Initially, the only way to handle faxing in Asterisk was through the spandsp library. spandsp provides a multitude of Digital Signal Processing (DSP) capabilities, but in this context all we are interested in is its fax functionality.

Asterisk has the hooks built in to make use of spandsp, but due to incompatible licenses, the spandsp libraries must be downloaded and compiled separately from Asterisk. Also, since spandsp was not written only for Asterisk, it will not assume that it is being installed for Asterisk. This means that a few extra steps will be required to ensure Asterisk can use spandsp.

Obtaining spandsp

As of this writing, the current version of spandsp is 0.0.6.

Download and extract the spandsp source code as follows:

$ mkdir ~/src/asterisk-complete/thirdparty
$ cd ~/src/asterisk-complete/thirdparty
$ wget
$ tar zxvf spandsp-0.0.6pre17.tgz
$ cd spandsp-0.0.6

Compiling and Installing spandsp

The spandsp software should compile and install with the following commands:

$ ./configure
$ make
$ sudo make install

This will install the library in the /usr/local/lib/ folder. On many Linux systems this folder is not automatically part of the library path (libpath), so it will need to be added manually.

Adding the spandsp Library to Your libpath

In order to make the spandsp library visible to all applications on the system, the folder where it is located must be added to the libpath for the system. This is typically done by editing files in the /etc/ directory. You simply need to ensure that one of the files in that directory has /usr/local/lib listed. If not, the following command will create a suitable file for you:

$ sudo cat >> /etc/

Press Ctrl+D to save the file, then run the ldconfig command to refresh the library paths:

$ sudo ldconfig

You are now ready to recompile Asterisk for spandsp support.

Recompiling Asterisk with spandsp Support

Since the spandsp library was probably not installed on the system when Asterisk was first compiled, you will need to do a quick recompile of Asterisk in order to have the spandsp support added:

$ cd ~/src/asterisk-complete/asterisk/1.8/
$ ./configure
$ make menuselect

Ensure that under the Resource Modules heading, the section for spandsp looks like this:

[*] res_fax_spandsp

If you see this instead:

XXX res_fax_spandsp

It means that Asterisk was not able to find the spandsp library.

Once you have verified that Asterisk can see spandsp, you are ready to recompile. Save and exit from menuselect, and run the following:

$ make
$ make install

You can verify that spandsp is working with Asterisk by issuing the following command from the Asterisk CLI:

*CLI> module show like

At this point the SendFAX() and ReceiveFAX() dialplan applications will be available to you.

Disabling spandsp (Should You Want to Test Digium Fax)

The spandsp library and the Digium fax library, discussed in the next section, are mutually exclusive. If you want to try out the Digium fax product, you will need to ensure that spandsp does not load. To disable spandsp in Asterisk, simply edit your /etc/asterisk/modules.conf file as follows:

noload =>

Save the changes and restart Asterisk.