Definition

FXS

Also known as: Foreign Exchange Subscriber, FXS port

FXS (Foreign Exchange Subscriber) is the interface that supplies dial tone, loop current, and ring voltage to an analog device — the side of the connection a telephone plugs into.

How it works

An FXS port behaves like the carrier's central office: it provides DC loop current, generates dial tone, applies ring voltage, and detects off-hook and DTMF from the attached device. In a POTS replacement appliance, FXS ports reproduce that behavior locally so analog equipment sees what it expects.

Why it matters

FXS port count is the practical measure of how many analog lines an appliance can replace. A device with two FXS ports can serve two analog circuits, regardless of how much data throughput it has — which is why FXS count, not bandwidth, drives sizing.

Common use cases

Connecting an analog telephone to a replacement appliance
Terminating a fire alarm panel's dial-out line
Sizing appliances against a site's analog line count

When sizing a deployment, FXS port count is the number that matters. Throughput rarely constrains an analog replacement; available ports almost always do.

Common misconceptions

“FXS and FXO are interchangeable.”

They are opposite ends of the same link. FXS provides dial tone; FXO consumes it. Connecting two FXS ports together, or two FXO ports, will not work.

Frequently asked questions

How many FXS ports do I need?

One per analog circuit you are replacing. Count the physical devices that need their own line — each alarm panel, elevator phone, fax machine, and terminal.

Does an FXS port provide power during an outage?

The port supplies loop current to the attached device, but the appliance itself needs power. That is why integrated battery backup matters for life-safety circuits.