Page 102 - Asterisk™: The Future of Telephony

If your Asterisk server has a compatible FXO port, you can plug a telephone line from
your telephone company (or “telco”) into this port. Asterisk can then use the telco line
to place and receive telephone calls. By the same token, if your Asterisk server has a
compatible FXS port, you may plug an analog telephone into your Asterisk server, so
that Asterisk may call the phone and you may place calls.
Ports are defined in the configuration by the signaling they use, as opposed to the
physical type of port they are. For instance, a physical FXO port will be defined in the
configuration with FXS signaling, and an FXS port will be defined with FXO signaling.
This can be confusing until you understand the reasons for it. FX_ cards are named not
according to what they are, but rather according to what is connected to them. An
FXS card, therefore, is a card that connects to a station. Since that is so, you can see
that in order to do its job, an FXS card must behave like a central office and use FXO
signaling. Similarly, an FXO card connects to a central office (CO), which means it will
need to behave like a station and use FXS signaling. The modem in your computer is
a classic example of an FXO device.

The older Digium X100P card used a Motorola chipset, and the X101P
(which Digium sold before completely switching to the TDM400P) is
based on the Ambient/Intel MD3200 chipset. These cards are modems
with drivers adapted to utilize the card as a single FXO device (the tel-
ephone interface cannot be used as an FXS port). Support for the X101P
card has been dropped in favor of the TDM series of cards.
These cards (or their clones) SHOULD NOT be used in production en-
vironments. They are $10 on eBay for a reason.
The X100P/X101P cards are poor cards for production use due to their
tendency to introduce echo into your telephone calls, and their lack of
remote disconnect supervision. Do yourself a favor and don’t waste your
time with this hardware. You will find that if you ask the community
for support of these cards, many responses will be hostile. You have
been warned.

Determining the FXO and FXS Ports on Your TDM400P

Figure 4-1 contains a picture of a TDM400P with an FXS module and an FXO module.
You can’t see the colors, but module 1 is a green FXS module, and module 2 is an
orange-red FXO module. In the bottom-right corner of the picture is the Molex con-
nector, where power is supplied from the computer’s power supply.

Plugging an FXS port (the green module) into the PSTN may destroy
the module and the card due to voltage being introduced into a system
that wants to produce voltage, not receive it!

74 | Chapter 4: Initial Configuration of Asterisk