Session Border Control - What is it?

A Session Border Controller (SBC) is a session-aware device that manages VoIP and MoIP calls at the borders of an IP network. Unlike most network devices session border controllers are aware of the relationship between the two parts of a VoIP call: signalling and media.

SBCs can be divided into two types of architectures:

1. The stand-alone Session Border Controller - this contains all of the intelligence and resources needed to process both the signalling and the media of the VoIP call.
2. The distributed Session Border Controller - In this case the signalling and media functions are divided between two systems that communicate with each other.

Newport Networks' session border controller supports both stand-alone and distributed deployments.
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What Do Session Border Controllers Do?

NAT Traversal:

One of the key functions of the Session Border Controller is the ability to provide SIP services across NAT and Firewalls devices located at a customer premise or within the network. The problem is actually twofold:

Whilst Firewalls are able to dynamically open and close multiple ports as required by VoIP signalling protocols such as SIP, they remain ineffective at securely supporting unsolicited incoming media flows.

NATs prevent two-way voice and multimedia communication, because the private IP addresses and ports inserted by client devices (IP phones, etc.) in the packet payload are not routable in public networks.

Thus incoming calls, essential to any service intended to replace the PSTN, are not possible with existing NAT/Firewalls.

SBCs provide traversal of NAT/Firewalls without additional customer premise equipment, and do not require the replacement of existing Firewalls and NATs.
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Security:

Session Border Controllers protect core networks elements, such as Softswitches, from signalling attacks by identifying malicious traffic before it reaches the core. SBCs also perform topology hiding, removing internal network information from the signalling stream thus preventing internal details from being propagated.
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Quality of Service

Session border controllers occupy a unique position in the network enabling them to control the quality of end-user communications and enforce operators' Service Level Agreements.

• Session (Call) Admission Control - SBCs can monitor the number of calls and bandwidth in use and can reject new calls in order to preserve the quality of established calls.
• Policing - SBCs can ensure that the signalled media requirements match the actual media being transmitted in a call, discarding excessive data. This prevents service theft and protects against a media DoS attack.
• Media release - often a call may be most effectively transported entirely within the subscribers' private network, in this case the SBC allows the media the flow directly between client devices.
• QoS Re-mapping - SBCs can monitor and optionally re-mark the quality settings of a user's data (Type of Service bits and DiffServ Code Point bits). This ensures the users receive the quality they pay for. When SBC are used in network interconnections they can map one service provider's quality settings onto another's.
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Regulatory:

It has become increasingly clear recently that VoIP services will be expected to provide Lawful Intercept and Emergency Call Handling services to the same level experienced in the PSTN. The FCC in North America for example has mandated that both emergency calls and Lawful Intercept must be available.

• Lawful Intercept - must be performed without the user being aware of any change in their normal communications. Session Border Controllers handle both media and signalling, intercept can be performed in a completely undetectable manner.
• Emergency Call Handling - SBCs must identify emergency calls and forward them on to a SIP Proxy that is able to route emergency calls. Emergency Calls must be carried under all traffic load conditions.
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Where are Session Border Controllers Used?

SBCs hold a unique position in VoIP networks. They form the border between (1) access and core networks and, (2) between core networks in the case of interconnect. In essence they form the gateway into and out of the Carriers core network.

• Interconnect - the session border controller forms the border between network operators. Here it secures the network border, enforces Quality of Service policies, ensure any intermediate NAT and firewalls can be traversed, and provides regulatory compliance.
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• Access - the session border controller enables the service provider to access the residential and corporate user across NAT and firewall devices whilst also providing Quality of Service, core network security and Regulatory compliance.
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SBC Solutions