Analysis of FXS, FXO, and E&M

FXS stands for Foreign Exchange Station. It is a voice interface that provides a line-side connection between a digital telephone switching system and a POTS telephone. It emulates a PABX extension interface (or a central office subscriber interface), enabling a standard telephone to connect to a multiplexer. In simple terms, it is the interface that directly connects to ordinary analog telephones, fax machines, and IP phones.
    FXO stands for Foreign Exchange Office. It is a voice interface that provides a trunk-side connection between a central office switch and a digital telephone switching system. Relative to the central office, it emulates a PABX extension, enabling a standard telephone to connect to a multiplexer. In other words, it is the interface that connects directly in analog mode to the program-controlled switch at the telephone exchange.
    FXS refers to Foreign Exchange Station, which is the interface on the Station side; FXO refers to Foreign Exchange Office, which is the interface on the Central Office side. In simple terms, on an analog line, FXS equipment sends ringing voltage to the Station side, while FXO equipment receives ringing voltage from the CO side (e.g., central office switches or commercial switches).

    E&M (RecEive and TransMit) is an analog trunk signaling method primarily used for PBX-to-PBX and PBX-to-central office interconnections. Unlike FXS/FXO, E&M ports interconnect directly, linking two PBXs together, which is commonly referred to as a tie trunk interface.
    Different signaling methods classify and identify control signals differently, and their application scenarios also vary.
    In practical applications, it can be simply understood as:
    FXO is the interface for ordinary telephones, requiring remote power feeding;
    FXS is the PBX internal extension interface, providing remote power feeding;
    E&M is a dedicated interface typically used for PBX trunk lines.

    In actual applications, the PBX conditions are as follows:
    PBX trunk lines require remote power feeding;
    Internal extensions provide power feeding.
    Trunk interfaces may select either ordinary analog trunk boards or E&M interfaces.
    
    Therefore, in usage:
    If the PBX is equipped with an E&M interface board, the E&M interface is the appropriate choice;
    The FXS interface is used to connect telephones, and it can also connect to ordinary PBX trunk lines;
    The FXO interface is used to connect to the PSTN, and it can also connect to PBX internal extensions, with the PBX providing power feeding signals to the FXO interface.
    The above analysis applies to user PBXs; for central office switches, the configuration is even more flexible.