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Fax Help


RobertoAchab
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Hello, I'm testing T38 for our costumers, a lot of them need to route a fax call through an AA.

I installed a Patton Gateway, configured to transform analog faxes in T38, then I have a Zoiper soft client on a machine, this client is an extension of our production pbx.

I switch the PAtton between two configurations, the first sends the fax directly to the IP of the Zoiper client, the second sends it to the pbx, this is the only difference between the two (infact, I only change manually the SIP proxy address).

pbxnsip has the inband detection enabled an AA with the "F" detection, and routing works well.

 

now:

If I send faxes directly to zoiper it receives them and wireshark shows a T38 communication.

If i send the same fax through pbxnsip zoiper tells me the phone is ringing, as if he can't understand it's a fax, or a T38 communication.

 

Unfortunately the first case is not a solution, because i'm sending ALL connections to Zoiper, so in a real environment my pbx couldn't receive any voice call.

I think that pbxnsip in some way "strips" the ced tone, or sends it where it can't be heard by zoiper (I tried sip-info and rtp mixing them on Patton and Zoiper both), please is there someone knowing which are the correct parameters?

Thank you

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Hello, I'm testing T38 for our costumer, a lot of them need to route a fax call through an AA.

I installed a Patton Gateway, configured to transform analog faxes in T38, then I have a Zoiper soft client on a machine, this client is an extension of our production pbx.

I switch the PAtton between two configurations, the first sends the fax directly to the IP of the Zoiper client, the seconds sends it to the pbx, this is the only difference between the two (infact, I only change manually the SIP proxy address).

pbxnsip has the inband detection enabled and an AA with the "F" detection, and routing works well.

 

now:

If I send faxes directly to zoiper it receives them and wireshark shows a T38 communication.

If i send the same fax through pbxnsip zoiper tells me the phone is ringing, as if he can't understand it's a fax, or a T38 communication.

 

Unfortunately the first case is not a solution, because i'm sending ALL connections to Zoiper, so in a real environment my pbx don't receive any call.

I think that pbxnsip in some way "strips" the ced tone, or sends it where it can't be heard by zoiper (I tried sip-info and rtp mixing them on Patton and Zoiper both), please is there someone knowing which are the correct parameters?

 

The CNG tone is repeated a couple of times, the receiver should be able to sense it... There must be something else.

 

Can you get a PCAP trace and put it somewhere where we can pull it down and take a look?

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In the attached zip file there is a sample of a successful transmission from Patton to Zoiper, and a sample of a failure through the pbx, same configuration of Zoiper and Patton.

Patton = 192.168.101.7

Zoiper's PC 192.168.101.213

PBX 192.168.100.3

I set the Patton to repeat CNG three times

 

Really strange. Attached is the RTP in the unsuccessful case. If Zoiper is supposed to detect the CNG? I can hear it loud and clear.

faxproblem.mp3

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Really strange. Attached is the RTP in the unsuccessful case. If Zoiper is supposed to detect the CNG? I can hear it loud and clear.

 

Yes, I knew, I think the problem is in negotiating the T38 protocol, Zoiper can't recognize an analog fax, but only a T38.

As I said beofre Patton->Zoiper works well, that's the reason why I think that pbxnsip pheraphs "translates" some tone from rtp to SIP info, or something like that, the initial connection of the two faxxes should be exctly the same if pns were "transparent"

:-(

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BTW, giving a better look at the two trace, I think the error is in the first"good" Vs. fourth"bad" packet, in the good one there is an "invite" that includes a T38 announcements in the last rows, then zoiper responds "200" and receives the call.

When pbxnsip sends the "same" invite it hasn't a T38 announcement, so Zoiper must accept it with a "ringing" state.

I wonder why pbxnsip can't recognize and announce T38...

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I wonder why pbxnsip can't recognize and announce T38...

 

T.38 is "not trivial"... If the PBX would have to advertize it after detecting the CNG signal, it would be responsible for converting G.711 into T.38 :huh: .

 

I totally agree, the whole situation is not very satisfactory. I believe almost the whole VoIP industry agrees... Modems were designed for analog lines, not for digital RTP packets.

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T.38 is "not trivial"... If the PBX would have to advertize it after detecting the CNG signal, it would be responsible for converting G.711 into T.38 :huh: .

 

I totally agree, the whole situation is not very satisfactory. I believe almost the whole VoIP industry agrees... Modems were designed for analog lines, not for digital RTP packets.

 

Well, I usually don't speak in english, so pheraphs I've missed the point, the communication is already in T38, the Patton already converts it when calling Zoiper "alone", if Pbxnsip would let all packet to pass, the fax should arrive correctly...

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