[asterisk-users] FAX Issues

[Eric Wieling]

> -----Original Message-----
> From: asterisk-users-bounces at lists.digium.com [mailto:asterisk-users-
> bounces at lists.digium.com] On Behalf Of Lee Howard
> Sent: Tuesday, August 09, 2011 7:22 PM
> To: Asterisk Users Mailing List - Non-Commercial Discussion
> Subject: Re: [asterisk-users] FAX Issues
> 
> Ryan McGuire wrote:
> > Unless your network is under load and you are seeing dropped packets
> > and high jitter, I would absolutely not do T.38. The cheapest and
> > easiest approach that I have found is to buy yourself an FXS gateway
> > and just make sure you are using ulaw.
> 
> As SIP is usually running over UDP/IP it doesn't take much to produce
> dropped packets.  Dropped packets mean lost audio which means lost data
> and possible demodulation difficulties for the modems.  If you're in an
> environment where dropped UDP packets don't occur you're in a very rare
> scenario.
> 
> For the most part people who claim success when faxing over SIP G.711 are
> being rescued by ECM (error correction) within the fax protocol.
> There are very, very few who really have mitigated UDP packet loss.
> 
> That said, all T.38 systems are not equal.  Certainly, the reliability of your
> T.38 provider may not be any better than that of G.711 fax over the SIP UDP.
> 
> I only recommend faxing over TDM everything else is at your own risk.
> 
Find a carrier, likely a CLEC, willing and able to quickly re-route numbers when you have an outage.  This is telecom, you are going to have an outages caused by something totally out of your control.  If you have a TN down for whatever reason, having a POTS fax machine or a couple of POTS lines into your PBX for your carrier to re-route the non-working TN to can save the day.  Carriers use a variety of methods to give you dialtone on a pair of wires.  Make sure the line is plain old boring analog all the way to your carrier's switch.  

Here is how I look at it.  Assume  1 in 10 faxes fail when using ulaw with SIP on a nice stable QoS'd connection to your carrier .  There is no specific percentage, read the mailing list archives; I don't think it is an outrageous assumption.   Error Correction (ECM) can only do so much.  For personal use, that might not be a big deal.  For business use, it is likely to be a big deal.  People get very annoyed when they don't get their faxes.   If you don't need a large number of fax numbers and don't need to handle a large number of faxes, then use standalone POTS lines and fax machines.  It is simple, reliable, and people are familiar with it.  POTS is generally easy to troubleshoot, if the telco who provides the POTS service tells you it is an inside wiring issue or a fax machine issue, then chances are it is an inside wiring issue, PBX issue, handset issue, or fax machine issue.  

If you need a large number of fax numbers or need to handle a large number of faxes then you really should consider a PRI to back up your VoIP service --  use the PRI for large numbers of fax numbers pointing to  app_fax, simple fax to PDF conversion scripts are on voip-info.org, e-mail it to a destination e-mail address.     This is quite reliable, relatively easy to implement, uses well known, reasonably mature technology, and protocols.  You can use something like NVFaxDetect or the built-in fax detection of DAHDI to do  "combined voice/fax" telephone numbers.  Enough people listen for the far end fax tone before pressing Send, iy can become an issue.   TNs on PRIs are often very cheap compared to DIDs on VoIP, consider a dedicated fax TN for each person and avoid the hassle of fax detecting.  There is a large community base and lots of documentation.  If you are budgeting for a PRI then a POTS line is not going to be a large expense, might as well have one or two around when all else fails. 

T.38 is a fairly new protocol compared to POTS and PRI and even Asterisk and app_fax.  The support community is much smaller, documentation is not as complete.  You will likely need plenty of T.38 support from your carrier to get it working.  If you need large numbers of fax numbers across a large area, or if you need fax numbers in places you have no presence , or any number of reasons, investigate T.38. Today T.38 seems to require a significant investment in research, trials, and failures compared to POTS or PRI fax.  You need to decide if the advantages of T.38 are worth the investment in time.

Others will have to comment on the option of Hylafax, IAXmodem, or BRI for fax.

* The opinions above are my own.