Editor’s Note: This article was originally posted in August 2010 and has been fully updated for accuracy and comprehensiveness in October 2015.
If you are researching into deploying a VoIP Telephony System on your own for the first time, you are probably seeing FXS and FXO acronyms all over the place. When you’re new to VoIP and learning it can be very difficult. Hopefully I can help to make a little sense of FXS/FXO for you.
Let’s start with FXS Ports
FXS stands for foreign exchange subscriber. Since that is so clear and makes so much sense I can stop there right? Not! So clearly the actual meaning of these words isn’t going to help us remember or understand what they mean.
FXS is an RJ11 port that connects internally to an analog office phone or fax machine. Think of the S as meaning a station or a cubicle. Any FXS port is going to connect to an analog DEVICE and the cable from the port to the device will never leave the building.
Now, on to FXO Ports
FXO stands for foreign exchange office designates a telephone signaling interface that receives POTS (plain old telephone service).
Um… ok. Let’s put it this way, FXO is a port that will connect a device to an outside telephone line. Think “O” for “Outside”. Picture an RJ11 wall jack that connects to a box in your basement which is connected to the line from your house to the nearest telephone pole on your street. Any RJ11 port on an device that is headed towards that wall jack is an FXO port. It connects your device to the “outside” world or your local area telephone “office”.
How about some visual representation?
I know for myself if I can visually see something it really helps to understand a lot better. Check out the video and hopefully it clears up the info above.
In the event of failure, the most critical phone systems should have a failover feature so phone calls or faxes may continue during the down time. You can still make telephone calls via the POTS line. Also, you can use the FXO port to make free local calls on your POTS line.