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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
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
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, itconnects the ring lead to ground. The FXS detects currentflow on the ring lead from battery at the FXS to ground at theFXO. It acknowledges the FXOs request for service byconnecting the tip lead to ground.The FXO detects this connection through its tip ground-detection circuitry. In response, it connects the tip lead to thering lead, which completes the call. Now the circuit looksexactly like a loop-start circuit in the active call state. 16
When Sending A DialToneLoopStart GroundStart 17