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An MSO’s voice network transformation with IMS

MSOs might find it a daunting prospect to evolve their legacy voice networks to IP. But as shown here, moving to an Internet Protocol Multimedia Subsystem (IMS) solution makes good business sense. As an example, let’s walk through a practical roadmap for a regional MSO that needs to move its legacy voice business to an IMS core solution.


As shown in Figure 1, the MSO’s present mode of operators (PMO) uses a variety of platforms, switches and management systems. This results in efficiencies, duplications and high OPEX. Furthermore, interoperability is a problem in this multi-vendor voice network environment.

Among the operational challenges facing the MSO are these:

  • Legacy call control platforms are not scalable
  • Voice network platforms are at or near end of life or end of support
  • Equipment obsolescence can result in service disruptions that potentially create liability issues for MSOs and put revenue at risk
  • Voice architecture is highly sub-regionalized
  • Circuit-based platforms exacerbate building space, energy consumption, and cooling issues
  • Any network transformation must minimize service interruptions to customers


There are compelling benefits to implementing a converged IMS network. With an all-IP network, the MSO can:

  • Mitigate revenue risk associated with platforms approaching end of life or end of service.
  • Simplify operations and lower OPEX by streamlining operational processes and managing a unified environment across sub-regions.
  • Position for service growth by modernizing the network. Scalable next-generation platforms can meet projected demands cost-effectively.
  • Offer improved experience, unified and seamless for voice end users across the serving region.
  • Coordinate Operation, Administration, Maintenance and Provisioning (OAM&P) strategy.  Focusing on a solution-wide management strategy is a key to simplifying IMS networks.


Network transformation to IMS is a multi-year commitment.  It requires a comprehensive end-to-end process for managing the migration and the hybrid network state during the transition. Typically, an IMS transformation is implemented in phases, with the new IMS solution being deployed as an overlay first. Meanwhile, growth and activities in the embedded base legacy network are minimized or capped. Finally, subscribers are migrated to the new IMS network and the legacy network is decommissioned. An overall sequencing strategy needs to guide the transformation process. The strategy is implemented within each sub-region, and consists of the following four phases:

  • Phase 0: Build the foundation for an overlay IMS deployment
  • Phase 1: Migrate off TDM access and switching technologies
  • Phase 2: Migrate the legacy packet switching technologies
  • Phase3: Upgrade/evolve the rest of the network to IMS compliant technologies

Since the voice platforms support both residential and enterprise customers, the MSO has two subscriber migration options:

  • Cap and Grow
    • Support all residential and enterprise subscriber growth in IMS
    • Migrate residential and SME customers to IMS during the transformation Phases 1 and 2
    • Migrate large business customers to IMS during Phase 3
  • Grow/Cap and Migrate
    • Grow enterprise customers on legacy platforms
    • Migrate residential customers in Phases 1 and 2
    • Migrate enterprise customers in Phase 3

In this example, the MSO chooses Option 1 for implementation. Option 1 offers a number of benefits:

  • Protects the enterprise revenues by delaying the migration of important and complex services to Phase 3.
  • Achieves a faster retirement of the legacy voice platforms and allows the MSO to direct all new investments to support the growth in the IMS network.
  • Avoids a two-step migration for the enterprise customers and minimizes risk for major service disruptions.

A drawback is that Option 1 requires more upfront investments in IMS to support high migration volumes. Figure 2 depicts a high-level roadmap for the MSO’s voice network transformation. It highlights a set of activities that need to take place at different layers--platform/control, OSS/BSS, access, and migration— for a successful transition.


MSOs can choose from various IMS deployment options. The scenarios listed here do not necessarily reflect Alcatel-Lucent’s current product capabilities and roadmap. Centralized scenario The MSO can deploy a centralized IMS core to cover the entire multi-country serving region. This deployment scenario leverages the IMS core along with one home subscriber server (HSS), a set of application servers, and a regional DNS/ENUM database to serve the MSO’s entire subscriber base. The Centralized scenario will help the MSO achieve a single integrated view of customer data and a single integrated management, call detail recording and billing. Distributed scenario In this scenario, the MSO deploys an IMS core in each sub-region to serve the local subscribers. Each sub-region has its own HSS, set of application servers, ENUM database and other key functions of the IMS solution. Depending on the size of the sub-region, the MSO might deploy a distributed IMS core or a lower-capacity compact IMS platform such as the Alcatel-Lucent 5060 IP Call Server. As for the centralized scenario, each sub-region requires call session control and application servers. Hybrid scenario In the hybrid scenario, some functions are centrally located to serve the entire region and others are deployed in the sub-regions. The extent to which IMS functions are centralized and distributed in sub-regions needs to be carefully planned by the MSO.


Challenges arise with IMS migration, particularly in meeting regulatory requirements in sub-regions. Following are a few capabilities MSOs must consider:

  • Emergency Calling - The centralized IMS operation needs to be able to identify and route calls to each sub-region’s local emergency service provider based on the calling subscriber. However, each sub-region might have different requirements for emergency call handling.
  • Lawful Intercept (LI) - Different sub-regions (or countries) might have different requirements and protocols to implement LI operation. This capability usually remains localized in a regional IMS deployment.
  • Number Portability (NP) - Different sub-regions (or countries) will use different NP schemes. IMS needs to support interoperability with existing NP schemes from other operators in each sub-region. Each sub-region might also have its own local NP database.
  • Virtualization - Virtualization and cloud computing lets system virtual machines share underlying physical machine resources. Virtualization can be a potential way to reduce cost and complexity in the network.
  • Consolidated OAM&P - The IMS solution consolidates OAM&P into a single element management system. It also includes a variety of “ease of operations” features across the entire IMS footprint. The result for the MSO is reduced OPEX from the initial IMS installation through each successive upgrade.


In the target converged IMS solution, the call and session control layer is centralized through an IMS core. It supports all residential and enterprise voice services and data applications. The main functions introduced by IMS at the applications, call control, and media layers are depicted in Figure 3:

  • Home Subscriber Server (HSS) – a central repository for user-related information. In the MSO’s network, the HSS is used only to support the NGN users. We assume a redundant configuration of the HSS as a single logical node in MSO’s network
  • Call Session Control Function (CSCF) -- processes SIP signaling in IMS. Four types of CSCF are deployed in the MSO’s network:
    • Proxy CSCF (P-CSCF)
    • Serving CSCF (S-CSCF)
    • Interrogating CSCF (I-CSCF)
    • Emergency CSCF (E-CSCF)
  • Telephony Application Server (TAS) --  enables fixed service providers to differentiate offers with enriched high definition voice and video calling. It also:
    • Integrates the New Conversation APIs transforming IMS into a platform for rapid web innovation
    • Provides a Personal Communication Manager function
    • Supports an API to enable a well-controlled and reliable migration process
  • Media Gateway Control Function/Signaling Gateway (GCF/SG) -- provides the control of the bearer path terminating on a media gateway (MGW). The SG provides the signaling interworking function with the PSTN Intelligent Network (IN) network.
  • Breakout Gateway Control Function (BGCF) -- provides the breakout function for calls destined to the PSTN/PLMN. The BGCF selects the appropriate MGCF to handle calls given their destinations.
  • Interconnection Border Control Function (I-BCF) -- provides an entry point for interconnection between the MSO IMS network and other IP networks.


The IMS architecture introduces new capabilities in several dimensions:

  • Multi-technology, multi-vendor, inter-network connections and convergence
  • Multi-service provider or multi-location interfaces and interactions
  • Multi-media, multi-services management as an integral part of network operations
  • End-to-end service security, availability, QoS and service level agreements

To fully leverage inherent IMS capabilities, the MSO will need to develop new methods and procedures documents, operations support systems, and business support systems for some functions. Moreover, as the number of vendors that support the various IMS functions increase in future networks,  the complexity of end-to-end solution integration for the MSOs, particularly during the transition, will go up substantially. MSOs will need to adopt industry best practices in order to de-risk the transformation process. For a more detailed description of the roadmap discussed in this article, please contact

Jean-Philippe Joseph

About Jean-Philippe Joseph

Jean-Philippe Joseph is a lead network transformation consultant in the Network Planning, Modeling and Optimization Department at Bell Labs in Murray Hill, New Jersey. He is also a member of the Alcatel-Lucent Technical Academy. He holds a B.S. in electronics engineering, summa cum laude, from La Faculté des Sciences de l’Université d’État d’Haïti in Port-au-Prince, and a M.S. degree in electrical engineering from Polytechnic Institute of New York University in New York City. In his present role, Mr. Joseph directly interfaces with major telecommunications service providers and customers worldwide delivering on network transformation consulting services. Mr. Joseph has deep and broad expertise in fixed broadband access architectures and technologies, PSTN and VoIP network transformation with IMS, and access network modeling and optimization for both cable operators and telcos.

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