5G Core Network Functions
02.03.2018
Introduction
As a follow up from our previous post on 5G Core Network (5GC) and system architecture,
today I’d like to touch upon the 5G network functions (NFs) that are related to core network. As
we’ll see, some of them look very similar to the corresponding functions in the previous
generations’ core networks (CN). This of course is not surprising, as the network always has to
carry out some basic functions such as: communicate with the UE, store its subscription and
credentials, allow access to external networks & services, provide security and manage the
network access and mobility. However, we will also see some new functions, that were not
present before, which are needed to enable the new network paradigms like slicing and servicebased networking.
Service-Based Architecture (SBA)
To describe the CN functions themselves, let’s firstly look at the system architecture from 3GPP
SA2 on the following figure.
Figure 1: 5G System: Service-based architecture (SBA) [1]
We can see that part of this architecture, looks like good old LTE/3G with similar nodes and
interfaces (lower part of the picture). However, the upper part of the figure (5GC Control Plane),
has a “bus” and service-based interface exhibited by individual function. This creates a so-called
Service Based Architecture (SBA), in which, one CP network function (e.g. SMF) allows any
other authorized NFs to access its services. According to [1], the NFs within 5GC Control Plane,
shall only use service-based-interfaces for their interactions.
Note: If you want to revisit the “good-old” reference point architecture representation for 5G,
read our other blog on the general 5GC aspects: [5G CN Overview]. Yes, the 5G system
architecture with reference points is still there.
Network Functions (NFs)
Ok, let’s discuss a bit of the 5GC network functions themselves and what functionalities do they
support (note that not all functionalities of individual functions are present). The main 5G NFs
are the following:










Access and Mobility Management function (AMF) supports: Termination of NAS
signalling, NAS ciphering & integrity protection, registration management, connection
management, mobility management, access authentication and authorization, security
context management. (AMF has part of the MME functionality from EPC world)
Session Management function (SMF) supports: session management (session
establishment, modification, release), UE IP address allocation & management, DHCP
functions, termination of NAS signalling related to session management, DL data
notification, traffic steering configuration for UPF for proper traffic routing. (AMF has
part of the MME and PGW functionality from EPC world)
User plane function (UPF) supports: packet routing & forwarding, packet inspection,
QoS handling, acts as external PDU session point of interconnect to Data Network (DN),
and is an anchor point for intra- & inter-RAT mobility. (UPF has part of the SGW &
PGW functionality from EPC world)
Policy Control Function (PCF) supports: unified policy framework, providing policy
rules to CP functions, access subscription information for policy decisions in UDR. (PCF
has part of the PCRF functionality from EPC world)
Authentication Server Function (AUSF) acts as an authentication server. (part of HSS
from EPC world)
Unified Data Management (UDM) supports: generation of Authentication and Key
Agreement (AKA) credentials, user identification handling, access authorization,
subscription management. (part of HSS functionality from EPC world)
Application Function (AF) supports: application influence on traffic routing, accessing
NEF, interaction with policy framework for policy control. (same as AF in EPC world)
Network Exposure function (NEF) supports: exposure of capabilities and events, secure
provision of information from external application to 3GPP network, translation of
internal/external information. (not present in EPC world)
NF Repository function (NRF) supports: service discovery function, maintains NF
profile and available NF instances. (not present in EPC world)
Network Slice Selection Function (NSSF) supports: selecting of the Network Slice
instances to serve the UE, determining the allowed NSSAI, determining the AMF set to
be used to serve the UE. (not present in EPC world)
Summary
As we can see from the description of the individual NFs, some of them are a modified, split or
merged version of the good old EPC functions (like MME, SGW, PGW, HSS & PCRF) namely,
AMF, SMF, PCF, UDM, AF, UPF or AUSF, but unlike EPC functions, AMF, SMF and UPF
should be access independent.
On the other hand, there are some functions that are new, related either to the concept of slicing
(i.e., NSSF), to support handling multiple slices, or to the SBA itself (i.e., NEF & NRF) to
support the concept of services subscription, exposure and access.
Another thing is that in this architecture, we have a full CP & UP split in the core network, i.e.
UPF supports UP data processing, while all other nodes act as CP functions (this is a bit different
compared to EPC, where, e.g. PGW was acting a bit as UP and CP node, having some DHCP
functionality and IP address allocation and PCEF functionality). This allows for independent
scaling of CP and UP functions for the operators and for handling network slices efficiently to
tailor them to support different services.
5G Core Network – a Short Overview
Author: Marcin Dryjanski
8
Category: 5G, Mobile Networks
05.06.2017
The 5G System (5GS) will have three main components as defined below:
1. 5G Access Network (5G-AN)
2. 5G Core Network (5GC)
3. Use Equipment (UE)
This blog is dedicated to describing the main components of the 5G Core Network as defined by
the ongoing efforts at 3GPP SA [1].
The 5G system is being designed to support data connectivity and services which would enable
deployment, by the industry, using new techniques such as Network Function Virtualization and
Software Defined Networking. The need for these new techniques rises due to the various different
profiles of data services that need to be supported by the 5G network. So far mobile networks had
been designed keeping the average smartphone user in the center but with 5G this is changing as
with the boom of data connectivity various use cases having completely different data
requirements have come up and the network operator needs to satisfy all these requirements as
efficiently as possible.
Having such requirements in mind the 3GPP has kept the basic idea of having a flat architecture
where the Control Plane (CP) functions are separated from the User Plane (UP) in order to make
them scaling independent allowing operators to use this functional split for dimensioning,
deploying and adapting the network to their needs easily. Another central idea in the design of 5G
has been to minimize dependencies between the Access Network (AN) and the Core Network
(CN) with a converged access-agnostic core network with a common AN – CN interface which
integrates different 3GPP and non-3GPP access types.
Network Functions of 5G Core Network
In order to facilitate the enablement of different data services and requirements the elements of the
5GC, also called Network Functions, have been further simplified with most of them being
software based so that they could be adapted according to need. The 5G System architecture
consists of the following network functions (NF) majority of which constitute the 5GC:














Authentication Server Function (AUSF)
Core Access and Mobility Management Function (AMF)
Data network (DN), e.g. operator services, Internet access or 3rd party services
Structured Data Storage network function (SDSF)
Unstructured Data Storage network function (UDSF)
Network Exposure Function (NEF)
NF Repository Function (NRF)
Policy Control function (PCF)
Session Management Function (SMF)
Unified Data Management (UDM)
User plane Function (UPF)
Application Function (AF)
User Equipment (UE)
(Radio) Access Network ((R)AN)
The modularity of the network functions also opens up the possibility to enable another new and
efficient feature i.e. Network Slicing.
The interaction between network functions in the current form is envisaged in the following two
ways as per [1]:
1. The first method is the service-based representation in which one network function (e.g. AMF)
within the Control Plane allows other network functions, which have been authorised, to access
its services. This representation also includes point-to-point reference points between the NFs
where necessary (see figure 1).
Figure 1: 5G System Service-based architecture [1]
2. On the other hand is the reference point representation which focuses on the interactions
between pairs of network functions defined by point-to-point reference point (e.g. N7) between
any two network functions (e.g. SMF and PCF). This kind of representation is used when some
interaction exists between any two network functions (see figure 2).
Figure 2: Non-Roaming 5G System Architecture in reference point representation [1]
Summary
With the development of 5G speeding up1, we can see a clearer picture of what the future
generation wireless network would like to be able to achieve. The 5G core network aims to be
flexible enough to adapt and satisfy the needs for Gbps seeking smartphone users as well as low
latency seeking critical services along with low speed IoT devices. It is also being designed to be
more open and modular than its predecessor allowing the different entities inside the core network
to interact with each other without any preconditions and allowing to define procedures for this
interaction. And this would be achieved with the help of new techniques like Network Function
Virtualization, Network Slicing, and Software Defined Networking2 etc.
References
[1] 3GPP TS 23.501 V0.4.0 (2017-04)