What Really Happens When Operators Merge EPC, 5G, CS, and IMS

Merging two mobile networks sounds simple until you step inside one. Across mature markets, telecom revenues have slowed while 5G rollout costs continue to rise. Firms like Deloitte and PwC expect operator mergers and core consolidation to accelerate because older business models can no longer support the cost of modern networks. For many operators, partnership is beginning to feel less like strategy and more like survival.

I learned this during a consolidation project early in my career. We were merging two national CS core networks built on MSSs, MGWs, HLRs, and legacy signalling domains. On paper, the plan looked clean. In reality, it was a live system carrying millions of connections that could not go dark. Every change carried weight. One misplaced route could affect emergency services. One mismatched subscriber profile could break lawful intercept.

What stood out was not just the technical challenge. It was the people involved. Voice, signalling, and radio engineers, billing analysts, and IP architects often looked at the same issue from completely different angles. And all of them were right in their own context. Our job was not only to merge systems. It was to translate between worlds and make those perspectives align.

That lesson followed me into EPC, IMS, and 5G Standalone integrations. The architectures evolve. The principle stays the same.

How Core Integration Has Evolved

In circuit-switched networks, integration meant aligning signalling and numbering. In EPC and 5G Core, everything becomes more complex. Subscriber databases multiply, IP layers spread across data centers, and service anchors shift into cloud environments that add new layers to the integration. Yet the goal remains the same: seamless service continuity. Operators must decide whether to keep both cores for stability or consolidate into one for long term efficiency.

How MOCN Lets Two Cores Share One RAN

In this model, both operators keep their own EPC or 5G Core or IMS Core while sharing the same radio network. Each cell broadcasts both PLMN IDs, and subscribers attached to their home network as described in 3GPP TS 23.251 and TS 23.501.

Figure 1: MOCN broadcasts two PLMN IDs on a shared RAN while each operator maintains its own core.

This setup is fast to deploy and minimizes disruption. It keeps lawful intercept domains intact and satisfies regulatory boundaries. The trade-off is duplication. MMEs, PGWs, PCRFs, and IMS cores all continue to operate separately. This increases operational cost and can create differences in user experience within the same shared RAN.

MOCN is best suited for short term coexistence right after a merger when regulation slows full consolidation It also raises a long-term question: how sustainable is it to operate two full brains of the same network?

Using National Roaming as a Bridge

Another way to bring two networks together is national roaming. In this setup, one PLMN provides nationwide coverage, but every subscriber still attaches back to their own home core. The visited RAN handles the radio side, while functions like MME or AMF, SGW or UPF, HSS or UDM, and PCRF or PCF continue to live in each operator’s original core. Traffic and signalling are tunnelled home through S8 or N9 or N32 interfaces, following GSMA IR.88 and IR.65, with support from bilateral agreements, PLMN allow lists, and TAC planning to avoid registration issues.

Operationally, national roaming is a quick way to unify coverage during RAN modernization or spectrum refarming. The trade-off is extra latency and more signalling since all traffic returns to the home core. Emergency routing, lawful intercept, and real time charging must stay aligned with each subscriber’s original network. Financially, it shifts cost from shared infrastructure to usage-based wholesale, making it practical mainly as a temporary step and bridge toward MOCN or a unified core.

Figure 2: National roaming connects subscribers to a visited RAN while all control and user-plane functions remain anchored in the home core.

Best suited for: short-term coverage unification during RAN modernization or refarming.

Why Operators Ultimately Move to a Unified 5G Core

The long-term goal is a single logical core that supports multiple PLMN IDs as described in 3GPP TS 23.501. This delivers consistent QoS, unified charging, a single network identity, and a simpler operational model. It removes duplicated capacity and standardizes customer experience across the footprint.

The challenge is the consolidation itself. Migrating subscriber data, retesting interfaces, aligning policy and charging, and validating orchestration tools all require careful coordination. Once complete, the unified core becomes a more efficient platform for future evolution.

Best suited for: operators aiming for long-term efficiency and a unified network identity.

The Roadmap That Gets Networks There

Core consolidation never happens overnight. It follows structured, overlapping phases:

Figure 3: Core integration progresses from dual cores to national roaming, MOCN, partial migration, and full unification.

Behind each phase are teams working in parallel: planners, regulatory specialists, and OSS engineers. The skill lies in aligning them toward a single end state.

The Hidden Complexity of Core Migration

Merging two cores is not a single task. It is a sequence of interdependent operations, each carrying risk. It starts with subscriber data. HLRs, HSSs, or UDMs need to merge in a way that keeps millions of devices authenticating without interruption.

Then comes session control and the user plane. MMEs, SMFs, and gateways must move without dropping live traffic. Policy and charging must line up so QoS stays consistent, and rating remains accurate. Voice and IMS elements such as CSCFs, SBCs, and TAS must be rehomed and SIP routing adjusted to avoid registration storms.

Beneath all of this is something more subtle. Each subscriber carries an IMSI, a digital identity that must be re-mapped, so devices still recognize their home. Every call and data session produces charging records. A mismatch here can derail billing or compliance. Lawful intercept systems must stay synchronized, and MSSs, MGWs, HLRs, and STPs reprogrammed carefully to avoid routing loops.

From the outside, it looks like connecting cables. In reality, it is open heart surgery while both hearts keep beating. Most incidents do not come from the nodes themselves. They come from how teams interpret the same data. I remember one meeting where security, billing, and core teams looked at the same call flow and each saw a different issue. One from a compliance angle. One from a revenue angle. One from a latency angle.

That is when it becomes clear. Technical alignment is hard. Human alignment is harder.

The Documentation That Holds It All Together

Figure 4: IR.21, IR.88, and IR.65 updates keep roaming partners aligned and ensure regulatory consistency.

Keeping the network running is only half the job. Keeping the paperwork alive is the other half. Every technical change has a twin in documentation, and if that twin falls behind, nothing lines up the way it should. The GSMA IR.21 file needs an update every time PLMN IDs, IMSI ranges, or routing contacts change. IR.88 and IR.65 also need attention whenever new interconnects or IMS nodes appear. And regulators expect to be told about anything that affects emergency routing or lawful intercept.

It sounds simple until you are in a merger and every team is updating something at the same time. Documentation becomes the glue that keeps partners aligned and keeps the merged network visible to the rest of the world. Get it right and roaming partners trust your network. Get it wrong and you spend weeks explaining why calls are landing in the wrong place.

The Expertise We Bring to Integrations

Core mergers demand a rare balance of deep technical understanding and day to day operational reality. That is where experience matters most. At Cyient we bring a complete view of the mobile chain, from the RAN to the Core, and from legacy networks to 5G Standalone. It is this end-to-end understanding that helps operators move through consolidation without losing sight of service continuity.

Breadth of Capabilities

Cyient teams work across every layer that matters in a core merger.

Key Activities

This experience translates into practical work that makes consolidation smoother.

• Audits and assessments, to identify configuration gaps, hidden dependencies, and optimization opportunities before migration.
  
  
• High level and low-level design, built for coexistence or consolidation depending on where operators are in their journey.
  
  
• Feature activation and proposals, aligned with vendor releases and 3GPP evolution so operators take advantage of new capabilities safely.
  
  
• Performance optimization, including KPI tracking, license utilization, and QoS tuning across RAN and Core boundaries.

What Sets Cyient Apart

Cyient’s strength in core integration comes from a systems view of how networks behave. Changes in one layer never stay in that layer. A shift in transport can reshape signalling. RAN behaviour can push unexpected load into the core. Policy decisions ripple into charging and customer experience. Nothing operates in isolation, and integrations only succeed when those connections are understood end-to-end.

The teams doing this work bring both engineering precision and operational awareness. A configuration update is not just a technical step. It affects live services. A feature rollout is not only about vendor releases. It is about readiness. A service migration is both a routing change and a moment that can influence revenue.

The aim is never just to bring a unified core online. It is to make sure the new core runs better than the two separate ones that came before it.

Where It All Connects

Every operator merger eventually arrives at the same question. Do we keep both cores, or merge into one. The answer unfolds in phases. Dual cores for safety. National roaming for reach. A shared RAN for alignment. A unified core for long-term efficiency.

The hardest part is not the technology. It is the culture. In one project we had to merge not only databases, but mindsets. When leadership stopped asking which core survives and started asking what kind of network they wanted to build together, everything changed.

Successful integrations are not measured only in migrated subscribers. They are measured in how well people listen, align, and trust the process. That is what truly connects networks.

About the Author

Pedro Xavier Core Network Technical Authority, Connectivity, Cyient

Pedro is a telecommunications expert with nearly 15 years of experience in mobile core networks (CS, PS, EPC, IMS). Passionate about innovation, he focuses on 5G Core evolution — particularly API exposure, network slicing, and service programmability. Starting his career at Nokia in 2011 and part of CYIENT’s Core+ team since 2019, he has led network design, optimization, and modernization initiatives. Pedro bridges deep technical expertise with emerging trends, helping operators unlock new business models and streamline service delivery in the 5G era. His work reflects a strong commitment to driving next-generation network transformation and practical, real-world implementation.