The limitations of softswitched architectures for real-time applications

In plane geometry, the shortest and therefore most cost-effective distance between two points is a straight line. This simple axiom describes the architecture of the 20th century's telecommunications industry: A business built on a network of copper cables and switches to send first analog and then time-division multiplexed (TDM) digital signals over end-to-end, hard-wired circuits.

But in 21st century telecommunications, the most cost-effective distance between two end points is no longer linear. Internet Protocol (IP) has displaced digital signaling as the preferred transport medium, and the ideal path for all telecommunications services is through the Internet "cloud."  Multi-Protocol Label Switching (MPLS) technology ensures that the Internet "cloud" provides deterministic performance for voice traffic and allows IP to transport a wide variety of additional telecom services, including video, virtual private networks, instant messaging, and multimedia. The versatility of IP and MPLS routing is driving the telecom business toward converged services.

But to date, convergence of services has been hampered by a stubborn divergence of transport technologies. This problem has been especially pronounced with respect to converged broadband data and voice over IP. In the past, CLECs tried to use IP based technologies to compete with traditional services providers. Financially hard-pressed, the CLECs needed a way to deliver voice services without sinking scarce capital funds into costly Class 5 switches. The eventual solution was the softswitch.

Combining feature servers, media gateways, media gateway controllers, and SS7 gateways, softswitches enabled CLECs to dramatically cut the cost of adding toll-quality voice to their service mixes. But softswitches were a less-than-ideal VoIP solution. They proved to be a halfway house on the road from TDM to VoIP, and they led carriers into a kind of technological cul de sac.

Softswitches allowed carriers to avoid the costs of investing in public switched telephone network switches to deliver toll-quality, IP-based voice services within their own networks. But they created VoIP islands within the wider telecommunications network, which was still TDM-centric. VoIP providers still needed media gateways and TDM connections at the edges of their networks to interface with enterprise customers and other carrier partners. Between endpoints, their VoIP streams went in and out of the Internet cloud. Carriers needed an IP-to-TDM-to-IP translation at every peering point and their voice services fell prey to the ultimate bugaboo of time-sensitive voice communication: latency.

Softswitches were an overlay on the IP/MPLS networks that carriers had relied on to enable broadband services. "Purpose-built" for the replacement of traditional PSTN switches, softswitches had limited functionality and did not support other revenue-enhancing IP-based services, such as video, enterprise instant messaging, and other multimedia applications. Carriers that wanted to add these kinds of services to their portfolios had to overlay still other service-specific solutions on top of their MPLS routers. Thus, they created islands of service positioned on top of the MPLS networks. Their investments in IP voice solutions didn't carry over to other IP services. They might be able to converge high-speed data, voice and other value-added broadband services for their customers, but they couldn't converge their own solutions into a unified networking platform.

To compete in a marketplace where traditional services could ultimately become commoditized, carriers required a better way to fully leverage the cost efficiencies of the Internet cloud while also delivering new and differentiating services and applications. They needed a solution that would provide seamless, end-to-end support for multiple, real-time IP services, including VoIP. Today, carriers are at last finding the solution they need to enable cost-effective converged services as they migrate their IP networks from decoupled service overlays to a tightly integrated solution, consisting of an underlying IP/MPLS network and a service management layer for real-time services.  

Packing a one-two IP punch, this solution combines a smart edge session controller working in tandem with an MPLS edge router. The session controller operates as a microflow manager, providing the underlying IP/MPLS network with session awareness. This combination of technology enables carriers to better manage their services and provides them with the capabilities to deliver any real-time service to their customers.

At the edge of the network, a smart edge session controller combines media routing, call admission control, and Session Initiation Protocol (SIP) and H.323 signaling elements on a single platform. Sitting "on top" of the MPLS router, this edge controller maintains SIP and H.323 state and uses this state information to control the flow of real-time media and control access to the network. Functioning as both an H.323 gatekeeper and a stateful SIP proxy server, the smart edge session controller enables carriers to seamlessly connect to or interoperate with other H.323 devices and function as a bridge between H.323 and SIP call setup protocols. Agnostic with respect to the protocol used by another endpoint to control a session or a call, the smart edge session controller resolves signaling conflicts at the edge of the carrier's network, enabling providers to interconnect successfully to any other carriers' VoIP networks. Since the edge session controller is stateful, it also provides a facility to record session details for billing.

In addition to enabling session routing and traffic management, the smart edge session controller provides advanced media routing capabilities that include network address translation traversal, topology hiding, route enforcement, and regulation of bandwidth consumption to manage media flows across carrier and enterprise boundaries. Other edge devices, like session border controllers (SBCs), also perform a number of these tasks, but their functionality is limited to interconnect issues between VoIP networks.

Over time, these media routing capabilities of the SBC will likely be subsumed by the edge MPLS router and there will be a tighter coupling between the session controller and the router. In current VoIP networks, session controllers and softswitch technologies form an overlay network on the MPLS foundation. Traffic management is based on service type, and tighter integration between the service layer and the MPLS network enables more granular traffic control on a per-user basis or even a session basis. 

The evolution of session controllers into microflow managers is fundamental to this networking approach. These microflow managers utilize call signaling to determine what QoS and security policies are needed to process a user's service request and convey this information to the edge router. The router in turn provides the appropriate levels of security, bandwidth, and QoS to the user session or indicates that the call cannot be serviced. In this way, the microflow manager makes the MPLS router session-aware without sacrificing router performance and scale.

Smart edge session controllers work with intelligent network-core session controllers to support seamless and scalable end-to-end VoIP services and enable cost-effective delivery of other high-value IP services. Intelligent core session controllers combine programmable routing, centralized collection of call detail records and SIP/H.323 internetworking on a single platform. Intelligent core session controllers not only work with smart edge session controllers, but they also function as route engines for all other IP endpoints at the network's edge, including application servers that provide value-added IP services and applications, and softswitches and media gateways that carriers may have already deployed to bridge the TDM divide between IP networks and the PSTN. 

Within this architecture, the softswitch becomes an edge device responsible for media gateway control and interfacing with the PSTN's signaling network. Operating as a stateful SIP proxy server, H.323 gatekeeper and providing SIP/H.323 internetworking, multi-vendor H.323 interoperability and policy and routing to handle call origination and termination, intelligent core session controllers eliminate the hassle and expense of deploying and managing multiple signaling and call routing platforms at the network's core. Moreover, by combining all of these capabilities on a single platform, intelligent core session controllers simplify the deployment of next-generation solutions, since there is no further need to overlay separate networks for each real-time service.

The evolution of edge session controllers into microflow managers is a key step in the convergence of applications and networks for service providers.  Together, microflow managers and MPLS edge routers provide a seamless, end-to-end, all-IP solution for the delivery of data, voice, streaming video, and other high-value, real-time services such as multimedia and VPN. Microflow managers give carrier's unprecedented control over their IP traffic by enabling granular traffic management on a user and call basis. 

This robust approach provides a new service delivery framework that captures revenues lost to flat-rate, unlimited-volume services, and enables new service offerings based on multimedia messaging and collaboration. Transport divergence no longer hampers service convergence, and the most cost-effective distance between any two end points--for VoIP as well as all other IP-based services--is finally executed entirely through the Internet cloud.

Dan Dearing is NexTone's vice president of marketing. He may be contacted at ddearing@nextone.com.

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