# FXS

> 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.

- Also known as: Foreign Exchange Subscriber, FXS port
- Last reviewed: July 29, 2026

## 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.

## Counting ports during an inventory

Because port count is the sizing constraint, the inventory has to count circuits rather than
devices or bills.

Three counting errors recur. **A device with multiple lines counts as multiple circuits** — an
alarm panel configured for a primary and secondary dial-out path needs two, and a multi-line
fax or key system needs one per line in service. **A shared line still needs its own port** in
most designs, because dedicating a port per endpoint avoids reproducing line-seizure behavior on
a shared one. **Lines nobody uses may still need to be counted** until someone confirms what is
on the far end; a circuit with no known purpose is an unfinished inventory item, not a
disconnect candidate.

Count on site, not from invoices. Billing records describe accounts rather than physical
circuits, and the routine outcome of a physical walk is finding equipment that appears nowhere
in the records.

Then size with margin. Adding a port later means a hardware change; ordering one spare per site
rarely does.

## Common misconceptions

**Claim:** FXS and FXO are interchangeable.

**Correction:** 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.

## Sources

- [Copper Retirement](https://www.fcc.gov/modernizing-telecommunications-networks-what-government-officials-need-know) — Federal Communications Commission


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Canonical page: https://dataremote.com/newsroom/definitions/fxs
Organization: DataRemote, Inc. — POTS line replacement, cellular failover, and fixed wireless access solutions. https://dataremote.com