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Managing lines
This section provides information on managing and configuring various types of subscriber lines.
FXS voice subscriber line
A foreign exchange station (FXS) interface uses a standard RJ-11 connector and a telephone cable
to directly connect with an ordinary telephone or a fax machine. An FXS interface accomplishes
signaling exchange based on the level changes on the Tip/Ring line and provides ring, voltage, and
dial tone.
FXO voice subscriber line
A foreign exchange office (FXO) interface uses a RJ-11 connector and a telephone cable to connect
local calls to a PSTN or PBX. Like an FXS interface, an FXO interface accomplishes signaling
exchange based on the level changes on the Tip/Ring line. An FXO interface can be connected only
to an FXS interface.
E&M subscriber line
E&M introduction
An E&M interface uses a RJ-48 telephone cable to connect a PBX. The PBX sends signals on the M
(M represents mouth) line and receives signals on the E (E represents ear) line. The voice router
receives M signals from the PBX and sends E signals to the PBX. An E&M interface can only be
connected to another E&M interface.
When E&M is applied in voice communication, two or four voice wires can be used. Besides, there
are two or four signaling wires. Therefore, 4-wire analog E&M actually has six wires at least. The
2-wire mode provides full duplex voice transmission and voice is transmitted in two directions on the
two wires. The 4-wire mode is equivalent to the simplex mode and every two wires are responsible
for the voice transmission in one direction.
E&M start mode
An E&M interface supports E&M signaling and divides each voice connection into trunk circuit side
and signaling unit side (similar to DCE and DTE).
An E&M interface provides on-hook/off-hook signals and minimizes the interference. Because an
E&M interface does not provide any dial tone, one of the following three signaling technologies is
used to start dialing:
• Immediate start—In this mode, the caller picks up the phone, and after a brief period, the
dialed number is sent to the called side. During this period, whether the called side has been
ready to receive the called number is not checked. After the called information is received, the
callee can pick up the phone to answer the call.
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