FXS or FXO?

[Tiago Santos]

Hi,

I am with the following question: My remote site uses Cisco CallManager Express with E1 for PSTN and IP phones. My question is at the central site where I need to integrate the PBX with a Cisco router 2801, for best operation, which interface to use? FXS or FXO?

Below is a picture of topology to help.

Thanks

Tiago

[Aaron Harrison]

Hi

Basically a choice of FXS or FXO is generally made for you:

- an FXS interface connects to a 'trunk' side connection on the PBX (i.e. one you would normally connect to a service provider, or an FXO in Cisco terms)

- an FXO interface connects to a 'line' side conection on the PBX (i.e. one you would normally  use to connect an analogue handset, or an FXS in Cisco terms)

- you can't connect an FXS to a line side interface or another FXS, and you can't connect an FXO to another FXO or trunk interface

But if the choice isn't already made, avoid FXO/FXS at all costs and install a digital circuit between the PBXs; such as an E1 or T1. Even if just running basic PRI, you will get lots of benefits:

- Far better reliability

- Greater number of channels (24 for T1, 30 for E1 -  compared to one channel per port for FXO/FXS)

- Better support for calling name, calling number etc.

Regards

Aaron

[Tiago Santos]

Thank Aaron,

But if I use FXO interfaces on the router and connects them into the PBX FXS interfaces, I can call an extension IP affiliate through a branch analog at the central site?

Tiago

[Aaron Harrison]

Hi

Absolutely - you'll be able to dial from the PBX to Cisco and from Cisco to PBX.

The problem would be that since it's not possible to send a 'dialled'/'called' number from the PBX to Cisco into an FXO port, it will work like this:

1) PBX has an extension assigned to it's FXS port

2) PBX 'rings' the FXS port

3) Cisco gateway detects the 'ring' signal on it's FXO port, and sets up a call to a predefined DN on the Cisco system - this is known as PLAR

The problem is that the FXO port must be configured to sent the call to a single DN; you can't dial any extension on the Cisco side as you would with a PRI.

You will also only get a single call through the FXS/FXO setup at once.

From the Cisco side, you send digits out the FXO and dial any extension on the PBX, or PSTN destinations (if the PBX is configured to allow this).

Regards

Aaron

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[Tiago Santos]

I understand it,
In case I use an FXS plugged in the PABX improve on something?

Tiago

[Aaron Harrison · Accepted solution]

If the Cisco gateway was FXS, and the PBX FXO, then the situation would be reversed:

PBX could dial any Cisco extension

Cisco could dial a preset PBX destination

All other restrictions would be the same.

Please rate helpful posts and mark answered questions that you've got a satisfactory response from to help identify useful content in the forums...
https://supportforums.cisco.com/docs/DOC-6212

[Tiago Santos]

Aaron,

Thank you for clarifying my doubts!

[ricardorojas123]

To complete the call flow (from the remote office IP phones to the analog telephones connected into PBX). It should add the gateway that is connected to the pbx on the CUCM (as h.323 GW), and create routes patterns with the numbers of the analog thelephones attached to the  pbx.

this is true?