What are FXS & FXO?
Posted by Elena Brambilla, Last modified by Daniel Lizaola on 22 November 2017 04:11 PM
In analog telephony there are two common types of interfaces: FXS and FXO. FXS stands for "Foreign eXchange Subscriber" interface is used to connect subscriber equipment such as telephones, modems and Fax machines. FXO stands for "Foreign eXchange Office" is used to connect to the Public Switched Telephone Network (PSTN) and can also be used to connect to a PABX or multiplexer FXS port. Another third interface, which we will not discuss here, is known as an E&M (Ear & Mouth) interface which is used to provide a leased line or tie-line interface connection between PABX systems. SmartNode products do not presently support the E&M interface.

An FXO device plugs always plugs into an FXS line. You cannot plug FXS into FXS, or FXO into FXO; it will not work.

FXS Information

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.

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

Typically FXS devices do not indicate when they want to clear a call down, they rely on the two parties noticing that the call has ended (through the other party saying goodbye or the line going quiet) and each end device clearing itself down.

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

FXO receives a call by:
  • 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.
Examples
  • 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
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