What’s the difference between FXS and FXO?

It’s easy to get lost in the confusion of FXS and FXO, especially if you’re a complete novice in matters of VoIP (Voice over Internet Protocol). These terms are often used in the telecom world, and understanding their difference is the first step in choosing the right VoIP system that best suits your needs. Here’s what you need to know.

FXS and FXO are the interfaces for analog telephony, also called POTS (Plain Old Telephone Service). Using these interfaces enables a call to be established - the ports provide the necessary electricity, dial tone, and call signal.

What is an FXS port?

FXS stands for Foreign Exchange Subscriber, a port that connects the router or access server to end-user equipment such as office phones, fax machines, or modems. In other words, it is a plug on the wall that delivers dial battery, loop current, and ringing voltage to the device, so that the analog signal can be transmitted.

What is an FXO port?

FXO stands for Foreign Exchange Office, a port on the end communication device, such as an office phone or fax machine. The FXO connects the device to the FXS port, as well as to the outside telephone line, requesting the dial tone needed to initiate a call.

How the connection works

In order for a call to work, a telecommunication line from an FXO port must be connected to an FXS port, and vice versa. The process for making calls is simple: when your FXS and FXO ports are connected, you will receive a signal from the telephone company through the FXS port in the wall. This signal is then transmitted to the FXO port connected to the device so that, when you pick up the phone, you hear the dial tone. Then you dial the phone number, which is passed as Dual-Tone Multi-Frequency (DTMF) digits to the FXS port, allowing you to make the call.

When you’re receiving an inbound call, on the other hand, the FXS port receives the call, then sends a ring voltage through the FXO port to your end device. The phone will ring, and you can pick it up to answer the call.

FXS, FXO and VoIP

The signal transmission process becomes more complex when you implement an additional network element, such as VoIP gateway. You need an FXS gateway to connect one or more lines to a VoIP system or provider. You’ll also need an FXO gateway to connect the VoIP system with analog phone lines, and to translate the analog phone line to a VoIP call.

In summary...

• FXS is a plug on the wall, and FXO is a plug on the phone
• FXS provides the dial tone, and FXO requests it
• FXS is a port that receives a call; FXO is a port that initiates it

Want to learn more about FXS, FXO, or VoIP? Give us a call and our telephony experts will be happy to help.