I have set up fax detection on one of my inbound routes. To get it to work I had to allow the incoming call from my SIP provider to be ULAW / G.711.
Normally I force the trunk to use G.726 for inbound/outbound, but fax detection does not seem to work with this codec. I’m forcing G.726 on the trunk by using the following in the PEER/USER details:
disallow=all
allow=g726
If the incoming call shows up in ULAW, fax detection works. G.726, no detection.
Is this expected, or is there anything I can do to work around this?
I dedicated a DID for faxing and sent all incoming calls on this line to the fax recipient. Line is still using G.726 and faxing is working.
So, I can receive faxes using G.726, however as stated above the fax detection does not work with G.726. I need to force all calls to the fax recipient extension to receive the fax.
Seems like a limitation of the fax detection routine not working with G.726.
I’m watching the incoming call on the asterisk channels page, it shows the G.726 codec in use. I’m also forcing the G.726 codec on the trunk and disallowing all others.
As well, I’m using an online fax provider to send the fax, which reports when complete that it negotiates a fax rate of either 9600 or 12200 baud. When using G.711 it would always negotiate the highest 14400 baud.
From what I’m reading, G.726 should be able to carry fax.
“The G.726 codec was, however, specifically designed to be able to carry medium speed modems, such as the V.29 modem used for FAX.”
“The dynamic switch allows the speed of voice band modem transmitted by G.726 at 32 kbit/s to be boosted to 9600 bit/s. At 40 kbit/s, modem speeds up to 14400 bit/s are possible.”
Yes, see my post above, G.726 should be able to carry fax/modem speeds. I was able to get it to work when forcing the incoming call to the fax extension.
My question really is about the Fax Detection not working when the call is using G.726.
Maybe in it’s day the support was added. If I recall g.726 used to be called “half rate” and was originally used in TDM pair gain technologies so some degree of data support was included in the original ITU specification.
How the Asterisk CODEC implementation supports it is entirely up to the implementer. My guess if you buried your nose in the FAX Detect application is it doesn’t even look at anything but full rate CODEC’s.