[Asterisk-Users] FXO vs FXS question

[Jim Van Meggelen]

The term "FX" refers to an old telecom service that provided a "Foreign
eXchange" circuit - basically an analog extension that the telco would
terminate at a remote location and connect back into your PBX.

The terms "FXS" and "FXO" came to refer to the respective ends of the
circuit, where "O" stands for "Office", and "S" stands for station.

The confusion that stems from all this is that cards which provide an
"FX" interface (basically an analog telephone interface) are named by
what they *connect* to, not by what they *do*. So, a card that you would
plug an analog phone into would be an FXS card, because it is connected
to a station. If you think about it, this can be confusing because an
FXS card actually needs to exhibit the behaviour of a central *O*ffice.
This is why, for example, a Zaptel FXS card uses FXO signalling. It
*connects* to a station, but it must *behave* like (or provide the
functions of) a central office.

Obviously, an FXO card is the exact opposite. An FXO card needs to
*connect* to a central office, but in order to successfully do so it
will need to *behave* like a station. A modem is a classic example of an
FXO device, so is a loop start trunk card in a PBX or KSU.

To summarize:

An FXS card emulates a central office. It:
- *connects* to a *S*tation
- provides dial tone
- supplies ringing voltage, loop current, battery, etc.
- receives touch tone digits
- sends CLID
- etc.

An FXO card emulates a telephone. It:
_ *connects* to an *O*ffice
- requests dial tone
- receives ringing voltage and is powered from the far end
- generates touch tone
- captures and interprets CLID
- etc.

The Terms FXS and FXO, when they refer to HARDWARE, always follow the
above naming convention. Be forewarned, however, that various software
may not follow this convention at all. In Asterisk, for example, an FXO
card is controlled by signalling=fxS_ks (et al), and an FXS card is
controlled by fxO_ks (et al). But other systems may not have such logic.
In another device, you may have a bunch of FXO commands that do indeed
program the FXO interface.

The trick for Asterisk is to understand that an FXS (which connects to a
*S*tation) needs to signal using FXO (because it needs to behave as an
*O*ffice), and vice versa.

Enjoy.

Cheers,

--
Jim




> -----Original Message-----
> From: asterisk-users-bounces at lists.digium.com 
> [mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of 
> Wolf Paul
> Sent: October 16, 2004 1:06 PM
> To: asterisk-users at lists.digium.com
> Subject: [Asterisk-Users] FXO vs FXS question
> 
> 
> Hello all,
> 
> as a relative newbie I obediently ordered the recently 
> recommended "Newton's Telecom Dictionary" but it did not 
> provide the answer to one of my question. Perhaps one of you 
> can do so.
> 
> What is the difference in functionality between an FXO and an 
> FXS port? I know that the line from telco goes into an FXO 
> and standard POTS phone goes into an FXS, but since both 
> types of ports must be able to both make and answer calls, 
> what is the actual functional difference?
> 
> Or in other words, what does an FXO to FXS converter do (I 
> have seen some mention but no explanation of those)?
> 
> Thanks.
> 
> Wolf Paul
> 
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