Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Fax server using t.38 and CNG tone

I have a fax server connected to my UCM 6.1.4 cluster via a sip trunk.  It is using T.38 protocol.  Most things are working OK but I have one customer's fax number that I am unable to connect when sending.  I can send a fax to this machine from an external fax machine, but not from the fax server.  If I call the machine from a regular phone, it answers but I get nothing but dead air.  This leads me to believe that the customer is using a line sharing device of some kind, and it is looking for a CNG tone before sending the call to the fax machine.  To test this, I placed a call from a regular phone and then hit the 1 button a couple of times.  I got a message stating "1 is not a valid option."

What is strange though, is that I can send faxes to this customer from my lab call manager / faxcom solution.  It goes through every time.  This would imply that the CNG tone is being sent from the lab solution.

Who generates this CNG tone?  Would the fax server do it, or does the gateway do it once it realizes that this is a T.38 call?  The gateway in the lab is a 2811 with a PRI connection.  The gateway in production is a 3845 with several PRI connections.

Any help will be greatly appreciated.




Re: Fax server using t.38 and CNG tone

The system originating the fax call would be the one who should send the CNG tone. The receiving station would respond with a CED tone (Called Tone). Then the receiving system would send a DIS (Digital Identification Signal) message. I believe this is where the speed and such is negotiated. Then you get into training and then sending the fax page(s).

When you have: FAX1>GW1>IPNetwork>GW2>FAX2. When FAX2 starts sending fax signals (v.21 preamble), GW2 picks up on it and starts initiating the switchover from standard voice to fax (i.e. T.38 relay). I assume that with a fax server that the GW2-->FAX2 is all "built-in" to the fax server and the T.38 is switched over automatically? I am not really sure as I haven't done this myself.




HTH -Bill (b) (t) @ucguerrilla

Please remember to rate helpful responses and identify

New Member

Re: Fax server using t.38 and CNG tone

Thanks for the reply Bill.

The issue is that the fax server is the originating party, not the answering party.  And unfortunately the called fax machine will not send the CED signal until it hears a CNG signal.

Just as a followup:

I had each of the fax servers call my phone.  The call from my production cluster / fax server did not send out a CNG tone.  My lab cluster / fax server does send out a CNG tone.

I'm still under the assumption that the fax server would be the one to send the CNG tone out, but I guess it's not impossible for the gateway to send it out.  After all, it does know that the call is a t.38 fax call because I see the MGCP message that changes it to a fax call.

The fax server is using a Brooktrout, a.k.a. Dialogics card.