Posted by: Mudassir Ali | May 26, 2010

How to understand and configure analog FXO or FXS

How to understand and configure analog FXO or FXS


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


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