Blog | Understanding the role of session border controllers

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By Pierre-Jean Chalon

Pierre-Jean Chalon

1) How do Session Border Controllers work within a network?

Session Border Controllers (SBCs) fulfill a number of roles, without which telecommunications networks would not be able to work effectively.

SBC do exactly what their name suggests: they control sessions — those voice, video, and conferencing applications — at the network border. 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 DoS attacks and directing traffic to the fastest, most efficient route.

As a translator, the SBC acts as intermediary of all the various ?flavors? of network protocols like VoIP and SIP that exist internal and external to your network, ensuring they can all ?understand? each other.

As a tour guide, an SBC manages policies to ensure the network doesn?t get overloaded.

As global service providers migrate away from end-of-life infrastructures based on Time Division Multiplexing (TDM) they are replacing them with high-capacity QoS-enabled IP networks. These IP networks carry voice, video instant messaging, presence and collaboration traffic using SIP signaling 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, ensuring security, call admission control, billing and interoperability.

2) What benefits can SBCs bring service providers?

When introduced in the early 2000s, SBCs were relatively ?dumb?, simply routing traffic around and passing it to more powerful media servers for any transcoding. Today, SBCs fulfill a variety of functions.

This includes ensuring that traffic only goes where it is allowed by policies to go; that it is only viewable by the intended recipients; and, if needed, 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 Denial-of-Service 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.

3) What market drivers are increasing the demand for SBCs?

The combined market for SBCs, (across both Service Providers and Enterprises) has a 15 percent CAGR, and is expected to double by 2017, growing to over $1 billion (Infonetics, March 2013).

The market is driven by the need for service providers to interconnect to other networks, as well as by the increasing move to SIP Trunking, growth in Enterprise Unified Communications (UC) and a growth in Residential VoIP.

As more traffic moves from TDM to SIP, the number of concurrent sessions will increase. The addition of video traffic with its higher bandwidth will also increase the session count and traffic volume through these interconnect SBCs. This will require increasingly numerous and more capable SBC devices.

Also, 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 (since the network infrastructure and equipment needed is greatly simplified) and the SBC plays a critical role in policing the border between the service provider and end user organisation.

4) How are SBC technology and practices likely to grow and evolve over the coming years?

As enterprises modernize their voice or UC 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.

Voice over LTE (VoLTE) will also impact the SBC market in the coming years. Mobile networks have used digital voice for a number of decades but the growing pervasiveness of 4G, with its large bandwidth, means that mobile providers can take real time IP communications down to the mobile handset. This allows another level of simplification of the carrier network as more and more traffic transition to IP and SIP.

Like other networks, VoLTE networks need border elements to police traffic between users and the core network, as well as between the core networks themselves. SBCs are evolving to meet this 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.

The author is the vice president and general manager for Asia Pacific at Sonus Networks

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