Core network is one of the main and important architectural component of mobile network systems. It has been developing since 3G because of packets are being send and received. In every version of core network, we can see some similarities and also some changes. However, main architecture is basically similar. In this document, we will investigate and compare differences and similarities between 3G, 4G and 5G core network.
3G Core Network
3G Core network has different structure compared to 4G and 5G Core networks. It is possible to divide core network into 2 with one part is packet switching and other part is circuit switching.
3G Core Network has 4 main functions as it can be seen from figure above. These are:
1. Circuit Switching: It uses Circuit Switched Network in which dedicated link or channel is provided for a particular time slot to set of users. The two functions which are related with circuit switching part is:
· MSC — Mobile Switching Centre manages circuit switched calls.
· GMSC — Gateway MSC acts as an intermediary between external and internal networks.
2. Packet-switching: It uses IP Network where IP’s are responsible for transmitting and receiving data among two or more devices. The two functions which are related with Packet Switching is:
· SGSN(Serving GPRS Support Node): The various functions provided by SGSN are mobility management, session management, billing, communication with other areas of the network.
· GGSN(Gateway GPRS Support Node): It can be considered as a very complex router and handles the internal operations between the external packet switched networks and UMTS packet switched network.
When we evolve from 3G to 4G, we can see big differences because circuit switching is no more used. All network is based on IP packets and packet switching so it is possible to say that this is a big evolution and change.
In addition to this, most of standard functions of a core network was held by just a single function, which is SGSN. It was doing mobility management, session management, billing and communication with other networks. However, when we look at it in a 4G Core network, this is separated to many other functions.
4G Core Network
After switching from 3G to 4G, we stopped using circuit switching and passed to packet switching completely. This was biggest innovation in new generation of core networks. This aims to have IP connectivity between UE and network.
Following figure shows the overall network architecture, including the network elements and the standardized interfaces. At a high level, the network is comprised of the CN (EPC) and the access network E-UTRAN. While the CN consists of many logical nodes, the access network is made up of essentially just one node, the evolved NodeB (eNodeB), which connects to the UEs. Each of these network elements is interconnected by means of interfaces that are standardized in order to allow multi-vendor interoperability. This gives network operators the possibility to source different network elements from different vendors. In fact, network operators may choose in their physical implementations to split or merge these logical network elements depending on commercial considerations.
4G core consists of 5 main core functions. These names and functionalities are following:
· Mobility Management Entity (MME): It is responsible for providing the Mobility and Session management to User Equipment. It is the control node that manages the signaling between UE and CN. The protocol running between UE and CN is know as NAS protocols. MME has 2 main functions which are bearer management and connection management. Bearer management is related with establishment and maintenance of bearers. Connection management is related with establishment of connection and security between network and UE.
· Serving Gateway(S-GW): It is responsible for exchanging the traffic between P-GW and 4G RAN. IP packets that are coming from user is passed from S-GW. It is like a local mobility anchor for the data bearers when UE changes eNodeBs. In addition to this, S-GW is responsible for some administrative functions like collecting information about charging from networks.
· PDN Gateway (P-GW): It is also responsible for data traffic between S-GW and other networks like IMS or internet. We can define this function as door to internet. One of main responsibility of P-GW is to assigning the IP to UE. It is responsible for the filtering of downlink user IP packets into the different QoS-based bearers.
· Home Subscriber Server (HSS): It is responsible for storing the data for customer profile and it creates authentication vectors that are sent to MME. It also holds information about the PDNs to which the user can connect. In addition to these, it has another responsibility that holding dynamic information such as identity of MME to which user is currently attached or registered. HSS may also integrate the authentication center (AUC), which generates the vectors for authentication and security keys.
· Policy and Charging Rules Functions (PCRF): It is responsible for providing the information of QoS to P-GW. This information may include charging rules, flow control rules and traffic priority.
5G Core Network
While 4G Core has evolved to 5G Core, it gained some new capabilities and some functions are divided in to more functions but general architecture is similar. One of the most significant difference between 4G and 5G core network is the separation of control and user plane functions from each other.
Because some functions are divided, 5G core network has more functions as it mentioned above. 5G Core Network Functions are like following:
- Access and Mobility Management function (AMF) supports: Termination of NAS signaling, NAS ciphering & integrity protection, registration management, connection management, mobility management, access authentication and authorization, security context management.
- Session Management function (SMF) supports: session management (session establishment, modification, release), UE IP address allocation & management, DHCP functions, termination of NAS signaling related to session management, DL data notification, traffic steering configuration for UPF for proper traffic routing.
- 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.
- Policy Control Function (PCF) supports unified policy framework, providing policy rules to CP functions, access subscription information for policy decisions in UDR.
- Authentication Server Function (AUSF) acts as an authentication server.
- Unified Data Management (UDM) supports: generation of Authentication and Key Agreement (AKA) credentials, user identification handling, access authorization, subscription management.
- Application Function (AF) supports application influence on traffic routing, accessing NEF, interaction with policy framework for policy control.
- 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.
- NF Repository function (NRF) supports: service discovery function, maintains NF profile and available NF instances.
- 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.
There are two representations of the 5G Core Network. One of them is reference point architecture and other one is service based architecture. They both almost tells the same functional elements.
In the reference point architecture, it is much more similar to traditional 3GPP architecture that defines functions and interfaces between them. Disadvantage of this kind of architecture is to adding a new network function to system because it will be required to reconfigure whole architecture again.
In the following years, it is expected to evolve more into service-based architecture. It has advantage that its APIs are reusable. Both architectures have same functional elements and same user-plane processing path between UE and other networks. The main difference between reference point architecture and service-based architecture is in the control plane. There is no predefined interfaces between control plane functions and NRF is used to discover and communicate with each other.
At this point, NFV becomes important because of cloud-native networking concept because it will be possible for core network to request new network functions from VNF catalog into E2E service chains. NFV will provide flexibility and time efficiency to system.
As we can see from figure above, control plane is upper part of architecture and lower part is for data (user) plane. Upper part is mostly responsible signaling between different core network functions and lower is for data traffic.
Let’s make a summary table for evolution of 4G functions to 5G functions.
As we can see from Table 1, AMF works as a part of MME from 4G Core. It is responsible from mobility management. It makes the NAS signaling connection with User Equipment and helps User Equipment to register.
HSS is turned into both AUSF and UDM. UDM sends the authentication vectors to AUSF in order to make the subscriber authentication during registration. In 4G, this was held by HSS.
Some part of MME is turned into SMF which provides the session management functionality of it. In addition to this, it also combines some control plane functions of the S-GW and P-GW. SMF also does the job of giving IP to User Equipment’s.
UPF has the some roles of S-GW and P-GW. S-GW and P-GW was responsible for data traffic and UPF does this traffic transport, which is done by these two. Data traffic comes from UE to gNB to UPF. Therefore, UPF can be called as gateway for internet or other networks.
As it is discussed in table above, NSSF is a new core network function in 5G. Its main functionality is to provide a required virtual slice of RAN, core and transport networks. In the future, operators will build some specific network slices depends on some requirements and NSSF function will have the information about these different network slices including their definitions, resources etc. UE may be able to ask operator to register different slice of a network and it will be able to request this while registering to network. At this point, NSSF will just provide the information and functionalities of slices but it will not authorize the UE. AMF will be responsible on this issue to authorize UE depending on the subscription information, which will come from UDM.
UDM is responsible for generating the authentication vectors (they are requested by AUSF) which is similar to one MME. However, he does this using the subscriber profiles that are stored in UDR. UDR is like a database to store subscriber information, application specific data and policy data.
Policy control is similar to one in 4G core network but this time it is controlled by PCF and PCF has some extra and new functions compared to PCRF, which is in 4G. AMF asks PCF for getting the access and mobility policies. One of the new functionality of PCF is to make resource reservation for other services using HTTP/XML based interface.
In the figure 10, we can see the classification of 5G Core Network functions:
SA and NSA Modes:
In the first stage of 5G Core Network deployment, it is thought that EPC(4G Core Network) and 5G Core Network will work together. This called stand alone and non stand alone. From the name of this system, we can understand that in the first stage, previous and new core networks will work together. However, we will switch to stand alone mode that 5G Core Network will work by self. In the following figure, we can see the architecture of this system:
We can see that in NSA Mode, EPC and 5G Core Network is working together and connected to LTE Radio(Access Network). In the other option, NSA and SA mode is working together and both core Networks are active but they are working separately. In the last option, we have only 5G Core network. It is working alone and it is connected to access network.
Cloud Native 5G Core:
One of the main improvement and innovation that will happen in 5G is the Cloud Native Architecture of Core Network. Services and functions will work as microservices, containers and VMs. In order to meet telco operators requirements, it will be important to deploy 5G Core as cloud native because it will require flexible, scalable and upgradable structure.
For container and microservice based architecture, 5G Core Functions will be formed by combining necessary micro services. Each microservice will runs container and they will be scalable and reusable.
On the other hand, similar architecture will continue in VMs too. Sometimes, it is port for a function to have high I/O bandwidth so VM based virtualization still be important. However, hybrid models will be the most common due to different requirements of different network functions. Therefore, services will be able to run in VMs or containers due to different situations and needs.
As we move from 3rd generation of mobile network to 5th generation, we can see many similarities in the core network sides. In the 3G, we were using circuit switching and less core network functions. However, when we move to 4G, we quit circuit switching and we just have the packet switching and so that IP based networks. We have more and much talented network functions compared to 3G Core Network Functions. In 5G, most important innovation is in the infrastructure of core network. We divided our main core network functions in to many others so that they can run as micro services in the virtualized infrastructure. Also, their talents and functionalities are also increased too.
3. Service-Oriented 5G Core Networks, Huawei, Gabriel Brown