How does T.38 do it's magic?

[northportphoneguy]

I have a general T.38 question. How does it work? Maybe that was too general. But seriously…

Let’s assume 2 MXes’ with 9.0 software and a four pack of T.38 licenses in each. One of the MXes’ has a PRI to the outside world and the 2 MXes’ are linked with IP trunking on their own VLAN. Both MXes’ have P-Nodes with ONS lines for the faxes. So here are my bunch of questions when the IP trunking is involved.

I assume that if a fax was send or received from the MXe with the PRI to the outside, the T.38 would not be involved at all, switched like the old SX2000. Correct?
Does the T.38 just listen for fax tone and then spool the data at the receiving 3300?
Or does a channel get set up between the MXes’ when crossing the IP trunks?
I’ve been told that with IP trunking, there is no programming required. After the licenses are in and the MXe is rebooted you’re good to go. Correct?
In general, how do the T.38 work? What do they do?

Thanks

 

[IrwinMFletcher]

You can think of a T.38 endpoint (license, port, whatever you want to call it) as a fax machine. Whenever you go from TDM to IP (or IP to TDM), you need one. It's basically a software fax machine, it decodes fax signals sent from a real fax machine and converts it to an message. That message is sent across the IP network to another T.38 instance that takes that message and regenerates the original signal to send to the fax machine local to it.

When you start a call with a fax enabled endpoint, across an IP trunk, the call starts off with both audio and fax streams. When the system detects either a fax tone on the audio channel or T.38 packets on the T.38 channel, the system will switch to use T.38 only.

Note the system always listen for fax tones, regardless of T.38 since it can do other things to help the non-T.38 faxes (i.e. ensure G.711 as opposed to G.729, and fixing the jitter buffers).

 

[IrwinMFletcher]

I should have answered your other questions, sorry.

1. Correct. A fax machine local to the gateway MXe doesn't not need/use T.38 as it is an all TDM call.
2. No programming should be required, as long as they are already fax capable endpoints. SIP trunking may require changes, most likely allowing multiple m-lines, although that is service provider specific.

 

[northportphoneguy]

IrwinMFletcher,

Thank you very much for taking the time to answer. I just wanted to understand the process and you have helped. Thanks again.

 

[Canuckvoip]

Perfect...
My understanding is also that if you have 4 T.38 licenses, the 5th simultaneous fax transmission at that controller will probably fail...


Dave

You can't believe anything you read... unless of course it's this sentence.

 

[IrwinMFletcher]

Canuck, probably, but not due to licensing. For example, if you have 4 T.38 calls in progress, the 5th call will not offer T.38 as an option, so it will default down to the non-T.38 case (i.e. switch to G711 if it was a G.729 call). So the call will not fail due to negotiation issues, but may due to network loss, etc.

If however you have 3 T.38 calls in progress and the 4th and 5th call start, both offering T.38, then the first one to switch will get the resource and the other call has to try to renegotiate back down. The 5th call may fail depending on the endpoints and timing.