Explaining 'FXS' and 'FXO' in VoIP Equipment

By Ed Silverstein, TMCnet Web Editor

When thinking about getting a VoIP Telephony System – there are many things to consider. One of the most basic things is understanding what all those acronyms mean.

Christina Smith, writing in a recent blog post for voip insider (for VoIP Supply) says newcomers to the field are probably seeing lots of “FXS” and “FXO” acronyms.

She admits it took her about two months to make sense of it all.

She explains that FXS, according to Wikipedia, stands for “foreign exchange station.” She adds that FXS is an RJ11 port that connects internally to an analog office phone or fax machine. The “S” means 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, she explains.

“FXO,”according to Wikipedia, means a foreign exchange office, which designates a telephone signaling interface that receives POTS, also known as plain old telephone service. FXO is a port that will connect a device to an outside telephone line, she adds. Think of the “o” for outside. The RJ11 wall jack 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, she adds. It connects your device to the outside world or your local area telephone office, she says.

FXS ports are often used on ATA, Analog Telephone Adapters, such as the Linksys (News - Alert) 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 will also have an FXO port. This connects to a 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, she explains. Users can employ the FXO port to make free local calls on your POTS line, she adds.

FXO ports are most often used on Gateways and PBXs to support failover or fallback, she said. Most critical phone systems should have a failover feature so phone calls or faxes can continue in the event of internet failure.

(source: http://www.tmcnet.com/channels/voip-equipment/articles/98076-explaining-fxs-fxo-voip-equipment.htm)