Survivable Branch Appliance for Microsoft Teams Direct Routing

This post describes what a SBA for Teams Direct Routing is and how it can increase your telephony reliability and service availability. At the time of writing this post, the SBA is in public preview.


What is a SBA?

A survivable branch appliance (SBA) is an on-premise service component to deal with an outage and keep (basic) telephony going for Microsoft Teams Direct Routing. Maybe you’ve already heard about this in the past in regards of Skype for Business Server and Enterprise Voice. A SBA provides voice resiliency towards the case that the sip trunk connection between Microsoft 365 Phone System and a Session Border Controller (SBC) fails. In anticipation of this worst case scenario the SBA keeps you telephony online as long as this affects only the link between SBC, Teams Clients and Microsoft 365 [Teams] Phone System.

The SBA code is provided by Microsoft to SBC vendors which embed it or provide it separately, e.g. for operation on a virtual machine.

Microsoft Teams Direct Routing – SBA Architecture Overview

In case the SIP trunk between SBC and PSTN provider fails, too, there is no more telephony possible. Except you have any re-routing and other high availability configurations in place.

Please note, you should have a holistic high availability concept in place if you heavily rely on these services. To do so you’ll have to start from the bottom up (building’s connections, building’s cabling, building power supply, provider connectivity (options), cloud & IT infrastructure …).

Architecture Overview

The following drawing depicts the SBA as a part of a Teams Direct Routing deployment and it’s capabilities in case of a temporary connection outage between SBC and Microsoft 365 [Teams] Phone System

Microsoft Teams Direct Routing – SBA Capability Overview

Assumptions, requirements and parameters

  • Teams Direct Routing for the (branch) site is configured for media bypass (SBC + Teams Phone System)
  • PSTN (SIP) Trunk is still online and fully operational
  • All (local) clients are still able to connect to the (local) SBC/SBA
  • SBA supports TLS1.2
  • Supported Microsoft Teams DR SBA clients: Windows and MacOS

Known issues and limitations

The most important issues or limitations which are listed on the products documentation are as follows:

  • No reverse number lookup against Azure AD contacts
  • No support for call forwarding settings
  • No support for for dynamic emergency calling (E911)

Conclusion, opinion and summary

At the moment the SBA capabilities are rather limited in particular the supported clients, which are Windows and MacOS only. So, no desk phones or lobby phones which to me might be more relevant because these devices are often used as emergency devices. However, in many cases I’ve seen that there are still analog devices deployed which removes the dependency towards the Microsoft 365 Phone System anyway. Or people have mobile devices with sufficient cell coverage for (emergency) calls.

It’s a start to have a SBA and has the potential to increase resiliency at branch sites for people in the office in front of a PC or Mac. My first thoughts on deploying a SBA – It could be placed where your internet link is very shaky and unreliable but on the second thought – asking – is there is a local PSTN connection (is it SIP and going over the same internet link)? Well, if you have a (single) shaky internet link which is used for the SBC to connect to the Microsoft 365 Phone System as well as the your PSTN SIP Trunk – there is no added value for a SBA because your local branch telephony will fail if your internet link fails.

In my humble opinion it might make sense to deploy a SBA in scenarios where your PSTN connection is not dependent on the same local shaky internet breakout, e.g. PSTN via (old) E1/T1 or other local dedicated PSTN connection (other internet breakout for voice-only).

Based on the above three more thoughts and questions came to my mind:

  1. Is there no way to get a more reliable internet connection to that (branch) site or are backup link via LTE, StarLink/Satelite or else?
  2. What’s the (cumulated) downtime for Microsoft 365 Phone System or Teams over one year impacting your business? … 8, 4, 1 hours/year? [I haven’t found any statistics regarding this.]
  3. Is there a real need for a SBA at that branch site?
    [I mean, if there is no internet, is your branch staff still able that they can work at all?]

Although I haven’t heard or ready anything regarding this, I hope that the SBA will get support for Teams IP phones, too, in the next several months.

Additional resources

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