How to understand and configure analog FXO or FXS
Resolution
FXS Foreign eXchange Subscriber interface (the plug on the wall) delivers
Plain Old Telephone Service (POTS) from the Central Office (CO) of
the local phone company and must be connected to subscriber equipment,
such as telephones, modems, and fax machines. In other words an FXS
interface points to the subscriber. An FXS interface provides these
primary services to a subscriber device:
Dial Tone
Battery Current
Ring Voltage
The FXS acronym can also be rendered as Foreign eXchange System.
FXO Foreign eXchange Office
interface (the plug on the phone) receives the POTS, typically from a CO
of the Public Switched Telephone Network (PSTN). In other words, an FXO
interface points to the Telco office. An FXO interface provides this
primary service to the Telco network device:
on-hook/off-hook indication (loop closure)
A registered jack (RJ)-11 connection is used in order to connect to
an FXS interface, such as the PSTN or a Private Branch eXchange (PBX).
This is the interface a standard telephone set provides. It uses only
two wires (tip and ring) for signaling and audio. It employs a relay
that is closed for off-hook and open for on-hook. FXO ports on Cisco IOS
Software platforms can support both loop-start and ground-start
signaling. Ground-start signaling is used primarily on trunk lines or
tie lines between PBXs.
An RJ-11 connection is used in order to connect common line side
equipment such as a phone. It uses only two wires (tip and ring) for
both signaling and the audio path. It supplies direct current, loop
current, dial-tone, and alternating current ring voltage. It must
connect to an FXO interface with loop-start or ground-start signaling.
Standard residential phone lines are configured as FXS loop-start.
Loop-start is the default signaling on Cisco IOS FXS and FXO voice ports. In order to change it, issue the signal ground-start voice-port command. Reset the voice-port after any changes with the shutdown/no shutdown command sequence.
Refer to these documents for more information:
Refer to Voice Network Signaling and Controlfor more information about loop-start and ground-start signaling.
Refer to the Configuring FXO or FXS Voice Ports section of Configuring Voice Portsfor information on how to configure FXS and FXO signaling.
Common issues with the analog FXO and FXS circuits
