System information

Traditional PSTN Trunks
There are two types of fundamental technology that phone carriers use to deliver tel-
ephone circuits: analog and digital.
Analog telephony
The first telephone networks were all analog. The audio signal that you generated with
your voice was used to generate an electrical signal that was carried to the other end.
The electrical signal had the same characteristics as the sound being produced.
Analog circuits have several characteristics that differentiate them from other circuits
you might wish to connect to Asterisk:
• No signaling channel exists—most signaling is electromechanical.
• Disconnect supervision is usually delayed by several seconds, and is not completely
reliable.
• Far-end supervision is minimal (for example, answer supervision is lacking).
• Differences in circuits means that audio characteristics will vary from circuit to
circuit, and will require tuning.
Analog circuits that you wish to connect to your Asterisk system will need to connect
to a Foreign eXchange Office (FXO) port. Since there is no such thing as an FXO port
in any standard computer, an FXO card must be purchased and installed in the system
in order to connect traditional analog lines.
‖
FXO and FXS
For any analog circuit, there are two ends: the office (typically the central office of the
PSTN), and the station (typically a phone, but could also be a card such as a modem
or line card in a PBX).
The central office is responsible for:
• Power on the line (nominally 48 VDC)
• Ringing voltage (nominally 90 VAC)
• Providing dial tone
• Detecting hook state (off-hook and on-hook)
• Sending supplementary signaling such as caller ID
‖ You would use the exact same card if you wanted to connect a traditional home telephone line to your Asterisk
system.
134 | Chapter 7: Outside Connectivity