FXO / FXS Programming and Hardware Reqs

Answered Question
Nov 26th, 2008


Not sure if I'm in the right area or not, but I'm looking for some help on old school fxo / fxs setup (NO IP PHONES).

The cisco sales department wasn't sure what exactly I was going to need, and an engineer never got back to me, but basically I'm going to have 2x of each of the following:

1760 router



I'm wanting some old school trunking. Simple analog extensions from a phone system in one office get fed to another office and terminated on the phone system as analog "CO" trunks, and visa versa with extensions fed back to the first office as "CO" lines.

The sales department was telling me I need the call manager express and all this other stuff (as far as I know that is only for IP phone services - which we will have none).

What I need is for someone to point me to a programming example and possibly some tips and tricks or something, and I need to know if any other hardware is required besides the routers, fxo, and fxs cards. Is 64mb ram and 32mb flash sufficient for this setup (keep in mind this is the only funtion these routers are going to be serving). I think even a 16mb flash would probably hold the ios this would need, but what would the ios version need to be? Something 12.3 or higher (package? - k9? ipbase?)

Thank you,


I have this problem too.
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Correct Answer by Nicholas Matthews about 7 years 12 months ago

For this type of setup to trunk two sets of analog connections together, you can use the voice port level command "connection trunk".

You will need PVDMs to do any analog-to-digital conversion for voice packets. Use the DSP calculator to find out what you need to get:


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Overall Rating: 5 (1 ratings)
jbayuka Tue, 12/02/2008 - 14:39

Voice requires a Cisco IOS Software Voice feature set on Cisco 1700 series routers. On the Cisco 1700 voice platforms, one or more packet voice and data modules (PVDMs) are needed to support Voice interface card (VICs,) or voice ports may be missing in the active configuration. The PVDMs hold digital signal processors (DSPs) that make the VICs fully functional, and are installed on the motherboard of the Cisco 1700 series.

sandman420 Tue, 12/02/2008 - 17:30

Are the PVDMs needed for non -V routers? Mine are regular 1760 routers, not 1750-V. If so, are they required for functionality? What do they add?

sandman420 Fri, 12/12/2008 - 08:53

Is no one able to tell me whether or not the PVDMs are required on my 1760 router for analog loop-start fxo/fxs trunking?

Also, I'm running into some issues trying to find an IOS version that supports easy vpn server and the voip features necessary for this setup. currently the routers are 64mb/32mb ram/flash respectively, which is not an issue to upgrade to 128-196mb easily.

sandman420 Tue, 12/16/2008 - 14:10

Thank you,

I do not have access to that tool, but the simple yes for the analog to digital conversion gives me what I need.

Thank you,



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