Analog Signaling (E & M, DID, FXS, FXO)


An FXS interface supplies ring, voltage, and dial tone for basic telephone equipment, keysets, and PBXs. An FXO interface is used for trunk, or tie line, connections to a PSTN CO or to a PBX that does not support E&M signaling (when local telecommunications authority permits). This interface is of value for off-premise station applications.

FXO and FXS interfaces indicate on-hook or off-hook status and the seizure of telephone lines by one of two access signaling methods: loop start or ground start. The type of signaling is determined by the type of service from the CO; standard home telephone lines use loop start, but business telephones can order ground start lines instead.