Siemens SLI-5310-I IAD manual Definitions of FXS and FXO, Call procedure of FXS-FXO

Chapter 4: Web Configuration

Definitions of FXS and FXO

FXS (Foreign eXchange Subscriber) interface delivers POTS service from the local phone company’s CD and connect to subscriber equipment (e.g., phone, fax, modem.. and so on). That is, an FXS interface will point to the subscriber directly. And FXS interface can provide primary service of dial tone, battery current and ring voltage to a subscriber device.

FXO (Foreign eXchange Office) interface receives POTS service, from a central CO of PSTN. That is, an FXO interface will point to the Telco office directly. And FXO interface can provide primary service of on-hook/off-hook indication to the Telco network device.

The following illustration shows the hardware connection for the FXS and FXO interfaces.

Call procedure of FXS-FXO

The call procedure for FXS and FXO differs for each other. Refer to the following for reference.

	For the side of FXS:		For the side of FXO:
z	Initiating a call by	z	Initiating a call by
	Presenting ring voltage over the		Going off-hook to seize the
	line to the attached FXO device.		telephone line and
	(Notice that the FXS device		Dialing the dual-tone
	cannot pass dialed digits.)		multi-frequency (DTMF) digits that
			can identify the destination to be
			called.
z	Receiving a call by	z	Receiving a call by
	Detecting the line that has been		Detecting the ring voltage supplied
	seized (the attached		by the FAX device
	telephone-FXO device-has gone		and
	off hook)		Going off-hook to answer the call.
	and
	Receiving dual-tome
	multi-frequency (DTMF) digits
	indicating how the call should be
	routed.

Notice that the FXS devices will supply DC power to the line.

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