Media and Fax Gateways: FXS vs. FXO

As someone looking at IP-based fax solutions like RightFax, you may keep running into terms like FXS and FXO. To help understand the differences between the two, we've developed as you move through the I would search on Google to learn all I could about these terms but it seemed the more I read the less I understood. As a result I think it might be worthwhile to discuss how these interfaces are used and their relevance to an Open Text Fax Server implementation.

Why should we care about FXS and FXO in the first place? If you have looked at media gateways to install in your VoIP network you know that they will probably have a T1, E1, BRI, FXS, or FXO interface. These interfaces are used to bridge between your IP network and the PSTN. Most of us have heard and dealt with T1, E1, and BRI but may never have heard of FXS, FXO. While you may not have heard of these interfaces we use them every day. Let’s start with FXS. FXS stands for Foreign eXchange Subscriber and in the simplest sense is the jack on the wall you plug your phone into. When we plug an analog phone into an FXS port we should hear dial tone when we pick up the receiver. That is because we will receive battery. In the case of a media gateway this is where we could add an analog fax machine into our FoIP environment. I have included a graphic to illustrate an FXS port in a VoIP environment.

So what is FXO? FXO stands for Foreign eXchange Office and is the connection between your device and the telco. If you have a media gateway with FXO ports you would connect these ports to the jack on the wall that is connected to your telephone company. You might also connect the ports to your internal PBX. The FXO ports do not supply battery. We can think of the RJ11 jack on your analog phone or fax machine as an FXO port. I have included a graphic to illustrate how an FXO port might be utilized in a VoIP environment.


So, how specifically would FXS and FXO ports be utilized in an Open Text Fax Server environment? If you have analog phone lines and wished to utilize FoIP to send and receive your faxes you would purchase a media gateway that has FXO ports. The FXO ports would connect to your telco or PBX. When you sent a fax from the Fax Server it would connect to the media gateway via a SIP or h.323 connection. The gateway would then make a connection to the remote fax machine via the FXO port connected to your telco or PBX and the fax will be transmitted. How the fax is translated from IP packets to an analog signal is a discussion for another blog.