Page 54 - Asterisk™: The Future of Telephony

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 inter-
face cards that evolved from the work of the Zapata Telephony Project (http://www.za
patatelephony.org).

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 web site for 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. Check out Digium’s web site (http://www.digium.com)
for more information about these cards.
Other companies that produce Asterisk-compatible analog cards include:
• Rhino (http://www.channelbanks.com)
• Sangoma (http://www.sangoma.com)
• Voicetronix (http://www.voicetronix.com)
• Pika Technologies (http://www.pikatechnologies.com)
These are all well-established companies that produce excellent products.

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

* Popularly called VoIP networks, although Voice over IP is not the only method of transmitting voice over
packet networks (Voice over Frame Relay was very popular in the late 1990s).
† 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. Chapter 7 discusses these in more detail.
§ T1 and E1 are digital telephony circuits. We’ll discuss them further in Chapter 7.

26 | Chapter 2: Preparing a System for Asterisk