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Collaboration, Voice & AV · Foundational

Foreign Exchange Station (FXS)

An analog interface that supplies dial tone, battery and ringing voltage to connect analog phones, faxes or modems.

Definition

An FXS port acts like the telephone company central office toward an attached analog device: it provides loop current, dial tone and ring voltage and collects dialed digits. FXS ports are found on voice gateways, ISR router voice modules and analog telephone adapters. An FXS port connects to an FXO device such as a phone, never directly to the PSTN.

Example

An ISR 4000 with an NIM-4FXS module connects four lobby analog phones to CUCM via MGCP or SCCP.

Exam tip

Memorize the pairing: FXS supplies service to stations (phones), FXO connects to the office (the PSTN or another FXS port).

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