The FreeSWITCH design – modular, scalable, and stable

The design goal of FreeSWITCH is to provide a modular, scalable system around a stable switching core, and to provide a robust interface for developers to add to and control the system. Various elements in FreeSWITCH are independent of each other and do not have much knowledge about how the other parts are working, other than what is provided in what are called exposed functions. The functionality of FreeSWITCH can also be extended with loadable modules, which tie a particular external technology into the core.

FreeSWITCH has many different module types that revolve around the central core, much like satellites orbiting a planet. The list includes:

Module type:	Purpose:
Endpoint	Telephone protocols like SIP/H.323 and POTS lines
Application	Performs a task such as playing audio or setting data
Application Programming Interface (API)	Exports a function that takes text input and returns text output, which could be used across modules or from an external connection
Automated Speech Recognition (ASR)	Interfaces with speech recognition systems
Chat	Bridges and exchanges various chat protocols
Codec	Translates between audio formats
Dialplan	Parses the call details and decides where to route the call
Directory	Connects directory information services, such as LDAP, to a common core lookup API
Event handlers	Allows external programs to control FreeSWITCH
File	Provides an interface to extract and play sound from various audio file formats
Formats	Plays audio files in various formats
Languages	Programming language interfaces used for call control
Loggers	Controls logging to the console, system log, or log files
Say	Strings together audio files in various languages to provide feedback to say things like phone numbers, time of day, spell words, and so on
Text-To-Speech (TTS)	Interfaces with text-to-speech engines
Timers	POSIX or Linux kernel timing in applications
XML Interfaces	Uses XML for Call Detail Records (CDRs), RADIUS, CURL, LDAP, RPC, and/or SCGI

The following image shows what the FreeSWITCH architecture looks like and how the modules orbit the core of FreeSWITCH:

By combining the functionality of the various module interfaces, FreeSWITCH can be configured to connect IP phones, POTS lines, and IP-based telephone services. It can also translate audio formats and interfaces with a custom menu system, which you can create by yourself. You can even control a running FreeSWITCH server from another machine. Let's start by taking a closer look at a pair of important module types.