Recently I came across an interesting question around why one would want to enable Siren using the command Set-CsMediaConfiguration –EnableSiren. By default this is set to FALSE. So while this seems like it might be handy to enable, if you have applied CU4 to Lync 2010 it really isn't.
With EnableSiren set to TRUE, three codecs will be offered to Lync by the mediation server. The codecs (in preferred order) will be Siren (wideband is always better), next G.711 (higher quality than RTA-NB), and the least-preferred is RTAudio narrowband. Since the client supports Siren and there would be little reason to switch to one of the other codecs, you can pretty much count on Siren being used for the duration of all non-bypass PSTN calls.
Of course, there will be higher CPU load on the mediation server as it transcodes Siren to G.711 but that’s the price of lower-then-G.711 bandwidth usage on the WAN.
This setting might have made some sense to use from Lync Server 2010 RTM through Lync Server 2010 CU3. In that timeframe, the only thing that would have kept Lync from using G.711 for PSTN calls was if there was a CAC bandwidth policy in place. If the CAC is not configured but wanted a lower-bandwidth codec to be used by default for PSTN calls, it is possible to enable Siren. (RTAudio narrowband was always an option but Lync only switches to it mid-call if congestion was detected.)
In Lync Server 2010 CU4, a round-trip-test between the Lync client and the mediation server was added. If the RTT is greater than a value expected on a LAN, the client will assume a WAN connection and switch to RTAudio narrowband to save bandwidth. RTA-NB requires less bandwidth than Siren. So best practice is to let RTT do its work enabling the us of RTA-NB or you’ll actually end up using more WAN bandwidth than necessary.
For more details on CU 4 update please refer to the KB article below:
These behavior changes were brought about due to feedback received from partners and companies deploying Lync. By enabling Lync to dynamically sense network conditions, Lync is not reliant on network components to make decisions. This enables companies to use any network vendor they see fit or is the case at most companies enables a multivendor network.
Thanks to Doug for the info in this post, a true Lync Master.