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T.38 Question

All: I'll apologize right up front for this newbie question, but I am new to the idea of faxing over IP networks.  I have been trying to do research on using T.38 for faxing, and while I have found some information, one question I have not yet been able to figure out is what role there is for T.38, if any, if you're using a fax machine connected to an analog FXS port on an ISR, which then calls outbound to the PSTN on either a PRI or FXO port?  All examples I have seen so far deal only with transporting the fax end to end over an IP network, what happens if you have a fax going part of the way over IP and then hitting the PSTN, what role is there for T.38 in that?  If somebody could point me to a document or two, I'd be glad to read up on it, so far I must be searching for the wrong stuff because I am not coming up with much useful info.  Thanks in advance.




I am not aware of any documents but here is where I see the benefits of T.38:

- it allows you to have "software" on the side of the fax machine (in other words you do not need old fashion boxes for faxing)

- it is "just" sip to sip endpoind comunication so easili managable on firwall side, and so "doable" even on complex networks

- also benefit over G.711 is the possible redundancy option

And you already have the gateway. However it has to have power to convert the T.38 to "voice" (quite resource demanding).

In your case, If your fax machine is connected to the FXS port, then I do not expect any T.38 in place at all. But if you do not want to replace tis device then probably you do not need any other solution, even t.38 based. I bet, it works



Thanks for your reply.  I am in fact having some issues with Brother fax machines.The issue seems to be negotiating between the Brother devices, which are G3, and other fax machines I have (most of Kyocera Mita all in one copy/fax devices) which are SG3.  It seems the SG3 devices can fax to each other, but the G3 and SG3 devices cannot.  If I take them back off the gateway (Cisco 2901 router), then they work fine.  I have read over and over again that T.38 is the preferred method of faxing over an IP network, so I was wondering if it might allieviate this issue, but I haven't found anything that would lead me to believe it would help in this instance, as the incoming fax arrives on an FXO port and is delivered to an FXS port with fax machine attached.


I am afraid that here the t.38 is not an option - I expect that these devices does not support T.38. Have you tried to send the Fax from SG3 to G3 with G3 speed? The problem could be that they cannot negotiate slower speed themselves ...

I would look here for inspiration (quick googling but it is mainly about T.38 SG3 to G3 switch) -


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