FXO vs FXS: which interface connects to what?
FXO and FXS describe opposite sides of a traditional analog telephone connection. Confusing them is easy because both may use familiar telephone connectors, but electrically and functionally they do different jobs.
A useful rule is:
- FXS provides a telephone line to an analog endpoint.
- FXO behaves like the analog endpoint and connects to a telephone line.
What is an FXS interface?
Foreign Exchange Station (FXS) is the side that provides line service to a telephone or other analog device. It typically provides functions such as:
- DC line/battery feed;
- ringing voltage;
- dial tone or related call-progress behavior;
- detection of on-hook/off-hook state;
- caller-ID signaling depending on implementation and region.
On an analog PBX or ATA, the port into which you plug a conventional telephone is normally an FXS port.
If the goal is to connect an analog telephone, fax machine or modem-like endpoint to SIP, an FXS gateway/ATA is usually the relevant device.
What is an FXO interface?
Foreign Exchange Office (FXO) is the side that connects to an FXS-provided line. It behaves electrically like the telephone-side device.
An FXO interface therefore needs to handle functions such as:
- line seizure/off-hook;
- ring detection;
- caller-ID reception;
- loop-current behavior;
- call-progress/tone detection;
- analog audio transmit and receive.
If the goal is to connect a PSTN line or analog PBX extension/trunk into a SIP system, the gateway needs FXO ports.
IRIS4000, for example, provides two FXO interfaces specifically for this direction of integration.
FXO/FXS in a SIP migration
Consider an organization replacing an old PBX with Asterisk or another SIP PBX but retaining one legacy analog PSTN line.
The path becomes:
PSTN line (FXS service)
↓
FXO port on gateway
↓
SIP/RTP over IP
↓
Asterisk / SIP PBX
↓
IP phones or applications
The FXO gateway terminates the electrical behavior of the analog line and presents the call to the IP system using SIP.
The reverse problem — connecting a traditional analog telephone to the new SIP PBX — uses an FXS gateway instead.
Why regional analog behavior matters
Unlike SIP signaling, analog telephone behavior is not completely uniform around the world. Deployment can vary in:
- impedance/termination;
- ringing cadence;
- caller-ID signaling method and timing;
- call-progress tones;
- disconnect supervision;
- line-voltage characteristics.
An FXO design therefore needs regional configuration and field validation. A gateway that works with one carrier can require different timing or line settings on another network even though its SIP side is unchanged.
Caller ID timing
Caller ID can be especially sensitive to local signaling rules. Depending on the network, caller-ID information may arrive between ring bursts or through different modulation/DTMF formats.
The gateway must avoid seizing the line too early if doing so would prevent caller-ID reception. This is one reason analog telephony firmware often includes configurable ring-count and caller-ID timing behavior.
Why FXO gateways still exist in an IP world
Even where most new communication systems are IP-based, analog interfaces remain in:
- legacy PSTN lines;
- older PBXs;
- building systems;
- alarm and emergency equipment;
- installed intercoms;
- remote sites where replacement is gradual rather than immediate.
An FXO gateway provides a migration boundary between that installed analog equipment and the IP application.
FXO gateway design considerations
A compact embedded FXO-to-SIP product normally includes:
- analog line-interface circuitry;
- audio conversion;
- ring/caller-ID/call-progress handling;
- a processor or embedded host;
- SIP signaling;
- RTP media;
- provisioning and diagnostics.
The analog line interface and the SIP component solve different problems, which is why a robust design keeps them separated behind clear application logic.