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Living With Lync: Where the SBC Fits In

Session border controllers (SBCs) connect Lync to the public switched telephone network (PSTN) via session initiation protocol (SIP) trunks. More simply, SBCs let Lync users make calls to and receive calls from ordinary phones. In truth, SBCs do many more things and I've found lots of confusion related to SBCs and SIP trunking in the context of Lync.

So let's take a look at some of the questions I'm often asked.

Q: Why would I want to use a SIP trunk as opposed to PRIs?
Using PRIs for Lync connectivity to the PSTN requires you to have gateways in every area code where your business operates -- or, technically, in every NPA-NXX range. A SIP trunking provider can offer points of presence (i.e., phone numbers) in almost every location across North America. You can manage a single SBC and a single connection to the PSTN as opposed to multiple PRIs and multiple gateways.

Q: Do I really need an SBC with Lync?
Technically, "No." You can connect a Lync-certified SIP trunk directly to your Lync Mediation Server. This is a great setup if you want to pilot SIP trunking. However, in a production environment many security departments take issue with a SIP trunk connected directly to a Lync Mediation Server.

Q: Which SBCs are compatible with Lync?
You can find the list of SBCs qualified for use with Lync Server 2013 and Lync Server 2010 at Microsoft's Lync TechCenter. Currently eight vendors offer Lync 2013-qualified SBCs. In my experience, most enterprise organizations choose devices from AudioCodes or Sonus while carriers favor the more expensive SBCs from Acme Packet (now Oracle). As always, document your specific requirements and then evaluate which device is best for you.

Q: Which SIP trunking providers offer Lync-compatible services?
By my count, and as provided on TechCenter, 53 providers offer SIP trunking services qualified for Lync 2013. Many of these providers offer services in specific geographies so you may not need to consider all the choices. If you already use PRI services, moving to SIP service with the same provider may be easiest (contractually).

Q: Do I really need to use a Lync-certified SIP trunking provider?
If you plan to connect the SIP trunk directly to your Lync Mediation Server then, "Yes."

If you plan to connect the SIP trunk to an SBC and then to your Mediation Server then, "No." The SBC will likely be able to do whatever signal and media transcoding is required to make Lync and the SIP trunk work well together. This is definitely more work and will require more testing. In my experience, the most tweaking comes into play when dealing with caller ID display during advanced call scenarios, such as SIM ringing a mobile phone.

Q: Do SBCs improve the security of Lync federation, remote access, or instant messaging?
No. While SBC vendors sometimes describe SBCs as "security firewalls for unified communications," with Lync voice is the only traffic to and from the PSTN that flows through the SBC.

Remote users and users at federated organizations connect through the Lync Edge Server, not the SBC.

Q: What else should I know about SBCs and SIP trunks?
SIP technology is newer and less well understand than PRIs, which have been around seemingly forever. This means fewer people understand SIP well, nuances are still being worked out, and your SIP implementation will likely not go as smoothly as you would like. An SBC can really help with smoothing out any signaling issues (even when working with a Lync-qualified SIP trunking provider) or manipulating inbound and outbound caller iDs.

While many No Jitter articles detail SIP challenges, the opportunity to centralize your communications architecture using SIP means that the gains outweigh the pains. Without an SBC, however, the manipulations you can do with Lync alone might sometimes prevent you from getting the job done.

Have questions or comments related to SBCs? If so, please ask your question or add your comment below or interact with me in real-time on Twitter at @kkieller.

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