Asterisk supports many different communications protocols from both the modern world of VoIP and from the legacy PSTN. This makes it a powerful tool for building gateways and protocol converters.
Below is a recipe for building a VoIP-to-PSTN gateway using Asterisk, an analog or digital telephony interface card and a standard PC server. The steps are as follows:
1. Select your telephony interface hardware.
Asterisk applications that connect with legacy telephony systems (PBXs or the PSTN) require telephony interface hardware. Small system generally use analog or ISDN BRI connections. Larger systems (more than 12 lines) frequently use T1, E1 or J1 digital connections.
2. Select your computer hardware.
Asterisk can run on virtually any modern computer, but when building a production telephony application server you should follow a few basic best-practice guidelines.
3. Install Asterisk
Once you have your Asterisk hardware the next step is software. You will either need to install Linux or use a ready-to-run distribution to install Linux, Asterisk and various related software packages. Since these application tutorials are intended to help you create custom telephony applications we will start with a generic installation of CentOS 5.3 and then install Asterisk from the Yum repository. This make it relatively easy to keep Asterisk up to date and avoids the complexities of hand compiling the Asterisk source code.
4. Configure your connections
Now that Asterisk is installed and running you need to edit the system configuration files to implement connections to VoIP and PSTN services. Since this step is common to all applications (Asterisk doesn't do much good if it is not connected to anything) it contains information on creating both service connections (connections to VoIP or PSTN services) and endpoint connections (connections to phones or terminal adapters). Some applications require both service and endpoint connections (PBX, ACD) while others may require only service connections.
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