Can Lync send audio/media direct to the PSTN using G.711?
Well sort of!
G.711 is a voice codec, used by the majority of IP PBXs to encode their audio.
OCS and Lync can use it in some circumstances, although Microsoft also has their own codec called RTAudio, which is an adaptive codec and thus aims to cope well with challenging network bandwidth.
In OCS there is the Mediation role which was used to convert from RTAudio to G.711 as media flows out to the PSTN.
In Lync the mediation role has been allowed to be collocated with the Front End server. This is possible because of something called Media Bypass, which allows calls to the telephone system (PSTN) to send the signalling through the Front End with collocated Mediation server, but not the media. So instead of the end Lync client talking RTAudio to the Mediation server and then having it converted to G.711 and sent out to the PSTN Gateway, the media goes direct to the gateway. This allows for less load on the Mediation server.
There are various caveats, such as what happens if the client is external. In this case the media will come in as RTAudio via the edge server and need translating via the Mediation server. Depending on the level of externally originated PSTN traffic, you may consider a separate Mediation server pool for load purposes.
Equally another point is that for large FE pools where SIP Trunking is used (Rather than E1 connections from gateways) it is suggested to have a Mediation server pool so as to simplify the relationship between the provider of PSTN connectivity and the pool. (Simply put to allow for perhaps 2 mediation servers each with individual trunks for HA, rather than a pool of 8 combined FE and Mediation boxes with all the required trunks!)
So to answer the question, yes in some ways, and with the right provider a Lync client could potentially send audio (media) traffic direct to the PSTN, however signalling still needs to go through the Front End/Mediation server. Therefore the only scenario where media would flow without the FE is where a call has already been setup. At that point if the FE went down, the call would still be able to continue.
Cheers
Nathan
