The Microsoft Lync Survivable Branch Office

Back in the dark ages of telephony (before the year 2000) it could be said that if you had a building you had a PBX.   It didn’t matter if that other building was across the street or across the country.  The wiring of an older PBX had distance limitations and although there were some tricks to extend them, those tricks were expensive and difficult to manage and most companies simply installed lots of standalone telephone systems.

That changed with IP telephony and the concept of the remote gateway.  You could now put an IP link between the PBX and a gateway and as long as you adhered to fairly lenient network guidelines for those links (packet loss, round trip delay, etc.), you could spread that PBX out across the country or potentially the world.

The major PBX vendors like Avaya and Nortel introduced gateway versions of their products years ago.  Avaya currently uses their G430 and G450 gateways and Nortel CS1000 systems use MG1000s and SRG 50s.  However, until recently Microsoft had a poor solution for branch offices and remote gateways.  That changed with Lync 2010, though, and that change has finally put Microsoft in the position of offering an enterprise quality communications system.

Depending on the number of people at the branch, Microsoft offers four different options.  They are as follows:

1 to 24 users.  Microsoft’s solution here is very simple.  There isn’t one.  They suggest that if you can live without survivability then you simply put Lync clients out at the branch.  Those clients will work fine when the connection to the Lync core is alive and kicking, but they become pretty worthless once that connection is lost.  Survivability means using your cell phone or wandering the streets until you find a pay phone.

 25 to 1000 users.  Microsoft recommends a Survivable Branch Appliance (SBA).  The SBA is a physical box (at least at this point in time) that contains PSTN interfaces along with Lync Registrar and Mediation Server functionality.   In the case of a loss of connection to the core, the SBA offers basic Lync voice functionality until the connection has been restored.  The PSTN modules in the SBA connect the branch to the outside world for incoming and outgoing calls.  While the PSTN functionality is not strictly required, without it Lync users will only be able to call other Lync users in the same branch office.

While in survivability mode, the branch users will lose all forms of conference since those services are provided by the Lync core.

Both Sonus and AudioCodes offer highly functional SBAs.  Other companies do, as well, but I have less familiarity with their products.

A branch can double up on SBAs to support up to 2000 users.

1000 to 2000 users.  For this situation Microsoft offers the Survivable Branch Server (SBS).  The SBS isn’t really anything new, but simply a Lync-lite server that you define in the topology and install on a regular Windows server.  In fact, you define it just as you would an SBA.

The biggest difference between an SBA and an SBS is that the SBS does not come with PSTN digital or analog connectivity.  That doesn’t mean that it cannot be provided, but you need to add an additional gateway.  You can, however, implement SIP trunking without a gateway device.  Of course, an SBC will be required if you do that.

Like the SBA, conference functionality is lost in survivability mode.

More than 2000 users.  In this case you need a full-blown Lync deployment at your branch site.  In other words, you need to install a Standard Edition of Lync.  Access to the PSTN is the same as if you deployed an SBS which means that you will need to use SIP trunks or buy a PSTN gateway.  Unlike the SBA and SBS, conference functionality is always available to the branch users.

Click on the graphic to see a pictorial view of all the branch options.


I hope this helps you understand the different options for branch office with Lync because typing them out sure helped me.  Win, win, win.


  1. “Survivability means using your cell phone or wandering the streets until you find a pay phone.” – That’s epic wording🙂

  2. Thanks, Zachary. Every once in a while I get lucky. 🙂

  3. Lucid article. I agree with Zachary. I read that line thrice and every time had a smile on my face.

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