The acronyms FXS and FXO are widely used terms in the analog telephony industry. FXS is an acronym for Foreign Exchange Subscriber and FXO is an acronym for Foreign Exchange Office. FXS and FXO are interfaces for POTS (Plain Old Telephone Service).
A FXS is the unit that is mounted in the wall often referred to as the wall jack to which the FXO devices or the POTS devices can be connected. Using the FXO to interface with the FXS a call can be established through an analog network.
In simple terms, the FXO is the plug on the phone and the FXS is the plug in the wall. The difference here being that the FXO is used to connect to an office, a FXS is used to connect to a station.
A fundamental difference between the two is in the mode of operation followed by them. While a FXS acts as the transmitter the FXO acts as a receiver.
In its role as a transmitter a FXS interface provides the following primary services to a customer device:
- Battery voltage
- Ring Voltage
- Dial Tone
This interface connects to the local telecom service provider’s Central Office to make the above mentioned services available. The FXS typically makes available the dial tone to the subscriber when the FXO device is taken Off-Hook for placing the call.
The FXO interface on the other hand, is the plug that is available on the device. This forms a male female pairing with the FXS interface and completes the connection for the POTS service to be provided to the subscriber. The FXO communicates “Off-hook/On-Hook” status to the local telecom service provider’s Central Office. The “Off-hook/On-Hook” status is also known as the loop closure information.
Thus it is mandatory for a FXS interface to be connected to an FXO device for the analog telephone connection to work.
Although the two FXS & FXO work in tandem, there is a marked difference in some of the procedures and protocols followed by the two, during the call set up and call reception time.
FXS Call Set-up: The attached FXO device is presented with the ringer voltage during the call set up.
FXO Call Set-up: “off-hook” signal is passed on to the line
FXS Call Reception: Firstly the FXS device detects that the line has gone off-hook by the change in the voltage levels. It then receives the DTMF (Dual Tone Multi Frequency) signal from the FXO device. This signal contains the required information for the FXS to route the call as per the digits dialed through the device.
The FXS interface also doubles up as a power back up. In the event of local power failure, the FXO devices manage to operate using the 50 Volts DC power supplied by the FXS interface to the line.
FXO Call Reception: Either by sensing the ringer voltage that is passed on by the FXS device or by going into an “off-hook” state.
In the presence of a PBX (Public Branch Exchange), the PBX is required to have both the FXS and the FXO ports and use these interfaces to connect to the devices and the local telecom service provider’s Central Office.
Incase IP phone systems are required to be connected to the analog phone lines then a FXO gateway is required. This FXO gateway then permits one to connect to the FXS port to the FXO port of the gateway. In effect this type of a connection converts the analog phone line to a VOIP call.
An FXS gateway on the other hand, allows a person to connect multiple traditional PBX lines to a VOIP system.