Specifications

Basic Telephony Terminology 191
• Foreign Exchange Office (FXO)
• Ear and Mouth (E&M)
The use of each depends upon the type of equipment and the signaling scheme
adopted. The following sections provide an overview of each type.
C.5 Foreign Exchange Subscriber (FXS)
The FXS interface is used on the switch (both PBX and central office switch) for
connecting a telephone instrument directly to the switch. This two-wire interface
is implemented by circuitry on a line card in the switch. It provides the necessary
battery potential (-48V DC) and also the ringing voltage to the analog telephone.
The signaling scheme is in-band, which means that both the signaling and the
speech path are established using the same pair of wires.
C.6 Foreign Exchange Office (FXO)
This interface derives its name from the application where two switches are
connected together, for example, a PBX to the central office. Since the PBX is
often called a foreign exchange office by the phone company, this was the name
that was given to the interface. In the FXO application, the trunk side of one
switch is connected to the line side of the other. In this case, the PBX would
normally be an FXO device. In fact, most analog Customer Premises Equipment
(CPE) that are connected to the PSTN are normally FXO devices. This includes
such equipment as analog telephones, fax machines, key systems, and in some
cases, PBXs. (PBXs can also use the E&M analog interface as well as digital
interfaces such as T1, E1, and J1.)
This interface does not provide any battery potential or ringing voltage. Instead,
these are provided to the FXO device by the FXS device at the other end of the
transmission line.
When used to attach a PBX or a key system to the Central Office as a trunk
connection, the PBX or key system looks like a normal business telephone line to
the CO switch. The switch can group multiple FXO ports into a hunt group so that
callers from the public network can reach the PBX by dialing a single telephone
number. The CO switch will find and use the first available line into the PBX.
While the FXO trunk costs less than an E&M trunk, it has fewer features. One
important feature that is not available with the FXO trunk is Direct Inward Dialing
(DID). This feature allows a caller to reach extensions of the PBX directly from the
PSTN without going through an attendant. Without this feature, the only phone
numbers that an outside caller can reach are the ones that are assigned to the
FXO lines themselves. This is because, with the FXO trunk, there is no way to
signal to the PBX which extension is desired.
C.6.1 Ear and Mouth (E&M) Interface
E&M is widely used today for interconnecting all types of long-haul telecom
equipment over both satellite and copper and microwave terrestrial links. It uses
separate leads for the signaling and voice paths of the circuit. (This type of
signaling is referred to as out-of-band.) The voice path uses AC coupling while
the signaling uses DC.