[Asterisk-Users] Application Faxing using SIP

[tim panton]

On 19 Feb 2006, at 06:04, Lee Howard wrote:

> 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.
>


Unless I've misunderstood the problem, your best bet is to take VOIP  
out of the picture, and keep your fax
purely digital up to the last possible moment. (in other words this  
isn't a problem for asterisk...)

Can't you move the architecture about a bit ? get the ROOT server to  
generate a suitable PDF or TIFF of the fax
then send it  over a reliable protocol (lpr/http post?) to a server  
that does have an analog or digital line
and that is running fax software?

All that said, I've had moderate success plugging an analog fax modem  
(in a Mac)
into an ATA talking G711 to an asterisk (on the same ethernet switch)
which sends it out over a PRI. This is low volume stuff. The only  
problems
I see are occasional retries and a total lockup when I put the Mac  
into sleep mode (That's an OSX bug I think).

OSX does have a really nice lp->fax engine, but that is straying way  
off topic.

T.


http://www.westhawk.co.uk/

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