FXS and FXO are the interfaces for analog telephony also called POTS (Plain Old Telephone Service). The expression was initially meant a joke but is used as common expression nowadays. FXS (Foreign Exchange Station) means the wall jack or the interface to the telephone system which FXO devices can be connected to. Using these interfaces a call can be established. The port provides the necessary electricity as well as the dialtone and the call signal. FXO (Foreign Exchange Office) is the port that receives the analog line on the telephone or fax machine. It establishes the connection to the analog line (FXS). FXS devices connected to phone system are referred to as endpoints (e.g. a phone).
Analog phone plugs are always pairs and appear as male or female plugs. 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 the phone system is used, the analog lines provided by the telephone company, are connected to the phone system. The phones on the other hand are connected to the phone system. Many traditional phone systems provide both, FXS and FXO ports. Many IP phone systems require additional gateways to provide FXS or FXO ports.
A phone call coming in via an analog line is initiated like this: Pick up the phone of the FXO device. The FXS port on the corresponding sides recognizes that the call should be established and expects the number, encoded as DTMF signals.
An incoming call works like this: The FXS port provides the necessary electricity. The phone starts ringing. By picking up the phone, the call is established.
When purchasing a phone system, it needs to be considered home many and analog ports are required eventually. 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.
Analog gateways are used to provide IP phone systems (PBXs) with analog ports. The gateway, provides the physical ports. It establishes a connection to the PBX using the local network. This way the IP phone system behaves the same way as an traditional phone system with analog ports.
Analog interface cards of doing the same job as analog gateways. In difference to gateways, they are built into the phone system instead of transferring data via the network.
AskoziaPBX IP phone systems are also available with built in analog FXS and FXO ports.