In this type of signaling, Exchange A sends each forward signal to Exchange B until the remote central office (exchange B) recognizes it and acknowledges it with a backward signal (from B to A). The exchange of signals always starts with the sending of a forward signal (signal offer). As soon as Exchange A detects the acknowledge signal, it deactivates the pending forward signal. As soon as Exchange B detects that the forward signal has been dropped, it deactivates the backward signal.