FXS vs FXO vs E&M

Gateways use different types of interfaces to connect to analog devices, such as phones,
fax machines, or PBX or public switched telephone network (PSTN) switches. Analog
interfaces used at the gateways include these three types:

FXS:
The FXS interface connects to analog end systems, such as analog phones or
analog faxes, which on their side use the FXO interface. The router FXS interface
behaves like a PSTN or a PBX, serving phones, answering machines, or fax machines
with line power, ring voltage, and dial tones. If a PBX uses an FXO interface, it can also
connect to a router FXS interface. In this case, the PBX acts like a phone.

FXO:
The FXO interface connects to analog systems, such as a PSTN or a PBX, which
on their side use the FXS interface. The router FXO interface behaves like a phone,
getting line power, ring voltage, and dial tones from the other side. As mentioned, a PBX
can also use an FXO interface toward the router (which will then use an FXS interface),
if the PBX takes the role of the phone.

E&M:
The E&M interface provides signaling for analog trunks. Analog trunks
interconnect two PBX-style devices, such as any combination of a gateway (acting as a
PBX), a PBX, and a PSTN switch. E&M is often defined to as "ear and mouth," but it
derives from the term "earth and magneto." "Earth" represents the electrical ground, and
"magneto" represents the electromagnet used to generate tones.
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