DEMISTIFYING FXO/FXS

FXO and FXs are analog interfaces that will continue to be used for some time to come.

You will come across these interfaces when connecting telephone systems to
the PSTN and also integrating IP PABX with analog PABX.

FXS - Foreign Exchange Station
FXO - Foreign Exchange Office.

Connecting Analog PABX to IP PABX

Most of the time, Voice Engineers will encounter a scenario to integrate an Analog PABX
with an IP PABX, the first questions to answer, is which card will i need, FXO or FXS?
The Answer is, u can either FXO or FXS card depending on how you want the setup to be.

1st scenario
You want the PABX users to be able to access all the extensions within the IP PABX,while the users within the IP PABX access the PABX users through the receptionist.

2nd scenario
You want the IP PABX users to be able to access all the extensions within the Analog PABX while the users within the Analog PABX access the IP PABX users through a receptionist.

3rd Scenario
You want both the IP PABX users and analog PABX users to be able to dial the extension numbers for all users within either PABX systems.



Solution
1st scenario, Connect FXS card on the IP PABX and FXO card on the Analog PABX. Configure a Calling prefix out through the FXO cards on the analog PABX to allow users access a secondary dial tone on the IP PABX, after which, the users will be able to dial any extension within the IP PABX.

For calls coming to the analog PABX, calls to the FXO port will be configured to ring on the
receptionist phone by the use of "connection plar settings"



2nd Scenario, Connect the FXS card on the Analog PABX and FXO card on the IP PABX.
Configure an outside calling prefix for the IP PABX users, to be directed out the FXO
port connecting to the analog PABX. This will provide the IP PABX users with a second dial tone, after which they will be able to dial any extension on the analog PABX users.

Calls landing on the FXO port from the analog PABX will be directed to a reception by the
use of "connection plar" settings.

3rd Scenario, Connect both FXO and FXS cards on both PABX, configure the calls to ensure that outgoing call for any PABX goes out through an FXO card.



points to note for FXO/FXS CARDS

FXS sends a dial tone, FXO receives a dial tone
FXS sends caller ID, FXO receives caller ID

For IP PABX and Analog PABX connections, its advisable to use ground-start signaling
for the voice interfaces.

Please note, When signaling is changed on the voice card interfaces, the ports must be restarted using shutdown/no shutdown commands for the configuration to take effect.

Failure do this will result in erroneous behavior, not even restarting the router/gateway can solve this.

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