Loop start line interfaces

This topic describes signaling under loop start interfaces.

Loop start transmit signaling

With loop start interfaces, the transmitted signaling A bit in the signaling timeslot causes the interface to seize the line (go off-hook) or release the line (go on-hook).

If the A bit is set to 1, the line goes off-hook. If the A bit is set to 0, the line goes on-hook.

Bits B, C, and D are reserved, and should be set to 0.

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

This table summarizes the transmit signaling for loop start line interfaces:

If you reset the switch, all bits are set to 0.

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: