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.


Connecting Lync 2010 or OCS 2007 R2 to the PSTN


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!!



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



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.



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.


Digicert being most impressive!


I’ve blogged about Digicert before, but over the last few days I’ve had yet another change to use them and been seriously impressed!

I was setting up an OCS 2007 R2 lab and got to the section where I needed to setup Forefront TMG to proxy connections to the OCS Web Components on my Standard Edition front end.

I created the cert request using the certificate wizard in the OCS administrative tools as it is the easiest way I know to mark the private key as exportable.

Having done that I submitted the request to Digicert and, because it was a domain I have already validated, the certificate was issued within five minutes.

I installed the certificate on the Front End, exported it, and installed it on the Forefront TMG box.

Then for testing I accessed the relevant website externally. Things looked like they worked but on one device I got a trust error. It was late, and I didn’t have time to investigate so I left it.

The next day I received an email from Digicert as below:

We just ran an installation check on the DigiCert SSL certificate that you installed on and it appears that the server needs to be configured for maximum compatibility. You will need to install the Intermediate certificates to the server in order to ensure compatibility with legacy browsers and mobile devices.

On Windows platforms, the easiest way to do this is to use our certificate utility. Just visit and download the Certificate Management Tool. After running it on the server, click the Repair button. Some servers require restarting the services or restarting the whole server after making this change.

You can verify that the problem is fixed at

If you have any problems correcting this issue, please contact our helpful support team and we will be happy to help.

Now I knew about the utility and have blogged about it before, but to be told this is the problem proactively was brilliant!

I ran the utility, it installed the intermediate CAs properly and all is well!

Thanks Digicert

Note: As an Exchange MVP, DigiCert has provided me certificates to use in test labs, without which I may not have had the opportunity to try their service.

Hosted Pilot for Microsoft Unified Communications – Features

As promised here is a post looking in a little more depth at the features of the Hosted UC Pilot from Microsoft.

Firstly I will look at some of my favourite features and then will take a look at what is not available.

The first thing that strikes you as you log into Outlook Web App, is that you now have a fully fledged Exchange 2010 mailbox.

This is accessed using the URL.

One of the nicest things here is that you can start sending IMs immediately by using the integrated IM functionality. Obviously this is not as feature rich as Office Communicator or even Communicator Web Access but it gets you started. Another new addition is the ability to see presence within OWA as can be seen in the email and top right of the screenshot below:


In my previous post on this subject I showed the process of logging into Office Communicator. Having done so I started poking around, and the first thing I tried was adding a federated user.

This gave me the following error:


The link for more info pointed here:

I then tried to add a friend at Microsoft – Brett Johnson

Surprisingly this worked as can be seen below, the globe icon shows up next to Brett which means that Federation is working for users at Microsoft but not external domains.


I then tried a bunch of other UC functionality such as desktop sharing, and peer to peer voice calls. Everything worked well! Sadly, one thing that is not available is Enterprise Voice which means that you can’t go making phone calls on the system.

One major thing that is really important to understand is that this is a multi tenant system on which you can see all the other accounts that people have created! There is NO segregation at all!!

So all in all this is a great service for anyone wanting to test out the full range of Microsoft UC functionality minus of course Enterprise Voice.



Opening the OCS Help File gives HTML blocked content error

I’ve been doing a bunch of work recently with OCS in various lab scenarios and am really pleased that the OCS product group have bundled all their documentation into a CHM file like the Exchange team have done for a while.

The only weird thing about using the downloaded help file (see link below) 

is that on opening it, it doesn’t show content instead giving an error message as shown below:


So what it turns out, is that this only happens when you download the CHM file directly because when doing that, you are prompted about the possible security threat from opening it.

To get around this, I uncheck the box to always prompt on opening this type of file;


at which point, the CHM file works correctly.


Hope that helps someone.



Updating Microsoft Office Communicator – April 2010 patch

Recently there have been a bunch of new updates for Office Communications Server 2007 R2 and it’s accompanying client Microsoft Office Communicator 2007 R2.

I will post later with screenshots of the server updates but for now here is the location and process to update MOC.

First download the patch from the link below:

You will notice that much more information is available from the following location –

For example from the above KB article we can see that there are many bug fixes included.

To install the update once simply double click the .msp file that you have downloaded and follow the steps below:

Click Next


Accept the license terms and click Next


If prompted accept (Click Yes) to the UAC request

Click Finish


All in all it’s really that simple!

Hosted Pilot for Microsoft Unified Communications

Whilst at Dimension Data there has long been a point of discussion about how we could offer a hosted pilot for the various Microsoft UC end user features. We were hoping to setup a multi tenant OCS and Exchange 2010 system by doing some segmentation work on the Address Book system and by using FaceTime IMAuditor to ensure that only people on the same trial from the same company could communicate.

However, it would now appear that Microsoft have gone and done it for us and the rest of the industry.

Here is the information from Microsoft

You and your colleagues are invited to experience first-hand how Microsoft’s Unified Communications solution can help you harness the power of software to streamline communications between people anytime, anywhere, free for 60-days.
Register for the Unified Communications Virtual Experience today at
The Virtual Experience allows you to test drive the end user capabilities delivered by Exchange Server 2010 and Office Communications Server 2007 R2 for 60-days, you will discover how you can:

Connect with the right person, right now from within familiar, easy-to-use Microsoft Office applications—with one click

Bring people together quickly and easily—anytime, anywhere

Break down communication silos—whether people are down the hall, across continents, or constantly on the road

So my plan is to try and trial this at home and then let you know all about the process.

Stay tuned for more!