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New Member

Advice about FXS Port configuration for connecting to modems

Hello all,

We have not confirm the problem, but we are having reports that analog devices like USR V.92 Modems and Power meters are not connecting or dropping the connection.  We have an ISR 2911 with the 4-port FXS-DID WIC (VIC3-4FXS/DID), configure to an internet SIP truck provider.  The Router is running IOS 15.3(3)M2.  If we connect an analog phone to the FXS port and call, voice is clear.  So the question is what configuration settings should we have for FXS ports and/or dialpeers for connections to modems?  Attached is the current configuration of the router.  

Do we have to be worried about Codec?  If so which Codec should we configure?
Any Physical FXS settings should be set?
Are my DSP setup correctly?  

Again, I'm at a loss, as voice sounds good and my searches for modem connections to FXS Ports on CUCME is coming up dry.  Any help would be great.  

As far as the "Power Meters", we are talking substation power meters.  

Thanks for any advice you can spare and your time,

Nick

Everyone's tags (4)
8 REPLIES
Cisco Employee

Hi Nick,I have went through

Hi Nick,

I have went through configuration, it all seems good. As FXS ports will always talk PCM to connected endpoint, so its DSPs which will convert it in to RTP, and DSPs should be able to convert it in to g711ulaw, g711alaw , etc.

 

Below said configuration should exists there to get modem calls working, as currently configuration is there for Fax, but not for modem.

 

So here is a bit dependency on ITSP, as cisco does modem relay using NSE events which are cisco proprietary and is likely that they will not be understood by Remote Device, causing , switchover never happens. 

 

So we need to create an specific dial-peer, for incoming modem / outgoing modem calls, so that specific configuration can be applied to those dial-peer, this is just to ensure, that normal fax transmission will not get affected.

 

dial-peer voice 1 voip
  fax protocol pass-through g711ulaw

 

As this will cause Gateway not to do any negotiation, and will keep whole call as voice call, so that end modems can do the negotiation.

 

Hope that helps.

 

Thanks,

Amit

 

New Member

Hi Amit,Thank you very much

Hi Amit,

Thank you very much for your post.  But the way you have laid down the information makes it hard to follow.  Are you saying the dial-peer for the port connecting to the modem should have this command?

dial-peer voice 1 voip
  fax protocol pass-through g711ulaw

Even though we're connecting the pots line to a modem we would use this fax protocol command? Am I getting you straight?  

Also why not: dial-peer voice 1 pots?

Thanks,

Nick

Cisco Employee

Hi Nick, Sorry for delay. As

Hi Nick,

 

Sorry for delay.

 

As we have ITSP SIP trunks, when we are talking to service provider. So it would be an VOIP-POTS connection.

 

Which mandates the use of some modem / fax transmission method in place. So that dial-peer 1, should be incoming voip dial-peer for an incoming call.

 

Similarly for an outbound call from Modem, the outgoing voip dial-peer matches should also have this CLI.

 

So it would be more safe to apply it globally instead on the dial-peers. As fax configuration will not exist on the POTS dial-peer, as for POTS - POTS connection there is no configuration required.

 

Hope that helps.

 

Thanks,

Amit

New Member

Hi Amit,

Hi Amit,

No problem on the delay.  Your help is very much appreciated.  

You're losing me a bit.  Because I don't know if your giving me dial-peer 1 as an example.  1 being just a random number.  Or your telling me to change my current dial-peer 1 configuration.  Which is currently a pots dial-peer:

dial-peer voice 1 pots
 description POTS port 0/1/0
 destination-pattern 111111111
 port 0/1/0

dial-peer 1000 voip is our outbound call leg.  Sorry call legs are not exactly clear to me yet.  In fact they haven't been in years.  :)  So change my dial-peer to dial-peer voice 1 voip?  

My other question still unknown to me is should we use:

 fax protocol pass-through g711ulaw

Even though we are trying to use a modem and not a fax?  Also this command is a dial-peer command and not a global command.  So what global command should i be using?

 

Again thank you for your time and help,

Nick

 

Cisco Employee

Hi Nick, Dial-peer tag 1, was

Hi Nick,

 

Dial-peer tag 1, was just an example. so you can use any number. You can locate incoming & outgoing dial-peers matched for an incoming and outgoing calls to / from modem.

 

About usage of fax pass-through command, is basically not specific to fax only, what this command does is that it will covert all PCM stream to RTP and transmit to another ip end point, after that remote modem is connected. So it can deal with both fax and Modem.

 

Here is the sample configuration for both the methods of Modem configuration on the router.

conf t

voice service voip

fax protocol pass-through g711ulaw

end

 

++++ Modem relay Method ( NSE Based ).

 

conf t

voice service voip

modem relay nse codec g711ulaw gw-controlled
  no fax-relay sg3-to-g3

end

 

However in NSE based method , cisco router will use NSE Events to indicate the switch over to modem call. Which may not be understood by remote SIP Server.

 

But you can give a try to both the methods.

 

Thanks,

Amit

 

Cisco Employee

debug voip ccapi inout, can

debug voip ccapi inout, can be used to find out the dial-peer matching for incoming & outgoing calls.

 

However i have also shared the method for applying command in global mode as well.

New Member

Hello Nick,

Hello Nick,

Can you please send me the config we also have to create the setup like you are having?

We have to connect 4 analog modems on the FXS port on the router and those modems will register through the SIP provider.

Rregards,

Mukesh Kumar

New Member

If you still need the config,

If you still need the config, message me.  I can dig them up.

My problem ended up being codec setting mainly, but we had to factory reset modem.

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