Loop start receive signaling

Depending on how the transmitted signaling A bit is set, the line has been placed on-hook or off-hook. Depending on the hook state, the received signaling A bit acts either as a ring signal detector or a loop current indicator. When the line is on-hook, monitoring the A bit tells you if the line is ringing. When the line is off-hook, monitoring the A bit indicates whether there is loop current flowing. The B bit indicates the polarity of tip and ring. If the B bit is set to 1, the loop current direction is reverse. Bits C and D are reserved, and should be ignored.

Regulations require that loop start equipment must function regardless of idle state polarity. The B bit normal state is undefined. The information in the B bit is in the change of state.

The following illustration shows receive signaling for loop start line interfaces:

The following table summarizes the receive signaling for loop start line interfaces:

Bit	Hex bitmask	If line is off-hook	If line is on-hook
A bit	0x08	Detects loop current: 0 = No loop current. 0x08 = Current is flowing.	A bit toggles with ring frequency. Idle state = 0.
B bit	0x04	Loop current direction: 0 = Tip positive with respect to ring. 0x04 = Tip negative with respect to ring.	0
C bit	N/A	Reserved (should be ignored).	Reserved (should be ignored).
D bit	N/A	Reserved (should be ignored).	Reserved (should be ignored).