October 2002

BY RAVI RAVISHANKAR

Just two short years ago, predictions about the imminent demise of the circuit-switched network abounded. Packet telephony was poised to transform the telecommunications infrastructure. New IP services promised to revolutionize communication and generate new revenue.

The landscape has changed dramatically since those optimistic forecasts. The dot.com bubble burst, the ranks of the competitive local exchange carriers (CLECs) have diminished, and the capital infusion that fueled the new innovation began its painful downward slide. 

In this tough economic environment, service providers are retrenching and casting a critical eye on expenditures. Cost cutting has become a primary focus. Carriers are expecting a short payback period before they will deploy any new technology.

Despite the gloom and doom of current market conditions, the promise of cost reduction and revenue generation using softswitch architecture remains real and proven. However, to deliver this promise, it is critical to fully leverage the existing telecom and networking infrastructure. 

Teaching New Dogs Old Tricks 
Deploying packet tandems in a geographically distributed network is a good case in point. In today�s typical telecom network, each circuit switch must be connected to every other switch creating a meshed-trunk architecture. As the network grows, it becomes increasingly expensive and cumbersome to add new switches. Consider a network with fifteen switches. The basic formula for calculating the trunk group ports, required for the network is n x n-1, where n is the number of switches (the switch ports on each side of the trunk are counted separately). This translates to 210 trunk group ports. The addition of a single new switch increases the port requirement by 30. 

Overengineering creates additional expense in mesh networks. Every trunk between every switch has to be engineered to peak network usage although the traffic load on each trunk will be much less. 

Introducing a packet tandem softswitch in such a network optimizes trunk utilization by aggregating traffic and significantly reducing the number of trunk ports required. In addition, it streamlines the network, centralizes several network operations, and reduces the operational expenses. Though some of these advantages can be gained by the deployment of a traditional circuit-switched tandem, the packet tandem softswitch provides unique advantages because of its distributed architecture and packet technology. The distributed packet tandem enables providers to leverage existing network infrastructure, take advantage of technological advancements to optimize resource utilization, and bridge legacy and new services. 

Leveraging The Data Network
For carriers with underlying data networks, there's a strong case for deploying distributed packet tandems. Many providers currently use ATM circuit emulation at the network core to interconnect local switches for their trunk connections. Typically, channelized T1/E1 links connect the Class 5 end offices to the ATM core through an ATM edge switch, which bridges the TDM channels to ATM permanent virtual channels (PVCs). On the ATM side, a nailed-up PVC is required for every incoming 56K/64 kbps DS0A channel. A T1, therefore, requires 24 PVCs. The same ATM infrastructure when controlled by a tandem softswitch eliminates the need for nailed-up ATM channels by switching calls between the TDM and ATM side of the network. This significantly improves the ATM bandwidth utilization and allows the network to grow without adding additional ATM resources. The same advantage can be realized with voice over ATM. 

Centralizing Call Control And Simplifying The Network
The question that arises when using circuit switches in a distributed network is where to locate tandems and how many will be required. With a circuit-based solution, all of the TDM trunks running between Class 5 switches in the mesh must be re-homed to the new tandems -- an expensive and cumbersome process. The deployment of centralized, media-independent softswitches dramatically simplifies the process. Separating call control from the underlying media architecture enables carriers to reuse existing resources with little or no investment in ports and hardware. In this scenario, TDM circuits from the end-offices can be terminated on the existing media gateways collocated with the local switches. The media gateways, acting as trunking gateways, convert the TDM flow to packets, which are delivered over a packet backbone. The softswitch tandem provides complete call control and manages the underlying gateway resources. The loadsharing and aggregation enabled by the centralized softswitch architecture delivers substantial savings in both operating and capital expenses.

The packet solution offers additional benefits. It reduces the number of ports required on the local switch and terminates the trunks closer to the end office. Efficiency is gained in long-haul transmission since the ATM resource is shared and the need for PVCs is eliminated. To further reduce bandwidth requirements, carriers can utilize voice compression and silence suppression. Silence suppression alone can theoretically save up to fifty percent on bandwidth requirements. Even greater savings can be realized with voice compression technology such as G.723.

This type of architecture also simplifies network expansion. CLECs providing service using unbundled network elements (UNEs) can deliver long-distance service by simply colocating media gateways at the incumbent local exchange carrier�s (ILEC�s) Class 5 switch. The approach has the same end result as collocating a tandem with an ILEC Class 5 switch, which cannot be cost effectively done in the circuit-switched world. 

Leveraging Signaling And Intelligent Network (IN) Resources
An added benefit of the centralized softswitch is that it enables carriers to maximize the use of their existing signaling and IN infrastructure. Triggers for IN services such as local number portability (LNP) and toll free can be loaded directly onto the softswitches. This is a much more efficient strategy than the alternative end-office triggers. Triggers, software upgrades, and translation table updates take place at centralized softswitch rather than at each and every end-office switch in the network, reducing operating expense and network complexity. IN triggers at the softswitch can also be used to develop new network applications such as on- and off-net call routing, prioritized routing, and private dialing plans. In the signaling network, if the signal transfer point (STP) supports SS7 over IP signaling, the softswitch can be connected directly to the STP without the need for external signaling gateways.

Bridging The Technology
The softswitch's ability to support traditional and next-generation signaling protocols and technologies from a single platform allows carriers to evolve their technology and provide the true integration of voice and Web resources. The softswitch architecture can support multiple transport technologies such as VoATM, VoIP over ATM, and VoIP over MPLS. It creates a smooth migration path from one technology to another without disturbing or requiring additional investment in elements in the network's other layers. While it supports IN for traditional services such as calling name delivery, number portability, and toll free, it also enables the next-generation services such as unified messaging and multimedia conferencing using new SIP-based architectures.

Conclusion
The telecom industry is going through a challenging time, which is driving both operational and capital cost reduction. A distributed, softswitch-based architecture helps carriers succeed in this environment by reducing costs, streamlining their networks, simplifying administration, and maximizing resource utilization. The separation of service logic, call control, and media transport enables the interoperability of multi-vendor networks with both legacy telecom and legacy data resources. A function at any one layer is truly independent of the technology and vendor solution that may already exist in other layers. Keys to successful softswitch architecture include multi-vendor support, granular scalability to avoid overprovisioning of capacity, and utilization of technological advancements to enable new services. It is these attributes that a circuit-switched solution simply cannot replicate and which will fuel the transition to packet technology and its benefits.

Mr. Ravi Ravishankar is director, Advanced Technology Planning, at Tekelec. His focus is on defining signaling solutions and products for the next-generation packet telephony and 3G wireless networks. Tekelec is a leading developer of telecommunications signaling infrastructure, softswitches, testing and diagnostic solutions, and service applications. Please visit their Web site at www.tekelec.com.

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