System information

Because of Asterisk’s open architecture (and open source code), it is ultimately possible
to connect any standards-compliant interface hardware. The selection of open source
telephony interface boards is currently limited, but as interest in Asterisk grows, that
will rapidly change.
†
At the moment, one of the most popular and cost-effective ways
to connect to the PSTN is to use the interface cards that evolved from the work of the
Zapata Telephony Project (http://www.zapatatelephony.org), which has evolved into
DAHDI.
Analog interface cards
Unless you need a lot of channels (or a have lot of money to spend each month on
telecommunications facilities), chances are that your PSTN interface will consist of one
or more analog circuits, each of which will require a Foreign eXchange Office (FXO)
port.
Digium, the company that sponsors Asterisk development, produces analog interface
cards for Asterisk. Check out its website (http://www.digium.com) for details on its
extensive line of analog cards, including the venerable TDM400P, the latest TDM800P,
and the high-density TDM2400P. As an example, the TDM800P is an eight-port base
card that allows for the insertion of up to two daughter cards, which each deliver either
four FXO or four FXS ports.
‡
The TDM800P can be purchased with these modules
preinstalled, and a hardware echo-canceller can be added as well.
Other companies that produce Asterisk-compatible analog cards include:
• Rhino (http://www.rhinoequipment.com)
• Sangoma (http://www.sangoma.com)
• Voicetronix (http://www.voicetronix.com)
• Pika Technologies (http://www.pikatechnologies.com)
Digital interface cards
If you require more than 10 circuits, or require digital connectivity, chances are you’re
going to be in the market for a T1 or E1 card.
§
Bear in mind, though, that the monthly
charges for a digital PSTN circuit vary widely. In some places, as few as five circuits can
justify a digital circuit; in others, the technology may never be cost-justifiable. The more
competition there is in your area, the better chance you have of finding a good deal. Be
sure to shop around.
† The evolution of inexpensive, commodity-based telephony hardware is only slightly behind the telephony
software revolution. New companies spring up on a weekly basis, each one bringing new and inexpensive
standards-based devices into the market.
‡ FXS and FXO refer to the opposing ends of an analog circuit. Which one you need will be determined by
what you want to connect to. Appendix A discusses these in more detail.
§ T1 and E1 are digital telephony circuits. We discuss them further in Appendix A.
654 | Appendix C: Preparing a System for Asterisk