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Community Member

VoIP test help

I have been given the task to test and implement VoIP on our exixting network. We have two remote offices and currently they are connected using point-to pint lease line,with the bandwidth of 128K and the other 64K.

We have a PBX at our main HQ (where Im currently based) and according to the installer, the PBX has capability of VoIP.

Im planning to upgrade the lease lines to our remote office to 2Mb each.

At our HQ, we have a 3661 with only one slot empty. We want to do a test for the time being where the PBX at our HQ act as the main PBX for our two remote office.

We want the setup to be something like this:

phone(extension 200) <---->HQ PBX <------>HQ Router<------------------------>Office branch Router <--->phone(extension 300)

2Mbps Data circuit

We are currently using encapsulation PPP on our serial links and static routing. Im currently using 1751's at the remote sites

I need advice on the following:

1. Router cards needed

2. IOS of the routers capable of VoIP

3. cables needed

4. Sample configuration

5. etc

Need your help



Re: VoIP test help

A bunch of this depends on what capabilities your PBX has? How are you connecting the PBX to the router. T1 CAS, ISDN PRI or analog E&M ports, FXS ports, FXO ports, considering that you will have to purchase a Network Module to insert into your 3661.

I am assuming its a T1 card on the PBX.

In that case, you can get a NM-HDV-1T1-24 for 1 t1 coming into it, and the cad will have enoug dsp's to support 24 cals.

On the rmeote side. the 1751's will support voice. Again you need to see are there gonna be just analog phones, or a t1. If analog, how many analog. The 1751 can support up to four Analog phones at a time.

The IOS you are going to need is anything in 12.2 and an IP PLUS feature set.

Once you figure out your design you can proceed with sample configurations etc.

Hope this helps.


Community Member

Re: VoIP test help

Im thinking of connecting the PBX to the router using analog E&M ports. What card will I have to buy for the Cisco 3661 router ? I have got only one slot left on the 3661 router. Is it enough ? If one slot is not enough, Im thinking of using 1751 for all the three sites including the HQ

At the remote sites we are planning to use analog phone and planning to put in 2 analog phone on each sites(might increase it in the future).

The PBX at our HQ will act as the main PBX that will serve the HQ and the 2 remote sites.


Re: VoIP test help

Your one slot in the 3661 can fit 4 E&M analog ports. If that is your requirement, then you should be able to. The 1751 can also fit a maximum of four analog E&M ports, so it will be the same.

On the remote side you cna have up to four analog fxs ports for the phones.


Re: VoIP test help

hi husain

can u just explain me in brief the difference in using FXO in place of E&M cards in this setup and also in general point of view ??



Community Member

Re: VoIP test help


If I understand:

As you know in VoIP if you use G.729 codec (which make sense for WAN) the bandwidth for each voice channel is 24 Kbps. If you do RTP compression you need only 13 Kbps (more less - for lines of 128 Kbps, do not use for 2 Mbps) for each voice channel. So I think you are upgrading the leases line only for data purpose, and not voice. As you can see if you have E&M at PABX and you can go to 4 E&M at 3661 then for voice you need +/- 52Kbps for each branch...

At branch you do not have a PABX, so you need to have at 1751 FXS, to connect analog phones and FAXs, you need one slot for WAN e two can be for voice. So you can have at each branch 4 or 8 FXS (for example 3 or 7 phones and 1 FAXs - without local PSTN connection!) - 2x VIC-2FXS or 2x VIC-4FXS.

If you need a local connection to PSTN at the then you can use one voice slot with 4 FXS (VIC-4FXS) and one slot for PSTN interface, which depends of what you have or what you pretende to have - it could be analog interface (FXO or E&M) or digital interface (ISDN BRI or PRI).



Community Member

Re: VoIP test help

Thank you all for your replys. I think I have gathered useful information from you.Correct me if Im wrong.From my understanding when reading your posts, I will need the following Hardware and IOS:

1) HQ - 1 x E&M Card for 3661/1751 to connect the Router to the HQ PBX, and 12.2 with IP Plus IOS

2) Remote Site 1 - 1 x FXS Card, Cisco 1751 Router with 12.2 and IP Plus IOS

3) Remote Site 2 - 1 x FXS Card, Cisco 1751 Router with 12.2 and IP Plus IOS

I will upgrade the link to 2Mbps for Data and VoIP

Community Member

Re: VoIP test help


the different configurations from hardware that they indicate to you estan very good.

But I want to you to indicate a Link that helped you with the configuration of your network.

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