What is a mobile core network?

A mobile core network is essential to every activity we perform through our mobile phones. It connects our mobile phones to external networks and allows us to access all the services that we are entitled to use.

A mobile core network is a central part of the overall mobile network that allows subscribers to get access to the services that they are entitled to use. It is responsible for critical functions such as subscriber profile information, location, service authentication and necessary switching tasks.

One of the most critical parts of a mobile network is its core network which is also sometimes referred to as the mobile core. Mobile networks are complex and consist of various key entities in their architecture. Depending on the network technology, e.g. GSM, UMTS, LTE, NR, etc., the architecture can look different.

These network entities are then grouped to form various parts of the overall mobile network, including the radio network, core network and transport network. The mobile network is then connected to external networks such as PSTN and Internet to communicate with the outside world.

What does a mobile core network do?

A mobile core network sits between the radio network and external networks. It performs switching functions for all services, including voice calls, text messages, and mobile data. It manages packet-switching in 2G, 3G, 4G and 5G networks and both circuit and packet-switching in 2G and 3G networks.

With the evolution of mobile networks from 2G to 3G, 4G and 5G, the mobile core network has also evolved considerably. In the early days of digital mobile communications, the core network for 2G GSM was called Network Switching System (NSS).

NSS only supported circuit-switching, which mainly enabled voice calls, SMS and limited data services. Have a look at this post from GSMA to learn more about the history of GSM.

Later, with GPRS (General Packet Radio Service), packet-switching was introduced into the mobile core network to support efficient data services (mobile internet). As a result, two nodes, SGSN (Serving GPRS Support Node) and GGSN (Gateway GPRS Support Node) became part of the 2G core network.

The 3G UMTS (Universal Mobile Telecommunication System) core networks followed the same approach for circuit-switched and packet-switched services. However, the 4G LTE networks (Long Term Evolution) use a more advanced core network called Evolved Packet Core or the EPC, which can support both 4G and 5G radio networks.

5G New Radio (NR) networks have a cloud-native core network (5GCN) that uses a flexible Service-Based-Architecture (SBA) to enable futuristic use cases. 5G core network can support 5G as well as 4G radio networks.

Mobile core network for 2G GSM and 3G UMTS

The mobile core network in the original 2G GSM is known as Network Switching System (NSS), which is circuit-switched only. It consists of the MSC, HLR, VLR, AuC and EIR. GPRS introduced two new core network nodes, SGSN and GGSN, that are also used by 3G UMTS networks for packet-switching.

In the original GSM networks in 1991-92, the core network architecture was called Network Switching System (NSS). NSS consists of Mobile Switching Centre (MSC), Home Location Register (HLR), Visitor Location Register (VLR), Authentication Centre (AuC) and Equipment ID Register (EIR).

MSC is a fundamental part of the core network, which uses other core network components to get subscriber information and enable different services for mobile subscribers. It performs functions such as switching of calls between mobile and fixed users, administration of handovers, authentication and location updates.

The original GSM networks used a circuit-switched technique for voice calls and SMS, which was supported by the MSC from a core network viewpoint. The GPRS networks employed a packet-switched method and introduced two additional nodes, SGSN and GGSN.

SGSN stands for Serving GPRS Support Node, and GGSN stands for Gateway GPRS Support Node (GGSN). SGSN is the packet-switched equivalent of the MSC and works alongside the MSC to enable mobile data services. It is responsible for mobility management, billing, and the management of data sessions.

GGSN, on the other hand, sits between the Serving GPRS Support Node (SGSN) and external data networks, e.g. the internet. I have written a dedicated post on SGSN and GGSN, which I would encourage you to read to get a better understanding of the packet-switched part of the 2G/3G core network.

When EDGE (Enhanced Data for Global Evolution) was introduced to enhance the existing GPRS networks, it used the same core network architecture. Likewise, the third-generation (3G) UMTS networks also use SGSN, GGSN and MSC in the core network. The high-level network diagram below shows the core network architecture for a 2G/3G network using GSM and UMTS.


Mobile core network in 2G/3G

Mobile core network for 4G LTE

Evolved Packet Core (EPC) is the 4G core network that consists of Home Subscriber Server (HSS), Mobility Management Entity (MME), Serving Gateway (S-GW), Packet Data Network Gateway (PDN-GW) and Policy & Charging Rules Function (PCRF). EPC is also integrated with 3G core network nodes SGSN and GGSN.

The 4G LTE core network is called Evolved Packet Core or EPC, which uses the packet-switched technique for mobile data as well as voice calls and SMS. The main network entities within the EPC are Home Subscriber Server (HSS), Mobility Management Entity (MME), Serving Gateway (S-GW), Packet Data Network Gateway (PDN-GW) and Policy & Charging Rules Function (PCRF). These entities are integrated with their 2G/3G counterparts so that inter-technology (e.g. 3G and 4G) handovers and communication can take place.

For packet-switched voice calls and text messages (SMS), EPC works alongside IMS (IP Multimedia Subsystem) to enable 4G calling or Voice over LTE (VoLTE) calls. VoLTE allows you to not only make high-quality voice calls using IP networks but also seamlessly switch between 4G and WiFi calling so you stay connected when the signal quality isn’t great.

If your handset doesn’t support VoLTE, there is a fallback system called Circuit Switched FallBack (CSFB), which allows you to make voice calls using 2G/3G even if you are on 4G. CSFB is also useful if your network doesn’t support VoLTE, e.g. when you are roaming and not entitled to use 4G calling by your operator. The diagram below provides a high-level view of the 3G/4G core network architecture.

Mobile core network in 3G/4G

Mobile core network for 5G New Radio (NR)

5G NR networks use a cloud-native (5GCN) mobile core network which employs a Service-Based Architecture (SBA). SBA makes the 5G core network highly flexible and enables network slicing and other advanced use cases. 5G networks can also operate using the LTE core network (EPC) in non-sand-alone 5G.

The core network architecture for 5G New Radio (NR) depends on the deployment model used for the implementation. 5G can be deployed in two ways, stand-alone (SA) and non-stand-alone (NSA). Non-standalone is currently the most common mode of deployment, which uses a new 5G radio network and an enhanced version of the 4G LTE core network (EPC).

This approach allows mobile operators to make the most of their existing LTE network infrastructure by using both 4G and 5G through dual-connectivity. For customers, that just means a bigger data pipe. On the other hand, the standalone mode is not as common yet but is expected to be the real game-changer for 5G. The standalone deployment will use a new 5G core network (5G Core) alongside the 5G radio network.

5G networks can deliver voice calls and text and multimedia messages through Voice over LTE (VoLTE) or Voice over New Radio (VoNR), depending on the core network. The non-standalone 5G deployments that use EPC enable voice calls and other real-time services through VoLTE. The standalone deployments use a dedicated 5G core network and deliver voice calls and messages through VoNR.

5G Core is a future-proof solution that makes use of cloud-based technologies optimised for cloud-native applications. The advanced use cases for 5G, e.g. critical IoT services, require ultra-low latency, which the 5G Core network will be able to support. Moreover, with a cloud-native core network, the introduction of new services or functionalities can be quicker and more in line with agile methodologies.

Conclusion

A mobile core network is a central part of the overall mobile network architecture. It allows mobile subscribers to access the services they are entitled to use, e.g. international calling. The mobile core network is responsible for critical functions such as subscriber profile information, subscriber location, authentication of services and the necessary switching functions for voice and data sessions.

Here are some helpful downloads

Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:

Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.

Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.

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