Controlling network borders

Controlling network borders

Roger Jones from IP networking company Sonus explains session border controllers (SBC), who they benefit, and how they are likely to evolve over the years.

What are session border controllers?

Session Border Controllers (SBC) essentially do exactly as their name suggests: they control sessions – such as voice, video and conferencing applications - at the network border. Put simply, they are the guardians, tour guides and translators for any given traffic moving across two or more telephone networks.

As a guardian, the SBC decides what can be admitted or denied, securing incoming and outgoing communications, protecting against DDoS attacks and directing traffic to the fastest, most efficient route. 

As a translator, it acts as intermediary of all the various ‘flavours’ of network protocols like VoIP and SIP that exist internally and externally in a network, ensuring they can all understand each other. As a tour guide, it manages policies to ensure the network doesn’t get overloaded.  The role of SBCs has evolved as carriers migrate away from legacy TDM networks and move increasingly towards IP-based network services. IP networks carry voice, video instant messaging, presence and collaboration traffic using SIP signalling and RTP media, and SBCs function either to peer or interconnect two networks together, or to enable access from a carrier to an enterprise network.


What benefits can SBCs bring operators?

Introduced in the early 2000s, SBCs fulfilled the simple role of routing traffic around and passing it to more powerful media servers for any transcoding. But today SBCs are said to fulfil a variety of functions, including ensuring that traffic only goes where it is allowed (by policies) to go, that it is only viewable by the intended recipients, and, if required, transcodes the information itself into a form that is easier for the receiving network to handle. 

Simply-put, service provider level SBCs are the first point in the network where the operator can influence incoming traffic, as well as the last opportunity to achieve their bi-lateral agreements with other service providers as the data heads out of their network. SBCs can also protect against DDOS attacks, regulate the number of inbound calls from a particular trunk, and, increasingly, process certain services at the edge of the network that would normally take up ‘core’ network capacity.

What drivers are helping to increase demand for SBCs?

According to Infonetics, the combined market for SBCs across both service providers and enterprises has a CAGR of 15%, which is estimated to double by 2017, growing from $503 million to over $1 billion. The market is driven by the need for Service Providers to interconnect to other networks, but also by the increasing move to SIP trunking, and the growth in both enterprise unified communications and residential VoIP.

As more traffic moves from TDM to SIP the amount of concurrent sessions will rise. The addition of video traffic with its higher bandwidth will also increase the session count and traffic volume through interconnect SBCs. This will require increasingly numerous and capable SBC devices. In addition, as networks evolve away from TDM to SIP, service providers will seek to shift their enterprise customers from ISDN trunks to SIP trunks. SIP trunks drive down costs for both the service provider and enterprise (as the network infrastructure and equipment needed is greatly simplified) and the SBC is said to play a critical role in policing the border between the service provider and end user organisation.

How are SBCs likely to grow and evolve over the coming years?

SIP trunking penetration in North America is estimated to be around 15%, with EMEA and Asia-Pacific significantly less. But as enterprises modernise their voice or unified communications platforms, the option of SIP trunking becomes much more attractive.


Lower costs, greater flexibility in terms of concurrent calls and, in many cases, greatly improved disaster recovery for phone lines will drive the uptake of SIP trunks and drive capacity requirements in the service providers’ access SBCs. Another emerging trend that will impact the SBC market over the coming years is Voice over LTE, (VoLTE). Mobile networks have used digital voice for a number of decades but the emergence of 4G, with its large bandwidth, means that mobile providers can start to take real time IP communications down to a mobile handset. This allows another level of simplification of the carrier network as more and more traffic transitions to IP and SIP. 

Like other networks, VoLTE networks need border elements to police traffic between users and the core network, and also between the core networks themselves. SBCs are further evolving to meet this growing requirement. The use of real time communications on 4G handsets will drive a massive increase in session counts and bandwidth as video is increasingly used and this, in turn, will increase the requirement for high performance SBCs.

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