IAXmodem FAQ

This is a set of anticipated questions and the associated answer.
Q1: IAX is VoIP.  Isn't it bad to run fax over VoIP?

A1: Yes, generally it is.  The conditions of jitter, packet loss,
and other latency issues cause problems for reassembling VoIP
audio streams on the receiver exactly as they were disassembled
on the sender.  However, these latency conditions can be
controlled.  For example, these conditions should not be present
when communicating over the localhost address ( in the
case where the Asterisk server and the system running IAXmodem
are the same.  Other possible arrangements could be to use a
dedicated network connection (e.g. a crossover cable) between
the Asterisk server and the system running IAXmodem.  IAXmodem
should also be fine to run over a local area network (LAN) that
is well-designed and controlled.

The basic point is this: if you can eliminate the mitigating factors
of jitter and packet loss, then you can reliably use VoIP channels
for faxing.
Q2: What about T.38?  Isn't T.38 the end-all solution for faxing
over the internet?

A2: T.38 is great, and yes, it is the end-all solution for faxing
over the internet.  However, IAXmodem isn't designed to be used
for faxing over the internet, as the internet is not a network
that can control the latency conditions enough for faxing to
work.  So T.38 and IAXmodem are really trying to solve two
different problems.

Someday the PSTN may be entirely replaced by VoIP.  Someday all
fax machines may have T.38 support.  In that day there will be
no need for fax DSPs.  But that day is not now, and it's not
coming in the foreseeable future.  When using VoIP, someone,
somewhere on that VoIP chain must have a "PSTN gateway".
Likewise, when using T.38, someone, somewhere on that chain must
have a "T.38 gateway" (to the PSTN).  And any time fax data
is passed through the PSTN it must be modulated, and that
requires DSPs.

IAXmodem was designed to give the fax application tight control
of the DSP behavior (through the Class 1 interface) and the
developers tight control of the DSP behavior (through access
to the source code).  As long as fax DSPs are needed and
someone using those DSPs wants tight control of how they work,
then there is a place for IAXmodem.
Q3: Why is IAXmodem open-source and licensed with the GPL?

A3: Firstly, much of the value in IAXmodem comes from being able
to control the DSP.  This cannot be done as desired without
having the source code to the DSP.

Second, having the source code allows all users the same
opporunity to improve and enhance it, and (hopefully) by
contributing that work back to the community IAXmodem and the
libraries that it uses also improve.

Third, IAXmodem depends heavily upon libiax2 and spandsp, both
of which are licensed with the GPL.  Licensing IAXmodem in some
other way would have been possible, although prohibitively
difficult, and may not have not fostered as much community
involvement and contribution.
Q4: IAX is bad and libiax2 is even worse.  Why didn't you use
SIP or some other channel driver?

A4: The availability of libiax2 and the small set of protocol in
IAX (compared to SIP) made it attractive for the purposes of 
getting IAXmodem running quickly.  Although libiax2 has needed
some bug fixes for IAXmodem to work properly, and although
libiax2 probably will need more bug fixing in the future, it
seemed like using it was the quickest path to a functional

If anyone is interested in enhancing IAXmodem to support SIP or
any other channel driver, development contributions are
always welcome.
Q5: What if I really want to use IAXmodem but my Asterisk server
is connected to my HylaFAX server only by a lossy (e.g.
internet) connection?

A5: The connection between IAXmodem and Asterisk must be clean
and clear.  However, the connection between HylaFAX and IAXmodem
need not be.  For example, you could run IAXmodem on the Asterisk
server communicating on the loopback adapter, and HylaFAX would
communicate with IAXmodem remotely using a "remote tty" service
such as termpkg's termnetd and ttyd programs.  (A patch is
included with IAXmodem source that will permit ttyd to be used
for fax communications.  You'll want to use a termnetd.conf entry
like: "64570:on:/dev/ttyIAX0:B19200 CLOCAL CREAD IGNBRK;")

An alternative to termpkg is interceptty, which is reported to
Q6: How do I set up my Asterisk dialplan for IAXmodem?

A5: This will vary depending upon what you want, but in many
instances it will be something like this:

  exten => fax,1,Dial(IAX2/ttyIAX/${EXTEN})

If you do not put ${EXTEN} there then IAXmodem will not receive
DID information.  In this example "fax" is your fax extension number.
If you use the Asterisk "fax" extension given by zaptel fax
detection then ${EXTEN} will lose its meaning, and you'll need
to use a workaround to get the correct called-number passed on
to IAXmodem.

If you want to operate multiple IAXmodem instances (to handle
more than one fax at a time) do something like this:

  exten => fax,1,Dial(IAX2/ttyIAX0/${EXTEN})
  exten => fax,2,Dial(IAX2/ttyIAX1/${EXTEN})
  exten => fax,3,Dial(IAX2/ttyIAX2/${EXTEN})
  exten => fax,4,Busy
  exten => fax,5,Hangup

Each IAXmodem instance can only handle one call at a time.  Thus
it will report as busy if it is already handling a call, and Asterisk
will then proceed to the next entry in the dialplan.

Passing ${EXTEN} or some related value in the Dial application is
important, as that provides the DID indication for iaxmodem.  If this
application parameter is omitted then iaxmodem will not receive DID.

Also, using the ChanIsAvail command prior to dialing is known not to work
since it issues a IAX "HANGUP" and puts the iaxmodem in a busy state.
Therefore, the following Dial command will be rejected with the cause
"Busy". Some Asterisk pre-packaged distributions dial extensions using
macros containg ChanIsAvail (ie. Voiceroute's Druid) so beware of them. 
Q7: I start iaxmodem and then place a call to it, and I see these
messages appear on output:

  "Unable to pass the full buffer onto the device file."

What's wrong?  Is this a bug?

A7: IAXmodem doesn't immediately know if anything is watching the
modem device or not.  So when you place a call to iaxmodem it will
instinctively pass RING messages and Call*ID to the pty.  If nothing 
is watching the modem device then eventually the pty buffer will
fill and the pty will begin refusing further data.  This is what is

So basically, it's nothing to worry about if you are aware that
you're not running any modem-watching process like faxgetty.  However,
you probably *should* be running something like faxgetty in anything
other than a test scenario.
Q8: How do I set up my Asterisk iax.conf for IAXmodem?

A8: Again, this will vary somewhat depending upon what you want.
However, here is an example:


The [context] name refers to the "peername" entry in the iaxmodem
config file.  The secret should match the "secret" entry in the same.
The context value refers to which dialplan (extensions.conf) context
should be used for handling calls which iaxmodem should initiate.
The disallow and allow values ensure that the server only uses uLaw
and alaw audio codecs when communicating with iaxmodem.  Disabling
the PBX-side jitterbuffer is quite important to prevent it from
disturbing the audio.


See the TODO file for information regarding bugs, needed
features, and enhancements.