DS-1 FXS Auto-Ringdown (ARD) Signaling
This mode of operation is referred to as Private Line Auto Ring (PLAR) within the carriers. This is essentially a 2-State signaling scheme, based upon the use of the ‘A-Bit’. An Auto-Ringdown circuit is one where you pick up the telephone, and the telephone at the other end starts ringing immediately.
The transmit ‘A-bit’ is either a 1 (LOOP OPEN) or a 0 (LOOP CLOSED). The received ‘A-bit’ controls ringing (1=NO RINGING, 0=RINGING).