Quality-of-Service - The SBC polices and prioritizes voice and video traffic in the network so that the underlying IP network does not become overloaded, using call admission control and rate limiting and implementing MPS overload prioritization and DSCP control.

Connectivity – The SBC provides SIP normalization so that devices from different manufacturers can coexist in the same network. It also handles interworking between IPv4 and IPv6 as well as between different VoIP and Video over IP protocols.

Media Manipulation – The SBC provides transcoding between different voice and video CODECs, encryption via TLS and IPsec, and for media with SRTP, fax interworking, and sometimes media insertion like tones and announcements.

Network Functions – The SBC provides underlying regulatory capabilities that support media recording, as well as lawful call interception. SBCs play an important role in SIP trunking and the routing of call traffic between SIP trunk endpoints.

Originally, session border controllers were implemented as a hardware appliance – much like a router or a firewall – that enterprises and service providers deployed as part of their overall IP network design. Recently, as the transition to the cloud has gained momentum across the industry, SBC vendors have launched cloud versions of their SBC product. Some of them simply pushed their SBC code into a virtual machine (VM), but other vendors have gone the full distance and virtualized their SBCs into a cloud-native format with Virtual Network Functions (VNFs) and VMs, together with support for cloud orchestration systems.

In all cases, the main purpose of the SBC is to sit at the border of an IP communication network and provide a controlled gate for voice and video media and signaling streams. It may be in place to control access from endpoint devices, such as VoLTE endpoints in a private LTE network, or to control interworking functions between network peers, such as for SIP trunking or to provide control over voice traffic to an external cloud-based service.

Typical SBC use cases

SIP trunking is one of the first applications requiring an SBC that an enterprise typically deploys. For many years, the standard interface between on-premises PBXs and IP-PBXs to the PSTN carrier was ISDN PRI, over a T1 trunk. As communications move to IP, these can be replaced with SIP trunks to the carrier at a much lower cost. In addition, SIP trunks can be defined internally to simplify the routing, reducing dropped calls and making it easier for call agents to direct customers to the right department or person. This directly impacts customer experience and how the customer perceives the business.

Another important use case is for the interconnection to cloud-based services that support voice or video. Many enterprises have an extensive set of endpoints, including soft phones for internal calling, with a premises-based call server. However, as part of their digital transformation, they may choose to move to a cloud-based communications solution. This results in a large amount of voice traffic that crosses the enterprise boundary, requiring an SBC to manage and protect that traffic. An example of this is enterprises that have premises-based Microsoft Skype for Business but are now moving to Microsoft Teams.

Unified Communications solutions also create a strong need for an SBC, especially when employees are remote and using smartphones or tablets to access the company voice network. In this case, the SBC provides the IP interface for the devices that are external to the enterprise network. Since this IP interface is directly exposed to the internet, it’s a prime target for attackers, and so a level of application-aware protection is needed.

SBCs and digital transformation

As enterprises deploy digital transformation programs, communications move to an all IP environment – whether it is voice, video or messaging. When enterprises combine this with a strategy that moves some or all of the infrastructure to the cloud (whether public, private or hybrid), it means that the sheer quantity of IP communications crossing the enterprise boundary vastly expands. This includes communication with customers – covering the full cycle of sales associates reaching out to prospective clients all the way through to the customer support call center.

The role of the session border controller is to manage and protect those IP voice and video streams, ensuring that the quality of service is maintained, calls are not dropped and customers receive the best experience possible. And that’s why the SBC is critical to the digital transformation process: it plays an important role in making sure that the primary goal – stellar customer experience – is achieved.