FXS vs FXO

Most people nowadays, especially those who are new to the VoIP / Asterisk / IP PBX industry often confuse an FXS for FXO, and vice-versa. These two module types are most commonly heard in Asterisk setups. The FXS and FXO are actually the interfaces or ports in analog telephony cards.

FXS Modules

FXS stands for Foreign eXchange Subscriber/System. FXS ports connect to the subscriber’s equipments – analog telephones, fax machines, modems, etc. to an IP network. FXS ports will deliver battery power, dial tone and ringing to the equipments. So if you have 15 analog telephones that you want to connect to your VoIP setup, you will need 15 FXS ports or modules. The FXS interfaces uses FXO signaling.

Important: FXS modules have a 1:1 ratio for analog devices (standard telephones with RJ-11 ports and FAX machines). This means every one analog device must have an equivalent FXS module assigned to it. Also, the power connector found on base cards should be connected to your power supply when using FXS modules.

FXO Modules

FXO stands for Foreign eXchange Office. FXO ports connect the Plain Old Telephone Service (POTS) or the telephone company’s line to the PBX system. FXO ports connect your computer to Public Switched Telephone Network (PSTN) telephone lines. An FXO adapter makes use of your telephone company’s subscription line to provide telephony functions to your computer – this is also a cheaper alternative to expensive VoIP systems.

FXO modules receive the signal coming from your POTS and transmit it to your network of IP telephones and VoIP applications.

It is very crucial to plug in the user equipments and telephone onto their proper FXS/FXO ports as they may irreparably damage the modules.