FXS is what is most commonly known as Plain Old Telephone Service (POTS). It is what your local phone company delivers to your home on a twisted pair. In other words, FXS looks a line from the telephone company switch (PSTN); it hooks to a telephone.

FXS interfaces provide to the subscriber:

• Battery current and ring voltage
• Dial tone (knows when to give dial-tome (seizure) when it sees current flowing from an FXO port closure.
• Optional: CallerID (both caller number and name)
• Optional: Call Waiting / Call Waiting ID
• Optional: Message waiting indicator

FXS interfaces receive:

• Hook Flash (to be notified of features, e.g, to set-up a three-way conference call or toggle between two incoming calls)
• DTMF (touch tones)

FXS "alterts" an incoming call by:

• Presenting ringing voltage to the line (attached device) – just like a PBX it does not and cannot pass any dialed digits.

• Loop closure - Identifying that the line has been seized by the attached telephone going off hook. It can then receiving dialed digits (via DTMF).

FXO Information

Your telephone is an FXO device and it connects to the FXS of the telephone company. Your phone provides on-hook/off-hook indication (loop closure) to the phone company. This is why you get a dial-tone when you pick up the phone.

FXO interfaces provide:

• onhook/off-hook indication (loop closure)
• HookFlash (to request features of PBX or PSTN, e.g., three-way conference calling) A quick loop closure or wink which is about a quarter of a second.
• DTMF (touch tones)

FXO interfaces receive:

• Dial tone as an indication from the FXS port that it achknowldeges the loop-closure.
• Optional: Ring indication (voltage to ring the phone)
• Optional: CallerID (both caller number and caller name)
• Optional: Call Waiting Indicator (tone indicating a second incoming call)
• Optional: Call Waiting ID (Caller ID of second incoming call)
• Optional: Message waiting indicator (blinking light to indicate voice mail)

FXO makes a call by:

• Seizing the telephone line (going off hook)
• Dialing DTMF digits to identify the destination to call
• Hanging up at the end of the call

• Identifying when ringing voltage is being supplied by the PBX / CO switch (ringing the telephone)
• Answers the call by “going off hook”. Call is then connected.

• A standard analog (plain old telephone) is FXO
• PBX/Switch lines from a PBX (that drive current) that you plug analog phones into are FXS
• The PBX analog ports lines that plug into the CO are FXO
• The SmartNode 4110 Analog VoIP Media Gateway provides up to eight FXS or FXO ports