What is reasons for bad vocie quality in FXO

Unanswered Question
Jul 12th, 2010

Hi everyone,

           I got a configuration that worked fine untill it upgraded. We have 16 fxo port configured in CUCM 6.1 (gateway ios is Version 12.4(15)T10) it all working fine. But when we upgraded the number of fxo ports 16 to 24 there comes the problems. There is some bad quality in three fxo ports, the IVR that we configured using CUE not work sometimes, also sometimes supervisory disconnect problems etc...

         My fxo gateway is configured in H 323, will it help me if  I changed it in to MGCP. I am attaching my configuration files along with this please go through it suggest me some solutions.

I have this problem too.
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Aaron Harrison Tue, 07/13/2010 - 00:51


I'm assuming here that the new FXO lines are from the same service provider etc? If so, then the same supervisory disconnect mechanisms should be used, so the configuration of the ports should be identical.

1) You should run through impedance matching using the steps detailed in this document; this can have a large effect on quality. Generally I would run it for one line, and then set the other x lines to the same impedance setting, and then run them individually if there is a problem reported. In your case I would start by running it on the problem lines. http://cco.cisco.com/en/US/tech/tk652/tk701/technologies_tech_note09186a0080477a06.shtml#thl

2) MGCP/H323 - switchign to MGCP wouldn't improve the quality of the FXO line, and with CM6 would also mean that you not get caller ID any more.

3) Bad lines:

If you are still experiencing problems after impedance matching the lines then it could be for a lot of reasons - it could be simply bad termination/connection of the wires on your premises, bad connections/cabling from the service provider presentation to your router, or problems further into the service provider network.

Anything like that could cause the tones being sent to your IVR to not be clear enough to be picked up, and the same for any supervisory disconect tones.

I would first rule out bad hardware by moving a problem line to one of the original 16 ports that was working well. If the problems move with the line, then you know your hardware is OK, and you should raise a call with the service provider.



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ullasmoopz Tue, 07/13/2010 - 05:50

Hi Aaron,

         Hearty thank you for your replay.

1) Yes the FXO line are from the same service provider (total of 24 FXO lines for one single number). And yes I gave same supervisory disconnect mechanisms that I gave to the earlier 16 FXO ports (which worked fine).

2) I asked about switching to H323  to MGCP gateway because I like to know which is better to handle and troubleshoot and also I don't know that MGCP doesn't support call id for CUCM6.

3)For the impedence match selection I need to test it and I'll let you know the result as soon as possible.

4)And for the bad voice quality I don't think its due to faulty hardware, because we checked it many times (including the cables).


Can go through my gateway configuration that I gave and suggest me some tips, it'll be much helpfull.

Aaron Harrison Tue, 07/13/2010 - 07:25


Config looks fine - given that your existing circuits aren't causing problems I wouldn't worry about that...

Do the impedance matching, and if that doesn't help report it to the SP as a fault.



ullasmoopz Tue, 07/13/2010 - 21:06

Hi Aaron,

       For the impedence matching I have to wait untill saturday. Only then I can test it and configure it. I'll let you know the result as soon as possible.


ullasmoopz Sat, 07/17/2010 - 04:36

Hi Aaron,

          I am really sorry for one my mistake. Actually the voice quality is fine. The problem is the FXO port is not connecting or forwarding the calls. Which means if "voice-port 1/1/0" is the faulty one and if the CUCM try to connect to it for an outside call, from the ip phone we can hear a loud noise (ssshhh...), sometime there is no sound at all and no call is connected to outside. Same case if some outside call comes it don't detecting the calls not forwarding to AA. I hope you understand my situation. These problems shows only for 3 FXO ports and there is no problem with incoming service provider lines.



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