ides two signaling states: one when the circuit is opened and one when the circuit is closed. A third signaling state is obtained by reversing the direction or changing the magnitude of the current in the circuit. Combinations of (1) open/close, (2) polarity reversal, and (3) high/low current are used for distinguishing signals intended for one direction of signaling (e.g., dial-pulse signals) from those intended for the opposite direction (e.g., answer signals). We describe the most popular method of supervision on metallic pair trunks below, namely, reverse-battery signaling. 7.9.2 Reverse-Battery Signaling Reverse-battery signaling employs basic methods (1) and (2) just mentioned, and takes its name from the fact that battery and ground are reversed on the tip and ring to change the signal toward the calling end from on-hook to off-hook. Figure 7.10 shows a typical application of reverse-battery signaling in a common-control path. In the idle or on-hook condition, all relays are unoperated and the switch (SW) contacts are open. Upon seizure of the outgoing trunk by the calling switch (exchange) (trunk group selection based on the switch or exchange code dialed by the calling subscriber), the following occur: SW1 and SW2 contacts close, thereby closing loop to called office (exchange) and causing the A relay to operate. Operation of the A relay signals off-hook (connect) indication to the called switch (exchange). Calling Called Calling Office Trunk Called Office Customer's Customer's +/1HII Customer's Customer's Instrument Instrument Originating Terminating Trunk Circuit Trunk Circuit Figure 7.10 Reverse-battery signaling. (From Figure 6-27 of Ref. 7, reprinted with permission.) SIGNALING Upon completion of pulsing between switches, SW3 contacts close and the called subscriber is alerted. When the called subscriber answers, the S2 relay is operated. Operation of the S2 relay operates the T relay, which reverses the voltage polarity on the loop to the calling end. The voltage polarity causes the CS relay to ope