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New Member

Modem Passthrough issues

Is anyone running a modem-passthrough configuration succsessfully?

We have a 2620XM with an NM-HDV, 12.2(15)T9, with a PCI modem card connecting via fxs. CallManager handles the call routing and send the call out a PRI on a 7206vxr.

The calls connect ok, and we do not see any packet loss or delay. The problems we are seeing are with the integrity of the modem call. The modem users see garbage on the screen, get long pauses of 20-30 seconds, recieve crc errors when transferring files, and intermittant disconnects.

We have tried different modems, IOS, tweaked the QoS, and hardcoded the modems to slower speeds.

We have a P2 case open with TAC, but so far that is getting us nowhere.

I am interested in hearing about other people's experiences with modem passthrough.


Tim Medley, CCNP, CCDP

Network Engineer

New Member

Re: Modem Passthrough issues

Afraid we have a similar issue (and a P3 case!), trying to dial out via FXS through a MGCP-controlled MRP on an ICS. We can dial out but the modems never train up.

We wish to use the same route group for the modem calls as the voice calls, and not dedicate an FXO just for the occasional modem call.

Would appreciate hearing any solution you find.

New Member

Re: Modem Passthrough issues

Had a similar issue. The problem was IOS. We were running 12.2(13)T something. It is a known bug. From what I remember, because it's a hairpin call, the router does not invoke a DSP. Anyway, it was first fixed in 12.2(15)T8. Modems work great now. Sorry, I don't have the exact bug ID handy!


Re: Modem Passthrough issues

We have seen a lot of issues with clocking on TDM capable devices such as the 7200VXR. The internal backplane and DSP's don't default their clocking to the E1/T1, so clock slips are seen. These don't affect voice calls as they are generally inaudible, but they kill fax and modem calls as there is a regular error in the data packets.

Do a sh controller t1 and check if there are any slips. If there are, add the following command on the 7206VXR -

frame-clock-select t1

And then add it again for each T1/E1.For the first port, set the priority to 1, second port to 2 etc ...

This should clear up the slips and may help the modem calls.

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