10. Loop Start
• When the FXO is idle, the two-wire local loop is
open. The tip and ring leads are not connected
to each other. The ring lead connects to a - 48
VDC battery at the FXS side. The tip lead
connects to ground at the FXS side. No current
is flowing in the idle state.
• To seize the circuit, the FXO simply connects
the ring wire to the tip wire by going off-hook.
The FXS side detects current flowing from
battery on the ring lead to ground on the tip
lead. The FXS returns dial tone to the FXO. The
FXO then sends the dialed digits.
If the FXS wants to seize the circuit to deliver
an inbound call, it applies a 90 VAC ring
voltage on top of the DC voltage on the ring
lead. After seeing this ring voltage, the FXO
may then connect the ring lead to the tip lead
to accept the inbound call.
The FXS detects that the FXO has closed the
circuit because current flows from battery to
ground. In response, the FXS removes the ring
voltage and completes the call.
10
13. Glare
• In the US for example, ring voltage is sent for
two seconds and not sent for the next four
seconds. When the FXS signals an inbound call,
the cadence does not always start with the
two-second 'on' phase.
• Consequently, the FXS may think it has seized
the circuit, but the FXO may not detect the
signal for up to four seconds. During this four-
second window, the FXO may attempt to seize
the circuit for an outbound call. If both sides
seize the circuit at the same time, the resulting
condition is called 'glare.'
When the FXO is a telephone, glare is not a big
problem. Often a residential phone user will
take the phone off-hook to place an outbound
call only to find someone already on the
phone. Glare is a problem on a residential
phone only if the end user is trying to avoid the
inbound caller.
When the FXO is a PBX or router trunk link, it
may be shared by many end users at different
times. Glare in this type situation may make a
business seem unprofessional. Inbound callers
will connect to unsuspecting outbound callers
who are not the intended called party.
13
16. Ground Start
When idle, the FXS attaches the ring lead to
battery, but does not attach the tip lead to
ground. The FXO connects the tip lead to a tip
ground detector circuit. During the idle state, the
ring lead in the FXO is not attached to anything.
• If the FXS side wants to seize a ground-start
trunk for inbound call, it connects the tip lead
to ground. The FXO detects this occurrence
through its tip ground detection circuitry. In
response, it connects the tip lead to the ring
lead, which completes the call. Now, the
circuit looks exactly like a loop start circuit in
the active call state.
• The ground-start process is essentially a
physical layer acknowledgment protocol. The
circuit is not seized until both sides say so.
If the FXO wants to seize the circuit for an outbound call, it
connects the ring lead to ground. The FXS detects current
flow on the ring lead from battery at the FXS to ground at the
FXO. It acknowledges the FXO's request for service by
connecting the tip lead to ground.
The FXO detects this connection through its tip ground-
detection circuitry. In response, it connects the tip lead to the
ring lead, which completes the call. Now the circuit looks
exactly like a loop-start circuit in the active call state.
16
