TDM to SIP Network Transformation with Metaswitch


There are thousands of central offices serving small and rural communities around the globe that are still dependent on aging TDM central office switches.  These older switches are expensive to operate, difficult to maintain and many are reaching their product “end-of-life”. At the same time, many of these same service providers are looking to broadband expansion to offer new high-value IP delivered services.

When considering a central office switch upgrade, planners need to consider the level of subscriber disruption and cost.  Truck rolls to install ATAs, media gateways or replacement telephones at the customer premises are expensive and disruptive.   CPE Self-installation programs are fraught with support issues.   Disruption from the subscriber perspective are likely to result in some loss to OTT providers, or a decision to eliminate the circuit altogether.  As a recent customer shared with us “If you must make a change to the central office switch, it is imperative that it is nearly invisible to the subscriber”.

To help service providers make this transformation with as little disruption as possible, TelcoBridges and an alliance partner, Metaswitch, have worked together to create a unique TDM to IP Network Transformation strategy.

The Network Transformation Strategy

As shown in Figure 1, the majority of existing TDM central office switches use a similar architecture,  interconnecting circuits from international carriers, national carriers or interconnect circuits using SS7 signaling and TDM voice equipment.   Customer subscriber lines are terminated via access equipment, connecting residential, business or enterprise-class circuits.   At the core of the central office is the switching equipment, providing intelligent routing and various Class 5 features found in residential and business telephony.

Figure 1: Simplified TDM Central Office Architecture

As shown in Figure 2, the TDM to IP transformation architecture replaces the legacy TDM switch with a seamless combination of TelcoBridges media gateways for connectivity and a Metaswitch CFS softswitch to provide call routing and subscriber features.  This allows the existing subscriber circuits and trunking SS7 circuits to remain, converting the signaling to SIP, while moving call routing and features to the pure software domain, on generic hardware, virtualize on -premises or in the cloud..

Figure 2: Transformed Central Office

As part of the network transformation, the user experience of legacy TDM class 5 features are emulated, creating a seamless conversion for the subscriber.   Once in place, additional features (voice mail to email, self-administration, and more) can be offered to subscribers as either value-add or upgrades.

When viewed from a real-world macro-level with multiple central offices as shown in Figure 3, multiple central offices are served from a consolidated and redundant Metaswitch softswitch.   A side benefit allows trunking services to be consolidated into fewer central offices where the termination fees can be minimized.    Interconnection traffic between central offices is carried over the data network, eliminating interconnection SS7 circuits.

Transformed central offices also benefit from significant savings in operational expenses by reducing power consumption, cooling requirements and maintenance costs.

Figure 3: Macro-level view with multiple central offices

Network Transformation Benefits:

The benefits of this architecture include:

  • Little (or no) disruption to the subscriber
  • Opportunity to offer new high-margin IP-based services to subscribers
  • Centralization of call switching and features in software
  • No additional CPE, fewer truck rolls
  • Migration to OPEX model for softswitch software
  • Availability of spare parts and ongoing support for both hardware and software
  • Significant reduction in power and cooling requirements

Next Steps:

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Case Study: HKT Transforms its Network
to IP with TelcoBridges and Metaswitch

  • HKT is one of the largest Telecommunications Service Provider in Hong Kong
  • Aging TDM equipment was becoming difficult to maintain with spare parts becoming scarce
  • Replaced TDM switches with cloud-based softswitch and TelcoBridges TMG Media Gateways
  • New platform is easier to maintain and manage, offering new IP-based services
  • Reduction in power usage will save over $2 million/year and earned government green awards