FAX on FXS ports not completing

[Erick Bergquist]

Hi,

Have a issue where FAX calls come in a PRI and go to FAX machine on a FXS port on the same router. So there isn't any VoIP involved, just a call in PRI and out the FXS port to FAX.

We areseeing where 20-30% of FAX's do not complete.

Anyone have any idea on what to look at?

[pwalenta]

Can you post the config of that device? It's possible than one of many things cound be causing the issue.

Off the top of my head, I'd say check input gain and output attenuation settings. I usually do -1 input and 3 output as a starting point.

[Erick Bergquist]

Right now we do not have any input or output gain applied, just echo cancel of 32 w/suppression of 10.

I will check out ERL, etc tomorrow on calls and adjust for it.

[pwalenta]

Echo cancellers shouldn't be in operation for fax or modem calls. The volume however can affect how well the faxes work. Also, is one of the connections a T1, or are both FXS/FXO ports?

[tom-holloway]

check for clock slippage on the e1/t1, sh controllers e1 . slips mean that you are not clocking properly from the pstn. have a look at this in the tac case collection K15397383

cheers Tom

[Erick Bergquist]

This is a PRI and both voice and FAX calls are received on it, destined to FXS ports (FAX) or CCM (voice).

There are no errors on T1 controller. All interfaces are clean. The FXS ports are on a NM-HDA module.

They do have echo-cancelling enabled.

I looked at the echo level (ERLLevel) and it is all over the place for active voice calls. They were mentioning a little echo now and then. I made some adjustments and it is better now but this is adjust & wait type of thing.

The 'show call act fax' however always reports ERLLevel of 0 for FAX calls. Does IOS Measure Echo, etc for FAX calls?

[maharris]

Here are two docs I have run across recently, dealing with issues when faxing within the same router. Maybe one of them will be pertinent, I am still trying to digest possible issues with faxing, they seem endless..

http://www.cisco.com/en/US/partner/products/sw/voicesw/ps556/products_configuration_example09186a0080093fbd.shtml

http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps1839/products_feature_guide09186a0080087c90.html

[dgahm]

I have been fighting the same issue. The TAC says this is a longstanding bug with MGCP that will require an extensive rewrite of the IOS to fix. They offered 2 suggestions; disable echo cancellers on both FXS and PRI and run passthrough, or convert to H323. Since disabling echo cancellers would cause lots of complaints on voice calls I went with H323. The fax calls now use fax relay at 14400 and are rock solid, with zero problems.

I was also having problems from branch offices that are using central 6608 PRIs. Most faxes were OK with fax relay, but all faxes to one location would fail 90% of the time. I converted the FXS port to H323, and all faxes now work great.

For IOS it seems H323 is the way to go.