OpalVoipWiki | Main / FaxSupport

Opal has extensive support for fax. This support can be divided into four areas

1. SIP

Opal can send and receive faxes using the T.38 protocol. This has been tested extensively. It will respond to incoming Re-INVITEs to switch into T.38 mode. Opal can also send Re-INVITEs to trigger T.38 mode. This can be triggered by the receipt of an in-band CED tone, the receipt of CED via RFC-2833, or by a timer.

This support requires the availability of the spandsp_util helper program.

2. H.323

Opal contains all of the code needed to negotiate T.38 over H.323. However, this code has not been tested for some time.

3. Connecting a fax to the internet.

There are four scenarios here based on the medium being used on the local and remote endpoints.

	T.38	Audio RTP, e.g. G.711
TIFF File, must be G.3 compatible.	Supported now by T38EndPoint class & spandsp_util.exe helper application	Supported now by FaxEndPoint class & spandsp_util.exe helper application
Raw PCM, e.g. via OpalLineEndPoint or OpalLocalEndPoint classes	Fax codec plug in. While this code works, it needs further work to separate the spandsp functions from the remainder of the code as they use mutually incompatible licenses. It is intended that this be done using the same spandsp_util helper application used by T38EndPoint and FaxEndPoint classes.	Basic OPAL routing and functionality, does not involve anything special

4. Building from source

Two external source code projects are needed in order to build spandsp_util from source

1. libtiff

libtiff is TIFF handling library and is required provides the ability to load and save fax image files, which are in TIFF format.

On most Linux systems, this library will be preinstalled. If not, it will almost certainly be available to install via yum/yast etc.

A pre-compiled Windows version is available from the following URL:

   http://gnuwin32.sourceforge.net/packages/tiff.htm

Downloading and installing the complete package is recommended.

Don't be tempted to use the Win32 version of libtiff. It won't work

Once installed, add "C:\Program Files\GnuWin32\include" to the Include directory list, and "C:\Program Files\GnuWin32\lib" to the Library directory list. The path "C:\Program Files\GnuWin32\bin" must also be added to the PATH environment variable, else the libtiff DLL files will not be found at runtime.

2. spandsp

Spandsp is a GPL library and suite of programs that implement a faxmodem.

On most Linux systems, this library will be preinstalled. If not, it can be downloaded from the site below, then compiled and installed as usual.

    http://www.soft-switch.org/downloads/spandsp

You will need to add "C:\Program Files\GnuWin32\include" to the Include directory list, and "C:\Program Files\GnuWin32\lib" to the Library directory list

Windows users will need to download and compile the library. A link to a suitably modified version of the library will be included here at a later date.

3. Common problems

If you see errors such as the following:

spandsp_if.cpp(912) : error C2664: 't38_core_rx_ifp_packet' : cannot convert parameter 2 from 'const unsigned char *__w64 ' to 'int' spandsp_if.cpp(1101) : error C2664: 't38_core_rx_ifp_packet' : cannot convert parameter 2 from 'const unsigned char *__w64 ' to 'int'

then uncomment the line '#define SPANDSP_VER3 1' near the top of spandsp_if.h