What is 5gCore ???

The purpose of this article is to inform readers about the Network Functions or network elements (4g term) available in 5gCore. Just like any other core in telecom network, 5g core is considered as the brain of the entire network.

Before Actually jumping into the 5g core functions directly , lets first make sure do we really need a separate core , are the current 4g networks not suffecient enough to run all the features which are required in 5g ?

The answer is simply NO , becasue 5g network deals with various aspects such as eMBB (enhance Mobile BroadBand), URLLC (Ultra-Reliable Low-Latency Communication) and mMTC (massive Machine Type Communications) with different requirements for each feature, for example eMBB required very high throughput , URLLC is low latency and mMTC is for the high capacity. So inorder to accommodate all these requirements a flexible system is needed.

5G core is also termed as System of Systems becasue it has to cover all the user requirements from various aspects. Therefore it can be said that 5g is more than a New Radio (NR) system. The 5g architecture uses a system of systems approach to integrate and align the many different and independent parts of a network to achieve higher performance with greater functionality as compared to prior generation.

Inorder to understand the 5gCore Network Functions let's first start from a top view of the architectures between 4g LTE and 5g-Core.

4g core is box driven , here each function has a separate server which means MME requires a separate hardware , SGW requires separate and same goes with PCRF / SAPC etc. These hardwares cannot be shared with each other which means more space is required and more resources will be utilized. Another thing also need to be noted here that each function has a separated session state. Although the User plane and Control plane can be separated in both PGW and SGW but they cannot be deployed in any other box. Each plane requires a specific function block.

However in 5gCore there is a first separation between a user plane and a control plane from 5g-NR , A User plane can is divided into a Local Gateway User plane and Centralized Gateway User plane which means in order to reduce the latency a local Gateway User plane can be placed in the Edge cloud or compute node near the radio and it can also be located as a Centralized Gateway user plane in core or in a centralized cloud depending on the requirement.

Some of the prominent features of 5gCore are:

1. Logical and physical separation of the User Plane and the Control plane. 
2. Session States are separated from the control plane which means unlike 4g Core there is a special storage in the 5gCore where all the states are stored and can be accessed upon the request by the functions in the Core. 
3. Consolidated Control plane , using Service Based Architecture (SBA).

Inorder to make all this happen 5gCore utilize couple of techniques first is NFV (Network Function Virtualization) for best scalability and optimum expansion. Which means all the 5gCore functions can be deployed by the software over the cloud or over any operator’s existing hardware without the need of any extra high end servers.

Second is SDN (Software Defined Networking) which enables the dynamic deployment of the user and control plane in the user defined location either centralized or in the Edge, meaning for such latency sensitive services the gateway (user plane) can be dynamically allocated close to the radio access network. All these above defined techniques are not possible in EPC (Evolved Packet Core).

Cloud and Virtualization enforce the 5g Core to undergo revolutionary changes. The underlying drivers behind the 5gC innovation are:

1. Service Based Architecture, 
2. Stateless,
3. Network Slicing & 
4. Common Core.

Service Based Architecture (SBA)

The Service based architecture concept according to the 3gpp R15 applies to the control plane of the core, where the elements in control plane are defined in terms of "Network Functions" (NFs) rather than "traditional" Network Entities or Network Elements in 4gCore.

The network functions consisted of network function services. This approach allows the addition of new services without any impact on the existing ones, making scalability simple and seamless. The interaction between network functions depends on how a service operates. Unlike traditional architecture where different point to point interfaces connect network entities,

Network Functions in SBA can request and provide service using a Service based interface (SBI), which according to 3gpp is a uniform interface based on HTTP/2 protocol. The interactions between the control plane and user plane continue using traditional point to point interfaces. All the network functions in the control plane communicate on the basis of the lookup table which is defined in Network Repository yet another function in the core known as NRF (Network Repository Function).

• 5g Architecture Model

The 3gpp R15 describes the architecture of the 5g system. The 5g architecture is defined as service based and the interconnection between the network functions is represented in two ways.

• A Service Based Representation, where network functions (e.g AMF or SMF) within the control plane enables other authorized network functions to access the services via lookup table, this representation also includes point-to-point reference points where necessary. 
• A Reference Point Representation defines the interaction between the Network Function services in the network functions described by point to point references point (e.g N11 and N10) between any two functions (e.g. AMF and SMF).

Note: Network Functions within the Core ontrol plane shall only use service based interfaces for their interactions.

• Network Functions Interactions

The interaction between the two network functions follows the two mechanisms.

1. Request-Response : Assuming there are 2 functions one is consumer and other one is producer. A control function NF_A is Consumer and NF_B is a producer. NF_B is requested by NF_A to provide a certain service , which either performs an action or provides information or both. In Request-Response mechanism communication is one to one between the NFs and a one-time response from the producer to a request from the consumer is expected within a certain timeframe. 

1. Subscriber-Notify : This a continuous message delivery , A control plane function NF_A subscribes to NF_B. NF_B notifies the results of this NF service to interested NF(s) that subscribed to this service.The subscription request may include notification request for periodic updates or notification triggered through certain events (example , the information requested get changed , reaches a certain threshold etc) , following is an example for subscriber notification.

In 5gCore the PCF can directly subscribe to location change service offered by the AMF rather than having to be sent via SMF. While in EPC 4g , similar information will flow multiple hops from MME to the SGW to PGW and finally to the PCR.

Stateless

In Conventional 4g the network elements are stateful and maintain the information about the user equipment context i.e. its an operating state for the duration of the user session for example  EMM-REGISTERED to EMM-DEREGISTERED etc.

 The 5G core breaks this combination of context and network element. The 5g Network function operates stateless which is a cloud concept where process and storage are information in separate databases defined as a standard Network Function. 

Any network function can store unstructured data in an Unstructured Data Storage Function (UDSF) and can store the data from Unified Data Management (UDM) , Policy Control Function and Network Exposure Function (NEF) in Unified Data Repository (UDR). These 5g stateless systems simplify the tasks performed by a network function and boosts its scalability.

Network Slicing

A network slicing can be better understood by a cake , for example there are multiple slices in a cake each slice is of different one is with M&M and chocolate and the other one is Kitkat chocolate with a caramel.

Same is the concept of network slicing applied here , which means the network is divided into logical networks over physical resources. These physical resources or network functions can be shared or dedicated based on the configuration and required requirement. So different can be configured with a desired configuration.

Each slice is of different configuration , For example in the figure it can be seen that Core-UP is configured in a URLLC Slice due to the requirement of lower latency. However in the massive IoT slice it is not necessarily needed. 

This grouping allows fast buildup of a network for new services without impacting the existing services. NSSF-Function (Network Slice Selection Function) provides the network slicing capabilities in 5g core. 

Common Core

This feature in a 5g network which allows devices to be connected to 5g Core via 3gpp and non-3gpp access like wifi or landline or sometimes using both access technology together depending upon the configuration and services it can be integrated via a common interface.

It is also known as 5g common core or access agnostics because 5g core is providing connection to UE regardless of access network. A unified authentication process supports multi-RAT (Radio access technology) access.

5G Core Network Functions

Since the high-level architecture of 5gCore has been explained earlier now it is good time to discuss the Network Functions one by one. Following are the functions available in 5gCore. 

AMF (Access and Mobility Function)

Following are the functions of AMF as per the definition of 3gpp R15. 

1. Termination of RAN Control Plane interface N2.
2. Termination of NAS (Non Access Stratum) (N1), NAS ciphering and integrity protection. 
3. Registration , connection , reachability for paging and mobility management.
4. Access authentication and authorization. 
5. Session management message transport and proxy between UE and Session Management Function (SMF).
6. Security anchor function and security context management for access security key handling.
7. SMS transport between UE and SMSF.
8. Lawful interception. 
9. Support of mandatory functionality for non-3gpp access.
10. N3IWF N2 interface termination and NAS signalling. 

User Plane Function (UPF)

Following are the functions of UPF as per the definition of 3gpp R15.

1. Anchor point for mobility procedures (when UE is moving from one gNB to another in the presence of Xn interface)
2. External PDU session point of interconnect to Data network. 
3. Packet routing and forwarding. 
4. Traffic usage reporting. 
5. QoS handling for user plane e.g. UL/DL rate enforcement, Reflective QoS marking in DL. 
6. Uplink traffic verification (SDF to QoS flow mapping).
7. Transport level packet marking in the uplink and downlink.
8. Downlink packet buffering and downlink data notification triggering.

Policy control function (PCF)

Following are the functions of PCF as per the definition of 3gpp R15.

1. Supports unified policy framework to govern network behaviour.
2. Provides policy rules to Control plane functions to enforce them implement it.
3. Provides the SMF with Service Data flow (SDF) templates. The SMF forwards the SDF templates to the UPF which uses them for DL QoS mapping. 
4. Accesses subscription information relevant for policy decisions in a Unified Data Repository (UDR).
5. The PCF provides the SMF with charging rules (e.g Data volumes etc…)

 Network Repository Functions (NRF)

Following are the functions of NRF as per the definition of 3gpp R15. This is the most important and newly introduced function in 5g-core which was not present in 4g core. In Fact this is among the key functions which is responsible for Service based architecture.

1. The NRF allows network functions to register their services and then allows other network functions to discover the services. 
2. Supports service discovery function. It receives Network Function discovery requests from Network Function instances and provides the information of the discovered Network functions instances (be discovered) to the Network functions instance. 
3. Maintains the Network function profile available Network functions instances and theis support services. Network Functions profile can include e.g. Network function ID, Network Function type , slice identifier, IP address of Network Function and Network Function specific service information. 
4. All network functions interact with Network Repository Functions.

Unstructured Data Storage Function (UDSF)

Following are the functions of UDSF as per the definition of 3gpp R15. 

1. Storage and retrieval of information as unstructured data by any Network function.
2. All network functions can interact with UDSF. 
3. Network Functions may share a UDSF for storing their respective unstructured data or may each have their own UDSF. 
4. Deployments can choose to co-locate UDSF with each network function.

Examples of unstructured data can be Audio , Video and bulk text etc , also 80% of data used in telecom is normally unstructured.

Unified Data Management Function (UDM)

Following are the functions of UDM as per the definition of 3gpp R15.This node acts like HSS in 4g legacy network.

1. It manages subscriber data and may also store subscriber data (in UDR).
2. User Identification handling. 
3. Access authorization based on subscription data (example, roaming restriction)
4. UE serving Network function registration Management (example, storing serving AMF for UE, storing SMF for UE’s PDU session)
5. Interacts with AUSF during the authentication procedure.

Unified Data Repository (UDR)

Following are the functions of UDR as per the definition of 3gpp R15.

1. Storage and retrieval of subscription data by the UDM. 
2. Storage and retrieval of policy data by the PCF. 
3. Storage and retrieval of structured data for exposure by NEF.
4. Deployment can choose to collocate UDR with UDM. 
5. UDR is a common backside for the UDM , NEF and PCF structured data.

Network Exposure Functions (NEF)

Following are the functions of NEF as per the definition of 3gpp R15.

1. Network Function capabilities and events may be securely exposed by NEF for example 3rd part or Application functions. 
2. Network Exposure function stores and retrieves information as structured data to the Unified Data Repository. 
3. It provides a means for Application functions to securely provide information to the 3gpp network.
4. It translates between information exchanged with the internal network functions.
5. It receives information from the network functions (based on exposed capabilities of other network functions). The stored information can be accessed and re-exposed by the NEF to other network functions and application functions and used for other purposes such as analytics. For example data from UDM for roaming purposes. 

Network Slice Selection Function (NSSF)

Following are the functions of NSSF as per the definition of 3gpp R15.

1. Verifies that the UE is subscribed to each of the S-NSSAI (Single - Network Slice Selection Assistance Information) belonging to the requested NSSAI , each UE as per 3gpp can support upto 8 slices maximum. 
2. Selects one for more network slices to serve the UE and generate the allowed NSSAI. 
3. Identifies a set of candidate AMF which can be used to serve the UE. 

Authentication Server Functions (AUSF)

Following are the functions of AUSF as per the definition of 3gpp R15.

1. Support authentication for 3gpp access and untrusted non-3gpp access.

Non-3gpp Internetwork Function (N3IWF)

Following are the functions of N3IWF as per the definition of 3gpp R15.

1. Support of IPSec tunnel establishment with the UE.
2. Termination of N2 and N3 interfaces to 5g core network for control plane and user plane respectively. 
3. Relaying uplink and downlink control plane NAS (N1) signalling between UE and AMF. 
4. Handling of N2 signalling from SMF (relayed by AMF) related to PDU sessions and QoS. 
5. Establishment of IPSec security association (IPSec SA) to support PDU session traffic. 
6. Relaying uplink and downlink user plane packets between the UE and UPF which involves decapsulation and encapsulation of packets for IPSec and N3 tunnelling. 
7. Enforcing QoS corresponding to N3 packet marking, taking into account QoS requirements associated to such marking received over N2. 
8. N3 user plane packet marking in the uplink. 
9. Local mobility anchor within untrusted non-3gpp access networks. 
10. Supporting AMF selection.

References :

1. 3gpp Release 15 and 16.
2. 5g Academy