System information

The station is responsible for:
• Providing a ringer (or at least being able to handle ringing voltage in some manner)
• Providing a dialpad (or some way of sending DTMF)
• Providing a hook switch to indicate the status of the line
A Foreign eXchange (FX) port is named by what it connects to, not by what it does. So,
for example, a Foreign eXchange Office (FXO) port is actually a station: it connects to
the central office. A Foreign eXchange Station (FXS) port is actually a port that provides
the services of a central office (in other words, you would plug an analog set into an
FXS port).
It is for this reason that the signaling settings in the Asterisk config files seem backwards:
FXO ports use FXS signaling; FXS ports use FXO signaling. When you understand that
the name of the physical port type is based on what it connects to, the signaling names
in Asterisk make a bit more sense: if an FXO port connects to the central office, it needs
to be able to behave as a station, and therefore needs FXS signaling.
Note that changing from FXO to FXS is not something you can simply do with a settings
change. FXO and FXS ports require completely different electronics. Most analog cards
available for Asterisk use some form of daughter card that connects to the main card
and provides the correct channel type, meaning that you have some flexibility in de-
fining what types of ports you have on your card.
Analog ports are not generally used in medium to large systems. They are most com-
monly used in smaller offices (less than 10 lines; less than 30 phones). Your decision
to use analog might be based on some of the following factors:
• Availability of digital trunks in your area
• Cost (analog is less expensive at smaller densities, but more expensive at higher
densities)
• Logistics (if you already have analog lines installed, you may wish to keep them)
From a technical perspective, you would normally want to have digital rather than
analog circuits. Reality does not always accommodate, though, so analog will likely be
around for a few more years yet.
Digital telephony
Digital telephony was developed in order to overcome many of the limitations of analog.
Some of the benefits of digital circuits include:
• No loss of amplitude over long distances
• Reduced noise on circuits (especially long-distance circuits)
• Ability to carry more than one call per circuit
• Faster call setup and teardown
PSTN Circuits | 135