User guide

A Foreign Exchange Station (FXS) is what is on the other side of a traditional telephone line. A FXS
delivers the dial tone and ring tone to the phones. In analogue lines, an FXS supplies the ring tone and
the power to the phones to work. To give you an idea, an FXS (telephone line) provides around 48 Volts
DC to the phone during conversation, and up to 80 Volts AC (20 Hz) when generating a ring tone.
A PBX that integrates FXO and FXS interfaces can connect to the PSTN or host telephones.
Telephone lines coming from an operator need to be connected to an FXO interface. Telephone(s) in
your office must be connected to the FXS interface(s) of the PBX.
Two simple rules that you should remember are:
1. An FXS needs to be connected to an FXO (like a telephone line needs to be connected to a
phone) or vice versa.
2. An FXS supplies power (active) to the phone FXO (passive).
a) A phone is a FXO connected to a FXS
(the PSTN)
b) A PBX can host both FXO's and FXS's.
c) An ATA acts as a FXS when connecting a
phone (FXO).
Image 1: a) A phone is a FXO connected to a FXS (the PSTN) b) A PBX can host both FXO's and FXS's. c) An
ATA acts as a FXS when connecting a phone (FXO).
4.3.1 Analogue signalling
A set of “signals” are sent every time you use a telephone line to provide feedback and status
information to the user. Among these signals are the dial and busy tones, the ring, and the on and off-
hook status. These signals are transmitted between the FXS and FXO using a signalling protocol.
Page 11 TRICALCAR | www.wilac.net/tricalcar – Version: February 2008