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Voice over IP

FreeSWITCH + ESL = Programmable Voice

An overview of FreeSWITCH’s ESL

No great secret, I’m a big Python fan.

Recently I’ve been working on a few projects with FreeSWITCH, and looking at options for programmatically generating dialplans, instead of static XML files.

Why not Static XML?

So let’s say I define a static XML dialplan.

It works great, but if I want to change the way a call routes I need to do it from the dialplan,

That’s not ideal if you’re using a distributed cluster of FreeSWITCH instances, or if you want to update on the fly.

Static XML means we have to define our dialplan when setting up the server, and would have to reconfigure the server to change it.

So what about mod_xml_curl?

When I’ve done this in the past I’ve relied on the mod_xml_curl module.

mod_xml_curl gets the XML dialplan using Curl from a web server, and so you setup a web server using Flask/PHP/etc, and dynamically generate the dialplan when the call comes in.

This sounds good, except you can’t update it on the fly.

mod_xml_curl means call routing decisions are made at the start of the call, and can’t be changed midway through the call.

So what’s ESL?

ESL is the Event Socket Library, essentially a call comes in, an ESL request is made to an external server.

For each step in the dialplan, an ESL request will be sent to the external server which tells it to do,

ESL allows us to use all FreeSWITCH’s fantastic modules, without being limited as to having to perform the call routing logic in FreeSWITCH.

So how do I use ESL?

You’ll need two create an ESL server,

Luckily there’s premade examples for popular languages;

Once you’ve got a basic server defined we’ll need to put some config in our XML Dialplan to say “transfer all your thinking to ESL!”;

<!-- Send everything that's numbers to ESL for Processing --> 
    <extension name="esl_route">
      <condition field="destination_number" expression="^\d*$">
        <action application="socket" data="10.0.1.252:5000"/>
        <action application="log" data="ERR Made it past ESL - Play error."/>
        <action application="playback" data="ivr/ivr-call_cannot_be_completed_as_dialed"/>
      </condition>
    </extension>

In the above example my ESL server is on 10.0.1.252 / port 5000.

Now any calls coming in will be transfered to ESL and where it goes from there, is something you define in your prefered programming language.

In the next post I’ll cover how I’ve been addressing this using Python and Greenswitch.

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