Smartphones are proliferating and so are SIP-based applications such as IM, presence and gaming. SIP signaling across an operator’s network will continue to grow rapidly as these exciting new applications multiply.  A key challenge that service providers face today is how best to manage this new SIP signaling traffic load in a cost effective manner - while at the same time ensuring security, service assurance and scalability. The session border controller (SBC) will be a critical network element for scaling SIP traffic in a secure manner.

Over the next decade or so, Infonetics is forecasting that service providers will be investing more than $5 billion in SBCs to support their buildout of IP services. To date, the majority of SBCs deployed have been optimized for voice-centric services and are customized hardware appliances which combine signaling and media in the same box.

Patrick FitzgeraldIn many current deployments, SBCs are not optimized to handle the large-scale SIP signaling requirement of tomorrow’s mobile applications. They lack IMS/3GPP compliance and they ultimately limit the flexibility of an operator’s network.

Today’s Challenge
The session border control function straddles the signaling and media planes in VoIP networks. In the signaling plane, SBCs perform complex filtering and transformation operations on SIP signaling messages. In the media plane, SBCs perform bandwidth policing, address translation and statistics collection on RTP streams.

SBCs are already well established in most service providers’ networks at both access and interconnect edges. However, total deployed SBC capacity will have to grow massively to address the complete transition of public fixed and mobile voice networks to VoIP. The great majority of these SBCs that have been deployed in the network to date are compact, integrated devices that handle both signaling and media. These SBCs are dimensioned in terms of a number of concurrent sessions, where a session is modeled as a voice call of typical average duration.

SBCs will need to evolve substantially to adapt to the changing SIP services landscape, where SIP-based messaging, presence and call forking services will have dramatic impacts on overall SIP signaling load, especially in VoLTE network buildouts. Current SBCs that integrate signaling and media are ill-suited to support this changing service mix for two main reasons.  First, these products embody signaling and media capacity in a fixed relationship that is unable to adapt to the changing service mix. Secondly, they offer rather modest overall capacity in the signaling plane, requiring them to be deployed in very large numbers to support the signaling loads that arise with new SIP-based services.

SIP Scalability to Support Mobile Services - Distributed SBC Architecture
A new solution for session border control is required that enables signaling and media capacity to be scaled independently of one another and that ultimately enables the service provider to scale the signaling plane to support IP services.

Today’s SBC vendors are beginning to introduce new platforms that are purpose-built to support a distributed architecture. The distributed SBC architecture separates the signaling plane functions of session border control from the media plane functions. This is exactly the same principle as the softswitch architecture, which has dominated the transition of the PSTN to VoIP for well over a decade. Independently scaling the signaling and media gives the service provider some important benefits compared to today’s model.

First, it provides the ability to design a SIP network with the appropriate amount of resources to support the desired service mix. Today’s model of a fixed ratio of media and signaling limits the architectural flexibility and results in a sub-optimal use of network resources, especially for the media path. Secondly, by enabling signaling to scale independently of media, a service provider will be following the architectural blue print of the IMS/3GPP architecture and will gain key functionality for LTE network deployments. Finally, a distributed architecture will be more cost effective for the operator with important savings in media capacity along with centralized management for the signaling plane.

Secure and Robust SIP Signaling
Service providers would do well to re-evaluate their SBC deployment plans to avoid over-investing in architectures that are ill-matched to their medium and longer term SIP services strategy.

The distributed SBC architecture offers a wide range of compelling advantages over traditional SBCs that integrate media and signaling. These advantages translate into lower capital costs for service providers that are faced with the need to massively scale their SBC deployments to optimize their SIP networks for mobile services.

As the changing service mix imposes ever-greater signaling requirements, service providers need a wider range of products and solutions to meet all their functional and quality requirements for session border control. In these circumstances, SBC architecture and scalability become critical differentiators – and the benefits of a distributed and highly-scalable architecture for session border control are so overwhelming that no service provider can afford to overlook them.

Patrick Fitzgerald is vice president of SIP Infrastructure at Metaswitch.