Foregn eXchange Service (FXS)

FXS stands for Foreign eXchange Service. I like to think of it as Foreign eXchange Set instead - set being short for phoneset or phone. That makes it an easy reminder as to what you should connect to a FXS port.

Plain Old Telephone Set (POTS)

FXS is an analog signaling port and you connect using dial-up POTS (Plain Old Telephone System/Set) phone. An important concept about FXS ports is that they provide Ringing Voltage and Dialtone. Ringing Voltage provides the voltage to ring the bell in the old POTS phone and can be quite large. Large enough to give you a good shock!

RJ11 POTS line connector

A typical FXS port uses an RJ11 connector which looks like the little 6 pin brother of an RJ45. Usually only two pins/wires are used: the center pair color-coded red (ring) and green (tip). Sometimes a second pair is used black/yellow to provide a second FXS line.

Signaling Methods

There are 3 main signaling methods used to initiate a call. Initiating a call is telling the Central Office (CO) that you want to make a call. The methods are:

  • Loop Start (ls) - When the handset goes off hook (lift the handset), a switch is closed and current flows - this informs the CO that a call is being initiated. This is a simple method, does not require a common ground but doesn't provide far end disconnect information (doesn't sense that the far end has disconnected well)

  • Ground Start (gs) - When the handset goes off hook (lift the handset), the FXS device momentarily grounds the ring. The CO responds by grounding the tip. The FXS device release the ring ground. Requires both FXS device and CO to have a common ground. Does provide far end disconnect information.

  • Kewl Start (ks) - is an extension of Loop start signaling which adds disconnect supervision through the use of an Open Switching Interval (OSI). The CO (central office) signals the end that the distant party has hung up by removing battery voltage from the loop for about 250 ms, within 6 seconds after the far-end party disconnects. This is the prefered signaling method for Asterisk

PC FXS card's Power Connection

You can purchase legacy analog cards for PCs for your VoIP PBX that provide FXS, FXO or a combination of both.

Legacy Analog Cards with both FXS (green Modules) and FXO (red modules) ports

FXS ports provide dialtone and ringing voltage. In order to do this, the PC FXS cards will need an extra power supply connector attached to provide unregulated +12V. Usually they accept a molex connector from the computer power supply. If the external power is not connected, the card will fail and you should see an error message during boot-up which will state something to the effect "Did you forget to connect the power cable?".

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TelecomWorld 101

Copyright July 2013 Eugene Blanchard