2G Core Network: What It Does and Nodes Used

The 2G core network, also known as the GSM core network, is the central part of a 2G mobile system that handles call routing, subscriber authentication, mobility management, and billing. It uses a set of specialized nodes, including the MSC, HLR, VLR, AuC, EIR, and SGSN, each with a distinct role. This article explains what each node does and how they work together.

Background: Why 2G Core Networks Still Matter

2G (second generation) mobile networks were introduced in the early 1990s as a digital replacement for analog 1G systems. The core network was designed around circuit-switched voice calls, which means a dedicated path is established for the duration of a call. While 3G, 4G, and 5G have largely replaced 2G in many regions, 2G networks remain active in parts of the world for basic voice calls, SMS, and machine-to-machine (M2M) communication such as smart meters and vehicle tracking systems. Many operators have announced shutdowns of their 2G networks, but others continue to maintain them because they cover large rural areas and support legacy devices.

Understanding the 2G core network is useful for telecom engineers, students, and anyone working with legacy mobile systems. The architecture also forms the foundation for later generations, so knowing it helps explain how mobile networks evolved.

Core Network Architecture Overview

The 2G core network is divided into two main domains: the circuit-switched (CS) domain for voice calls and the packet-switched (PS) domain for data. The CS domain uses the Mobile Switching Center (MSC) and related databases, while the PS domain uses the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). Both domains share the Home Location Register (HLR), Authentication Center (AuC), and Equipment Identity Register (EIR).

The core network connects to the radio access network (RAN), which in 2G is called the Base Station Subsystem (BSS). The BSS includes the Base Transceiver Station (BTS) and the Base Station Controller (BSC). The BSC connects to the MSC for voice and to the SGSN for data. The core network also connects to external networks: the Public Switched Telephone Network (PSTN) via the MSC and the internet via the GGSN.

Here is a simple list of the main nodes and their roles:

• MSC (Mobile Switching Center) – switches voice calls, handles handovers between BSCs, and manages call setup and teardown.
• HLR (Home Location Register) – stores subscriber data, including the phone number, subscribed services, and current location.
• VLR (Visitor Location Register) – temporarily stores data about subscribers currently in the area served by an MSC.
• AuC (Authentication Center) – generates security keys and authenticates subscribers.
• EIR (Equipment Identity Register) – checks whether a mobile device is allowed on the network using its IMEI number.
• SGSN (Serving GPRS Support Node) – handles data sessions for GPRS (2.5G), tracking the location of data users.
• GGSN (Gateway GPRS Support Node) – connects the GPRS network to external packet networks like the internet.

How the Circuit-Switched Domain Works

When a subscriber makes a voice call, the process involves several core network nodes. First, the mobile phone sends a call setup request to the BSS, which forwards it to the MSC. The MSC queries the VLR to check if the subscriber is allowed to make calls and to get the subscriber’s profile. If the call is to another mobile subscriber, the MSC uses the HLR to find the location of the called party. The HLR points to the VLR that currently serves the called subscriber, and the call is routed to the appropriate MSC.

During the call, the MSC manages the connection, including handovers if the subscriber moves between BSC areas. The MSC also generates call detail records (CDRs) for billing. The VLR is important because it reduces the load on the HLR: the HLR only needs to know which VLR area the subscriber is in, not the exact cell. The VLR holds the precise location and the subscriber’s service profile.

The AuC plays a security role. When a subscriber registers or makes a call, the MSC requests a set of authentication triplets from the AuC. These triplets include a random number (RAND), a signed response (SRES), and a cipher key (Kc). The phone and the network use these to verify the SIM card and encrypt the radio link.

How the Packet-Switched Domain (GPRS) Works

GPRS (General Packet Radio Service) was introduced as an extension to 2G, often called 2.5G. It allows data to be sent in packets rather than requiring a dedicated circuit. The SGSN is the key node for GPRS. It tracks the location of data users, manages the radio resources for data, and handles authentication and encryption for data sessions. The SGSN connects to the BSC via the Gb interface.

The GGSN acts as a gateway between the GPRS network and external packet networks, such as the internet. It assigns IP addresses to mobile devices, routes packets to and from the internet, and enforces quality of service policies. The GGSN also generates billing data based on data volume.

When a subscriber wants to browse the internet, the phone sends a PDP (Packet Data Protocol) context activation request to the SGSN. The SGSN authenticates the subscriber and requests an IP address from the GGSN. Once the context is active, the phone can send and receive IP packets through the SGSN and GGSN. The SGSN and GGSN together form the GPRS core network, which is a precursor to the evolved packet core used in 4G and 5G.

Interfaces Between Nodes

The nodes in the 2G core network communicate over standardized interfaces. These interfaces are defined by the 3GPP (Third Generation Partnership Project) and ensure interoperability between equipment from different vendors. Some of the most important interfaces are:

• A interface – between the BSC and the MSC (for voice).
• Gb interface – between the BSC and the SGSN (for data).
• B interface – between the MSC and the VLR (usually the VLR is integrated with the MSC).
• C interface – between the MSC and the HLR (for location and routing).
• D interface – between the VLR and the HLR (for subscriber data).
• E interface – between two MSCs (for handover and call transfer).
• F interface – between the MSC and the EIR (for equipment check).
• Gn/Gp interfaces – between the SGSN and the GGSN (for data).

These interfaces use signaling protocols such as SS7 (Signaling System No. 7) for circuit-switched signaling and GTP (GPRS Tunneling Protocol) for packet data. Understanding these interfaces is essential for troubleshooting network issues and for planning network upgrades.

Common Problems and Troubleshooting

2G core networks are often considered legacy, but they still require maintenance. Common problems include:

• Subscriber authentication failures – often caused by incorrect HLR or AuC data, or by SIM card issues.
• Call setup failures – can result from congestion in the MSC or VLR, or from signaling errors between the BSC and MSC.
• Data session drops – in GPRS, this can happen due to SGSN overload or incorrect GGSN configuration.
• Handover failures – typically due to poor radio conditions or misconfigured neighbor lists.

When troubleshooting, network engineers use protocol analyzers to capture and decode signaling messages on the A and Gb interfaces. They also check the HLR and VLR databases for consistency. A common issue is that the HLR and VLR become out of sync, causing a subscriber to be unreachable. This can be resolved by re-registering the subscriber or by manually updating the databases.

Another common misconception is that the 2G core network is entirely obsolete. While it is true that 4G and 5G use a different core architecture (the Evolved Packet Core and 5G Core), many operators still run 2G for voice fallback in areas where 4G coverage is poor. In fact, some smartphones still use 2G for emergency calls when no other network is available.

Comparison with 3G and 4G Core Networks

The 2G core network is circuit-switched for voice, while 3G introduced a parallel packet-switched core and also supported circuit-switched voice. 4G (LTE) eliminated the circuit-switched domain entirely, using Voice over LTE (VoLTE) for calls. The 4G core, called the Evolved Packet Core (EPC), is all-IP and uses nodes like the MME (Mobility Management Entity), SGW (Serving Gateway), and PGW (Packet Data Network Gateway).

Compared to 2G, the 4G core is simpler in some ways because it handles voice and data over the same IP network. However, 2G core networks are still used for fallback in many networks. For example, when a 4G phone makes a voice call and VoLTE is not available, it may fall back to 2G or 3G. This is called CSFB (Circuit-Switched Fallback). In such cases, the 2G core network still plays a role in providing voice services.

Frequently Asked Questions

What is the difference between the MSC and the VLR? The MSC is the switch that handles call routing and connection control. The VLR is a database that stores temporary information about subscribers currently in the MSC’s area. In most modern networks, the VLR is integrated with the MSC, so they are often considered as a single node.

Why do some networks still use 2G? 2G networks are still used because they provide wide coverage, especially in rural areas, and they support basic voice and SMS. Many M2M devices, such as smart meters and car trackers, rely on 2G because it is cheap and has good building penetration. Some operators have shut down 2G to free up spectrum for 4G and 5G, but others keep it running.

Can a 2G phone work on a 4G network? No, a 2G phone cannot connect to a 4G network because the radio access technology is different. However, a 4G phone can fall back to 2G for voice calls if VoLTE is not available. This requires the network to have a 2G core and radio access.

What is the role of the AuC? The AuC (Authentication Center) generates security parameters used to authenticate the SIM card and to encrypt the radio link. It is usually co-located with the HLR. Without the AuC, the network cannot verify that a subscriber is legitimate, which would allow cloning and fraud.

Is GPRS the same as 2G? GPRS is an extension of 2G, often called 2.5G. It adds packet-switched data capabilities to the existing circuit-switched 2G network. GPRS uses the SGSN and GGSN nodes, which are not part of the original 2G core but are added to support data.

What happens when a 2G network is shut down? When a 2G network is shut down, all devices that rely on 2G will lose service. This includes older phones and M2M devices that only support 2G. Users must upgrade to 3G, 4G, or 5G devices, and M2M modules must be replaced. Operators typically announce shutdowns well in advance to allow migration.

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