FXO and FXS

In modern day usage, "foreign exchange office" (FXO) and "foreign exchange station" (FXS) refer to the different ends of a telephone line in the context of voice over IP (VoIP) systems and its interconnection with analog telephony equipment.

The telecommunication circuit between central offices that implements foreign exchange service has complementary interface types at each end.

The Bell System established a unified set of central offices prefixes after World War II.

The prefixes often still reflected the geography and had value in user's perception of the number, beyond the pure technical function of uniquely identifying the central office.

For instance, a suburban business may want to market extensively to Toronto, a large city with flat-rate local calling: The "FX line" is usually treated as part of the distant city when originating calls to N11-style numbers, such as information or emergency telephone numbers.

[6] A similar "FCO" service provided no difference in local calling area (the distant exchange is in the same rate centre).

Historically, it was a means to obtain features not available on the local exchange (such as DTMF tone dialling when first introduced in 1963)[7] or keep an existing business telephone number operational after a cross-town move.

It generates the off-hook and on-hook indications through loop closure and non-closure of a direct current (DC) circuit powered by the serving central office switch.

An FXS interface utilizes a line protocol, most commonly loop start, to detect when the terminating device (telephone) goes on-hook or off-hook, and can send and receive voice signals.