fixed-function, hard-wired, appliance-based architecture as was the case for 4G LTE Core (or Evolved Packet Core (EPC)), fully realizing the potential of 5G means moving to a software and cloud-based open platforms.
• EPC (4G Core) elements were architected to be implemented on physical nodes that were virtualized, but not designed to be virtualized from the outset.
• Network elements in 5G core are cloud native; referred to as “functions” vs. “nodes.”
• With the flexibility, virtualization and programmability, the new architecture would better support possibility for diverging architectures for new service.

4G Control and User Plane Separation (CUPS) EPC

The separation of Control and User Plane for the 4G architecture was introduced with 3GPP Release 14. It separated the packet gateways into control and user planes allowing for more flexible deployment and independent scaling achieving benefits in both, CapEx and OpEx.

The next step in the evolution to 5G was to rename core network entities and either split or merge them depending on the functions that fall within the user or control plane in the 5G architecture. For those of you with the 4G background, some 4G CUPS Core elements can be easily mapped to renamed 5G Core elements. Here are the few key ones:

Next generation NB (gNB)

The new radio access technology is called New Radio (NR) and replaces LTE. The new base radio station is called next generation NB (gNB) (or gNodeB). It replaces the eNB (or eNodeB or Evolved Node B) in 4G-LTE, or NodeB in 3G-UMTS.

The gNB handles radio communications with the 5G capable User Equipment (UE) using the 5G New Radio (NR) air interface. Although, some types of gNB may connect to the 4G EPC instead of 5G Core.

The Control Plane – AMF and SMF

The Mobility Management Entity (MME) in LTE is the signaling node for UE access and mobility, establishing the bearer path for UE’s, and mobility between LTE and 2G/3G access networks. Mobility Management Function in LTE is now replaced with:

• Access & Mobility Management Function (AMF) – oversees authentication, connection, mobility management between network and device. It receives connection and session related information from the UE.
• Session Management Function (SMF) – handles session management, IP address allocation, and control of policy enforcement.

The Data Plane – User Plane Function (UPF)

As CUPS decouples Packet Gateway (PGW) control and user plane functions. This enables the data forwarding component (PGW-U) to be decentralized which is mapped to the UPF for the 5G Core.

• The user plane function consists of a single entity User Plane Function (UPF)
• It combines functionality from previous EPC Serving-Gateway (S-GW) and PDN-Gateway (P-GW).
• UPF is responsible for packet routing and forwarding and Quality of Service (QoS).

Network Slicing in 5G

A 5G network is geared towards supporting multiple use cases / applications. Examples of these uses cases include:

• enhanced Mobile Broadband (eMBB) which entails supporting user throughputs in the Gbps range (x Gbps)
• Industrial Internet of things that requires the Ultra-Reliable and Low Latency Communications (URLLC) capabilities (~ 1ms latency)
• massive Machine Type Communications (mMTC)– a network that can support millions of IoT devices

5G supports these multitude of services by leveraging the SBA to support multiple virtual networks that operate on the same physical hardware. The slices that occupy a single physical network are separated, meaning traffic and security breaches from one slice cannot interfere with another slice.

In 5G, a Network Slice includes the Core Network Control Plane and User Plane Network Functions as well as the 5G Access Network (AN). The 5G Access Network may be:

• A Next Generation (NG) Radio Access Network (gNB)
• A non-3GPP Access Network where the terminal may use any non-3GPP access to reach the 5G core network via a secured IPSec/IKE tunnel terminated on a Non-3GPP Interworking Function (N3IWF).

5G Core Service-Based Architecture (SBA)

Service-Based Architecture for core 5G networks is defined in 3GPP Technical Specification (TS) 23.501 — “System Architecture for the 5G System”. It uses service-based interfaces between control-plane functions, while user-plane functions connect over point-to-point links.

A good introduction to the SBA is the “Service-Based Architecture in 5G” paper by NGMN Alliance.

For those of you from IT and cloud background, you can imagine the 5G SBA as a hybrid of Service-Oriented Architecture (SOA) and microservices.

In short, it is an architectural approach that enables 5G network functionality to become more granular and decoupled. This allows individual services to be updated independently with minimal impact to other services and deployed on demand allowing for vendor independence, automation and agile operational processes, reduction in delivery and deployment time, and enhanced operational efficiencies.

Basic principles are:

• A Control Plane Network Function can provide one or more NF Services
• A NF Service consist of operations based on either a request-response or a subscribe-notify model
• Common control protocol using e.g. HTTP based API, replacing protocols like e.g. Diameter

The major building blocks of the 5G Core Service-Based Architecture are simplified as follows:

1. Network and Resource Management
2. Signaling
3. Subscriber Data
4. Application Function and Network Exposure Function
5. Location Services
6. Subscriber Management
7. Policy
8. Control Plane
9. User Plane
10. Access Network

The IMS Core Functionality is the same as for 4G.

Network and Resource Management

Network Repository Function (NRF)

• Allows every network function to discover the services offered by other network functions.
• It serves as a repository of the services;
• supports discovery mechanisms that allows 5G elements to discover each other; and
• enable status updates of the 5G elements.

Network Slice Selection Function (NSSF)

• Selects the Network Slice Instance (NSI) based on information provided during UE attach.
• Redirects traffic to a network slice.
• A set of Access and Mobility Management Function (AMF) are provided to the UE based on which slices the UE has access to.

Network Data Analytics Function (NWDA)

• Responsible for providing network analysis information upon request from network functions.

Security Edge Protection Proxy (SEPP)

• Protects control plane traffic that is exchanged between different 5G operator networks.

Service Communication Proxy (SCP)

• SCP is a decentralized solution and composed of control plane and data plane.
• SCP is deployed along side of 5G Network Functions (NF) for providing routing control, resiliency, and observability to the core network.

Binding Support Function(BSF)

• BSF is used for binding an application-function request to a specific Policy Control Function (PCF) instance.
• It is comparable to Policy and Charging Rules Function (PCRF) binding function provided by a 4G Diameter Routing Agent (DRA), for VoLTE and VoWiFi.

Unified Data Repository (UDR)

• A converged repository of subscriber information that can be used to service a number of network functions.
• Stores structured data that can be exposed to an NF.

Unstructured Data Storage Function (UDSF)

• Repository for storage and retrieval of unstructured data by a suitable network function.
• Network Functions (NFs) can store/retrieve “unstructured” data from UDSF.

Application Function (AF)

• Supports application influence on traffic routing, accesses NEF, interacts with policy framework for policy control.

Network Exposure Function (NEF)

• Provides a means to securely expose the services and capabilities provided by 3GPP network functions.
• It exposes APIs from/to external systems.

Policy Control Function (PCF)

• Governs the network behavior by supporting a unified policy framework.
• Accesses subscription information for policy decisions taken by the UDR.
• Supports the new 5G QoS policy and charging control functions.

Charging Function (CHF)

• Allows charging services to be offered to authorized network functions.

Authentication Server Function (AUSF)

• Is in a home network and performs authentication with a UE.
• Relies on backend service authenticating data and keying materials when 5G-AKA or EAP-AKA is used.
• Performs the authentication function of 4G Home Subscriber Server (HSS) – a database that contains user-related and subscriber-related information.

Unified Data Management (UDM)

• Is a converged repository of subscriber information; used to service a number of network functions.
• The 5GUDM (Unified Data Management) can use the UDR to store and retrieve subscription data.

Equipment Identity Register (5G-EIR)

• Enables authentication of devices in the network.
• Protects networks and revenues against the use of stolen and unauthorized devices.

Home Subscriber Server (HSS)

• Is in 4G networks fills a similar function to the UDM for 5G.
• It stores customer profile data and authentication information along with encryption keys.