4.3. FXO and FXS Channels
The difference between an FXO channel and an FXS
channel is simply which end of the connection provides the dial
tone. An FXO port does not generate a dial tone; it accepts one. A
common example is the dial tone provided by your phone company. An
FXS port provides both the dial tone and ringing voltage to alert
the station user of an inbound call. Both interfaces provide
bidirectional communication (i.e., communication that is
transmitted and received in both directions simultaneously).
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 that 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
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.
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The older 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 telephone interface cannot be used as an FXS port).
Support for the X101P card has been dropped in favor of the TDM
series of cards. Use of these cards (or their clones) is not
recommended in production environments.
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4.3.1. 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 connector, where power is supplied from computer's power
supply.
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Plugging an FXS port (the green module) into the
PSTN may destroy the module and the card!
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Be sure to connect your computer's power supply
to the Molex connector on the TDM400P if you have FXS modules, as
it is used to generate the voltage to produce ringing on the phone.
The Molex connector is not required if you have only FXO
modules.
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