[Asterisk-Users] Application Faxing using SIP

[Lee Howard]

J Poz wrote:

> Using an analog line is not an option for my service. My application 
> runs on a ROOT SERVER of an ASP. So I can do anything I want to the 
> server but I can't connect to or get external analog lines. So my 
> options are doing faxing via the Internet (VOIP/SIP) or use a faxing 
> service. But my experience with faxing services has not been too good 
> as I've mentioned.


Traditional faxing (not T.38) pretty much requires a lossless audio 
channel.  Normally the best way to get this is with PSTN channels/lines 
through a Zap device.  That said, VoIP channels can be configured such 
that they are also lossless.  IAXmodem, for example, functions on the 
premise that an IAX2 channel passing over the loopback device will be 
lossless.  I have also seen lossless SIP and IAX channels running over a 
WAN, but they were very specificially configured, and I wouldn't expect 
most connections with traditional VoIP providers to be anything near the 
kind of losslessness that is required for this to work well.

For the most part, I suspect that those VoIP providers that promise fax 
support (over VoIP G.711) are doing so on a type of gamble... that ECM 
support of most fax machines will compensate, that they can control 
enough of the communication to mitigate the problem substantially, and 
that the remaining (say, 10%) error rate will not cause significant 
enough complaints from the users to cause it to be unprofitable.

So, be forewarned that faxing over VoIP channels is usually not going to 
work extremely well for you... not unless you can mitigate the problem 
by creating near-lossless connections between you and the endpoint with 
the PSTN connection.

Lee.