Foregn eXchange Office (FXO)

FXO stands for Foreign eXchange Office. These are ports on a device like an Asterisk server that are connected to a phone line from a Central Office (CO). An important concept is that an FXO port accepts dialtone and ringing voltage.

Connect an FXO port to a Phone Outlet

From that definition and knowledge that an FXS port provides dialtone and ringing voltage, it becomes clear that you always connect an FXO port to an FXS port. You never connect a FXS port to a FXS port OR a FXO port to a FXO port. For reference, you can review FXS Lines.

RJ11 POTS line connector

A typical FXO 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

It is important that both ends of the communication agree on the signaling method otherwise the line will not work.

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

Since an FXO port accepts dialtone and ringing voltage and does not generate either, there is no requirement for an external power supply connection from the PC's power supply if your card only has FXO ports.

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Copyright July 2013 Eugene Blanchard