FXO, FXS confusion

Hello all,

I’ve heard about Asterisk for a long time, but I never really read much into it until someone passed me a link to a Kevin Rose’s video on Asterisk. With the additional information that the video has given me, I have a clearer picture now and would like to start implementing an Asterisk system at home.

For starters I’d like to just setup a PSTN line -> Asterisk -> Analog Phone.

I understand that FXO = Foreign Exchange Office and FXS = Foreign Exchange Station.

From reading up, FXO basically is the line between your PSTN network and your server and FSX is the line from your server to your Analog Phone.

So if I’d like to have Asterisk process all my incoming calls and then re-route them to my analog phone, I’d need a FXO and FXS card connected to my Asterisk server?

And that if I only have an FXO port connected to my server, all my incoming call will be processed by Asterisk and just end there?

Is this true?

You pretty much hit the nail on the head. If you want the call to be able to “do something” you would either need an ATA to convert the digital call back to an analogue phone, check this out voip-info.org/wiki-ATA
Or you would need to buy some VoIP phones, you could get away with the el-cheapo grandstream phones (I have one and don’t like it) or go with something a little better, such as the Polycom 301 (I have 15 of them, love them)

As for ATA’s go, I have a SPA-2002 and it works well for the job I use it for, however if I was planning on using an ATA on the road, I would go with an IAXy, the IAX protocol is much easier to use when it comes to things like NAT.

Hope something in there was useful.


also keep in mind that an ATA is a sip device wtih two FXS ports. You can get a similar effect for more money and less setup by buying cards with FXS ports on them.

Also each individual extension needs its own FXS port. You can plug two phones into one port, but if one of them is talking and you pick up the other one it will intrude on the conversation. If each phone is plugged into its own FXS port then each phone gets its own dialtone etc.

hope that helps