> -----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.