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Modem Passthrough

I have a 3661 running (H.323)12.4.4T with an NM-2V 2 FxS with a modem attached. I am calling a 2821 running (H.323) 12.4.4T with a PRI connecting to the PSTN. I have configured the following on the 3661:

modem passthrough nse codec g711ulaw redudancy.

I am unable to get the modem to connect on a regular basis.

I have turned off error correction, flow control, and auto speed on the modem. Looking for some guidance.


Re: Modem Passthrough

Is the 2821 also configured for passthrough? Both sides need to be setup.

When a modem call is active, do a show call active voice brief. The call should be modem, rather than G711Ulaw. If it is G711Ulaw, your config is probably not correct.

If it is modem, make sure you are error free on your PRI T1 controller.

You can alse try disabling the echo canceller on the FXS port, sometimes this helps.

Cisco Employee

Re: Modem Passthrough

Can you post a "show run" from the 3660 and the 2821? Also, from the 2821 could you post "show controllers t1"?

You will want to remove the "redundancy" keyword from the modem passthrough command. This sends redudant packets and can break things if the end devices are not expecting it.

Disabling the echo canceller is not something you want to do. It needs to be on for modems to work.


Re: Modem Passthrough

For modem passthrough to work the echo cancellers have to be disabled. This should happen dynamically, but if a port is only used by a modem you can statically disable it.

When the gateway detects a data modem, both the originating gateway and the terminating gateway roll over to G.711. The roll over to G.711 disables the high-pass filter, disables echo cancellation, and disables VAD.