More SIP question and faxing

[Mitelpassion]

Hi,
I have a 3300 Mxe MCD 4 setup with an service provider using SIP.

I'm using faxing from linksys ATA units, also connected to the 3300.

I'm able to send outgoing faxes but can't receive.
A trace reveals that among the 'talk', the controller is asking to use T.38. The ATA doesn't support this, hence the fax does not complete. Why is the controller asking for T.38 in the first place. Should the SIP SP border gateway not ask for this?

How do I turn off the controller's T.38 support, without taking off licenses and taking out any cards.
I'm not 100% clued up on how T.38 works yet but the helpfile would inidicate T.38 is used when a certain fax compression zone is selected.

thanks in advance ('I can attach the trace if need be)

 

[tlpeter]

Use ATA's who support T.38 !!!
Without you will not have a good faxing solution !



ACSS - SME (IP Office)
ACA - Implement IP Telephony -- ACA - Design IP Telephony
ACA - Voice Services Management
______________
Women and cats can do as they please and men and dogs should relax and get used to the idea!

 

[IrwinMFletcher]

The 3300 will request T.38 if it detects a fax tone, it's not a sole function of the SIP gateway. Regardless, if the ATA endpoint doesn't support T.38, the system should fail back to G711 passthrough. That however is provided that the ATA responds correctly.

 

[Mitelpassion]

Yeah,

This is what I'm getting. When faxtone is detected T.38 comes into play however the ATA can't do T.38 and the fax essentially fails it seems.

Issue with G711 is that the provider doesn't suppport this whatsoever.

So my question, if T.38 what codec is used? Can T.38 run over G729 or is T.38 a codec of sorts in it's own way? I've come to the point using the embedded analogue on the system, where T.38 is negotiated. Before this though it appears that the service provider is not liking something we are asking for.

in short we send SDP request with the codec info. Service providers responds back with not available here. I can't figure out what is not available there. after this the T.38 starts negotiation.

A little lost here
Thanks for the help so far.

 

[IrwinMFletcher]

To be clear, T.38 IS the codec. So no T.38 can't 'run over G729'. You can fax over G729, but not use T.38 over G729 as they are both separate codecs.

I find it surprising that your provider doesn't support G711, this is the bread and butter default codec.

If you can get a SIP trace and post it it would help to understand what side isn't behaving properly.

 

[Mitelpassion]

thanks for the reply,

bandwidth over here is a problem so G711 is just not allowed at this point.

Traces attached. inbound 3rd from top
outbound 5th from top

the number to and from is 011 915 1716 (seperate trace for inbound and outbound). Call says complete but fax is never actually sent or received.

 

[IrwinMFletcher]

So if I see this correctly, looking at the file outbound fax not working.pcap, the 3rd sip call has 10.236.212.2 (the 3300) starting out as G729, then after 4 secs it requests a switch to T38. The far end 41.0.26.166 responds with a 200 OK, then 10.236.212.2 sends a T.38 CNG message and the far end never responds. Correct? Then the far end is not behaving properly, they should NACK the request if they can't do it.

 

[IrwinMFletcher]

Same goes for the 5th call.

 

[Mitelpassion]

ok so CNG get sent in whatever direction, other side must respond?

what does no signal mean? tried to google it but none the wiser at the moment.

far end definately supports T.38. from what I can see the trace shows this right?

 

[IrwinMFletcher]

From the trace that I looked at, 10.236.212.2 (this is the 3300 correct?) ask to renegotiate to T.38, the other end said 'Ok'. So 10.236.212.2 sent a T.38 CNG message, to which the far end should respond with a CED message. It doesn't, so the 10.236.212.2 side gives up, same as if you were calling a real fax machine and it doesn't answer.

'No signal' means just that, no signal is on the line. It's just a default message that gets pumped out after a preset amount of time to keep the firewalls open.

T.38 is simply a translation of the real modem signals into a message. Think of it this way, in a T.38 call, there are really 4 fax machines. The real physical sending machine sends it's modulated data to the T.38 'fax machine' on the 3300. It decodes the signal (say CNG for example) and creates a message that can be sent reliably over IP to the far end T.38 'fax machine'. That endpoint then takes the message and regenerates the modulated tone and sends it to the far end real fax machine. So when you see a 'No signal', it just means that for that period of time, the fax machine that that T.38 endpoint is representing has been silent.

The far end 'says' it supports T.38, as it sends an OK. Since T.38 was the only codec offered, it should immediately switch and start transmitting packets. The trace doesn't show that, so I would say No, it doesn't fully support it.

Do you have any traces of T.38 calls that work?

 

[Mitelpassion]

nope not to this particular provider. I'll ask them if they have some for their other customers.

thanks for clarifying the protocol.