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FXO/FXS PBX System
FXS and FXO are the names of the ports used by analog phone lines (also known as POTS – Plain Old Telephone Service) or phones. The expression “POTS” was initially intended as a joke but is now used as a common expression in the telecommunications industry.
FXS (Foreign Exchange Subscriber) is the port that actually delivers the analog line to the subscriber. In other words, it is the “plug in the wall” that delivers a dial tone, battery current and ring voltage. This is the jack or interface to the phone system which FXO devices can be connected to.
FXO (Foreign Exchange Office) is the port that receives the analog line. It is the plug on the phone or fax machine or the plug(s) on your analog phone system. It delivers an on-hook / off-hook indication (loop closure). Since the FXO port is attached to a device, such as a fax or a phone, the device is often called the “FXO device.” This port establishes the connection to the analog line (FXS).
FXO and FXS are always paired, i.e similar to a male / female plug. If no phone system is used the telephone (FXO) is directly connected to the FXS port. The port is provided by the telephone company.
If you have a PBX, then you connect the lines provided by the telephone company to the PBX and then the phones to the PBX. Therefore, the PBX must have both FXO ports (to connect to the FXS ports provided by the telephone company) and FXS ports (to connect the phone or fax devices). Many traditional phone systems provide both, FXS and FXO ports while many IP Phone systems require additional gateways to provide FXS or FXO ports.
FXS & FXO & VoIP
You will come across the terms FXS and FXO when deciding to buy equipment that allows you to connect analog phones to a VoIP Phone System or traditional PBXs to a VoIP service provider or to each other via the Internet.
If you want to connect an analog fax machine to the phone system, you will need at least one FXS port. If you want to use analog telephone lines with your phone system, you need one FXO port per line.
