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epaschalFlag for United States of America

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Configuring Microsoft Lync to use Cisco 3925 as a PSTN Gateway


We presently have a 100 percent Lync/Exchange based phone system, with all of our PSTN calls going to a gateway device that's connected to our 1 PRI and FXS ports. We run Polycom CX600's as our endpoints.

We need to replace the current PSTN gateway device (extremely buggy, unreliable) with a certified gateway for Lync. The gateway device we use now has poor support and there is no on-site or after hours support.

When looking for a replacement, I see the Microsoft has certified our Cisco 3925E/K9 as a gateway for Lync. We have a killer Smartnet agreement with 24/7 support and 4 hour onsite, so this seems the perfect fit.

Presently the Cisco device sits outside of our Sonicwall firewall and simply acts as the connection to our Fiber internet link. So at present the only add-on is the SFP module for the fiber connection.

My question is this:

What all is required to make the 3925 Lync friendly? Mainly I know we need to at least add some sort of card for the PRI and FXS port. Are other modules recommended/suggested?

This Cisco device sits outside the firewall - whereas our current PSTN gateway device (and lync) are inside the network. Is it still possible do this config, or will it create a networking nightmare?

Lastly, since we still have a fax line (hence the need for the FXS port) - is it possible to have the Cisco device intercept calls to our fax DID's (we have 18) and route those to the FXS port? That way they don't pass to Lync, which wouldn't be supported.

I had also look at Dialogic as a solution, but given the mission critical nature of our voice service - the Cisco device seemed best. Are there any big tradeoffs going this route? I don't know Cisco IOS, however I do have a contractor who usually helps with config.

Thanks for any help/advice/warnings you can provide.
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Thank you Alexander,

Presently the cisco device already routes traffic from our internet connection to the firewall, so wouldn't it have to stay outside the firewall?

Also, for the DSP modules, do those perform better/worse/same as the audio hardware provided by the likes of AudioCodes and Dialogic?

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OK, it looks like you want to use an existing Cisco 3925 router.
If so, it's place, of course, is outside the FW.

But in this case there are two ways:
1. Add an additional network interface and connect it into your inside network. On the router strongly separate VoIP traffic, generated by router, from data traffic from the Internet. VoIP interface can be connected to the inside network. It will be secure enough, but not for some "paranoid" Security officer :)
2. Use the existing connection trhrough you FW and write some rules on it to aloow VoIP traffic from the router to the Lynx server. Something like this, your network guy can make it easy.

Analog lines from the faxes - just connect to FXS ports on the 3925 in any way.

DSP modules - of course, they are the same. May be even literally the same - Audiocodes produces voice processors as OEM manufacturer for many vendors, but I'm not sure about Cisco.

Another solution is to buy a specialized gateway, for instance Audiocodes Mediant 1000, and it will be place for 20 FXS lines left after you install a PRI card in it. In this case you physically separate two pieces of hardware - Internet router and PSTN gateway.

There are pros and cons in both solutions,  jus take a calculator and count the money :)

The main problem are faxes - it's a little bit expensive to convert 19 analog lines to IP.
Thanks - this worked perfectly.
You are welcome!