Session Border Controllers

Anytime SIP trunking is discussed, the question about Session Border Controllers (SBCs) appears. An SBC is in the same vein as a media gateway, and in many cases the gateways qualified for Lync can also act as an SBC, but their purposes are very different. The difference with an SBC is that it’s exclusively used for SIP-to-SIP connections as opposed to interacting with a TDM infrastructure. These devices are used by the ITSP, but it is entirely optional for an organization to deploy its own SBC. When an SBC is being deployed, there will be a SIP trunk between the ITSP and the SBC, and another between the SBC and the Lync Server environment. There are several advantages to using an SBC, but the necessity of using one will depend on the situation.

• Flexibility—Microsoft supports a number of ITSPs with Direct SIP connections to the Mediation Server role through a qualification process. In some cases a business might want or need to use an ITSP that isn’t directly qualified with Lync. This doesn’t mean the service won’t work, but using an SBC between the ITSP and Lync will usually allow remediation for any differences between the two sides. Manipulations or conversions in the SIP messaging can be performed at the SBC to allow for the service to function properly.

• Security—SBCs can perform the NAT and SIP message inspection for firewalls that cannot handle the conversion themselves. This allows an organization to hide any internal server names or IP addresses from being sent to the ITSP.

• Transcoding—In some cases a provider cannot deliver a G.711 audio stream to the business, or a different codec mighty be desired for bandwidth reasons. An SBC can act similarly to the Mediation Server role and transcode audio streams between differing codecs.

• Authentication—Some ITSPs offer the capability for authentication on the SIP trunk in the form of a username/password or shared secret. The Mediation Server role cannot perform this function so an SBC can used for terminating the SIP trunk and authenticating the business.