Posts tagged: PSTN

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

Connecting Lync 2010 or OCS 2007 R2 to the PSTN

Hi,

Having just written a long (ish) post on a forum, I thought I would post it here for future reference.

These are high level methods of connecting OCS 2007 R2 or Lync 2010 to the PSTN.

There are three options;

In terms of capability of connection to the PSTN (Telephone network) functionally nothing much has changed in Lync 2010 compared to OCS 2007 R2 other than the need for SRTP support in gateways.

The lack of change is not a bad thing, as you really already have plenty of options!!

 

1.

Traditionally people have ISDN connections to the telephone network which in the USA are called T1 lines which give you a potential 24 channels (calls). Lync 2010 Server has no way of connecting to these ISDN lines without some form of interface. This could be putting an ISDN interface card into your server, but in general is by using a gateway device from the likes of Audiocodes or NET Quintum.

The gateway terminates the ISDN lines and then translates the audio into RTP streams (SRTP for Lync) and SIP for signalling (i.e. setting up who is calling who).

Generally this is the most used option as by terminating the ISDN lines on the gateway it is then possible to route calls either to Lync or to an existing PBX system

 

2.

The next most common option is to use an existing PBX to terminate the ISDN lines and have it talk to Lync, either through a gateway (kind of a reverse of the order above) or, the PBX might be able to talk SIP, and use TCP to talk over Ethernet to Lync.

 

3.

Instead of dealing with traditional ISDN lines the Lync server will connect over IP (TCP or UDP) Port 5060, to an ITSP (Internet Telephony Service Provider). For example people like Verizon, BT, Global Crossing etc.

This allows OCS to route and receive calls directly to the PSTN without the need for any legacy telephony equipment.

Sometimes, even with the above solution, a gateway can be useful, which can be used more like a session border controller, giving options to manipulate the traffic as it passes from Lync to the ITSP.

 

Hope that clears things up.

Cheers
Nathan

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