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Cisco FXO/FXS connection

Posted on 2009-04-04
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Last Modified: 2012-05-06
This might seem like a silly question, but if I want to connect my Cisco 2811 gateway router to the PSTN through a FXO/FXS port, what am I PHYSICALLY connecting to?

In other words, do the cables go from the router's FXO port(s) to an Adtran? (if I have a T1 circuit that will handle voice traffic). A socket in the wall?

I know it can connect directly to a PBX, but I want to connect directly to the PSTN.
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Question by:AsenathWaite
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by:ehatchell
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A FXO/FXS port is an analog port, its for dialtone (its not a modem).
I'm not sure what role the Adtran is playing in this, and why you're talking about a PBX also, along with a T1 as your explanation is quite confusing.
Please clarify what you have in more detail, and please clarify your question so we can help.
Thanks.
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by:AsenathWaite
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It is my understanding that an FXS port is for things like analog phones, fax machines, etc.

Isn't the FXO port for a trunk connection to the PSTN?
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by:ehatchell
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An FXO device plugs into an FXS line...
FXS puts power on the line, FXO is powered by the line voltage from FXS...
Here's a good explanation from an Asterisk guide:
"The difference between an FXO channel and an FXS channel is simply which end of the connection provides the dial tone. An FXO port does not generate a dial tone; it accepts one. A common example is the dial tone provided by your phone company. An FXS port provides both the dial tone and ringing voltage to alert the station user of an inbound call. Both interfaces provide bidirectional communication (i.e., communication that is transmitted and received in both directions simultaneously)."
So, for your question, if you're wanting to take in a POTS line, you'll use FXO as the carrier will provide it.  If you're connecting to a PBX, it can go both ways so get out your meter and see if there is voltage. :)
Cheers.
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by:ehatchell
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This wasn't clear:  "So, for your question, if you're wanting to take in a POTS line, you'll use FXO as the carrier will provide it"
what I mean by provide "it" is power to make it work (on and off hook functions)
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by:AsenathWaite
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O.K. I think I am starting to understand--however, let's say the provider (PSTN) has provided me with a t1 line for voice communication (no data, just voice). A FXO card does not have an integrated t1 controller, so I assume that I would connect the FXO card to another intermediate device (maybe telephone company equipment, like an Adtran) yes?

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by:ehatchell
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You'll want to install a T1 card in your router... VWIC2-1MFT-T1 is probably the card you want.
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by:AsenathWaite
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Yes, but doesn't the t1 card have a different physical interface than the FXO (which has RJ11 connections)?
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by:ehatchell
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Yes, a T1 will more than likely come from your telco via a RJ48 (it'll usually be off of a smartjack)... and is 24 channels (or 23B and 1D)... an RJ11, is NOT a T1, a RJ11 will be an analog service.
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ged125 earned 50 total points
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You mentioned Adtran a couple times, do you already have one in place, if so, what is the model number?   The problem with FXO is that it only supports a single analog trunk where as a T-1 can do up to 24 as ehatchell has articulated.   Some models of ADTRAN allow you to insert modules to "break out" a single B channel into an analog port that you could hand off to the router.  However, ehatchell is also correct in recommending the  VWIC2-1MFT-T1 card as it would allow you to terminate the T-1 directly to the router.

One thing that he left out is that you will also need DSP resources in order to terminate all 24 (or 23 for PRI) B-Channels as voice capable.  Depending on the codec you use, the number of required DSP resources may vary.  There used to be  a DSP calculator on the web but I can't find a working link to it.

Hope that helps.
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by:ehatchell
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The DSP calc from Cisco is here:
http://www.cisco.com/cgi-bin/Support/DSP/cisco_prodsel.pl
You'll need your CCO login to access it. Ged125 is correct, DSP's will be needed if your connecting voice, so just use the calc and enter in the type of transcoding you'll be doing, etc. and also a voice capable IOS...
Cheers.
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by:ged125
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I tried that link before I posted.  Looks to be a broken link.  Are you able to get to it?
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by:ehatchell
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Yes, I am able to get to it, but you need a CCO login.
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by:ged125
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Is this issue still open?
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