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. If you haven’t figured it out yet, this can be very frustrating. It took me about 2 months to “get it”. Maybe I can help you make sense of it all.
FXS – According to Wikipedia, FXS stands for foreign exchange station. 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.
FXO – According to Wikipedia, 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”.
FXS ports are often used on ATA (Analog Telephone Adapters) such as the Linksys PAP2T or Grandstream HT502. The FXS port connects the analog telephone to the ATA which connects to the internet via a Router or modem. Some ATA’s also have an FXO port. This connects to the wall jack by the computer to provide failover. In this case, in the event of internet failure, 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.
FXO ports are most often used on Gateways and PBX’s to support Failover or Fallback as mentioned above. Most critical phone systems should have a failover feature so phone calls or faxes may continue in the event of internet failure.
So you see, FXS and FXO are super easy, as long as you don’t rely on their given names for an explanation!