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What is the difference between the FXS and FXO voice cards?

I have just inherited a network which contains a small group of sites using VoIP. They are using Cisco 1750s with the VIC cards and Frame Relay. Some of the routers contain the FXS VIC cards and some contain the FXO cards. The documentation I have found explaining the differences is unclear to me so I would appreciate a detailed explanation of when you would use each.


Re: What is the difference between the FXS and FXO voice cards?

There's not much to explain except

1. FXS cards in a vic slot in cisco router represent an analog telephone line

rj11 jack/socket, like pacbell, at&t or BT terminates at your house. You can plug a telephone direct to it

and whenever you pick up your phone itgenerates a dialtone to prompt you todigit collection readiness. It also gerates a ring voltage that ring you phone to alert

you of an incoming calls.

The number assigned to it can be configured on the router as explained in the url.

2. FXO port in the other hand are used to simulate a phone at a PSTN/PBX fxs line termination. The router can make calls out this port to

the pstn/pbx via the fxo port. When a number is dialed say a PSTN number and the proper configuration is inplace to route pstn calls out the fxo port.

The port goes off-hook and get a dial-tone from the pstn/PBX, it then pulse out the dialed digit from an FXS analog phone or contained int the

h225 call set received over ip.

The FXO only connect to PSTN/PBX fxs port. It does not generated dialtone itself and can't plug a phone to it.

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Re: What is the difference between the FXS and FXO voice cards?


the 'S' in FXS stands for station, meaning telephones or other client end units such as fax machines. FXO are trunks, so you use them to connect to either the public telephone switch (PSTN) or to another telephone system in your network.

In other words, you use the FXO ports between your frame relay network and your FXS ports for telephones in your branch offices. Also, remember that FXS ports are used for analog devices, so you don't put Cisco IP phones on them.

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