Analog ports FXS-FXO-E&M

Who can detaily explain where we should use FXS, FXO and E&M? What are the differencies between them?

For example: If two of WAN sides routers have got only FXS ports that directly connected to PBXs, can we not call other sides PBX outside? So we need at least one FXO port?

Hi Mkugu! Welcome to VoipUser.

You really don't want the detailed explanation of the difference between FXO and FXS, do you? Cisco do one of those nice 60 page white papers on it, perfect for leaving on a coffee table to blot up spilt coffee and impress visitors!

The practical difference is that an FXS card supplies line voltage to an analogue phone circuit. The FXO receives line voltage. Which means you cannot plug two FXS cards together (something breaks) or two FXO cards together (nothing happens).

To plug an FXS port into a PBX, one would normally attach it to a PBX trunk port (same ports as the PSTN lines are attached to). An FXO would attach to an extension port.

As the PSTN is expected to supply line voltage, you can only use an FXO port for direct connection.

E&M is a signaling protocol used in private ccts and connections to routers. The E stands for EAR and the M mouth
It comes in 2 and 4 wire alog variaties as well as a Digital version used mainly on T1 links. Cisco does use alog E&M connections to their routers. Of the Alog connections for PBX to Routers its the most reliable, On par with AC15 which is not as widely avalible outside the UK.

To not to get confused about FXS and FXO, I heard the following "definition":

* If you look into the wire, and see the "S"tation on the another end, it's FXS.

* If you look into the wire, and see ph"O"ne - it's FXO.

You will get used to...

Mike
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Hi Mike

Quote:

To not to get confused about FXS and FXO, I heard the following "definition": * If you look into the wire, and see the "S"tation on the another end, it's FXS. * If you look into the wire, and see ph"O"ne - it's FXO.

Well the first one makes sense the second one doesnt and would realy confuse.

Its far simple to understand the an FXS stands for "Foreign eXchange Station" Ie you connect a Station(Phone) to this port and FXO stands for "Foreign eXchange Office" Ie it connects to an central Office line.

Simple realy

Ian

I liked rgower's explanation best. And I expanded it in my mind like this: S is the power supply. O is zero power. Makes sense to me. All the other explanations only confuse me.

Now I know what to plug to what. The phone has zero power (FXO), so I plug it to the power supply (FXS). I even remember that it starts with FX because you have to be a Fone Xpert to understand it all. :lol:

Quote:

I even remember that it starts with FX because you have to be a Fone Xpert to understand it all

Ah the "FX" comes when trying to decipher the different makers qwerks. I will never forget trying to explain to Cisco TAC how to connect an E&M, As for some reason what they call the M is actually the E and vice versa . The same also went fo TX and RX on some G703 interfaces, Their reasoning was that it simplified it to label connections as what they are to connect to and not what they are !!!

Ian

Or you could just remember that telephones plug in to an FXS port?

may i know what is the signaling for fxo-fxs connection?
is it just loop-start?, ground-start? or wink start?

help is highly appreciated.. thanks in advaced..mwah! lolz

It depends on the system

3 wire systems are generally ground start, the circuit is activated by earthing and mostly obsolete.

Loop disconnect is probably the most common, so if unsure start with that. You won't break anything